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SDLRC - Scientific Articles all years by Author - Ho+
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Abstract: New evidence from seismic tomography reveals a unique mineral fabric restricted to the thick mantle lithosphere beneath ancient continental cratons, providing an important clue to the formation of these prominent and influential features in Earth’s geological history. Olivine, the dominant mineral of Earth’s upper mantle, has elastic properties that differ along its three crystallographic axes, and preferential alignment of individual olivine grains during plastic deformation can affect the bulk nature of seismic-wave propagation. Surface-wave tomography has shown that over most of Earth, deformation of the mantle lithosphere has oriented olivine crystals with the fast axis in the horizontal plane, but at depths centered at ?150 km within cratonic continental-lithosphere roots, the fast crystallographic axis is preferentially aligned vertically. Because of the high viscosity of the cratonic roots, this fabric is likely to be a vestige from craton formation. Geochemical and petrological studies of upper-mantle garnet-peridotite nodules demonstrate that the cratonic mantle roots are stabilized by their reduced density, which was caused by melt removal at much shallower depths than those from which the nodules were subsequently extracted. The mineral fabric inferred from surface-wave tomography suggests that horizontal shortening carried the depleted zone downward after the melt-depletion event to form the thick continental roots, stretching the depleted material in the vertical dimension by pure shear and causing the fast crystallographic axis to be aligned vertically. This seismological fabric at ?150 km is evidence of the shortening event that created the cratonic roots.
Abstract: In the Archaean, global surface heat flow was substantially higher than today because of greater internal radiogenic heat production and primordial heat content within the Earth. Nonetheless, the lithospheric roots of Archaean cratons were apparently surprisingly cool, recording similarly low ambient temperatures to those inferred today, allowing e.g. for the stabilisation of diamond. This finding is seemingly in conflict with a generally ‘hotter’ Archaean mantle, as is widely postulated, but the paradox could be explained if the sub-cratonic lithospheric mantle was substantially thicker in the Archaean than today. Here, we report a re-investigation of the thermal structure of the Archaean Kalahari lithosphere using published and unpublished petrological data of diamond inclusions indicated to be of Archaean age. Our thermobarometric calculations agree with earlier findings that the Archaean cratonic mantle root was surprisingly cool. Importantly, the shape of the inclusion-derived P-T array deviates from the modern geotherm recorded by peridotite xenoliths. Specifically, diamond inclusions define a systematically steeper geothermal gradient than is observed in cratonic xenoliths. We find that Archaean diamond inclusion and modern xenolith P-T data cannot be reconciled by a single steady-state geotherm. The P-T conditions recorded in diamond inclusions are incompatible with the current characteristically low present-day heat-production of the overlying crust. Instead, the steeper geotherm implies high heat production in the crust during diamond formation and the distinctive geothermal gradient recorded in the studied diamond inclusions could reflect ancient mantle conditions. We modelled a suite of ‘fossil’ geotherms, with increased radiogenic heat production within the crust during the Archaean. Solutions providing very good fits with the diamond inclusion geotherm all require that the Archaean lithosphere must have extended to far greater depths than is preserved today. The required depth ranges from ~ 300 km to ~ 450 km depth, for a modern (~ 1350°C) and a significantly hotter (~ 1600°C) mantle potential temperature, respectively. In either case, it is clear that the Kalahari lithosphere must have experienced significant (at least 100 km) basal erosion subsequent to its formation.
Abstract: It is well established that the cratonic subcontinental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond ?20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene; however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic, and chemical data for an exceptionally well preserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogeneously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are subcalcic Cr-pyrope, and the orthopyroxene host is high-Mg enstatite; these phases equilibrated at 4.4 GPa and 975 °C. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of ?6 GPa and 1750 °C. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most subcalcic garnets in depleted peridotites.
Abstract: It is now well established that the cratonic sub-continental lithospheric mantle (SCLM) represents a residue of extensively melted fertile peridotite. The widespread occurrence of garnet in the Archaean SCLM remains a paradox because many experiments agree that garnet is exhausted beyond c. 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene [1,2,3]. However, the few examples of putative garnet exsolution in cratonic samples remain exotic and have not afforded a link to garnet that occurs as distinct grains in granular harzburgite. We present crystallographic (EBSD), petrographic and chemical (SEM-EDS and LA-ICP-MS) data for an exceptionally well-preserved orthopyroxene megacryst juxtaposed against granular harzburgite. Garnet lamellae within the megacryst show crystallographic continuity and have a strong fabric relative to the host orthopyroxene, strongly indicating that the megacryst formed by exsolution. Garnet lamellae are sub-calcic Cr-pyropes with sinusoidal rare earth element patterns, while the orthopyroxene host is high-Mg enstatite; the reconstructed precursor is clinoestatite. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. The compositions of the exsolved Cr pyrope and enstatite are typical of harzburgites and depleted lherzolites from the SCLM. Furthermore, garnet inclusions within orthopyroxene in several granular peridotites exhibit the same fabric as those in the exsolved megacryst. We hypothesise that clinoenstatite was a common phase in cratonic SCLM and that exsolution is the likely origin of many sub-calcic garnets in depleted peridotites.
Formation of platiniferous sulfide horizons by crystal fractionation and magma mixing in the Munni Munni layered intrusion, west Pilbara block, WesternAustralia
Economic Geology, Vol. 84, No. 7, November pp. 1775-1804
Use of geochemistry as a guide to platinum group element potential of mafic ultramafic rocks- examples the West Pilbara block and Halls Creek Mobile Zone:
Precambrian Research, Vol. 50, No. 102, April pp. 1-35
Burgess, R., Cartigny, P., Harrison, D., Hobson, E., Harris, J.
Volatile composition of Micro inclusions in diamonds from the PAnd a kimberlite, Canada: implications for chemical and isotopic heterogeneity in the mantle.
Geochimica et Cosmochimica Acta, Vol. 73, 6, pp. 1779-1794.
Abstract: We propose to use high-purity lab-grown diamond for the detection of sub-GeV dark matter. Diamond targets can be sensitive to both nuclear and electron recoils from dark matter scattering in the MeV and above mass range, as well as to absorption processes of dark matter with masses between sub-eV to 10's of eV. Compared to other proposed semiconducting targets such as germanium and silicon, diamond detectors can probe lower dark matter masses via nuclear recoils due to the lightness of the carbon nucleus. The expected reach for electron recoils is comparable to that of germanium and silicon, with the advantage that dark counts are expected to be under better control. Via absorption processes, unconstrained QCD axion parameter space can be successfully probed in diamond for masses of order 10 eV, further demonstrating the power of our approach.
Abstract: Mining activities over several millennia have resulted in a legacy of environmental contamination that must be mitigated to minimize ecosystem damage and human health impacts. Designing effective remediation strategies for mining and processing wastes requires knowledge of nano- and molecular-scale speciation of contaminants. Here, we discuss how modern nano- and molecular-level concepts and methods can be used to improve risk assessment and future management of contaminants that result from mining activities, and we illustrate this approach using relevant case studies.
A semi-quantitative model for fractionation of rhyolite from rhyodacite in a compositionally altered Archean volcanic complex, Superior Province, Canada
Precambrian Research, Vol. 50, No. 1-2. April pp. 49-67
Abstract: Reconstructions of past ice-flow provide useful insights into the long-term behaviour of past ice sheets and help to understand how glaciated landscapes are shaped. Here, we present reconstruction of a 10-phase ice-flow history from southwestern Hudson Bay in northeastern Manitoba (Canada), a dynamic region situated between two major ice dispersal centres of the Laurentide Ice Sheet. We utilize a diverse geologic dataset including 1900 field-based erosional indicators, 12 streamlined-landform flowsets, esker and meltwater corridor orientations, 103 till-fabrics analyses, and 1344 till-clast lithology counts. Our reconstruction suggests that both pre-MIS 2 and MIS 2 glaciations followed similar growth patterns, where ice advanced into study area from ice centered to the east (probably in northern Quebec), followed by a switch in ice-flow direction indicating flow from the Keewatin ice centre to the northwest and north. The cause for this switch in ice-flow orientation is uncertain, but the youngest switch may relate to retreat of ice during MIS 3 that then left space for Keewatin-sourced ice to advance over the study area. While modelling experiments indicate widespread cold-based conditions in the study area during the last glacial cycle, uniformly relict landscapes are not common. Instead, the glaciated landscape is palimpsest and commonly fragmented, forming a subglacial bed mosaic of erosional and depositional assemblages that record both shifting ice-flow direction through time and shifting subglacial conditions. Each assemblage formed, or modified, during times of dynamic (warm-based) ice, and later preserved under conditions below or close to the pressure melting point (slow and sluggish, or cold-based).
Abstract: Canada exhibits many of the challenges involved with exploring for coloured stones in countries with very low population densities, temperate-to-arctic climates and a lack of infrastructure hindering access to most prospective areas. Despite this, a number of discoveries have occurred, mainly during the past two decades. These include emeralds from Northwest Territories (1997) and Yukon (1998); sapphire (2002) and spinel (from 1982)—including cobalt-blue stones—from Baffin Island in Nunavut; and ruby and pink sapphire (2002) from British Columbia. Such discoveries can be assisted by undertaking scientific research into gem formation, as well as by applying exploration criteria developed elsewhere to uncharted territory. Future exploration in Canada and other countries facing similar challenges will likely benefit from additional geological studies to identify prospective areas and features; innovative means of transportation, such as boats instead of aircraft; drones for exploring rugged terrain; hyperspectral imaging for mineral sensing; surveying with UV lamps to identify minerals associated with gem mineralisation; and careful prospecting (including field mapping and collecting heavy mineral concentrates) by experienced individuals. Quaternary geology fieldwork was conducted at a reconnaissance-scale in the Kaskattama highland area to document the Quaternary stratigraphy and till composition. The diamond potential of this region was investigated using kimberlite-indicator-mineral (KIM) counts from till samples. Indicator mineral results are the focus of this report and are combined with ice-flow and till-clast-lithology data to provide a context to interpret provenance. Kimberlite-indicator minerals were recovered from glacial sediments (till) in the Kaskattama highland area and KIM counts are elevated relative to data from the surrounding area. The lowest KIM counts were from till with a high Hudson Bay Basin (carbonatedominated) and low undifferentiated greenstone and greywacke (UGG) provenance signature. The highest KIM counts are associated with till samples that have a relatively elevated UGG or elevated granitoid provenance signature. Till samples with relatively elevated UGG concentration have an interpreted east or southeast provenance, which is supported by ice-flow data and the recovery of distinct east-sourced erratics. Till samples with a relatively elevated granitoid clast concentration have a correlation with the southwest- trending Hayes streamlined-landform flowset. Considering the likely provenance for granitoid clasts is to the northwest, the presence of relatively high concentrations of granitoid clasts in the Hayes flowset could be indicative of a higher inheritance from previous ice-flow events or a palimpsest dispersal pattern. Interpretation of till-composition and ice-flow data has indicated there are likely multiple sources for the KIMs recovered during this study. Detailed work is recommended to clarify local-scale dispersal patterns.
Abstract: The goal of this work is to determine the effect of multiple glaciations on till composition, in a zone of transition from a multi-till stratigraphy within the Hudson Bay Lowland (HBL) to a single till stratigraphy over the Precambrian shield. The study area, in NE Manitoba, has access to numerous sections that expose multiple tills, in addition to interglacial and postglacial sediments. Sequences of thick till are not easily separated into different units, despite previous field attempts to define four named tills. The compositional transition to thin till overlying the Precambrian Shield in the west is also not well understood. Yet, the two different settings were affected by the same 3+ glacial cycles. The wide range in eastern- and/or northeastern-sourced calcareous clast concentrations, and ‘locally’-sourced shield clast concentrations, combined with variable concentrations of northern-sourced clasts, suggests that the tills of northeastern Manitoba are ‘provenance’ hybrids. Local tills result from the net effect of multiple glacial processes that underwent spatiotemporal variability. Mixed provenance applies not only to surface tills, but to the subsurface tills as well. Preliminary results suggest that carbonate transport across the shield was continuous throughout several glacial cycles, but the bulk of transport likely occurred prior to the most recent glacial cycle. Current work has established a northern-Manitoba ice-flow history using the erosional and depositional record, which encompasses 5 to 7 phases. This new compilation is used in conjunction with ‘till-clast’ stratigraphy and ‘till-geochemistry’ stratigraphy, to identify a new provenance framework for tills in northeastern Manitoba.
Geostandards and Geoanalysis Research, doi.org/10.1111/GGR.12419 34p. Pdf
Australia
geochemistry
Abstract: To promote a more efficient and transparent geochemistry data ecosystem, a consortium of Australian university research laboratories called the AuScope Geochemistry Network (AGN) assembled to build a collaborative platform for the express purpose of preserving, disseminating, and collating geochronology and isotopic data. In partnership with geoscience-data-solutions company Lithodat Pty Ltd, the open, cloud-based AusGeochem platform (https://ausgeochem.auscope.org.au) was developed to simultaneously serve as a geosample registry, a geochemical data repository, and a data analysis tool. Informed by method-specific groups of geochemistry experts and established international data reporting practices, community-agreed database schemas were developed for rock and mineral geosample metadata and secondary ion mass spectrometry U-Pb analysis, with additional models for laser ablation inductively-coupled mass spectrometry U-Pb and Lu-Hf, Ar-Ar, fission-track and (U-Th-Sm)/He under development. Collectively, the AusGeochem platform provides the geochemistry community with a new, dynamic resource to help facilitate FAIR (Findable, Accessible, Interoperable, Reusable) data management, streamline data dissemination and advanced quantitative investigations of Earth system processes. By systematically archiving detailed geochemical (meta-)data in structured schemas, intractably large datasets comprising thousands of analyses produced by numerous laboratories can be readily interrogated in novel and powerful ways. These include rapid derivation of inter-data relationships, facilitating on-the-fly data compilation, analysis, and visualisation.
Multistage exhumation and juxaposition of lower continental crust in the western Canadian Shield: linking high resolution U Pb and 40 Ar / 39 Ar thermochronometry with pressure temperature deformation paths.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Abstract: Establishing relationships between the long-term landscape evolution of drainage basins and the fill of sedimentary basins benefits from analysis of bedrock river terrace deposits. These fragmented detrital archives help to constrain changes in river system character and provenance during sediment transfer from continents (source) to oceans (sink). Thick diamondiferous gravel terrace deposits along the lower Orange River, southern Namibia, provide a rare opportunity to investigate controls on the incision history of a continental-scale bedrock river. Clast assemblage and heavy mineral data from seven localities permit detailed characterisation of the lower Orange River gravel terrace deposits. Two distinct fining-upward gravel terrace deposits are recognised, primarily based on mapped stratigraphic relationships (cross-cutting relationships) and strath and terrace top elevations, and secondarily on the proportion of exotic clasts, referred to as Proto Orange River deposits and Meso Orange River deposits. The older early to middle Miocene Proto Orange River gravels are thick (up to 50 m) and characterised by a dominance of Karoo Supergroup shale and sandstone clasts, whereas the younger Plio-Pleistocene Meso Orange River gravels (6-23 m thick) are characterised by more banded iron formation clasts. Mapping of the downstepping terraces indicates that the Proto gravels were deposited by a higher sinuosity river, and are strongly discordant to the modern Orange River course, whereas the Meso deposits were deposited by a lower sinuosity river. The heavy minerals present in both units comprise magnetite, garnet, amphibole, epidote and ilmenite, with rare titanite and zircon grains. The concentration of amphibole-epidote in the heavy minerals fraction increases from the Proto to the Meso deposits. The decrease in incision depths, recorded by deposit thicknesses above strath terraces, and the differences in clast character (size and roundness) and type between the two units, are ascribed to a more powerful river system during Proto-Orange River time, rather than reworking of older deposits, changes in provenance or climatic variations. In addition, from Proto- to Meso-Orange River times there was an increase in the proportion of sediments supplied from local bedrock sources, including amphibole-epidote in the heavy mineral assemblages derived from the Namaqua Metamorphic Complex. This integrated study demonstrates that clast assemblages are not a proxy for the character of the matrix, and vice versa, because they are influenced by the interplay of different controls. Therefore, an integrated approach is needed to improve prediction of placer mineral deposits in river gravels, and their distribution in coeval deposits downstream.
The stability of priderite, lindsleyite-mathiasite andyimengite-hawthornite under lower continental lithosphere conditions:experiments at 35 to 50 Kbar
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 106-108
Geochemistry International, Vol. 56, 8, pp. 766-783.
Russia, Siberia
carbonatite
Abstract: In this study we present a fresh isotopic data, as well as U-Pb ages from different REE-minerals in carbonatites and phoscorites of Guli massif using in situ LA-ICPMS technique. The analyses were conducted on apatites and perovskites from calcio-carbonatite and phoscorite units, as well as on pyrochlores and baddeleyites from the carbonatites. The 87Sr/86Sr ratios obtained from apatites and perovskites from the phoscorites are 0.70308-0.70314 and 0.70306-0.70313, respectively; and 0.70310-0.70325 and 0.70314-0.70327, for the pyrochlores and apatites from the carbonatites, respectively. Furthermore, the in situ laser ablation analyses of apatites and perovskites from the phoscorite yield ?Nd from 3.6 (±1) to 5.1 (±0.5) and from 3.8 (±0.5) to 4.9 (±0.5), respectively; ?Nd of apatites, perovskites and pyrochlores from carbonatite ranges from 3.2 (±0.7) to 4.9 (±0.9), 3.9 (±0.6) to 4.5 (±0.8) and 3.2 (±0.4) to 4.4 (±0.8), respectively. Laser ablation analyses of baddeleyites yielded an eHf(t)d of +8.5 (± 0.18); prior to this study Hf isotopic characteristic of Guli massif was not known. Our new in situ ?Nd, 87Sr/86Sr and eHf data on minerals in the Guli carbonatites imply a depleted source with a long time integrated high Lu/Hf, Sm/Nd, Sr/Rb ratios. In situ U-Pb age determination was performed on perovskites from the carbonatites and phoscorites and also on pyrochlores and baddeleyites from carbonatites. The co-existing pyrochlores, perovskites and baddeleyites in carbonatites yielded ages of 252.3 ± 1.9, 252.5 ± 1.5 and 250.8 ± 1.4 Ma, respectively. The perovskites from the phoscorites yielded an age of 253.8 ± 1.9 Ma. The obtained age for Guli carbonatites and phoscorites lies within the range of ages previously reported for the Siberian Flood Basalts and suggest essentially synchronous emplacement with the Permian-Triassic boundary.
Geochronological and geochemical constraints on the formation and evolution of the mantle beneath the Kaapvaal craton: Lu Hf and Sm Nd systematics of subcalcic garnets from highly depleted peridotites.
Geochimica et Cosmochimica Acta, Vol. 113, pp. 1-20.
Mantle eclogites and garnet pyroxenites - the meaning of two point isochrons, Sm-Nd and Lu-Hf closure temperatures and the cooling of the subcratonic mantle.
Earth and Planetary Science Letters, Vol. 389, pp. 143-154.
Contributions to Mineralogy and Petrology, Vol. 171, 11, 24p.
Africa, South Africa
Deposit - Bellsbank
Abstract: An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and ? 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4 6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.
Abstract: An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and ?18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.
Genesis of ultramafic lamprophyres and carbonatites at Aillik Bay, Labrador: a consequence of incipient lithospheric thinning beneath the North Atlantic Craton
Abstract: In the early 1990s several rebel groups turned to natural resource extraction to pay for war. A key form of this is rebel diamond production, commonly referred to as conflict diamonds, which is widely perceived as being highly beneficial to insurgent organisations. Yet in the Angolan Civil War (1992-2002), the use of conflict diamonds by the National Union for the Total Independence of Angola (UNITA) resulted in a decisive insurgent defeat. How can this outcome be explained? Offering a nuanced understanding of how conflict diamonds affect civil war, this article shows that although diamonds generated considerable revenue for UNITA, they were not an effective method for them to take on the Angolan government. This was for two reasons: internally, the rebels greatly struggled to convert their diamond proceeds into sufficient goods and services; and externally, it left the group highly vulnerable to international countermeasures in the form of United Nations Security Council sanctions. Natural resource extraction may therefore not be as useful to rebel groups as is frequently believed.
Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr Nd, Pb and O isotope geochemistry of quartz diorites, Damara Orogen
Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr Nd Pb O isotope geochemistry of diorites from Damara Origin.
Contributions to Mineralogy and Petrology, Vol. 143, 5, pp.551-66.
Portnyagin, M., Hoernie, K., Plechov, P., Mironov, N., Khubunaya, S.
Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles ( H2) S Cl F) and trace elements in melt inclusions from the Kamchatka Arc.
Earth and Planetary Science Letters, Vol. 255, 1-2, pp. 53-69.
Abstract: Oceanic basalts reflect the heterogeneities in the earth's mantle, which can be explained by five mantle end members. The HIMU end member, characterized by high time-integrated ? (238U/204Pb), is defined by the composition of lavas from the ocean islands of St. Helena, South Atlantic Ocean and Mangaia and Tubuai (Cook-Austral Islands), South Pacific Ocean. It is widely considered to be derived from a mantle reservoir that is rarely sampled and not generally involved in mixing with the other mantle components. On the other hand, the FOZO end member, located at the FOcal ZOne of oceanic volcanic rock arrays on isotope diagrams, is considered to be a widespread common component with slightly less radiogenic 206Pb/204Pb and intermediate Sr-Nd-Hf isotopic compositions. Here we present new major and trace element, Sr-Nd-Pb-Hf isotope and geochronological data from the Walvis Ridge and Richardson Seamount in the South Atlantic Ocean and the Manihiki Plateau and Eastern Chatham Rise in the southwest Pacific Ocean. Our new data, combined with literature data, document a more widespread (nearly global) distribution of the HIMU end member than previously postulated. Our survey shows that HIMU is generally associated with low-volume alkaline, carbonatitic and/or kimberlitic intraplate volcanism, consistent with derivation from low degrees of melting of CO2-rich sources. The majority of end member HIMU locations can be directly related to hotspot settings. The restricted trace element and isotopic composition (St. Helena type HIMU), but near-global distribution, point to a deep-seated, widespread reservoir, which most likely formed in the Archean. In this context we re-evaluate the origin of a widespread HIMU reservoir in an Archean geodynamic setting. We point out that the classic ocean crust recycling model cannot be applied in a plume-lid dominated tectonic setting, and instead propose that delamination of carbonatite-metasomatized subcontinental lithospheric mantle could be a suitable HIMU source.
Abstract: The Rae craton is an important part of the Canadian Shield and was amalgamated to the Slave craton at ?? 1.9 Ga [1]. Recent geophysical and geochemical data indicate a protracted geodynamic history [1, 2]. Even though the oxidation state of the Earth’s mantle has an important influence of fluid compositions and melting behavior, no data on the oxidation state of the Rae’s mantle are available. The aims of this study were to 1) determine the oxidation state (ƒO2) of the lithosphere beneath the Rae craton, 2) link these results to potential metasomatic overprints and 3) compare the geochemical evolution with the Slave craton. We studied 5 peridotite xenoliths from Pelly Bay (central craton) and 22 peridotites from Somerset Island (craton margin). Pelly Bay peridotites give T < 905°C and depths of ??80- 130 km. Garnets have depleted or “normal” REE patterns, the latter samples recording fO2 values ??0.5 log units higher. The deeper samples are more enriched and oxidised. Peridotites from Somerset Island record T ??825-1190°C, a ?logfO2 ranging from ?? FMQ - FMQ-3.6 from a depth interval of ??100-150 km. Garnets exhibit two REE signatures - sinusoidal and “normal” - indicating an evolutionary sequence of increasing metasomatic re-enrichment and a shift from fluid to melt dominated metasomatism. Compared to the Slave craton, the Rae mantle is more reduced at ??80km but becomes up to 2 log units more oxidised (up to ??FMQ-1) at ??100-130 km. Similar oxidising conditions can be found >140 km in the Slave mantle [3]. Especially under Somerset Island, the lithospheric mantle has contrasting fO2 and metasomatic overprints in the same depth range, which may represent juxtaposed old and rejuvenated domains [2].
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Girnis, A.V., Bulatov, V.K., Brey, G.P., Gerdes, A., Hofer, H.E.
Trace element partitioning between mantle minerals and silico-carbonate melts at 6-12 Gpa and applications to mantle metasomatism and kimberlite genesis.
The lithospheric mantle underneath the Gibeon kimberlite field ( Namibia): a mix of old and young components - evidence from Lu-Hf and Sm-Nd isotope systematics.
Contributions to Mineralogy and Petrology, Vol. 170, 25p.
Technology
Experimental petrology
Abstract: Melting of carbonated sediment in the presence of graphite or diamond was experimentally investigated at 7.5–12 GPa and 800–1600 °C in a multianvil apparatus. Two starting materials similar to GLOSS of Plank and Langmuir (Chem Geol 145:325–394, 1998) were prepared from oxides, carbonates, hydroxides and graphite. One mixture (Na-gloss) was identical in major element composition to GLOSS, and the other was poorer in Na and richer in K (K-gloss). Both starting mixtures contained ~6 wt% CO2 and 7 wt% H2O and were doped at a ~100 ppm level with a number of trace elements, including REE, LILE and HFSE. The near-solidus mineral assemblage contained a silica polymorph (coesite or stishovite), garnet, kyanite, clinopyroxene, carbonates (aragonite and magnesite-siderite solid solution), zircon, rutile, bearthite and hydrous phases (phengite and lawsonite at <9 GPa and the hydrous aluminosilicates topaz-OH and phase egg at >10 GPa). Hydrous phases disappear at ~900 °C, and carbonates persist up to 1000-1100 °C. At temperatures >1200 °C, the mineral assemblage consists of coesite or stishovite, kyanite and garnet. Clinopyroxene stability depends strongly on the Na content in the starting mixture; it remains in the Na-gloss composition up to 1600 °C at 12 GPa, but was not observed in K-gloss experiments above 1200 °C. The composition of melt or fluid changes gradually with increasing temperature from hydrous carbonate-rich (<10 wt% SiO2) at 800-1000 °C to volatile-rich silicate liquids (up to 40 wt% SiO2) at high temperatures. Trace elements were analyzed in melts and crystalline phases by LA ICP MS. The garnet-melt and clinopyroxene-melt partition coefficients are in general consistent with results from the literature for volatile-free systems and silicocarbonate melts derived by melting carbonated peridotites. Most trace elements are strongly incompatible in kyanite and silica polymorphs (D < 0.01), except for V, Cr and Ni, which are slightly compatible in kyanite (D > 1). Aragonite and Fe-Mg carbonate have very different REE partition coefficients (D Mst-Sd/L ~ 0.01 and D Arg/L ~ 1). Nb, Ta, Zr and Hf are strongly incompatible in both carbonates. The bearthite/melt partition coefficients are very high for LREE (>10) and decrease to ~1 for HREE. All HFSE are strongly incompatible in bearthite. In contrast, Ta, Nb, Zr and Hf are moderately to strongly compatible in ZrSiO4 and TiO2 phases. Based on the obtained partition coefficients, the composition of a mobile phase derived by sediment melting in deep subduction zones was calculated. This phase is strongly enriched in incompatible elements and displays a pronounced negative Ta-Nb anomaly but no Zr-Hf anomaly. Although all experiments were conducted in the diamond stability field, only graphite was observed in low-temperature experiments. Spontaneous diamond nucleation and the complete transformation of graphite to diamond were observed at temperatures above 1200-1300 °C. We speculate that the observed character of graphite-diamond transformation is controlled by relationships between the kinetics of metastable graphite dissolution and diamond nucleation in a hydrous silicocarbonate melt that is oversaturated in C.
Abstract: A comprehensive petrographic and in situ major and trace element study of rutile, ilmenite and Ti-magnetite was undertaken in six lower crustal xenoliths of metabasaltic (?underplate) and metasedimentary (subduction) origin from the Diavik kimberlites (central Slave Craton, Canada). The aims of the study were to improve our understanding of trace element incorporation into these Ti-minerals, and to use these systematics to obtain insights into lower continental crust formation and evolution. Abundant (oxy)exsolution of titanomagnetite lamellae, blocky rutile, as well as minor pleonaste and zircon in ilmenite from metabasaltic granulites are proposed to reflect cooling from magmatic or metamorphic temperatures and subsequent secular mantle cooling. This explains the large spread in Zr-in-rutile temperatures (>200°C) and may partly be responsible for the substantial heterogeneity of other trace element concentrations in rutile and ilmenite. Even after accounting for trace element heterogeneity and modal uncertainties, mass-balance calculations indicate that both Ti and Nb in lower crustal granulites are largely controlled by rutile and ilmenite. Rutile U-Pb data define discordia arrays that yield upper intercept ages broadly coincident with the 1•27 Ga giant Mackenzie dike swarm event, suggesting reheating of the lower crust above the rutile U-Pb closure temperature, whereas lower intercept ages roughly correspond to the age of Cretaceous to Eocene kimberlite magmatism. Subsequent cooling led to partial resetting and data spread along the concordia. Closer inspection reveals that inter-grain concentrations of elements that are compatible in rutile (Nb, Ta, W, U), but highly incompatible in the abundant silicate minerals (in equilibrium with melt), are heterogeneous and contrast with the more homogeneous concentrations of the transition metals (NiO, V). This may indicate that local reaction partners for diffusive homogenization of these element concentrations were absent. Nb/Ta is also highly variable at the sample scale. This may be explained by prograde growth from high-Nb/Ta mineral precursors (e.g. biotite) in the metasedimentary granulites and crystallization of the protoliths to the metabasaltic granulites from a mafic magma that had experienced fractionation of ilmenite with low Nb/Ta in a crustal magma chamber. Thus, (Fe)-Ti minerals represent high field strength element ‘islands’ in the granulite silicate matrix. The lack of homogenization and persistence of high-energy grain boundaries, such as exsolution lamellae, further indicate that the lower continental crust remained essentially dry and did not recrystallize, possibly since Neoarchaean metamorphism.
Abstract: The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ?100-170?km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150?Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ?90-110?km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550?Ma extended to >210?km depth, and suggests loss of ?40?km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (?90-110?km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.
Abstract: Diamond nucleation and growth were investigated experimentally at 7.5 and 10.5?GPa and temperatures up to 1500?°C. Samples consisted of two layers: i) H2O- and CO2-bearing model sediment and ii) graphite-bearing garnet harzburgite comprising natural minerals. Two experimental series were conducted, one under a controlled temperature gradient with the sedimentary layer usually in the cold zone and the other under isothermal conditions. In the latter case, diamond seeds were added to the sedimentary mixture. During the experiments, the sedimentary layer partially or completely melted, with the melt percolating and interacting with the adjacent harzburgite. The graphite-to-diamond transition in the peridotite was observed above 1300?°C at 7.5?GPa and 1200?°C at 10.5?GPa in the temperature-gradient experiments, and at temperatures ~100?°C lower in the isothermal experiments with diamond seeds. Newly formed diamond occurs mostly as individual grains up to 10??m in size and is separate from graphite aggregates. In some cases, an association of diamond with magnesite was observed. Diamond nucleation occurs in hydrous and CO2-bearing silicate melt following graphite dissolution and recrystallization. In the case of the diamond-magnesite association, diamond was probably formed through carbonate reduction coupled with graphite oxidation. The composition of the melts ranged from “carbonatitic” with ~10?wt% SiO2 and?>?50?wt% volatiles to hydrous silicate with ~40?wt% SiO2 and?
Mineralogy and Petrology, in press available, 10p.
South America, Brazil, Juina
deposit - Sao Luiz
Abstract: The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg?+?Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a “D” layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from ?7Li?+?9.6 ‰ to ?3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than ‘primitive upper mantle’ Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This mélange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.
Abstract: Bimineralic eclogites and pyroxenites (n?=?75?±?accessory rutile, ilmenite, sulphide, apatite) from the Koidu kimberlite (West African Craton) were investigated for mineral major and trace elements and mineral Sr-Nd isotope compositions to constrain (1) the nature and age of their basaltic to picritic protoliths, and (2) the effect, timing and source of mantle metasomatism. Consistent with published work, samples are grouped into low-Mg eclogites with Mg# from 0•49 to 0•73 (median 0•59; n?=?40) and high-Mg eclogites with Mg# from 0•60 to 0•88 (median 0•75; n?=?14), plus pyroxenites [clinopyroxene Na/(Na + Ca) <0•2; n?=?8] and gabbroic eclogites and pyroxenite (Eu/Eu* of reconstructed bulk-rocks >1•05; n?=?8), with five unclassifiable samples. Reconstructed low-Mg and gabbroic eclogites have major and trace element systematics (Eu/Eu*-heavy rare earth elements-Y) indicating crustal protolith crystallisation, confirming an origin as subducted oceanic crust. Their high FeO contents at MgO >?10?wt % require an Fe-rich source, the high melt productivity of which led to the formation of thicker crust, perhaps in a plateau-like setting. This is consistent with SiO2-MgO relationships indicating differentiation at ?0•5?GPa. Unradiogenic Sr in some clinopyroxene (87Sr/86Sr of 0•7010-0•7015), combined with light rare earth element (LREE) depletion relative to normal mid-ocean ridge basalt (N-MORB) for the majority of samples (average N-MORB-normalised Nd/Yb of unmetasomatised samples = 0•51), suggests eclogitisation and partial melt loss in the Neoarchaean, possibly coeval with and parental to 2•7?Ga overlying continental crust. Most reconstructed high-Mg eclogites and some pyroxenites formed by metasomatic overprinting of low-Mg eclogites and gabbroic eclogites, as indicated by the preservation of positive Eu anomalies in some samples, and by the Mg-poorer composition of included versus matrix minerals. Coupled enrichment in MgO, SiO2 and Cr2O3 and in incompatible elements (Sr, LREE, Pb, Th and U) is ascribed to metasomatism by a kimberlite-like, small-volume, carbonated ultramafic melt, mediated by addition of clinopyroxene from the melt (i.e. stealth metasomatism). Strontium-Nd isotope systematics suggest a Neoproterozoic age for this metasomatic event, possibly linked to Rodinia break-up, which facilitated intrusion of asthenospheric carbonated melts with an ocean island basalt-like 87Sr/86Sri of ?0•7035. Cretaceous kimberlite magmatism (including Koidu), with more radiogenic 87Sr/86Sr (?0•7065, intermediate between Kaapvaal kimberlites and orangeites), may have been partially sourced from associated Neoproterozoic metasomes. The presence of diamonds in low-Mg eclogites, but absence in high-Mg eclogites, indicates the diamond-destructive nature of this event. Nevertheless, the moderate proportion of affected eclogites (?35%) suggests preservation of a sizeable diamond-friendly mantle eclogite reservoir beneath Koidu.
Nature Scientific Reports, 10.1038/s41598-021-83261-6 11p. Pdf
Canada, Northwest Territories
metasomatism
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
International Journal of Earth Science, Vol. 111, 2, 16p.
Europe, Germany
carbonatite
Abstract: The accessories perovskite, pyrochlore, zirconolite, calzirtite and melanite from carbonatites and carbonate-rich foidites from the Kaiserstuhl are variously suited for the in situ determination of their U-Pb ages and Sr, Nd- and Hf-isotope ratios by LA-ICP-MS. The 143Nd/144Nd ratios may be determined precisely in all five phases, the 176Hf/177Hf ratios only in calzirtite and the 87Sr/86Sr ratios in perovskites and pyrochlores. The carbonatites and carbonate-rich foidites belong to one of the three magmatic groups that Schleicher et al. (1990) distinguished in the Kaiserstuhl on the basis of their Sr, Nd and Pb isotope ratios. Tephrites, phonolites and essexites (nepheline monzogabbros) form the second and limburgites (nepheline basanites) and olivine nephelinites the third. Our 87Sr/86Sr isotope data from the accessories overlap with the carbonatite and olivine nephelinite fields defined by Schleicher et al. (1990) but exhibit a much narrower range. These and the ?Nd and ?Hf values plot along the mantle array in the field of oceanic island basalts relatively close to mid-ocean ridge basalts. Previously reported K-Ar, Ar-Ar and fission track ages for the Kaiserstuhl lie between 16.2 and 17.8 Ma. They stem entirely from the geologically older tephrites, phonolites and essexites. No ages existed so far for the geologically younger carbonatites and carbonate-rich foidites except for one apatite fission track age (15.8 Ma). We obtained precise U-Pb ages for zirconolites and calzirtites of 15.66, respectively 15.5 Ma (±?0.1 2?) and for pyrochlores of 15.35?±?0.24 Ma. Only the perovskites from the Badberg soevite yielded a U-P concordia age of 14.56?±?0.86 Ma while the perovskites from bergalites (haüyne melilitites) only gave 206Pb/238U and 208Pb/232Th ages of 15.26?±?0.21, respectively, 15.28?±?0.48 Ma. The main Kaiserstuhl rock types were emplaced over a time span of 1.6 Ma almost 1 million years before the carbonatites and carbonate-rich foidites. These were emplaced within only 0.32 Ma.
Geophysical Research abstracts, EGU, EGU2019-9348, 1p. Pdf
Canada
geodynamics
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Contributions to Mineralogy and Petrology, Vol. 171, 45p.
Mantle
Magmatism
Abstract: The origin and source rocks of alkali-rich and SiO2-undersatured magmas in the Earth’s upper mantle are still under debate. The garnet signature in rare earth element patterns of such magmas suggests a garnet-bearing source rock, which could be garnet lherzolite or garnet pyroxenite. Partial melting experiments were performed at 2.8 GPa and 1345-1445 °C in a piston-cylinder using mixtures of natural lherzolite with either 0.4 wt% H2O and 0.4 wt% CO2 or 0.7 wt% H2O and 0.7 wt% CO2. Different designs of AuPd capsules were used for melt extraction. The most successful design included a pentagonally shaped disc placed in the top part of the capsule for sufficient melt extraction. The degrees of partial melting range from 0.2 to 0.04 and decrease with decreasing temperature and volatile content. All samples contain olivine and orthopyroxene. The amounts of garnet and clinopyroxene decrease with increasing degree of partial melting until both minerals disappear from the residue. Depending on the capsule design, the melts quenched to a mixture of quench crystals and residual glass or to glass, allowing measurement of the volatile concentrations by Raman spectroscopy. The compositions of the partial melts range from basalts through picrobasalts to foidites. Compared to literature data for melting of dry lherzolites, the presence of H2O and CO2 reduces the SiO2 concentration and increases the MgO concentration of partial melts, but it has no observable effect on the enrichment of Na2O in the partial melts. The partial melts have compositions similar to natural melilitites from intraplate settings, which shows that SiO2-undersaturated intraplate magmas can be generated by melting of garnet lherzolite in the Earth’s upper mantle in the presence of H2O and CO2.
28th. De Beers Alex du Toit memorial lecture, 2004. On cryogenian (Neoproterozoic) ice sheet dynamics and the limitations of the glacial sedimentary record.
South African Journal of Geology, Vol. 108, pp. 557-577.
Abstract: A geometrically quantitative plate-kinematic model, based on paleomagnetism, for the initial assembly of Laurentia has taken form in the past few decades. Within this framework, there remains but one problematic interval of data predominantly from the Slave craton, which is the 1.96-1.87?Ga Coronation apparent polar wander path (APWP). The Coronation APWP shows large (?110°) back-and-forth oscillations that are difficult to explain in terms of plate motion. Nonetheless, poles from the Coronation APWP have been incorporated in various paleogeographic reconstructions of Laurentia and the supercontinent Nuna, pointing to the importance of testing its veracity. In this study, we conducted a detailed paleomagnetic and rock magnetic study of the ca. 1.87?Ga Pearson Formation, East Arm of Great Slave Lake, Northwest Territories, Canada. Our results show that Pearson Formation yields a characteristic remanent magnetization carried by single-domain or small pseudo-single-domain magnetite. The age of the magnetization is constrained to be older than Paleoproterozoic deformation and is interpreted as primary. Paleomagnetic declinations reveal a one-to-one correlation with local structural attitudes, indicating that some small blocks in the fold belt likely experienced significant (?60°) vertical-axis rotations, presumably related to large dextral displacements along the McDonald Fault system. Alternative explanations, such as true polar wander or a non-dipole magnetic field, are considered less parsimonious for the data presented here. It is suspected that some existing Christie Bay Group poles (the Stark and Tochatwi Formations), which were sampled in areas with anomalous structural attitudes and differ from time-equivalent poles obtained from areas of the Slave craton far from major transcurrent faults, may similarly suffer from vertical-axis rotation. We suggest further study before using possibly rotated Christie Bay Group poles for paleogeographic reconstructions.
Annual Reviews of Earth and Planetary Sciences, Vol. 47, pp. 2-19.
Global
plate tectonics
Abstract: The Proterozoic Eon was once regarded as the neglected middle half of Earth history. The name refers to early animals, but they did not appear until the eon (2.5-0.54 Ga) was nearly over. Eukaryotic cells and sexual reproduction evolved much earlier in the eon, as did chloroplasts. Molecular dioxygen, the presence of which altered the geochemical behavior of nearly every element essential to life, rose from negligible to near-modern levels, and then plummeted before rising fitfully again. Plate tectonics took on a modern form, and two supercontinents, Nuna and Rodinia, successively congregated and later dispersed. Climate regulatory failures, i.e., Snowball Earth, appear to be a uniquely Proterozoic phenomenon, having occurred twice in rapid succession near the end of the eon (from 717 to 660 Ma and from 650 to 635 Ma) and arguably once near its beginning (ca. 2.43 Ga). Dynamic sea glaciers covered Snowball Earth oceans from pole to pole, and equatorial sublimation drove slow-moving ice sheets on land. Ultimately, the gradual accumulation of CO2 triggered rapid deglaciation and transient greenhouse aftermaths. Physically based and geologically tested, Neoproterozoic Snowball Earth appears to have molecular legacies in ancient bitumens and modern organisms. This is the story of my love affair with an eon that is now a little less neglected.
Annual Review of Earth and Planetary Sciences, Vol. 47, pp. 1-17. pdf
Mantle
plate tectonics
Abstract: The Proterozoic Eon was once regarded as the neglected middle half of Earth history. The name refers to early animals, but they did not appear until the eon (2.5-0.54 Ga) was nearly over. Eukaryotic cells and sexual reproduction evolved much earlier in the eon, as did chloroplasts. Molecular dioxygen, the presence of which altered the geochemical behavior of nearly every element essential to life, rose from negligible to near-modern levels, and then plummeted before rising fitfully again. Plate tectonics took on a modern form, and two supercontinents, Nuna and Rodinia, successively congregated and later dispersed. Climate regulatory failures, i.e., Snowball Earth, appear to be a uniquely Proterozoic phenomenon, having occurred twice in rapid succession near the end of the eon (from 717 to 660 Ma and from 650 to 635 Ma) and arguably once near its beginning (ca. 2.43 Ga). Dynamic sea glaciers covered Snowball Earth oceans from pole to pole, and equatorial sublimation drove slow-moving ice sheets on land. Ultimately, the gradual accumulation of CO2 triggered rapid deglaciation and transient greenhouse aftermaths. Physically based and geologically tested, Neoproterozoic Snowball Earth appears to have molecular legacies in ancient bitumens and modern organisms. This is the story of my love affair with an eon that is now a little less neglected.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Abstract: An important issue in Earth’s earliest history is the timing and mixing history of the late accreted material that supplied highly siderophile elements to Earth’s mantle after core segregation. Previously, constraints on ancient mantle processes could only be obtained indirectly from mantle-derived magmas such as basalts or komatiites. Relics of Eoarchean (older than 3.8 Ga) mantle were proposed to occur within the Eoarchean terrains of western Greenland. Here we provide geochemical evidence, including combined platinum group element (PGE) and Re-Os isotope data, showing that modern mantle-like peridotites occur at two localities in southwest Greenland. Rhenium-depletion model ages of these peridotites are mostly of Eoarchean age, in accord with U-Pb zircon ages of crosscutting granitoid intrusives. PGE abundances and patterns are similar to those of modern depleted mantle peridotites. For the first time, such patterns provide conclusive evidence for preservation of Eoarchean depleted mantle rocks that are clearly distinguishable from magmatic cumulates or komatiites. Abundances of Os, Ir, and Ru combined with Os isotope compositions in the Greenland peridotites reveal that primitive late accreted material appears to have been efficiently mixed into the sampled mantle domains by Eoarchean time.
Abstract: The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2?Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20?Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2?Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2?Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2?Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
Hofmann, M., Linnemann, U., Hoffmann, K-H., Gerdes, A., Eckelmann, K., Gartner, A.
The Namuskluft and Dreigratberg sections in southern Namibia ( Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana.
International Journal of Earth Sciences, Vol. 103, pp. 1187-1202.
Eiler, J.M., Berquist, B., Bourg, I., Cartigny, P., Farquhar, J., Gagnon, A., Guo, W., Halevy, I., Hofman, A., larson, T.E., Levin, N., Schauble, E.A., Stolper, D.
Geochimica et Cosmochinica Acta, Vol. 215, pp. 432-446.
Africa, Zimbabwe
craton
Abstract: Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9–2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon ?18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In ?Hf-time space, 3.8–3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ?0.015) that separated from chondritic mantle at ?3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35 Ga.
Lithospheric and asthenospheric sources of lamprophyres in the Jiadong Peninsula: a consequence of rapid lithospheric thinning beneath the North Chin a craton?
Geochimica et Cosmochimica Acta, Vol. 124, pp. 250-271.
Geochimica et Cosmochimica Acta, Vol. 232, pp. 82-100.
Mantle
geochemistry
Abstract: We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3?Ga) to Paleozoic (404?Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30-40‰ u?1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3?Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9?±?1.2 (1?) ‰ u?1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u?1 at 3.5?Ga to 12.9‰ u?1 at 3.3?Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7?Ga. However, further studies will be needed to confirm this observation. After 2.7?Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1?Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3?Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth's surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3?Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time.
Abstract: The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2?Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20?Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2?Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2?Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2?Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
Abstract: The North China Craton is a classic case for the destruction of an ancient craton, in that it records the loss of more than 100 km of ancient refractory lithospheric mantle during the late Mesozoic and early Cenozoic. However, the mechanisms for this lithospheric thinning remain controversial in large part due to the lack of any systematic investigations of the Mesozoic asthenospheric mantle via its derived mafic rocks, which are key to understand the thinning processes. In this paper, we present detailed zircon U-Pb geochronology, elemental geochemistry, and Sr-Nd-Hf isotopic data for lamprophyres and diabase-porphyries of the Jiaodong Peninsula, in the eastern North China Craton in order to place constraints on models for lithospheric thinning. Our results show that the lamprophyres and diabase-porphyries are derived from the convective asthenospheric mantle via different degrees of partial melting, and that this mantle source was previously modified by carbonatitic liquids. Zircon LA-ICP-MS U-Pb dating suggests an emplacement age for these rocks of 123-121 Ma, the earliest evidence for asthenospherically-derived melts in the Jiaodong Peninsula so far. This emplacement age indicates that the thickness of the lithosphere in the Jiaodong Peninsula was relatively thin at that time. Co-occurrence of the asthenospheric and lithospheric mantle-derived mafic rocks as well as high-Mg adakites record a rapid transition from lithospheric to asthenospheric mantle sources, indicating that the lithosphere beneath the Jiaodong Peninsula was rapidly detached just prior to ca. 120 Ma. Lithospheric thinning of the North China Craton may have been initiated from the Jiaodong Peninsula and Bohai Sea and then propagated towards the interior of the craton.
The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U-Pb ages of zircons from recent river sands.
International Journal of Earth Sciences, Vol. 103, 2, pp. 579-595.
Hofmann, M., Linnemann, U., Hoffmann, K-H., Gerdes, A., Eckelmann, K., Gartner, A.
The Namuskluft and Dreigratberg sections in southern Namibia ( Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana.
International Journal of Earth Sciences, Vol. 103, pp. 1187-1202.
Journal of Asian Earth Science, Vol. 200, 104432, 18p. Pdf
India
deposit - Munnar
Abstract: Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.
Strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens: implications of temperature dependent thermal
Journal of Geophysical Research, Vol. 115, B 12 B12417
International Journal of earth Sciences, Vol. 110, pp. 1593-1609.
Canada, Ontario
carbonatite
Abstract: Igneous intrusions are important to the thermomechanical evolution of continents because they inject heat into their relatively cold host rocks, and potentially change the distribution of radiogenic heat production and thermal properties within the crust. To explore one aspect of the complex evolution of the continental crust, this paper investigates the local thermal effects of two intrusive rock types (carbonatites and anorthosites) on the Archean Superior Province of the Canadian shield. We provide new data on their contrasting properties: rock density near 298 K, thermal diffusivity, and heat capacity up to 800 K (which altogether yield thermal conductivity), plus radiogenic element contents. The volumetrically small carbonatites have widely varying radiogenic heat production (2–56 µW m?3) and moderate thermal conductivity at 298 K (~?1 to 4 W m?1 K?1) which decreases with temperature. The massive Shawmere anorthosite has nearly negligible radiogenic heat production (
Migration of elements from carbonatites into dolostone at Carillon Dam, southeastern Ontario
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A59. Abstract
Abstract: Near Meech Lake, Québec, the edges of Mesoproterozoic carbonatite dikes are composed of calcite, dolomite, fluorapatite, phlogopite, amphibole, and pyrochlore. The carbonatite is separated from amphibole-fenite by a narrow, fine-grained reaction selvage of phlogopite pierced with long prisms of amphibole. The amphibole is mainly richterite, but it extends to magnesio-arfvedsonite (overgrowth, crystal rim). Uranium-rich pyrochlore is metamict and ranges from calciopyrochlore to kenopyrochlore with Ta-U enrichment in crystal rims. Chemical characteristics of the suite are: (1) F and Nb highest in the selvage, and (2) decline of Sr and Ce outwards from the carbonatite. A similar pattern (this research) is found at Fen, Norway. Rare earths are enriched in LREE with smooth downward-sloping patterns, in chondrite-normalized curves, to HREE. Two major surges of mineralization are suggested: (1) early, metasomatic-alkalic, creating fenites with enrichment in Mg, Na, and K; and (2) later igneous depositing carbonatites and introducing first F, P, and Nb, then Ca, Sr, and Ce. Thermochemical and geochronological data place carbonate equilibration at 700 °C and the emplacement at 1026 Ma b.p. Calciocarbonatites, in monzonitic orthogneiss, are enriched in Ba and Ce. They are composed of baryte, calcite, phlogopite, fluorapatite, magnesio-riebeckite, and non-metamict allanite-(Ce). A mica selvage is present, but amphibole fenite is almost completely lacking. Magnesiocarbonatite has a well-developed selvage against granite but lacks significant amphibole fenite. In breccia cement at nearby Fortune Lake, pyrochlore is associated with abundant fluorapatite but lacks carbonates. The Cambro-Proterozoic calciocarbonatite near Fen, Norway is particularly Nb-rich in breccia zones, and pyroxene fenite takes the place of amphibole fenite at Meech Lake. In contrast to a relatively anorogenic regime during carbonatite petrogenesis at Fen, metamorphism has obscured pyrochlore zonation and enhanced amphibole growth at Meech Lake
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
Abstract: The Archean (~2.6?Ga) Siilinjärvi carbonatite complex in east-central Finland is crosscut by a few ultramafic lamprophyre dykes, together with a broad array of more evolved mafic dykes that range in composition from foidites to various types of alkali basalts. A possible genetic link between the primitive lamprophyres and the carbonatite complex has previously been hypothesised, but their exact relations have been unclear due to the regional metamorphic overprint (i.e., greenschist facies). Here we focus on the petrology and petrography of the mafic dykes, and integrate the data to present a coherent model that can explain the genesis of the Siilinjärvi carbonatite complex. Field-relations, in combination with petrography and geochemistry, indicate that there are at least three generations of mafic dykes present. The oldest dykes (Generation I) are strongly deformed, and inferred to have been emplaced shortly after the formation of the complex itself. These dykes can be divided into two groups (i.e., ultramafic lamprophyres and Group A), where Group A comprises foidites characterised by low SiO2 (41.4-51.5?wt%) and high alkali (>10?wt% K2O) content. We interpret the foiditic magmas to have evolved from primitive ultramafic lamprophyres by fractionating a clinopyroxene-olivine dominated mineral assemblage that was devoid of feldspar. This fractionation path forced alkali-enrichment in the magmas belonging to Group A, which pushed them into the miscibility gap, and resulted in liquid immiscibility that produced moderately alkaline conjugate carbonatite(s). Subsequent fractionation of the conjugate carbonatite by predominantly calcite and apatite produced the mineralogically homogeneous carbonatite cumulate that is exposed at Siilinjärvi. Younger, less deformed, mafic dykes (belonging to Generations II and III) exhibit trace element characteristics, broadly similar to basaltic dyke swarms in the region. The younger dykes are characterised by the presence of large plagioclase crystals in thin sections. Crystallisation of a feldspar-bearing mineral assemblage resulted in only moderate enrichment of alkalis with increased fractionation, which caused the younger dykes to evolve along the more common basalt-to-trachyte series. Thus, the magmas belonging to Generations II and III at Siilinjärvi never fulfilled the conditions required to produce carbonatites by liquid immiscibility.
Abstract: The West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoulé-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present?day thermal lithosphere?asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoulé-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoulé-Mossi domain, later enrichment of the lithosphere cannot be excluded.
Abstract: The James Bay Lowlands is a large remote area of Northern Ontario with very limited access. The Archean basement rocks lie beneath a layer of Paleozoic limestone, up to 300 m thick, that is topped by glacial till and bog. This setting, without outcrop or hard geological knowledge, presented a blank slate well suited to airborne geophysical exploration. This paper presents the aeromagnetic survey methodology and analysis techniques that evolved from the initial kimberlite aeromagnetic program carried out by Selco in 1979 through the 1989 DeBeers discovery of the Victor kimberlite and the 1993 Spider/KWG discovery of the older sub-Paleozoic Kyle series kimberlites and eventually the Ring of Fire. Without property constraints the exploration methodology was a cycle of survey-interpret-drill then move on and repeat as discoveries and finances permitted. After 3 cycles of kimberlite discovery a Spider/KWG/DeBeers partnership encountered VMS copper mineralization in 2001. An airborne EM survey in 2003 identified a number of excellent prospects and the most technically promising became the Noront Eagles Nest MMS nickel deposit that began the Ring of Fire saga. These greenfield discoveries, in a blind geological environment beneath limestone cover, illustrate the potential effectiveness of geophysically directed exploration.
Abstract: Convective circulation of the Earth’s mantle maintains some fraction of surface topography that varies with space and time. Most predictive models show that this dynamic topography has peak amplitudes of about ±2?km, dominated by wavelengths of 104?km. Here, we test these models against our comprehensive observational database of 2,120 spot measurements of dynamic topography that were determined by analysing oceanic seismic surveys. These accurate measurements have typical peak amplitudes of ±1?km and wavelengths of approximately 103?km, and are combined with limited continental constraints to generate a global spherical harmonic model, the robustness of which has been carefully tested and benchmarked. Our power spectral analysis reveals significant discrepancies between observed and predicted dynamic topography. At longer wavelengths (such as 104?km), observed dynamic topography has peak amplitudes of about ±500?m. At shorter wavelengths (such as 103?km), significant dynamic topography is still observed. We show that these discrepancies can be explained if short-wavelength dynamic topography is generated by temperature-driven density anomalies within a sub-plate asthenospheric channel. Stratigraphic observations from adjacent continental margins show that these dynamic topographic signals evolve quickly with time. More rapid temporal and spatial changes in vertical displacement of the Earth’s surface have direct consequences for fields as diverse as mantle flow, oceanic circulation and long-term climate change.
Earth and Planetary Science Letters, Vol. 496, 1, pp. 142-158.
Asia, Borneo
convection
Abstract: Most, but not all, geodynamic models predict 1-2 km of mantle convective draw-down of the Earth's surface in a region centered on Borneo within southeast Asia. Nevertheless, there is geomorphic, geologic and geophysical evidence which suggests that convective uplift might have played some role in sculpting Bornean physiography. For example, a long wavelength free-air gravity anomaly of +60 mGal centered on Borneo coincides with the distribution of Neogene basaltic magmatism and with the locus of sub-plate slow shear wave velocity anomalies. Global positioning system measurements, an estimate of elastic thickness, and crustal isostatic considerations suggest that regional shortening does not entirely account for kilometer-scale regional elevation. Here, we explore the possible evolution of the Bornean landscape by extracting and modeling an inventory of 90 longitudinal river profiles. Misfit between observed and calculated river profiles is minimized by smoothly varying uplift rate as a function of space and time. Erosional parameters are chosen by assuming that regional uplift post-dates Eocene deposition of marine carbonate rocks. The robustness of this calibration is tested against independent geologic observations such as thermochronometric measurements, offshore sedimentary flux calculations, and the history of volcanism. A calculated cumulative uplift history suggests that kilometer-scale Bornean topography grew rapidly during Neogene times. This suggestion is corroborated by an offshore Miocene transition from carbonate to clastic deposition. Co-location of regional uplift and slow shear wave velocity anomalies immediately beneath the lithospheric plate implies that regional uplift could have been at least partly generated and maintained by temperature anomalies within an asthenospheric channel.
Abstract: Earth’s surface topography is a direct physical expression of our planet’s dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a substantial proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth’s deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth’s oceanic residual topography power spectrum. This indicates water-loaded power of 0.5?±?0.35?km2 and peak amplitudes of up to ~0.8?±?0.1?km at long wavelengths (~104?km), decreasing by roughly one order of magnitude at shorter wavelengths (~103?km). We show that geodynamical simulations can be reconciled with observational constraints only if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-wavelength) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth’s lithosphere.
Earth and Planetary Science Letters, Vol. 542, 116317 16p. Pdf
Mantle
plumes, geothermometry
Abstract: Mantle convection is the principal mechanism by which heat is transferred from the deep Earth to the surface. Cold subducting slabs sink into the mantle and steadily warm, whilst upwelling plumes carry heat to the base of lithospheric plates where it can subsequently escape by conduction. Accurate estimation of the total heat carried by these plumes is important for understanding geodynamic processes and Earth's thermal budget. Existing estimates, based upon swell geometries and velocities of overriding plates, yield a global heat flux of ?2 TW and indicate that plumes play only a minor role in heat transfer. Here, we revisit the Icelandic and Hawaiian plumes to show that their individual flux estimates are likely to be incorrect due to the assumption that buoyancy is mainly produced within the lithosphere and therefore translates at plate velocities. We develop an alternative methodology that depends upon swell volume, is independent of plate velocities, and allows both for decay of buoyancy through time and for differential motion between asthenospheric buoyancy and the overlying plate. Reanalysis of the Icelandic and Hawaiian swells yields buoyancy fluxes of Mg s?1 and Mg s?1, respectively. Both swells are used to calibrate a buoyancy decay timescale of ?45 Myr for the new volumetric approach, which enables buoyancy fluxes to be estimated for a global inventory of 53 swells. Estimates from magmatic hotspots yield a cumulative lower bound on global plume flux of 2 TW, which increases to 6 TW if amagmatic swells are also included and if all buoyancy is assumed to be thermal in origin. Our results suggest that upwelling plumes play a significant role in the transfer of heat into the uppermost mantle.
Abstract: Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10?megatonnes of metal), occur within 200?kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two?billion?years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone.
Physics of the Earth and Planetary Interiors, Vol. 247, pp. 11-16.
Mantle
Experimental Petrology
Abstract: Lattice thermal conductivity of ferropericlase and radiative thermal conductivity of iron bearing magnesium silicate perovskite (bridgmanite) - the major mineral of Earth’s lower mantle- have been measured at room temperature up to 30 and 46 GPa, respectively, using time-domain thermoreflectance and optical spectroscopy techniques in diamond anvil cells. The results provide new constraints for the pressure dependencies of the thermal conductivities of Fe bearing minerals. The lattice thermal conductivity of ferropericlase Mg0.9Fe0.1O is 5.7(6) W/(m * K) at ambient conditions, which is almost 10 times smaller than that of pure MgO; however, it increases with pressure much faster (6.1(7)%/GPa vs 3.6(1)%/GPa). The radiative conductivity of a Mg0.94Fe0.06SiO3 bridgmanite single crystal agrees with previously determined values for powder samples at ambient pressure; it is almost pressure-independent in the investigated pressure range. Our results confirm the reduced radiative conductivity scenario for the Earth’s lower mantle, while the assessment of the heat flow through the core-mantle boundary still requires in situ measurements at the relevant pressure-temperature conditions.
Abstract: Felsic to intermediate igneous rocks from the Cuchilla Dionisio (or Punta del Este) Terrane (CDT) in Uruguay and the Várzea do Capivarita Complex (VCC) in southern Brazil were emplaced in the Tonian and experienced high-grade metamorphism towards the end of the Cryogenian. Geological and geochemical data indicate an S-type origin and formation in a continental within-plate setting by recycling of lower crustal material that was initially extracted from the mantle in the Palaeoproterozoic. Similar felsic igneous rocks of Tonian age occur in the Richtersveld Igneous Complex and the Vredefontein and Rosh Pinah formations in westernmost South Africa and southern Namibia and have been correlated with their supposed equivalents in Uruguay and Brazil. Geochemical and isotope data of the largely unmetamorphosed felsic igneous rocks in southwestern Africa imply a within-plate origin and formation by partial melting or fractional crystallization of mafic rocks that were extracted from the mantle in the Proterozoic. The parental melts of all of these Tonian igneous rocks from South America and southwestern Africa formed in an anorogenic continental setting at the western margin of the Kalahari Craton and were emplaced in, and/or contaminated by, Namaqua Province-type basement after separation from their source region. However, the source regions and the time of extractions thereof are different and, moreover, occurred at different palaeogeographical latitudes. New petrological data of CDT high-grade gneiss indicate a geothermal gradient of c. 20-25 °C/km, implying continental collisional tectonics following subduction and ocean basin closure at an active continental margin at the eastern edge of present-day South America in the late Cryogenian to early Ediacaran. The associated suture may be traced by the high-grade gneiss and amphibolite-facies mafic rocks in the CDT and probably continues northwards to the Arroio Grande Complex and the VCC in southern Brazil.
Abstract: The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution-reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite.
Three dimensional mantle convection simulations with a low viscosity asthenosphere and the relationship between heat flow and the horizontal length scale
Geophysical Research Letters, Vol. 35, 10, May 28, L10304
Physics of the Earth and Planetary Interiors, in press avaialable, 72p. Pdf
Mantle
plate tectonics
Abstract: The solid Earth system is characterized by plate tectonics, a low viscosity zone beneath plates (the asthenosphere), and long wavelength flow in the convecting mantle. We use suites of numerical experiments to show: 1) How long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere, and 2) How an asthenosphere can maintain long wavelength flow and plate tectonics. Plate subduction generates a sub-adiabatic temperature gradient in the mantle which, together with temperature-dependent viscosity, leads to a viscosity increase from the upper to the lower mantle. This allows mantle flow to channelize in a low viscosity region beneath plates (an asthenosphere forms dynamically). Flow channelization, in turn, stabilizes long wavelength convection. The degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet. The plate margin strength needed to initiate that transition increases for long wavelength cells (long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength). The coupled feedbacks allow for a linked causality between plates, the asthenosphere, and the wavelength of mantle flow, with none being more fundamental than the others and the existence of each depending on the others. Under this hypothesis, the asthenosphere is defined by an active process, plate tectonics, which maintains it and is maintained by it and plate tectonics is part of an emergent, self-sustaining flow system that bootstraps itself into existence.
Physics of the Earth and Planetary Interiors, in press available, 57p. Pdf
Mantle
plate tectonics
Abstract: The solid Earth system is characterized by plate tectonics, a low viscosity zone beneath plates (the asthenosphere), and long wavelength flow in the convecting mantle. We use suites of numerical experiments to show: 1) How long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere, and 2) How an asthenosphere can maintain long wavelength flow and plate tectonics. Plate subduction generates a sub-adiabatic temperature gradient in the mantle which, together with temperature-dependent viscosity, leads to a viscosity increase from the upper to the lower mantle. This allows mantle flow to channelize in a low viscosity region beneath plates (an asthenosphere forms dynamically). Flow channelization, in turn, stabilizes long wavelength convection. The degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet. The plate margin strength needed to initiate that transition increases for long wavelength cells (long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength). The coupled feedbacks allow for a linked causality between plates, the asthenosphere, and the wavelength of mantle flow, with none being more fundamental than the others and the existence of each depending on the others. Under this hypothesis, the asthenosphere is defined by an active process, plate tectonics, which maintains it and is maintained by it and plate tectonics is part of an emergent, self-sustaining flow system that bootstraps itself into existence.
Physics of the Earth and Planetary Interiors, in press 10.1016/j.pepi.2019.106299 18p. Pdf
Mantle
convection
Abstract: The solid Earth system is characterized by plate tectonics, a low viscosity zone beneath plates (the asthenosphere), and long wavelength flow in the convecting mantle. We use suites of numerical experiments to show: 1) How long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere, and 2) How an asthenosphere can maintain long wavelength flow and plate tectonics. Plate subduction generates a sub-adiabatic temperature gradient in the mantle which, together with temperature-dependent viscosity, leads to a viscosity increase from the upper to the lower mantle. This allows mantle flow to channelize in a low viscosity region beneath plates (an asthenosphere forms dynamically). Flow channelization, in turn, stabilizes long wavelength convection. The degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet. The plate margin strength needed to initiate that transition increases for long wavelength cells (long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength). The coupled feedbacks allow for a linked causality between plates, the asthenosphere, and the wavelength of mantle flow, with none being more fundamental than the others and the existence of each depending on the others. Under this hypothesis, the asthenosphere is defined by an active process, plate tectonics, which maintains it and is maintained by it and plate tectonics is part of an emergent, self-sustaining flow system that bootstraps itself into existence.
Perraki, M., Proyer, A., Mposkos, E., Kaindl, R., Hoinkes, G.
Raman micro spectroscopy on diamond, graphite and other carbon polymorphs from the ultrahigh pressure metamorphic Kimi Complex of the Rhodope metamorphic province.
Earth and Planetary Science Letters, Vol. 241, 3-4, pp. 672-685.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Aug. pp. 745-753.
Africa, South Africa
Deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 745-752.
Africa, South Africa
deposit - Cullinan
Abstract: In 2012, Cullinan Diamond Mine began an expansion programme with the shaft deepening and development of access to the C-Cut 1 block at approximately 839 m below surface. The expansion programme is funded by a combination of bank loans and retained operating profit generated by the mine. Continuous production during deepening of the No. 1 Shaft, which is the rock hoisting shaft, was therefore critical for sustainability and efficiency as well as overall funding of the project. The deepening method, support design and verification, as well as learning outcomes pertaining to the extension of the No. 1 Shaft underneath the existing operating shaft are summarized, with emphasis on the importance of gaining some understanding of the shaft's host rock mass.
Abstract: Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics1. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science1,2,3,4. Metamorphic rocks—rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)—record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth’s tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts5, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones5,6. Here we show that Earth’s modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago9,10,11,12) at the latest.
Abstract: Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics1. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science1,2,3,4. Metamorphic rocks—rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)—record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth’s tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts5, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones5,6. Here we show that Earth’s modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago9,10,11,12) at the latest.
Geochimica et Cosmochimica Acta, Vol. 251, pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Examination of diamond stability phase mantle indicator minerals from Leucite Hills, Sweetwater County, Wyoming and Crater of Diamonds State Park, Pike Count, Arkansas.
Geological Society of America, Vol. 48, 3, 1p. Abstract
An improved and extended internally consistent thermodynamic data set for phases of petrological interest, involving a new equation of state for solids.
Journal of Metamorphic Geology, in print available
An improved and extended internally consistent thermodynamic data set for phases of petrological interest, involving a new equation of state for solids.
Multiple reaction oxygen barometry for mantle peridotite: an internally consistent thermodynamic model for reactions and garnet solid-solutions, with applications to the oxidation state of lithospheric mantle.
Volcanic and Magmatic Studies Group meeting, Poster Held Jan. 6-8. See minsoc website
Abstract: New thermodynamic data for skiagite garnet (Fe3Fe23+Si3O12) are derived from experimental phase-equilibrium data that extend to 10 GPa and are applied to oxybarometry of mantle peridotites using a revised six-component garnet mixing model. Skiagite is more stable by 12 kJ mol-1 than in a previous calibration of the equilibrium 2 skiagite = 4 fayalite + ferrosilite + O2, and this leads to calculated oxygen fugacities that are higher (more oxidized) by around 1-1•5 logfO2 units. A new calculation method and computer program incorporates four independent oxybarometers (including 2 pyrope + 2 andradite + 2 ferrosilite = 2 grossular + 4 fayalite + 3 enstatite + O2) for use on natural peridotite samples to yield optimum logfO2 estimates by the method of least squares. These estimates should be more robust than those based on any single barometer and allow assessment of possible disequilibrium in assemblages. A new set of independent oxybarometers for spinel-bearing peridotites is also presented here, including a new reaction 2 magnetite + 3 enstatite = 3 fayalite + 3 forsterite + O2. These recalibrations combined with internally consistent PT determinations for published analyses of mantle peridotites with analysed Fe2O3 data for garnets, from both cratonic (Kaapvaal, Siberia and Slave) and circumcratonic (Baikal Rift) regions, provide revised estimates of oxidation state in the lithospheric mantle. Estimates of logfO2 for spinel assemblages are more reduced than those based on earlier calibrations, whereas garnet-bearing assemblages are more oxidized. Importantly, this lessens considerably the difference between garnet and spinel oxybarometry that was observed with previous published calibrations.
Abstract: Evidence for chemical and lithological heterogeneity in the Earth’s convecting mantle is widely acknowledged, yet the major element signature imparted on mantle melts by this heterogeneity is still poorly resolved. In this study, a recent thermodynamic melting model is tested on a range of compositions that correspond to potential mantle lithologies (harzburgitic to pyroxenitic), to demonstrate its applicability over this compositional range, in particular for pyroxenite melting. Our results show that, despite the model’s calibration in peridotitic systems, it effectively reproduces experimental partial melt compositions for both Si-deficient and Si-excess pyroxenites. Importantly, the model accurately predicts the presence of a free silica phase at high pressures in Si-excess pyroxenites, indicating the activation of the pyroxene-garnet thermal divide. This thermal divide has a dominant control on solidus temperature, melt productivity and partial melt composition. The model is used to make new inferences on the link between mantle composition and melting behaviour. In silica-deficient and low-pressure (olivine-bearing) lithologies, melt composition is not very sensitive to source composition. Linearly varying the source composition between peridotite and basaltic pyroxenite, we find that the concentration of oxides in the melt tends to be buffered by the increased stability of more fusible phases, causing partial melts of even highly fertile lithologies to be similar to those of peridotite. An exception to this behaviour is FeO, which is elevated in partial melts of silica-deficient pyroxenite even if the bulk composition does not have a high FeO content relative to peridotite. Melt Al2O3 and MgO vary predominantly as a function of melting depth rather than bulk composition. We have applied the thermodynamic model to test the hypothesis that Fe-rich mantle melts such as ferropicrites are derived by partial melting of Si-deficient pyroxenite at elevated mantle potential temperatures. We show that the conspicuously high FeO in ferropicrites at a given MgO content does not require a high-Fe mantle source and is indeed best matched by model results involving around 0-20% melting of silica-deficient pyroxenite. A pyroxenite source lithology also accounts for the low CaO content of ferropicrites, whereas their characteristic low Al2O3 is a function of their high pressure of formation. Phanerozoic ferropicrites are exclusively located in continental flood basalt (CFB) provinces and this model of formation confirms that lithological heterogeneity (perhaps recycled oceanic crust) is present in CFB mantle sources.
Abstract: Extensive partial melting of the middle to lower crustal parts of orogens, such as of the current Himalaya-Tibet orogen, significantly alters their rheology and imposes first-order control on their tectonic and topographic evolution. We interpret the late Proterozoic Araçuaí orogen, formed by the collision between the São Francisco (Brazil) and Congo (Africa) cratons, as a deep section through such a hot orogen based on U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages and Ti-in-zircon and Zr-in-rutile temperatures from the Carlos Chagas anatectic domain. This domain is composed of peraluminous anatexites and leucogranites that typically exhibit interconnected networks of garnet-rich leucosomes or a magmatic foliation. Zirconium-in-rutile temperatures range from 745 to 820 °C, and the average Ti-in-zircon temperature ranges from 712 to 737 °C. The geochronologic and thermometry data suggest that from 597 to 572 Ma this domain was partially molten and remained so for at least 25 m.y., slowly crystallizing between temperatures of ?815 and >700 °C. Significant crustal thickening must have occurred prior to 600 Ma, with initial continental collision likely before 620 Ma, a time period long enough to heat the crust to temperatures required for widespread partial melting at middle crustal levels and to favor a "channel flow" tectonic behavior.
Journal of Volcanology and Geothermal Research, in press available 11p.
South America, Brazil
Alkaline rocks
Abstract: We report the first high-precision ID-TIMS U-Pb baddeleyite/zircon and 40Ar/39Ar step-heating phlogopite age data for diabase and lamprophyre dykes and a mafic intrusion (José Fernandes Gabbro) located within the Ponta Grossa Arch, Brazil, in order to constrain the temporal evolution between Early Cretaceous tholeiitic and alkaline magmatism of the Paraná-Etendeka Magmatic Province. U-Pb dates from chemically abraded zircon data yielded the best estimate for the emplacement ages of a high Ti-P-Sr basaltic dyke (133.9 ± 0.2 Ma), a dyke with basaltic andesite composition (133.4 ± 0.2 Ma) and the José Fernandes Gabbro (134.5 ± 0.1 Ma). A 40Ar/39Ar phlogopite step-heating age of 133.7 ± 0.1 Ma from a lamprophyre dyke is identical within error to the U-Pb age of the diabase dykes, indicating that tholeiitic and alkaline magmatism were coeval in the Ponta Grossa Arch. Although nearly all analysed fractions are concordant and show low analytical uncertainties (± 0.3-0.9 Ma for baddeleyite; 0.1-0.4 Ma for zircon; 2?), Pb loss is observed in all baddeleyite fractions and in some initial zircon fractions not submitted to the most extreme chemical abrasion treatment. The resulting age spread may reflect intense and continued magmatic activity in the Ponta Grossa Arch.
Katanga: mineral wealth, human challenges. The mineral wealth of the Congo is a phrase much used nowadays, often in condemnation of those endeavouring to develop
Nature Communications, 10.1038/s41467-022-28633-w 12p. Pdf
Mantle
magmatism
Abstract: Volcanism on Earth and on other planets and satellites is dominated by the eruption of low viscosity magmas. During explosive eruption, high melt temperatures and the inherent low viscosity of the fluidal pyroclasts allow for substantial post-fragmentation modification during transport obscuring the record of primary, magmatic fragmentation processes. Here, we show these syn-eruption modifications, in the form of melt stripping and agglutination, to be advantageous for providing fundamental insights into lava fountain and jet dynamics, including eruption velocities, grain size distributions and melt physical properties. We show how enigmatic, complex pyroclasts termed pelletal lapilli form by a two-stage process operating above the magmatic fragmentation surface. Melt stripping from pyroclast surfaces creates a spray of fine melt droplets whilst sustained transport in the fountain allows for agglutination and droplet scavenging, thereby coarsening the grain size distribution. We conclude with a set of universal regime diagrams, applicable for all fluidal fountain products, that link fundamental physical processes to eruption conditions and melt physical properties.
The Canadian Geographer, doi.org/10.1111/ cag.12521
Global
GIS
Abstract: Higher?education geographic information system (GIS) curricula largely marginalize and separate instruction of critical GIS and open GIS, paralleling a divide between GIS and non?GIS in geography. GIS is typically represented as a singular, infallibly objective, and universally applicable technology. GIS generally dismisses the critiques from human geography, while critical human geography dismisses GIS for its association with positivism and unethical applications. Teaching critical open GIS may bridge this divide, creating a transformative pedagogical space for human geography to affectively and effectively engage with open GIS technology at the level of code. Critical open GIS students practice and critique GIS as conflicted insiders, bridging the divide between GIS and non?GIS in their geography education. Reviews of GIS curricula find support for teaching critical and open GIS, but reviews of texts and syllabi confirm their marginalization and separation. A new critical open GIS course is introduced, using GIS in development and political ecology as integrative frameworks.
Abstract: Geological mapping and zircon U-Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820 Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610 Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcano-sedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725 Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740 Ma, with additional detrital sources 2820 and 2940 Ma. Thus, 200 Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism. In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630 Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran "Pan-African" isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675 Ma, coeval with previous dates of the "Eastern Granulites" of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730 Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa "block" have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920 Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870-1900 Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique. In SW Tanzania, rocks from the Mgazini area gave Ubendian protolith ages of ca. 1980-1800 Ma, but these rocks underwent Late Mesoproterozoic high-grade metamorphism between 1015 and 1040 Ma. One granitoid gave a crystallisation age of ca. 1080 Ma correlating with known Mesoproterozoic crust to the east in SE Tanzania and NE Mozambique. However, while the crust in the Mgazini area was clearly one of original Ubendian age, reworked and intruded by granitoids at ca. 1 Ga, the crust of SE Tanzania is a mixed Mesoproterozoic terrane and a continuation from NE Mozambique. Hence the Mgazini area lies at the edge of the Ubendian belt which was re-worked during the Mesoproterozoic orogen (South Irumide belt), providing a further constraint on the distribution of ca. 1 Ga crust in SE Africa. Hf data from near-concordant analyses of detrital zircons from a sample from the Tanzania Craton lie along a Pb-loss trajectory (Lu/Hf = 0), extending back to ?3.9 Ga. This probably represents the initial depleted mantle extraction event of the cratonic core. Furthermore, the Hf data from all igneous samples, regardless of age, from the entire study area (including the Neoproterozoic granulite nappes) show a shallow evolution trend (Lu/Hf = 0.028) extending back to the same mantle extraction age. This implies the entire Tanzanian crust sampled in this study represents over 3.5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the Tanzanian Craton that was subsequently reworked was composed of a very depleted, mafic source with a very high Lu/Hf ratio. Our study helps to define the architecture of the Tanzanian Craton and its evolution from a single age-source in the early Eoarchaean.
Geochemistry and radiogenic isotope characteristics of xenoliths in Archean Diamondiferous lamprophyres: implications for the Superior Province cratonic keel.
Journal of Petrology, Vol. 62, 7, 10.1093/petrology/ega036
Canada
deposit - Coldwell
Abstract: Mafic intrusions on the NE shoulder of the Midcontinent Rift (Keweenawan LIP), including Cu-PGE mineralized gabbros within the Coldwell Complex (CC), and rift parallel or radial dykes outside the CC are correlated based on characteristic trace element patterns. In the Coldwell Complex, mafic rocks are subdivided into four groups: (1) early metabasalt; (2) Marathon Series; (3) Layered Series; (4) Geordie-Wolfcamp Series. The Marathon Series are correlated with the rift radial Abitibi dykes (1140?Ma), and the Geordie-Wolfcamp Series with the rift parallel Pukaskwa and Copper Island dykes. U-Pb ages determined for five gabbros from the Layered and Marathon Series are between 1107•7 and 1106•0?Ma. Radiogenic isotope ratios show near chondritic (CHUR) ?Nd(1106?Ma) and 87Sr/86Sri values that range from -0•38 to +1•13 and 0•702537 to 0•703944, respectively. Distinctive geochemical properties of the Marathon Series and Abitibi dykes, such as Ba/La (14-37), Th/Nb (0•06-0•12), La/Sm (3•8-7•7), Sr/Nd (21-96) and Zr/Sm (9-19), are very different from those of the Geordie-Wolfcamp Series and a subset of Copper Island and Pukaskwa dykes with Ba/La (8•7-11), Th/Nb (0•12-0•13), La/Sm (6•7-7•9), Sr/Nd (5-7•8) and Zr/Sm (18-24). Each unit exhibits covariation between incompatible element ratios such as Zr/Sm and Nb/La or Gd/Yb, Sr/Nd and Ba/La, and Nb/Y and Zr/Y, which are consistent with mixing relationship between two or more mantle domains. These characteristics are unlike those of intrusions on the NW shoulder of the MCR, but resemble those of mafic rocks occurring in the East Kenya Rift. The results imply that an unusual and long-lived mantle source was present in the NE MCR for at least 34?Myr (spanning the 1140?Ma Abitibi dykes and the 1106?Ma Marathon series) and indicate potential for Cu-PGE mineralization in an area much larger than was previously recognized.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Fundamental Research , 10.1016/j.fmre.2022.04.004 34p. Pdf
Mantle
REE
Abstract: Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles from the peralkaline granites show evolutionary trends from calcic through sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index (Na#, defined as molar Na/(Na+Ca) ratios). The Na-amphiboles (i.e., arfvedsonite) can be subsequently altered, or breakdown, to form Na-clinopyroxene (i.e., aegirine) during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the alkali amphiboles have high and variable REE (1295-2218 ppm) and HFSE (4194-16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles. Compared to amphiboles, the early replacement aegirine (Aeg-I, Na#?=?0.91-0.94) has notably lower REE (577-797) and HFSE (4351-5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na#?=?0.92-0.96) has significantly lower REE (127-205 ppm) and HFSE (6.43-72.2 ppm) contents. Given that the increasing Na# from alkali amphibole to aegirine likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by alkali amphiboles during Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The Na-index of alkali amphibole-aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.
Earth and Planetary Science Letters, Vol. 557, doi.org/10.1016/ j.epsl.2020.116730 9p. Pdf
Europe, Greenland
meteorite
Abstract: Large meteorite impacts have a profound effect on the Earth's geosphere, atmosphere, hydrosphere and biosphere. It is widely accepted that the early Earth was subject to intense bombardment from 4.5 to 3.8 Ga, yet evidence for subsequent bolide impacts during the Archean Eon (4.0 to 2.5 Ga) is sparse. However, understanding the timing and magnitude of these early events is important, as they may have triggered significant change points to global geochemical cycles. The Maniitsoq region of southern West Greenland has been proposed to record a ?3.0 Ga meteorite impact, which, if confirmed, would be the oldest and only known impact structure to have survived from the Archean. Such an ancient structure would provide the first insight into the style, setting, and possible environmental effects of impact bombardment continuing into the late Archean. Here, using field mapping, geochronology, isotope geochemistry, and electron backscatter diffraction mapping of 5,587 zircon grains from the Maniitsoq region (rock and fluvial sediment samples), we test the hypothesis that the Maniitsoq structure represents Earth's earliest known impact structure. Our comprehensive survey shows that previously proposed impact-related geological features, ranging from microscopic structures at the mineral scale to macroscopic structures at the terrane scale, as well as the age and geochemistry of the rocks in the Maniitsoq region, can be explained through endogenic (non-impact) processes. Despite the higher impact flux, intact craters from the Archean Eon remain elusive on Earth.
Earth and Planetary Science Letters, Vol. 534, 8p. Pdf
Europe, Greenland
kimberlite genesis
Abstract: Archean cratons are composites of terranes formed at different times, juxtaposed during craton assembly. Cratons are underpinned by a deep lithospheric root, and models for the development of this cratonic lithosphere include both vertical and horizontal accretion. How different Archean terranes at the surface are reflected vertically within the lithosphere, which might inform on modes of formation, is poorly constrained. Kimberlites, which originate from significant depths within the upper mantle, sample cratonic interiors. The North Atlantic Craton, West Greenland, comprises Eoarchean and Mesoarchean gneiss terranes - the latter including the Akia Terrane - assembled during the late Archean. We report U-Pb and Hf isotopic, and trace element, data measured in zircon xenocrysts from a Neoproterozoic (557 Ma) kimberlite which intruded the Mesoarchean Akia Terrane. The zircon trace element profiles suggest they crystallized from evolved magmas, and their Eo-to Neoarchean U-Pb ages match the surrounding gneiss terranes, and highlight that magmatism was episodic. Zircon Hf isotope values lie within two crustal evolution trends: a Mesoarchean trend and an Eoarchean trend. The Eoarchean trend is anchored on 3.8 Ga orthogneiss, and includes 3.6-3.5 Ga, 2.7 and 2.5-2.4 Ga aged zircons. The Mesoarchean Akia Terrane may have been built upon mafic crust, in which case all zircons whose Hf isotopes lie within the Eoarchean trend were derived from the surrounding Eoarchean gneiss terranes, emplaced under the Akia Terrane after ca. 2.97 or 2.7 Ga, perhaps during late Archean terrane assembly. Kimberlite-hosted peridotite rhenium depletion model ages suggest a late Archean stabilization for the lithospheric mantle. The zircon data support a model of lithospheric growth via tectonic stacking for the North Atlantic Craton.
Abstract: The jigsaw fit of Earth’s continents, which long intrigued map readers and inspired many theories, was explained about 60 years ago when the foundational processes of plate tectonics came to light. Topographic and magnetic maps of the ocean floor revealed that the crust—the thin, rigid top layer of the solid Earth—is split into plates. These plates were found to shift gradually around the surface atop a ductile upper mantle layer called the asthenosphere. Where dense oceanic crust abuts thicker, buoyant continents, the denser crust plunges back into the mantle beneath. Above these subduction zones, upwelling mantle melt generates volcanoes, spewing lava and creating new continental crust.
Diamonds from nepheline mugearite- a discussion of garnet web sterites and associated ultramafic inclusions from nepheline mugearite in the Malcha area New South WalesAust
Mineralogical Magazine, Vol. 49, No. 354, December pp. 748-751
Australian Gemmologist, Vol. 25, 3, July-Sept. pp.
Technology
Diamond features
Abstract: This study describes how visual properties determine the perception of a diamond’s appearance and its performance attributes of brilliance, scintillation and fire, and how these influence beauty. Further articles will describe other parts of our cut study project. This research enables the development of methods and instruments for diamond performance analyses, shifting from current diamond cut rejection based tools, to diamond performance scoring systems, and the introduction of a new consumer language for communication between diamond buyers and sellers. The proposed Performance Scoring System is consumer friendly and can be used to design and manufacture new diamond cuts with improved optical appearance.
Graphite melt equilibration temperatures during mantle melting: constraints on CO2 in Mid Ocean Ridge Basalt (Mid Ocean Ridge Basalt (MORB))magmas and carbon content..
Chemical Geology, Vol. 147, No. 1-2, May 15, pp. 89-98.
Marathon Dikes: Rubidium-strontium and Potassium-argon Geochronology of ultrabasic Lamprophyres from the Vicinity of Mckellar Harbour, Northwestern ontario, Canada.
Canadian Journal of Earth Sciences, Vol. 20, No. 6, PP. 961-967.
Abstract: The Earth’s cratons are traditionally regarded as tectonically stable cores that were episodically buried by thin sedimentary covers. Cratonic crust in southern Finland holds seven post-1.7 Ga tiered unconformities, with remnants of former sedimentary covers. We use the geometries of the tiered unconformities, along with previously dated impact structures and kimberlite and carbonatite pipes, to reconstruct the erosion and burial history of the craton and to derive estimates of depths of erosion in basement and former sedimentary rocks. The close vertical spacing (<200 m) of the unconformities and the survival of small (D ? 5 km) Neoproterozoic and Early Palaeozoic impact structures indicate minor later erosion. Average erosion rates (<2.5 m/Ma) in basement and cover are amongst the lowest reported on Earth. Ultra-slow erosion has allowed the persistence in basement fractures of Phanerozoic fracture coatings and Palaeogene groundwater and microbiomes. Maximum thicknesses of foreland basin sediments in Finland during the Sveconorwegian and Caledonide orogenies are estimated as ~1.0 km and <0.68-1.0 km, respectively. Estimated losses of sedimentary cover derived from apatite fission track thermochronology are higher by factors of at least 2 to 4. A dynamic epeirogenic history of the craton in Finland, with kilometre-scale burial and exhumation, proposed in recent thermochronological models is not supported by other geological proxies. Ultra-slow erosion rates in southern Finland reflect long term tectonic stability and burial of the craton surface for a total of ~1.0 Ga beneath generally thin sedimentary cover.
Geochimica et Cosmochimica Acta, Vol. 251. pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Tectonic imbrication and foredeep development in the Penokean Orogeny, east central Minnesota: an interpretation based on regional geophysics and drill
United States Geological Survey (USGS) Bulletin, No. 1904 C-D, 17p. and 10p
PluS One, Vol. 13, 8, doi:10.1371/journal.pone.0200351
Asia, Thailand
garnets
Abstract: Garnets from disparate geographical environments and origins such as oxidized soils and river sediments in Thailand host intricate systems of microsized tunnels that significantly decrease the quality and value of the garnets as gems. The origin of such tunneling has previously been attributed to abiotic processes. Here we present physical and chemical remains of endolithic microorganisms within the tunnels and discuss a probable biological origin of the tunnels. Extensive investigations with synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveal morphological indications of biogenicity that further support a euendolithic interpretation. We suggest that the production of the tunnels was initiated by a combination of abiotic and biological processes, and that at later stages biological processes came to dominate. In environments such as river sediments and oxidized soils garnets are among the few remaining sources of bio-available Fe2+, thus it is likely that microbially mediated boring of the garnets has trophic reasons. Whatever the reason for garnet boring, the tunnel system represents a new endolithic habitat in a hard silicate mineral otherwise known to be resistant to abrasion and chemical attack.
Abstract: Slab?slab interaction is a characteristic feature of tectonically complex areas. Outward dipping double?sided subduction is one of these complex cases, which has several examples on Earth, most notably the Molucca Sea and Adriatic Sea. This study focuses on developing a framework for linking plate kinematics and slab interactions in an outward dipping subduction geometry. We used analog and numerical models to better understand the underlying subduction dynamics. Compared to a single subduction model, double?sided subduction exhibits more time?dependent and vigorous toroidal flow cells that are elongated (i.e., not circular). Because both the Molucca and Adriatic Sea exhibit an asymmetric subduction configuration, we also examine the role that asymmetry plays in the dynamics of outward dipping double?sided subduction. We introduce asymmetry in two ways; with variable initial depths for the two slabs (geometric asymmetry), and with variable buoyancy within the subducting plate (mechanical asymmetry). Relative to the symmetric case, we probe how asymmetry affects the overall slab kinematics, whether asymmetric behavior intensifies or equilibrates as subduction proceeds. While initial geometric asymmetry disappears once the slabs are anchored to the 660 km discontinuity, the mechanical asymmetry can cause more permanent differences between the two subduction zones. In the most extreme case, the partly continental slab stops subducting due to the unequal slab pull force. The results show that the slab?slab interaction is most effective when the two trenches are closer than 10-8 cm in the laboratory, which is 600-480 km when scaled to the Earth.
Abstract: The thermal evolution of subducting slabs controls a range of subduction processes, yet we lack a robust understanding of how thermal structure develops over a subduction zone's lifetime. We investigate the time-dependence of slab thermal structure using dynamically consistent, time evolving models. Pressure-temperature (P-T) conditions along the slab Moho and slab top exhibit substantial variability throughout the various phases of subduction: initiation, free sinking, and mature subduction. This variability occurs in response to time-dependent subduction properties (e.g., fast vs. slow convergence) and thermal structure inherited from previous phases (e.g., due to upper plate aging). At a given depth, the slab cools rapidly during initiation, after which slower cooling occurs. In the case of the Moho, additional cooling occurs during the free sinking phase. We explore the implications of time-dependent thermal structure on exhumed rocks and slab dehydration. Modeled slab top P-T paths span much of the P-T space associated with exhumed rocks, suggesting a significant component of recorded variability may have dynamic origins. Coupling our P-T profiles with thermodynamic models of oceanic lithosphere, we show that dehydrating ultramafic rocks at the slab Moho provide the bulk of hydrous fluid at subarc depths during the earliest phases. Over subsequent phases, these rocks carry fluids into the deeper mantle, and it is mafic crust along the slab top that releases water at subarc depths. We conclude that varying subduction conditions, and non-steady-state thermal structure, challenge the utility of kinematically driven models with constant subduction parameters, particularly for investigating thermal structure in the geological past.
The dynamics of western North America: stress magnitudes and the relative role of gravitational potential energy, plate interaction, boundary and basal
Geophysical Journal International, Vol. 169, 3, pp. 866-896.
Geophysical Research Letters, Vol. 43, 1, pp. 144-153.
United States
Geophysics - gradiometry
Abstract: In order to better understand the tectonic evolution of the North American continent, we utilize data from the EarthScope Transportable Array network to calculate a three-dimensional shear velocity model for the continental United States. This model was produced through the inversion of Rayleigh wave phase velocities calculated using ambient noise tomography and wave gradiometry, which allows for sensitivity to a broad depth range. Shear velocities within this model highlight the influence of orogenic and postorogenic events on the evolution of the lithosphere. Most notable is the contrast in crustal and upper mantle structure between the relatively slow western and relatively fast eastern North America. These differences are unlikely to stem solely from thermal variations within the lithosphere and highlight both the complexities in lithospheric structure across the continental U.S. and the varying impacts that orogeny can have on the crust and upper mantle.
Journal of Geophysical Research,, Vol. 120, 12, pp. 8633-8655.
Mantle
Geodynamics
Abstract: With the advances in technology, seismological theory, and data acquisition, a number of high-resolution seismic tomography models have been published. However, discrepancies between tomography models often arise from different theoretical treatments of seismic wave propagation, different inversion strategies, and different data sets. Using a fixed velocity-to-density scaling and a fixed radial viscosity profile, we compute global mantle flow models associated with the different tomography models and test the impact of these for explaining surface geophysical observations (geoid, dynamic topography, stress, and strain rates). We use the joint modeling of lithosphere and mantle dynamics approach of Ghosh and Holt (2012) to compute the full lithosphere stresses, except that we use HC for the mantle circulation model, which accounts for the primary flow-coupling features associated with density-driven mantle flow. Our results show that the seismic tomography models of S40RTS and SAW642AN provide a better match with surface observables on a global scale than other models tested. Both of these tomography models have important similarities, including upwellings located in Pacific, Eastern Africa, Iceland, and mid-ocean ridges in the Atlantic and Indian Ocean and downwelling flows mainly located beneath the Andes, the Middle East, and central and Southeast Asia.
Earth and Planetary Science Letters, Vol. 479, pp. 43-49.
Mantle
perovskite
Abstract: Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (?1.1 W/m/K) is almost two times smaller than that of bridgmanite (?2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.
Abstract: The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.
Earth and Planetary Science Letters, Vol. 578, 9p. Pdf
Mantle
bridgmanite
Abstract: The Earth has been releasing vast amounts of heat from deep Earth's interior to the surface since its formation, which primarily drives mantle convection and a number of tectonic activities. In this heat transport process the core-mantle boundary where hot molten core is in direct contact with solid-state mantle minerals has played an essential role to transfer thermal energies of the core to the overlying mantle. Although the dominant heat transfer mechanisms at the lowermost mantle is believed to be both conduction and radiation of the primary lowermost mantle mineral, bridgmanite, the radiative thermal conductivity of bridgmanite has so far been poorly constrained. Here we revealed the radiative thermal conductivity of bridgmanite at core-mantle boundary is substantially high approaching to ?5.3±1.2 W/mK based on newly established optical absorption measurement of single-crystal bridgmanite performed in-situ under corresponding deep lower mantle conditions. We found the bulk thermal conductivity at core-mantle boundary becomes ?1.5 times higher than the conventionally assumed value, which supports higher heat flow from core, hence more vigorous mantle convection than expected. Results suggest the mantle is much more efficiently cooled, which would ultimately weaken many tectonic activities driven by the mantle convection more rapidly than expected from conventionally believed thermal conduction behavior.
Earth and planetary Science Letters, Vol. 578, 117328, 9p. Pdf
Mantle
bridgmanite
Abstract: The Earth has been releasing vast amounts of heat from deep Earth's interior to the surface since its formation, which primarily drives mantle convection and a number of tectonic activities. In this heat transport process the core-mantle boundary where hot molten core is in direct contact with solid-state mantle minerals has played an essential role to transfer thermal energies of the core to the overlying mantle. Although the dominant heat transfer mechanisms at the lowermost mantle is believed to be both conduction and radiation of the primary lowermost mantle mineral, bridgmanite, the radiative thermal conductivity of bridgmanite has so far been poorly constrained. Here we revealed the radiative thermal conductivity of bridgmanite at core-mantle boundary is substantially high approaching to ?5.3±1.2 W/mK based on newly established optical absorption measurement of single-crystal bridgmanite performed in-situ under corresponding deep lower mantle conditions. We found the bulk thermal conductivity at core-mantle boundary becomes ?1.5 times higher than the conventionally assumed value, which supports higher heat flow from core, hence more vigorous mantle convection than expected. Results suggest the mantle is much more efficiently cooled, which would ultimately weaken many tectonic activities driven by the mantle convection more rapidly than expected from conventionally believed thermal conduction behavior.
Mineralogy and Petrology, doi.org/101007/ s00710-018-0623-6 10p.
Europe, Sweden
alkaline
Abstract: Potassic-richterite, ideally AKB(NaCa)CMg5TSi8O22W(OH)2, is recognized as a valid member of the amphibole supergroup (IMA-CNMNC 2017-102). Type material is from the Pajsberg Mn-Fe ore field, Filipstad, Värmland, Sweden, where the mineral occurs in a Mn-rich skarn, closely associated with mainly phlogopite, jacobsite and tephroite. The megascopic colour is straw yellow to grayish brown and the luster vitreous. The nearly anhedral crystals, up to 4 mm in length, are pale yellow (non-pleochroic) in thin section and optically biaxial (?), with ??=?1.615(5), ??=?1.625(5), ??=?1.635(5). The calculated density is 3.07 g•cm?1. VHN100 is in the range 610-946. Cleavage is perfect along {110}. EPMA analysis in combination with Mössbauer and infrared spectroscopy yields the empirical formula (K0.61Na0.30Pb0.02)?0.93(Na1.14Ca0.79Mn0.07)?2(Mg4.31Mn0.47Fe3+0.20)?5(Si7.95Al0.04Fe3+0.01)?8O22(OH1.82F0.18)?2 for a fragment used for collection of single-crystal X-ray diffraction data. The infra-red spectra show absorption bands at 3672 cm?1 and 3736 cm?1 for the ? direction. The crystal structure was refined in space group C2/m to R1?=?3.6% [I >?2?(I)], with resulting cell parameters a?=?9.9977(3) Å, b?=?18.0409(4) Å, c?=?5.2794(2) Å, ??=?104.465(4)°, V?=?922.05(5) Å3 and Z?=?2. The A and M(4) sites split into A(m) (K+), A(2/m) (Na+), A(2) (Pb2+), and M(4?) (Mn2+) subsites, respectively. The remaining Mn2+ is strongly ordered at the octahedrally coordinated M(2) site, possibly together with most of Fe3+. The skarn bearing potassic-richterite formed at peak metamorphism, under conditions of low SiO2 and Al2O3 activities and relatively high oxygen fugacities.
Abstract: The contribution of magmatic and hydrothermal processes to rare earth element (REE) mineralization of carbonatites remains an area of considerable interest. With the aim of better understanding REE mineralization mechanisms, we conducted a detailed study on the petrology, mineralogy and C-O isotopes of the Bachu carbonatites, NW China. The Bachu carbonatites are composed predominantly of magnesiocarbonatite with minor calciocarbonatite. The two types of carbonatite have primarily holocrystalline textures dominated by dolomite and calcite, respectively. Monazite-(Ce) and bastnäsite-(Ce), the major REE minerals, occur as euhedral grains and interstitial phases in the carbonatites. Melt inclusions in the dolomite partially rehomogenize at temperatures above 800?°C, and those in apatite have homogenization temperatures (Th) ranging from 645 to 785?°C. Oxygen isotope ratios of the calciocarbonatite intrusions (?18OV-SMOW?=?6.4‰ to 8.3‰), similar to the magnesiocarbonatites, indicate the parental magma is mantle-derived, and that they may derive from a more evolved stage of carbonatite fractionation. The magnesiocarbonatites are slightly enriched in LREE whereas calciocarbonatites have higher HREE concentrations. Both dolomite and calcite have low total REE (TREE) contents ranging from 112 to 436?ppm and 88 to 336?ppm, respectively, much lower than the bulk rock composition of the carbonatites (371 to 36,965?ppm). Hence, the fractional crystallization of carbonates is expected to elevate REE concentrations in the residual magma. Rocks from the Bachu deposit with the highest TREE concentration (up to 20?wt%) occur as small size (2?mm to 3 cm) red rare earth-rich veins (RRV) with barite + celestine + fluorapatite + monazite-(Ce) associations. These rocks are interpreted to have a hydrothermal origin, confirmed by the fluid inclusions in barite with Th in the range 198-267?°C. Hydrothermal processes may also explain the existence of interstitial textures in the carbonatites with similar mineral assemblages. The C-O isotopic compositions of the RRV (?13CV-PDB?=??3.6 to ?4.3‰, ?18OV-SMOW?=?7.6 to 9.8‰) are consistent with an origin resulting from fluid exsolution at the end of the high temperature fractionation trend. A two-stage model involving fractional crystallization and hydrothermal fluids is proposed for the mineralization of the Bachu REE deposit.
Abstract: On Earth, broken, rigid tectonic plates lie atop slowly flowing mantle rock (over millions to billions of years). A basic understanding of the global variation in thickness of this rigid lid provides the foundation to many geodynamical predictions. However, using different techniques to estimate its thickness, for example, seismic wave propagation (acting on timescales of seconds), to the warping of plates under the weight of volcanoes (acting on timescales of millions of years) reveals many inconsistencies. At the heart of these inconsistencies is the fact that rock deforms differently to forces acting on different timescales. At very fast timescales rock deforms like an elastic solid, but at much longer timescales, rock flows. To resolve these inconsistencies, we attempt to coherently tie these disparate observations together to reach a more holistic understanding of plate thickness, accounting for these timescale effects. By incorporating current understanding on rock deformation from laboratory experiments, we demonstrate that on fast timescales (of the seismic waves used to image the Earth's interior), tectonic plates appear significantly thicker than the true thickness at million? to billion?year timescales of plate tectonics. This demonstration involves a new theoretical and conceptual framework for interpreting distinct observations acting on vastly different timescales.
Nature Communications, doi.org/10.1038/s41467-019-11065-4 pdf 10p.
Mantle
subduction
Abstract: Ore deposits are loci on Earth where energy and mass flux are greatly enhanced and focussed, acting as magnifying lenses into metal transport, fractionation and concentration mechanisms through the lithosphere. Here we show that the metallogenic architecture of the lithosphere is illuminated by the geochemical signatures of metasomatised mantle rocks and post-subduction magmatic-hydrothermal mineral systems. Our data reveal that anomalously gold and tellurium rich magmatic sulfides in mantle-derived magmas emplaced in the lower crust share a common metallogenic signature with upper crustal porphyry-epithermal ore systems. We propose that a trans-lithospheric continuum exists whereby post-subduction magmas transporting metal-rich sulfide cargoes play a fundamental role in fluxing metals into the crust from metasomatised lithospheric mantle. Therefore, ore deposits are not merely associated with isolated zones where serendipitous happenstance has produced mineralisation. Rather, they are depositional points along the mantle-to-upper crust pathway of magmas and hydrothermal fluids, synthesising the concentrated metallogenic budget available.
Nature Communications, doi.org/10.1038/ s41467-020-18157-6 11p. Pdf
Mantle
carbon
Abstract: Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.
Journal of African Earth Sciences, Vol. 129, pp. 202-223.
Africa, Morocco
Alkaline rocks
Abstract: The Jbel Boho complex (Anti-Atlas/Morocco) is an alkaline magmatic complex that was formed during the Precambrian-Cambrian transition, contemporaneous with the lower early Cambrian dolomite sequence. The complex consists of a volcanic sequence comprising basanites, trachyandesites, trachytes and rhyolites that is intruded by a syenitic pluton. Both the volcanic suite and the pluton are cut by later microsyenitic and rhyolitic dykes. Although all Jbel Boho magmas were probably ultimately derived from the same, intraplate or plume-like source, new geochemical evidence supports the concept of a minimum three principal magma generations having formed the complex. Whereas all volcanic rocks (first generation) are LREE enriched and appear to be formed by fractional crystallization of a mantle-derived magma, resulting in strong negative Eu anomalies in the more evolved rocks associated with low Zr/Hf and Nb/Ta values, the younger syenitic pluton displays almost no negative Eu anomaly and very high Zr/Hf and Nb/Ta. The syenite is considered to be formed by a second generation of melt and likely formed through partial melting of underplated mafic rocks. The syenitic pluton consists of two types of syenitic rocks; olivine syenite and quartz syenite. The presence of quartz and a strong positive Pb anomaly in the quartz syenite contrasts strongly with the negative Pb anomaly in the olivine syenite and suggests the latter results from crustal contamination of the former. The late dyke swarm (third generation of melt) comprises microsyenitic and subalkaline rhyolitic compositions. The strong decrease of the alkali elements, Zr/Hf and Nb/Ta and the high SiO2 contents in the rhyolitic dykes might be the result of mineral fractionation and addition of mineralizing fluids, allowing inter-element fractionation of even highly incompatible HFSE due to the presence of fluorine. The occurrence of fluorite in some volcanic rocks and the Ca-REE-F carbonate mineral synchysite in the dykes with very high LREE contents (Ce ?720 ppm found in one rhyolitic dyke) suggest the fluorine-rich nature of this system and the role played by addition of mineralizing fluids. The REE mineralization expressed as synchysite-(Ce) is detected in a subalkaline rhyolitic dyke (with ?LREE = 1750 ppm) associated with quartz, chlorite and occasionally with Fe-oxides. The synchysite mineralization is probably the result of REE transport by acidic hydrothermal fluids as chloride complex and their neutralization during fluid-rock interaction. The major tectonic change from compressive to extensional regime in the late Neoproterozoic induced the emplacement of voluminous volcaniclastic series of the Ediacran Ouarzazate Group. The alkaline, within-plate nature of the Jbel Boho igneous complex implies that this extensional setting continued during the early Cambrian.
Geophysical Research Letters, Vol. 46, 16, pp. 9509-9518.
Africa, South Africa
geophysics - seismics
Abstract: P and S waves travel times from large, distant earthquakes recorded on seismic stations in Botswana and South Africa have been combined with existing data from the region to construct velocity models of the upper mantle beneath southern Africa. The models show a region of higher velocities beneath the Rehoboth Province and parts of the northern Okwa Terrane and the Magondi Belt, which can be attributed to thicker cratonic lithosphere, and a region of lower velocities beneath the Damara?Ghanzi?Chobe Belt and Okavango Rift, which can be attributed a region of thinner off?craton lithosphere. This finding suggests that the spatial extent of thick cratonic lithosphere in southern Africa is greater than previously known. In addition, within the cratonic lithosphere an area of lower velocities is imaged, revealing parts of the cratonic lithosphere that may have been modified by younger magmatic events.
Abstract: Oceanic basalts reflect the heterogeneities in the earth's mantle, which can be explained by five mantle end members. The HIMU end member, characterized by high time-integrated ? (238U/204Pb), is defined by the composition of lavas from the ocean islands of St. Helena, South Atlantic Ocean and Mangaia and Tubuai (Cook-Austral Islands), South Pacific Ocean. It is widely considered to be derived from a mantle reservoir that is rarely sampled and not generally involved in mixing with the other mantle components. On the other hand, the FOZO end member, located at the FOcal ZOne of oceanic volcanic rock arrays on isotope diagrams, is considered to be a widespread common component with slightly less radiogenic 206Pb/204Pb and intermediate Sr-Nd-Hf isotopic compositions. Here we present new major and trace element, Sr-Nd-Pb-Hf isotope and geochronological data from the Walvis Ridge and Richardson Seamount in the South Atlantic Ocean and the Manihiki Plateau and Eastern Chatham Rise in the southwest Pacific Ocean. Our new data, combined with literature data, document a more widespread (nearly global) distribution of the HIMU end member than previously postulated. Our survey shows that HIMU is generally associated with low-volume alkaline, carbonatitic and/or kimberlitic intraplate volcanism, consistent with derivation from low degrees of melting of CO2-rich sources. The majority of end member HIMU locations can be directly related to hotspot settings. The restricted trace element and isotopic composition (St. Helena type HIMU), but near-global distribution, point to a deep-seated, widespread reservoir, which most likely formed in the Archean. In this context we re-evaluate the origin of a widespread HIMU reservoir in an Archean geodynamic setting. We point out that the classic ocean crust recycling model cannot be applied in a plume-lid dominated tectonic setting, and instead propose that delamination of carbonatite-metasomatized subcontinental lithospheric mantle could be a suitable HIMU source.
Honda, M., Phillips, D., Harris, J.W., Matsumoto, T.
He, Ne and Ar in peridotitic and eclogitic paragenesis diamonds from the Jwaneng kimberlite, Botswana - implications for mantle evolution and diamond formation ages.
Earth and Planetary Science Letters, Vol. 301, 1-2, pp. 43-51.
Mineralogy and Petrology, 10.1007/ s710-018- 0603-x 15p.
Africa, South Africa
deposit - Finsch, De Beers Pool, Koffiefontein
Abstract: Fibrous diamond growth zones often contain abundant high-density fluid (HDF) inclusions and these provide the most direct information on diamond-forming fluids. Noble gases are incompatible elements and particularly useful in evaluating large-scale mantle processes. This study further constrains the evolution and origin of the HDFs by combining noble gas systematics with ?¹³C, N concentrations, and fluid inclusion compositions for 21 individual growth zones in 13 diamonds from the Finsch (n = 3), DeBeers Pool (n = 7), and Koffiefontein (n = 3) mines on the Kaapvaal Craton. C isotope compositions range from ?2.8 to ?8.6‰ and N contents vary between 268 and 867 at.ppm, except for one diamond with contents of <30 at.ppm N. Nine of the thirteen studied diamonds contained saline HDF inclusions, but the other four diamonds had carbonatitic or silicic HDF inclusions. Carbonatitic and silicic HDFs yielded low He concentrations, R/Ra (³He/?Hesample/³He/?Heair) values of 3.2–6.7, and low ??Ar/³?Ar ratios of 390–1940. Noble gas characteristics of carbonatitic-silicic HDFs appear consistent with a subducted sediment origin and interaction with eclogite. Saline HDFs are characterised by high He concentrations, with R/Ra mostly between 3.9 and 5.7, and a wide range in ??Ar/³?Ar ratios (389–30,200). The saline HDFs likely originated from subducted oceanic crust with low He but moderate Ar contents. Subsequent interaction of these saline HDFs with mantle peridotite could explain the increase in He concentrations and mantle-like He isotope composition, with the range in low to high ??Ar/³?Ar ratios dependent on the initial ³?Ar content and extent of lithosphere interaction. The observed negative correlation between ?He contents and R/Ra values in saline HDFs indicates significant in situ radiogenic ?He production.
Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids.
Geochimica et Cosmochimica Acta, in press available 29p.
Africa, South Africa, Botswana
deposit - Koffiefontein, Letlhakane, Orapa, Finsch, De Beers Pool
Abstract: Ten individual gem-quality monocrystalline diamonds of known peridotite/eclogite paragenesis from Southern Africa (Koffiefontein, Letlhakane, Orapa) were studied for trace element concentrations and He and Ar abundances and isotopic compositions. In addition, two samples, consisting of pooled fragments of gem-quality peridotitic diamonds from Finsch and DeBeers Pool respectively, were analysed for noble gases. Previous studies (Richardson et al., 1984; Pearson et al., 1998; Gress et al., 2017; Timmerman et al., 2017) provided age constraints of 0.09, 1.0-1.1, 1.7, 2.3, and 3.2-3.4?Ga on mineral inclusions in the studied diamonds, allowing us to study trace elements and noble gases over 3 Gyr of geological time. Concentrations of trace elements in the diamonds are very low - a few hundred ppt to several tens of ppbs - and are likely dependent on the amount of sub-micron inclusions present. Trace element patterns and trace element/3He ratios of the studied monocrystalline diamonds are similar to those in fibrous diamonds, suggesting that trace elements and stable noble gas isotopes reside within the same locations in diamond and track the same processes that are reflected in the trace element patterns. We cannot discern any temporal differences in these geochemical tracers, suggesting that the processes generating them have been occurring over at least the past 2.3?Ga. 3He/4He ratios decrease and 4He and 40Ar* contents increase with increasing age of peridotitic and some eclogitic diamonds, showing the importance of in-situ radiogenic 4He and 40Ar ingrowth by the decay of U-Th-Sm and K respectively. For most gem-quality monocrystalline diamonds, uncertainties in the 3He/4He evolution of the continental lithospheric mantle combined with large analytical uncertainties and possible spatial variability in U-Th-Sm concentrations limit our ability to provide estimates of diamond formation ages using 4He ingrowth. However, the limited observed 4He ingrowth (low U?+?Th/3He) together with a R/Ra value of 5.3 for peridotitic diamond K306 is comparable to the present-day sub-continental lithospheric mantle value and supports the young diamond formation age found by Re-Os dating of sulphides in the same diamond by Pearson et al. (1998). After correction for in-situ radiogenic 4He produced since diamond formation a large variation in 3He/4He remains in ?1?Ga old eclogitic diamonds that is suggested to result from the variable influence of subducted altered oceanic crust that has low 3He/4He. Hence, the 3He/4He isotope tracer supports an origin of the diamond-forming fluids from recycled oceanic crust for eclogitic diamonds, as indicated by other geochemical proxies.
Abstract: The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, ?-implantation, ?-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71?Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.
Abstract: He-Ne-Ar compositions were determined in diamonds from the Argyle lamproite, Western Australia, to assess whether subducted material affects the noble gas budget and composition of stable old sub-continental lithospheric mantle (SCLM). Twenty diamonds (both peridotitic and eclogitic) were characterized for their carbon isotopic compositions and N abundance and aggregation from which 10 eclogitic growth zones and 5 peridotitic growth zones were analysed for their He-Ne-Ar compositions. The eclogitic diamonds have ?13C values of ?4.7 to ?16.6‰ indicating a subduction signature, whereas the peridotitic diamonds have mantle-like compositions of ?4.0 to ?7.8‰. Mantle residence temperatures based on N-in-diamond thermometry showed that the eclogitic diamonds were mainly formed at 1260-1270?°C or above 1300?°C near the base of the lithosphere, whereas the peridotitic diamonds generally formed at lower temperatures (mostly 1135-1230?°C). A noble gas subduction signature is present to various extents in the eclogitic diamonds and is inferred from a hyperbolic mixing relationship between R/Ra and 4He and ?13C values concentrations with a predominance of low R/Ra values (<0.5; R/Ra?=?3He/4Hesample/3He/4Heair). In addition, low 40Ar/4He and 40Ar/36Ar ratios, high nucleogenic 21Ne/4He and low 3He/22Ne ratios are characteristic of subducted material and were found in the eclogitic diamonds. The peridotitic diamonds show generally higher R/Ra values (median 1.1?±?1.1) and lower 4He/40Ar ratios compared to eclogitic diamonds (median 0.1?±?0.8 R/Ra; with 7/10 samples having an average of 0.13?±?0.14 R/Ra). The studied peridotitic diamond growth zones showed a negative correlation between R/Ra and 4He concentrations over 2 orders of magnitude and limited variation in 3He, that can be largely explained by radiogenic 4He ingrowth. At low 4He concentrations the R/Ra value is around 2.8 for both paragenesis of diamonds and is significantly lower than present-day SCLM values, suggesting (1) a more radiogenic helium isotope composition beneath the Halls Creek Orogen than those for typical SCLM from other cratons and/or (2) that the peridotitic diamonds are formed from fluids that also had a subduction input. The high mantle residence temperature and low R/Ra value in the core and low temperature and higher R/Ra value in the rim of a single peridotitic diamond indicate multiple growth events and that part of the lherzolitic diamond population may be genetically related to the eclogitic diamonds. Combining the diamond mantle residence temperatures with noble gas compositions shows that noble gas subduction signatures are present at the base of the lithosphere below 180?km depth beneath Argyle and that fluid migration and interaction with the SCLM occurred over scales of at least 15?km, between 180 and 165?km depth.
Abstract: Isotope compositions of basalts provide information about the chemical reservoirs in Earth’s interior and play a critical role in defining models of Earth’s structure. However, the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly. This information is a vital baseline for understanding helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers) unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr isotope variability, with high 3He/4He ratios related to higher helium concentrations. This indicates that a less degassed, high-3He/4He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts.
International Journal of Remote Sensing, Vol. 41, 14, pp. 5549-5572.
Global
hyperspectral
Abstract: The classification of hyperspectral images (HSIs) is one of the most popular topics in the remote sensing community. Numerous feature extraction methods have been proposed to improve the classification accuracy of HSIs. Recently, deep features extracted by convolution neural network (CNN) have been introduced into the classification process of HSIs. Due to the nonlinear and invariant advantages of the features, CNN methods provide a powerful tool for representing geographic objects and classifying HSIs. However, traditional deep features only extracted at pixel-level and often neglect multiscale characteristics of geographic objects. In this study, a new deep feature extraction method is proposed, which takes advantage of multi-scale object analysis and the CNN model. Firstly, multiscale image objects are obtained by the multiscale segmentation algorithm and multiscale low-level features of objects are extracted. Secondly, the CNN is devoted to obtain deep features from low-level object features at each scale, respectively. Thirdly, the obtained deep features at all scales are stacked and fed to one fully connected layer to extract the multiscale deep learning features for classification. Finally, the logistic regression classifier is applied to hyperspectral image (HSI) classification based on object-oriented multiscale deep features. The proposed method was carried out on three widely used hyperspectral data sets: University of Pavia, Salinas, and Washington DC. The results reveal that the proposed method provides better results than other state-of-the-art methods.
Abstract: North America provides an important test for assessing the coupling of large continents with heterogeneous Archean- to Cenozoic-aged lithospheric provinces to the mantle flow. We use the unprecedented spatial coverage of the USArray seismic network to obtain an extensive and consistent data set of shear wave splitting intensity measurements at 1436 stations. Overall, the measurements are consistent with simple shear deformation in the asthenosphere due to viscous coupling to the overriding lithosphere. The fast directions agree with the absolute plate motion direction with a mean difference of 2° with 27° standard deviation. There are, however, deviations from this simple pattern, including a band along the Rocky Mountain front, indicative of flow complication due to gradients in lithospheric thickness, and variations in amplitude through the central United States, which can be explained through varying contributions of lithospheric anisotropy. Thus, seismic anisotropy may be sourced in both the asthenosphere and lithosphere, and variations in splitting intensity are due to lithospheric anisotropy developed during deformation over long time scales.
Physics of the Earth and Planetary Interiors, Vol. 287, pp. 37-50.
Mantle
plate tectonics
Abstract: Is the present-day water-land ratio a necessary outcome of the evolution of plate tectonic planets with a similar age, volume, mass, and total water inventory as the Earth? This would be the case - largely independent of initial conditions - if Earth’s present-day continental volume were at a stable unique equilibrium with strong self-regulating mechanisms of continental growth steering the evolution to this state. In this paper, we question this conjecture. Instead we suggest that positive feedbacks in the plate tectonics model of continental production and erosion may dominate and show that such a model can explain the history of continental growth. We investigate the main mechanisms that contribute to the growth of the volume of the continental crust. In particular, we analyze the effect of the oceanic plate speed, depending on the area and thickness of thermally insulating continents, on production and erosion mechanisms. Effects that cause larger continental production rates for larger values of continental volume are positive feedbacks. In contrast, negative feedbacks act to stabilize the continental volume. They are provided by the increase of the rate of surface erosion, subduction erosion, and crustal delamination with the continental volume. We systematically analyze the strengths of positive and negative feedback contributions to the growth of the continental crust. Although the strengths of some feedbacks depend on poorly known parameters, we conclude that a net predominance of positive feedbacks is plausible. We explore the effect of the combined feedback strength on the feasibility of modeling the observed small positive net continental growth rate over the past 2-3 billion years. We show that a model with dominating positive feedbacks can readily explain this observation in spite of the cooling of the Earth’s mantle acting to reduce the continental production rate. In contrast, explaining this observation using a model with dominating negative feedbacks would require the continental erosion and production rates to both have the same or a sufficiently similar functional dependence on the thermal state of the mantle, which appears unreasonable considering erosion to be largely dominated by the surface relief and weathering. The suggested scenario of dominating positive feedbacks implies that the present volume of the continental crust and its evolution are strongly determined by initial conditions. Therefore, exoplanets with Earth-like masses and total water inventories may substantially differ from the Earth with respect to their relative land/surface ratios and their habitability.
Abstract: Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnäsite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globally, rare-earth mineral deposits commonly show evidence of polyphase development and mineralisation processes, which need to be better understood to improve exploration strategies. The Norra Kärr alkaline complex (Sweden) contains a globally significant deposit of rare-earth elements, hosted in the mineral eudialyte. In this study, we focussed on eudialyte crystals in undeformed, cross-cutting pegmatoid veins from Norra Kärr. In order to determine their age, we refined an established micromilling method to enable sampling of minerals rich in rare-earth elements for precise analysis of major and trace elements, Nd isotope ratios, and Sm-Nd geochronology down to a scale of <200??m. Mineral samples were subjected to detailed textural and chemical characterisation by backscattered electron imaging and laser ablation inductively coupled plasma mass spectrometry, by which precise and accurate Sm/Nd ratios were determined to steer subsequent micromill sampling for small-aliquot Sm-Nd isotope analysis by isotope dilution thermal ionisation mass spectrometry. Given enough internal spread in Sm/Nd ratios, reliable Sm-Nd isochrons can be derived from discrete textural domains within a single crystal. This provided an age of 1.144?±?0.053?Ga (95% confidence); approximately 350?million?years younger than the magmatic intrusion of the alkaline complex (ca. 1.49?Ga). Primary compositional sector and oscillatory zoning in these eudialyte crystals shows core-to-rim enrichment in rare-earth elements and significant fractionation of K/Rb, Y/Ho, Zr/Hf, and Nb/Ta, which we attribute to crystallisation under influence of complexing ligands in a confined volume. We argue that these mineralised pegmatoid veins formed by low-temperature (<550?°C) partial melting of the agpaitic host rock during an early Sveconorwegian (Grenvillian) metamorphic overprinting event. Given the challenge of directly dating rare-earth ore minerals by conventional methods, modification of rare-earth mineral deposits may be more widespread than already assumed, which shows the importance of investigations that date the rare-earth minerals themselves.
Brazil Journal of Geology, Vol. 44, 2, pp. 325-338.
South America, Brazil, Minas Gerais
Deposit - Coromandel
Abstract: Important diamond deposits southeast of Coromandel and the local geology have been studied in an attempt to understand what surface source provided the stones. River gravels of Pleistocene to Recent age from this region have supplied most of Brazil’s large diamonds over 100 ct. The upper cretaceous Capacete Formation of the Mata da Corda Group, composed of mafic volcanoclastic, pyroclastic and epiclastic material, has been worked locally for diamonds, nevertheless considered non-economic. The authors present results of their study of a deactivated small mine, representing the first report with description and analyses of two gem diamonds washed from this material. Hundreds of kimberlites, discovered in the last half century in the region, are sterile or non-economic. We propose that the surface source of the diamonds is the Capacete “conglomerado”. The volume of this material is enormous representing a potential resource for large-scale mining. The authors suggest detailed studies of the volcanic facies of this unit focusing on the genesis, distribution and diamond content. As to the question concerning the origin of these diamondiferous pyroclastic rocks, the authors exclude the kimberlites and point towards the large Serra Negra and Salitre alkaline complexes which are considered the primary source for the pyroclastic units of the Mata da Corda Group. They propose that early eruptive phases of this alkaline complex brought diamonds from a mantle source to the surface, much as happens with traditional kimberlites, to explain the association of such huge carbonatite complexes and diamonds.
Neues Jahrbuch fur Geologie und Palaontologie , Vol. 277, 2, pp. 237-250.
South America, Brazil, Minas Gerais
Deposit - Coromandel
Abstract: The origin of diamonds in the Coromandel area has been an enigma for many years, in spite of high investment in conventional and high tech prospecting methods by major mining companies for over half a century. The authors review the history, and then discuss the two principal hypotheses to explain the source of these alluvial diamonds. After mapping the headwater region of one of the richest alluvial diamond rivers, the Santo Antônio do Bonito River, they reject both principal hypotheses and conclude that the surficial source can be only the Upper Cretaceous Capacete Formation, composed of pyroclastics and epiclastics. Based on geophysical data from the literature, combined with field observations the authors suggest that the largest alkaline complex, situated within the diamond producing area, the Serra Negra/Salitre Complex has been the primary source for those pyroclastics of the Capacete Formation and the diamonds. The plugs of this complex are 15-30 times deeper than average kimberlites and other alkaline complexes in the region, and its excess of volume of the intrusive is three orders of magnitude larger than a typical kimberlite. With an intrusive volume of over 1000 km3 the complex is suggested to be a possible supervolcano. This explains the vast areal distribution of the pyroclastics and diamonds. This new hypothesis has advantages and disadvantages, some of them discussed in the paper and leading to the conclusion that further research is needed.
Brazil Journal of Geology, Vol. 44, 1, pp. 91-103.
South America, Brazil
Coromandel district
Abstract: The diamond bearing district of Coromandel is located in the northwestern part of Minas Gerais, within the Alto Paranaíba Arch, famous for the discovery of most of Brazil's large diamonds above 100 ct. Detailed mapping, aimed at characterizing the Mata da Corda Group of Upper Cretaceous age of Coromandel, has been carried out. This Group was divided into the Patos Formation, composed of kimberlitic and kamafugitic rocks, and the Capacete Formation, presented by conglomerates, pyroclastic rocks, arenite and tuffs. Exposures of the latter Formation have been studied in detail at the small abandoned mine called Canastrel, as well as in the headwater of Santo Antônio do Bonito River. The results have been compared to studies of the kimberlite bodies in the nearby Douradinho River. Kimberlite indicator minerals from these localities show the same compositional trend. Moreover, in the basal conglomerate of the Garimpo Canastrel two diamonds diamonds have been recovered and described. The Garimpo Wilson, situated in the headwater of the river Santo Antônio do Bonito in paleo-alluvium, is composed of material exclusively derived from the erosion of the Capacete Formation and Precambrian (sterile) Canastra quartzites and schists. These detailed investigations suggest that the basal conglomerates of the Capacete Formation represent the main source rock of the alluvial diamond deposits in the Coromandel region.
GSA Annual Meeting, Paper 300-1, 1p. Abstract only Booth
South America, Brazil, Minas Gerais
Deposit - Coromandel
Abstract: The origin of diamonds in the Coromandel area has been an enigma for many years, in spite of high investment in conventional and high tech prospecting methods by major mining companies for over half a century. The authors review the history, and then discuss the two principal hypotheses to explain the source of these alluvial diamonds. After mapping the headwater region of one of the richest alluvial diamond rivers, the Santo Antônio do Bonito River, they reject both principal hypotheses and conclude that the surficial source can be only the Upper Cretaceous Capacete Formation, composed of pyroclastics and epiclastics. Based on geophysical data from the literature, combined with field observations the authors suggest that the largest alkaline complex, situated within the diamond producing area, the Serra Negra/Salitre Complex has been the primary source for those pyroclastics of the Capacete Formation and the diamonds. The plugs of this complex are 15-30 times deeper than average kimberlites and other alkaline complexes in the region, and its excess of volume of the intrusive is three orders of magnitude larger than a typical kimberlite. With an intrusive volume of over 1000 km3 the complex is suggested to be a possible supervolcano. This explains the vast areal distribution of the pyroclastics and diamonds. This new hypothesis has advantages and disadvantages, some of them discussed in the paper and leading to the conclusion that further research is needed.
The Australian Gemmologist, Vol. 26, 5&6, pp. 88-99.
South America, Brazil, Minas Gerais
deposit - Alto Paranaiba
Abstract: The authors, in a paper in this journal in 2009, note a puzzle, that in spite of extensive exploration for diamonds by major producers in the Alto Paranaiba region of West Minas Gerais State, Brazil, no primary source, such as kimberlites, for the many diamonds produced since their discovery over 250 years has been found. To answer this puzzle we propose that the diamonds are present within a large extrusive volcanic unit probably derived from the Serra Negra alkaline-carbonatitic complex which comprises a super volcano. This origin fits with the 1995 prediction of Nixon on the future direction of diamona-exploration that extrusive units may contain very large volumes of ore, and that carbonatitic emplacement sources need to be considered. The authors argue, based on available evidence from geology and geophysics, that such an origin is compatible with the known data, but that much additional information is needed to substantiate these ideas. Diamonds of the Alto Paraniaba, Brazil: Nixon's prediction verified?
Journal of Petrology, in press available, 52p. Pdf
United States, Colorado Plateau
eclogite
Abstract: The Colorado Plateau has undergone as much as 1.8?km of uplift over the past 80?Ma, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for ?18O by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3‰. All samples have core ?18O values greater than that of the pristine mantle (?5.3‰, mantle garnet as derived from mantle zircon; Valley et al., 1998; Page et al., 2007) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim recording interaction with a low-?18O fluid at high temperature, likely derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a ?18O value of ?6‰, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best-estimates of peak metamorphic conditions (620ºC, 3.7?GPa) yield durations of?
An analytical method for hydrogeochemical surveys: inductively coupled plasma atomic emission spectrometry after using enrichment coprecipitation with cobalt and amM.
Journal of Geochemical Exploration, Vol. 41, No. 3, November pp. 349-362
Hopp, J., Trieloff, M., Brey, G.P., Woodland, A.B., Simon, N.S.C., Wijbrans, J.R., Siebel, W., Reitter, E.
40 Ar 39 Ar ages of phlogopite in mantle xenoliths from South African kimberlites: evidence for metasomatic mantle impregnation during Kilbaran orogenic cycle.
Buikin, A.I., Trieloff, M., Korochantseeva, E.V., Hopp, J., Kaliwood, M., Meyer, H-P.,Altherr, R.
Distribution of mantle and atmospheric argon in mantle xenoliths from western Arabian Peninsula: constraints on timing and composition of metasomatizing agents....
Geochemistry International, Vol. 55, 5, pp. 457-464.
Russia
carbonatite - Guli
Abstract: For better understanding of the fluid phase sources of carbonatites of Guli alkaline-ultrabasic intrusion (Maymecha-Kotuy complex) we have studied isotope composition of He and Ne in the carbonatites of different formation stages. The data definitely point to the subcontinental lithospheric mantle (SCLM) as a primary source of fluid phase of Guli carbonatites. The absence of plume signature in such a plume-like object (from petrological point of view) could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.
Noble gas composition of Indian carbonatites ( Amba Dongar, Siriwasan): implications on mantle source compositions and late stage hydrothermal processes.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 10.
India
deposit - Amba Dongar, Siriwasan
Abstract: Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He* and nucleogenic 21Ne* imprint related to insitu production from U and Th in mineral impurities, most likely minute apatite grains. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. We conclude that previously derived models of a lithospheric mantle source containing recycled components in generation of the carbonatitic magmas from Amba Dongar are obsolete. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions, similar to Kilauea Volcano, Hawai’i. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component likely had been added at shallow levels during the emplacement process. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.
Earth Planetary Science Letters, Vol. 492, pp. 186-196.
India
carbonatite
Abstract: Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He and nucleogenic 21Ne imprint related to in situ production from U and Th in mineral impurities, most likely minute apatite grains, or late incorporation of crustal fluids. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. This transition from a mantle plume signal in first crushing step to a more nucleogenic signature with progressive crushing indicates the presence of an external (crustal) or in situ nucleogenic component unrelated and superposed to the initial mantle neon component whose composition is best approximated by results of first crushing step(s). This contradicts previous models of a lithospheric mantle source of the carbonatitic magmas from Amba Dongar containing recycled crustal components which base on nucleogenic neon compositions. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component had been added at shallow levels during the emplacement process or later during hydrothermal alteration. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.
Geochemistry, Vol. 80, doi.org/10.1016 /j.chemer.2019 .125534 11p. Pdf
India
deposit - Amba Dongar
Abstract: Wakefieldite-(Ce,La) and vanadinite in coarse-grained calciocarbonatites (sovites) are for the first time reported from the northeastern part of the worldwide largest fluorite deposit at the Amba Dongar carbonatite ring dike, India. Sovite in this part of the carbonatite ring dike is rich in pyrochlore, calcite and magnetite. Pyrochlore makes up almost 50% of some sovite samples and shows core-to-rim compositional changes. The core of pyrochlore consists of primary fluorcalciopyrochlore with high F and Na contents while the margins gained elevated amounts of Pb, La and Ce with the associated loss of F and Na due to circulation of hydrothermal solutions. The presence of wakefieldite-(Ce,La) and vanadinite points to an exceptionally high V abundance in hydrothermal solutions formed towards the end of the carbonatite magma activity. This investigation thus opens new promising areas for Nb and REE prospection in the eastern part of the Amba Dongar carbonatite body.
Abstract: Here we present summarizing of isotopic compositions and element ratios of noble gases, nitrogen, carbon and hydrogen in carbonatites of different generations of the Guli massif (West Siberia, Russia) obtained by stepwise crushing. The data point to the subcontinental lithospheric mantle (SCLM) as a primary source of the fluid phase in Guli carbonatites. However, the estimated 40Ar/36Ar ratio in the Guli mantle source of about 5400 is similar to the Kola plume value of 5000 ± 1000 (Marty et al., 1998). One explanation of such a low estimated 40Ar/36Ar ratio in the mantle end-member with SCLM type helium (4??/3?? ~ 120000) and neon (21N?/22N?mantle ~ 0.7) is an admixture of atmospheric argon to the local mantle source. This assumption is supported by the Ar-Ne systematics as well as by the data for hydrogen isotopic composition. Early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotopic compositions of carbon (CO2), argon, and hydrogen (H2O). The mantle component dominated in fluids at the early formation stages of the Guli massif rocks, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon and neon, and most likely a high portion of CO2 with isotopically heavy carbon. The argon-neon-hydrogen isotope systematics suggest that the most plausible source of these late stage fluids are high temperature paleometeoric waters. The absence of a plume signature could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.
Earth and Planetary Science Letters, Vol. 494, 1, pp. 50-59.
Mantle
peridotites
Abstract: Elevated abundances of highly siderophile elements in Earth's mantle are thought to reflect the late accretion of primitive material after the cessation of core formation, but the origin of this material, and whether or not it can be linked to specific types of meteorites remain debated. Here, mass-dependent Ru isotopic data for chondrites and terrestrial peridotites are reported to evaluate the chemical nature and type of the late-accreted material. After correction for nucleosynthetic Ru isotope anomalies, enstatite, ordinary and carbonaceous chondrites all have indistinguishable mass-dependent Ru isotopic compositions. Thus, neither distinct formation conditions in the solar nebula nor parent body processes resulted in significant mass-dependent Ru isotope fractionation. All five terrestrial peridotites analyzed have mass-dependent Ru isotopic compositions that are indistinguishable from each other and from the composition of chondrites. The chondritic mass-dependent Ru isotopic composition of Earth's mantle is difficult to reconcile with prior suggestions that the late accretionary assemblage was a mixture of chondrites with a chemically evolved metal component. Although this mixture can reproduce the suprachondritic Ru/Ir inferred for Earth's mantle, it consistently predicts a heavy Ru isotopic composition of Earth's mantle with respect to chondrites. This is because metal components with elevated Ru/Ir are also enriched in heavy Ru isotopes, resulting from isotope fractionation during core crystallization. Thus, if late accretion involved impacts of differentiated protoplanetary bodies, then the projectile cores must have been either homogenized upon impact, or added to Earth's mantle completely, because otherwise Earth's mantle would have inherited a non-chondritic mass-dependent Ru isotopic composition from the unrepresentative sampling of core material.
Geophysical Research Letters, Vol. 43, 23, pp. 12,054-62.
United States
Geophysics - seismics
Abstract: To constrain crustal structures in the southern Appalachians and the suture zone with the Gondwanan-affinity Suwannee terrane, we applied the 2-D generalized Radon transform wavefield migration method to the scattered incident P wavefield recorded by the EarthScope Southeastern Suture of the Appalachian Margin Experiment and adjacent Transportable Array stations. We resolve the root of thickened crust beneath the high topography of the Blue Ridge Mountains and estimate its density contrast with the mantle to be only 104?±?20?kg/m3. A weak velocity contrast across the crustal root Moho is observed and may be related to an ongoing crustal delamination event, possibly contributing to local tectonic rejuvenation. Beneath the Suwannee terrane, we confirm prior observations of a gently south-southeastward dipping crustal suture, indicating the terminal collision of Laurentia and Gondwana involved several hundred kilometers of overthrusting.
Geochemistry, Geophysics, Geosystems, Vol. 19, 8, pp. 2593-2614.
United States
geophysics - seismic
Abstract: Juxtaposed terranes of highly varied tectonic history make up the contiguous U.S.: the tectonically active western U.S., the largely quiescent Archean and Proterozoic cratons of the central U.S., and the Phanerozoic orogen and rifted margin of the eastern U.S. The transitions between these regions are clearly observed with Sp converted wave images of the uppermost mantle. We use common conversion point stacked Sp waves recorded by EarthScope's Transportable Array and other permanent and temporary broadband stations to image the transition from a strong velocity decrease at the lithosphere?asthenosphere boundary (or LAB) beneath the western U.S. to deeper, less continuous features moving east that largely lie within the lithosphere. Only sparse, localized, weak phases are seen at LAB depths beneath the cratonic interior. Instead, we observe structures within the cratonic lithosphere that are most prominent within the Archean lithosphere of the Superior Craton. The transition from west to east is clearly revealed by cluster analysis, which also shows eastern U.S. mantle velocity gradients as more similar to the western U.S. than the ancient interior, particularly beneath New England and Virginia. In the western U.S., the observed strong LAB indicates a large enough velocity gradient (an average velocity drop of 10?±?4.5% distributed over 30?±?15 km) to imply that melt has ponded beneath the lithosphere.
Abstract: How much of Earth's compositional variation dates to processes that occurred during planet formation remains an unanswered question. High-precision tungsten isotopic data from rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of tungsten isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of hafnium-182 in mantle domains with high hafnium/tungsten ratios, were created during the first ~50 million years of solar system history, indicating that portions of the mantle that formed during Earth’s primary accretionary period have survived to the present
PNAS, Vol. 118, no. 23, doi.org/10.1073/pnas .e2020680118 8p. Pdf
Mantle
deep source, genesis
Abstract: Globally distributed kimberlites with broadly chondritic initial 143Nd-176Hf isotopic systematics may be derived from a chemically homogenous, relatively primitive mantle source that remained isolated from the convecting mantle for much of the Earth’s history. To assess whether this putative reservoir may have preserved remnants of an early Earth process, we report 182W/184W and 142Nd/144Nd data for "primitive" kimberlites from 10 localities worldwide, ranging in age from 1,153 to 89 Ma. Most are characterized by homogeneous ?182W and ?142Nd values averaging ?5.9 ± 3.6 ppm (2SD, n = 13) and +2.7 ± 2.9 ppm (2SD, n = 6), respectively. The remarkably uniform yet modestly negative ?182W values, coupled with chondritic to slightly suprachondritic initial 143Nd/144Nd and 176Hf/177Hf ratios over a span of nearly 1,000 Mya, provides permissive evidence that these kimberlites were derived from one or more long-lived, early formed mantle reservoirs. Possible causes for negative ?182W values among these kimberlites include the transfer of W with low ?182W from the core to the mantle source reservoir(s), creation of the source reservoir(s) as a result of early silicate fractionation, or an overabundance of late-accreted materials in the source reservoir(s). By contrast, two younger kimberlites emplaced at 72 and 52 Ma and characterized by distinctly subchondritic initial 176Hf/177Hf and 143Nd/144Nd have ?182W values consistent with the modern upper mantle. These isotopic compositions may reflect contamination of the ancient kimberlite source by recycled crustal components with ?182W ? 0.
Rates of hornblende etching in soils in glacial deposits of the northern Rocky Mountains (Wyoming-Montana); United States (US): influence of climate/parent
Chemical Geology, Vol. 105, pp. 17-29
Wyoming, Montana
Geomorphology, Wind River Range, Tobacco Root Range
Journal of African Earth Sciences, Vol. 134, pp. 10-23.
Africa, Malawi
carbonatite - Songwe Hill
Abstract: Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.
Abstract: Geological mapping and zircon U-Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820 Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610 Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcano-sedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725 Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740 Ma, with additional detrital sources 2820 and 2940 Ma. Thus, 200 Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism. In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630 Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran "Pan-African" isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675 Ma, coeval with previous dates of the "Eastern Granulites" of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730 Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa "block" have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920 Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870-1900 Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique. In SW Tanzania, rocks from the Mgazini area gave Ubendian protolith ages of ca. 1980-1800 Ma, but these rocks underwent Late Mesoproterozoic high-grade metamorphism between 1015 and 1040 Ma. One granitoid gave a crystallisation age of ca. 1080 Ma correlating with known Mesoproterozoic crust to the east in SE Tanzania and NE Mozambique. However, while the crust in the Mgazini area was clearly one of original Ubendian age, reworked and intruded by granitoids at ca. 1 Ga, the crust of SE Tanzania is a mixed Mesoproterozoic terrane and a continuation from NE Mozambique. Hence the Mgazini area lies at the edge of the Ubendian belt which was re-worked during the Mesoproterozoic orogen (South Irumide belt), providing a further constraint on the distribution of ca. 1 Ga crust in SE Africa. Hf data from near-concordant analyses of detrital zircons from a sample from the Tanzania Craton lie along a Pb-loss trajectory (Lu/Hf = 0), extending back to ?3.9 Ga. This probably represents the initial depleted mantle extraction event of the cratonic core. Furthermore, the Hf data from all igneous samples, regardless of age, from the entire study area (including the Neoproterozoic granulite nappes) show a shallow evolution trend (Lu/Hf = 0.028) extending back to the same mantle extraction age. This implies the entire Tanzanian crust sampled in this study represents over 3.5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the Tanzanian Craton that was subsequently reworked was composed of a very depleted, mafic source with a very high Lu/Hf ratio. Our study helps to define the architecture of the Tanzanian Craton and its evolution from a single age-source in the early Eoarchaean.
The Canadian Mineralogist, Special Publication 14, 634p.
Canada, Quebec
Book
Abstract: This paper introduces a special section of the Canadian Journal of Development Studies, "The Africa Mining Vision: A Manifesto for More Inclusive Extractive Industry-Led Development?" Conceived by African ministers "in charge of mineral resources" with inputs and guidance from African Union Heads of State, the Africa Mining Vision (AMV) was officially launched in February 2009. The papers presented in this special section reflect critically on progress that has since been made with operationalising the AMV at the country level across Africa; the general shortcomings of the manifesto; and the challenges that must be overcome if the continent is to derive g Taking over 20 years of meticulous preparation, László and Elsa Horváth, a duo of dedicated and dynamic amateur mineralogists, along with two researchers, Robert Gault, a mineralogist, and Glenn Poirier, a geologist, have produced the ultimate book "Mont Saint-Hilaire: History, Geology, Mineralogy". The photography captures the colors of Vásárely, the symmetry of Escher, the form of Bartók and the intricate patterns of Mandelbrot, all found here, in this miracle of nature. One cannot but marvel at how this single, small quarry contains such mineral diversity. At last count, over 434 mineral species have been found at Mont Saint-Hilaire, representing 9% of all known mineral species. The 66 type minerals first described from this locality represent 1.3 % of all mineral species, placing the Poudrette quarry in an extremely rarified class for worldwide mineral localities. Almost half, 47, of all known chemical elements are included in this mineral mix. Beginning some 124 million years ago, several million years and a variety of geological processes were needed to accomplish this assemblage. Be captivated, learn and, most of all, enjoy!reater economic benefit from its abundant mineral wealth.
Abstract: Coupled oxygen-hafnium isotope and trace element geochemical data were obtained using thirty eight previously dated zircon grains extracted from five mafic to intermediate crustal xenoliths of the Wyoming Craton (Montana, USA). Xenoliths include mid to lower crustal (642-817?°C and 3.5-9.4?kbar) mafic granulites and amphibolites with dominantly Mesoproterozoic (1772-1874?Ma) and minor Paleoproterozoic to Late Archean (2004-2534?Ma) 207Pb/206Pb zircon ages. Zircon oxygen isotope data indicate derivation from melts in equilibrium with a mantle source that interacted with limited supracrustal material (?18O?=?4.4-5.7‰), as well as the incorporation of supracrustal fluids or melts into mantle source regions (?18O?=?6.0-8.1‰). The small within-sample isotopic variability suggests that primary zircon did not exchange with isotopically distinct fluids or melts after initial formation. Initial zircon Hf isotopic values are highly variable across all xenoliths (?Hf?=?+3.7 to ?17.6), consistent with protolith derivation from mantle sources that incorporated evolved, unradiogenic material or were modified by subduction-related fluids. Within a single granulite xenolith, two zircon types are recognized based on CL imagery, Hf isotopes and U-Pb ages (Type I and Type II). Type I magmatic zircons show dispersed ages (ca. 1700-2534?Ma) and unradiogenic initial Hf (?Hf?=??17.6 to ?1.5, 176Hf/177Hf?=?0.281074-0.281232). The spread in ages and initial ?Hf, but narrow range in initial 176Hf/177Hf, points to variable Pb loss in response to dissolution-recrystallization of pre-existing zircon. Type II metamorphic zircon yields a younger Proterozoic population (ca. 1700-2155?Ma) with more radiogenic initial Hf relative to Type I zircon (?Hf?=??7.9 to +1.4, 176Hf/177Hf?=?0.281427-0.281578); this type represents newly grown metamorphic zircon that formed in the solid-state and incorporated Zr and Hf from pre-existing zircon and silicate matrix/metamorphic phases. REE patterns from all xenoliths are steep and positively sloping without discernible HREE depletion relative to LREE, implying zircon crystallization/recrystallization in the absence of garnet. Negative Eu anomalies signify simultaneous zircon and feldspar crystallization. Solid-state recrystallization may have lead to variations in LREE, Eu and Ce in certain xenoliths. Xenoliths containing magmatic zircon (1834?±?19?Ma) with mantle-like ?18O (4.4-5.5‰) and radiogenic initial ?Hf (?2.3 to +3.7) likely formed through crystallization of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874?±?8?Ma) xenoliths with elevated ?18O (6.0-7.0‰) and unradiogenic initial ?Hf (?8.2 to ?9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism of the overlying mantle wedge in the vicinity of the northern GFTZ. Older (ca. 2534?Ma) xenoliths containing zircons with elevated ?18O (6.4-7.2‰) and unradiogenic ?Hf (up to ?17.6) represent crystallization of protolith magmas extracted from a mantle source metasomatized by subduction-derived fluids and melts in the Late Archean or earlier. Zircon geochronology and isotope systematics within Mesoproterozoic xenoliths support a model of ocean-closure and subsequent continental collision between the Medicine Hat Block and Wyoming Craton, resulting in the formation of subduction-related melts at ca. 1834-1874?Ma, followed by ca. 1770?Ma collision-related metamorphism thereafter.
Geochemistry: Exploration, Environment, Analysis, Vol. 19, pp. 414-430.
Canada, British Columbia
geochemistry
Abstract: Using Rock Canyon Creek REE-F-Ba deposit as an example, we demonstrate the need for verifying inherited geochemical data. Inherited La, Ce, Nd, and Sm data obtained by pressed pellet XRF, and La and Y data obtained by aqua regia digestion ICP-AES for 300 drill-core samples analysed in 2009 were compared to sample subsets reanalysed using lithium metaborate-tetraborate (LMB) fusion ICP-MS, Na2O2 fusion ICP-MS, and LMB fusion-XRF. We determine that LMB ICP-MS and Na2O2 ICP-MS accurately determined REE concentrations in SY-2 and SY-4, and provided precision within 10%. Fusion-XRF was precise for La and Nd at concentrations exceeding ten times the lower detection limit; however, accuracy was not established because REE concentrations in SY-4 were below the lower detection limit. Analysis of the sample subset revealed substantial discrepancies for Ce concentrations determined by pressed pellet XRF in comparison to other methods due to Ba interference. Samarium, present in lower concentrations than other REE compared, was underestimated by XRF methods relative to ICP-MS methods. This may be due to Sm concentrations approaching the lower detection limits of XRF methods, elemental interference, or inadequate background corrections. Aqua regia dissolution ICP-AES results, reporting for La and Y, are underestimated relative to other methods.
Sano, A., Ohtani, E., Litasov, K., Kubo, T., Hosoya, T., Funakoshi, K., Kikegawa, T.
In situ x-ray diffraction study of the effect of water on the garnet perovksite transformation in MORB and implications for the penetration of oceanic crust...
Physics of the Earth and Planetary Interiors, Vol. 159, 1-2, pp. 118-126.
Geochemistry, Geophysics, Geosystems, Vol. 19, 5, pp. 1464-1483.
Mantle
geophysics - seismic
Abstract: SubMachine is a collection of web-based tools for the interactive visualisation, analysis, and quantitative comparison of global-scale, volumetric (3-D) data sets of the subsurface, with supporting tools for interacting with other, complementary models and data sets as listed below. In short, SubMachine is a computational engine (Machine) to visualize models and datasets of the sub-surface (Sub).
Delamination and ultra deep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultra high pressure metamorphic rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Abstract: Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E-92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377-653 Ma). The western ultrapotassic rocks (79°E-89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714-0.734; peridotites: 87Sr/86Srt = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.
Abstract: The Paleo-Mesoproterozoic Zhongtiao aulacogen in the North China Craton and Cuddapah basin in the Indian Craton, have both been interpreted as intra-continental rift formed by a mantle plume that led to the breakup of Columbia supercontinent, but the mechanism has not been completely deciphered. In this paper, the mechanism of the Zhongtiao aulacogen and Cuddapah basin related to initial breakup of Columbia has been evaluated with 2D elastic finite element models of the North China Craton and the Indian Craton. The trajectories of the horizontal maximum principal compressive stress of the best-fit model fit well with the trends of dyke swarms in the North China Craton and the Indian Craton. When the other three models generated were compared with the best-fit model, it can be found that a mantle plume beneath the Zhongtiao and Cuddapah areas played the most vital role in developing the Zhongtiao aulacogen, Cuddapah basin and initial breakup of Columbia supercontinent. The boundary subduction forces, including the northern margin of the NCC, the northwest and southwest margins of the Indian Craton are indispensable factors for the rifting and breakup, whereas the mechanical properties have little influence on these modeling results. The initial breakup of Columbia supercontinent might have been resulted from the coupling between a mantle plume upwelling and some plate tectonic forces.
Environmental Forensics, Vol. 22, 3-4, pp. 340-350. abstract only
China
synthetics
Abstract: In this study, soil and sediment samples along with groundwater samples were collected and analyzed from an abandoned synthetic diamond production plant in Anhui Province, South China. Chemical analysis, pollution characteristics analysis, and correlation analysis were conducted to assess and to determine the source(s) of the toxic metal and organic pollutions in the study sites. The Co and Ni concentrations of soil samples collected from the production area exceed the risk screening value for contaminated development land in Soil Environment Quality Standards for soil pollution risk control on construction land (Trial) of China, while the concentrations of other toxic elements such as Cr, Cu, and Zn are lower than the screening value. The PCA and HCA results are consistent with the correlation coefficient analysis and indicate that industrial activities are the main sources of Co and Ni. The chemical composition and source analysis results of soil and groundwater show that toxic metals originating from catalyst and low pH value from acid waste water should be the main point of concern in the synthetic diamond production plant.
Lithospheric and asthenospheric sources of lamprophyres in the Jiadong Peninsula: a consequence of rapid lithospheric thinning beneath the North Chin a craton?
Geochimica et Cosmochimica Acta, Vol. 124, pp. 250-271.
Abstract: The North China Craton is a classic case for the destruction of an ancient craton, in that it records the loss of more than 100 km of ancient refractory lithospheric mantle during the late Mesozoic and early Cenozoic. However, the mechanisms for this lithospheric thinning remain controversial in large part due to the lack of any systematic investigations of the Mesozoic asthenospheric mantle via its derived mafic rocks, which are key to understand the thinning processes. In this paper, we present detailed zircon U-Pb geochronology, elemental geochemistry, and Sr-Nd-Hf isotopic data for lamprophyres and diabase-porphyries of the Jiaodong Peninsula, in the eastern North China Craton in order to place constraints on models for lithospheric thinning. Our results show that the lamprophyres and diabase-porphyries are derived from the convective asthenospheric mantle via different degrees of partial melting, and that this mantle source was previously modified by carbonatitic liquids. Zircon LA-ICP-MS U-Pb dating suggests an emplacement age for these rocks of 123-121 Ma, the earliest evidence for asthenospherically-derived melts in the Jiaodong Peninsula so far. This emplacement age indicates that the thickness of the lithosphere in the Jiaodong Peninsula was relatively thin at that time. Co-occurrence of the asthenospheric and lithospheric mantle-derived mafic rocks as well as high-Mg adakites record a rapid transition from lithospheric to asthenospheric mantle sources, indicating that the lithosphere beneath the Jiaodong Peninsula was rapidly detached just prior to ca. 120 Ma. Lithospheric thinning of the North China Craton may have been initiated from the Jiaodong Peninsula and Bohai Sea and then propagated towards the interior of the craton.
Zhao, Z., Niu, Y., Christensen, N.I., Zhou, W., Hou, Q., Zhang, Z.M., Xie, H., Zhang, Z.C., Liu, J.
Delamination and ultradeep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh pressure met. rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Cheng, Z., Zhang, Z., Santosh, M., Hou, T., Zhang, D.
Carbonate and silicate rich globules in the kimberlitic rocks of northwestern Tarim large igneous province, NW China: evidence for carbonated mantle source.
Journal of Asian Earth Sciences, Vol. 95, pp. 114-135.
Abstract: The Bayan Obo deposit is a world-class Fe-REE-Nb deposit, and its reserves of rare earth element (REE) resources rank the first over the world. In the face of the current situation of insufficient utilization rate of rare earth resources and scarcity of middle-heavy rare earth elements (M?HREE) resources, the Bayan Obo deposit with such a huge amount of M?HREE cannot be underestimated. In this paper, the occurrence characteristics of M?HREE in different types of iron ore in the Bayan Obo main ore body are studied by using field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS) and advanced mineral identification and characterisation system (AMICS), and the enrichment mechanism is also discussed. The results show that both Sm and Y are the most abundant M?HREE in each type of iron ore in the main ore body, and the content of M?HREE accounts for 1.41%-5.57% of total REE, among which the content of M?HREE in aegirine type Nb-REE-Fe ore (824.47 ppm) and fluorite type Nb-REE-Fe ore (794.82 ppm) are higher, and the content of M?HREE in massive type Nb-REE-Fe ore is lower (318.49 ppm). The main minerals containing M?HREE are bastnasite, parisite, Huanghoite, monazite, aeschynite and fergusonite, among which the content of M?HREE in fergusonite and aeschynite are the highest. According to the characteristics of mineral paragenetic association of REE in this ore district, it is believed that the REE migrates mainly in many different forms of complexes. Heavy rare earth elements (HREE) mainly experienced carbonatite magmatism stage, sodium-fluorine metasomatism stage and late vein mineralization stage, and finally got enrichment.
Contributions to Mineralogy and Petrology, in press available 19p.
Asia, Tibet
Melting
Abstract: Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (?Hfi +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon ?18O (+8.73-8.97 ‰) values, and extremely low ?Hfi (?12.7 to ?9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low ?18O (+6 to 7.5 ‰), intermediate (?18O +8.5 to 9.0 ‰), and high ?18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with ?18O 10-12 ‰. Both the low and intermediate ?18O groups show significant variation in Fe content, whereas the two high ?18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate ?18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high ?18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust-mantle hybridization in the deep Tibetan crust.
Journal of Asian Earth Sciences, in press available, 134p.
China, Tibet
Carbonatite
Abstract: The Cenozoic Mianning-Dechang (MD) rare earth element (REE) belt in eastern Tibet is an important source of light REE in southwest China. The belt is 270 km long and 15 km wide. The total REE resources are >3 Mt of light rare earth oxides (REO), including 3.17 Mt of REO at Maoniuping (average grade = 2.95 wt.%), 81,556 t at Dalucao (average grade = 5.21 wt.%), 0.1 Mt at Muluozhai (average grade = 3.97 wt.%), and 5764 t of REO at Lizhuang (average grade = 2.38 wt.%). Recent results from detailed geological surveys, and studies of petrographic features, ore-forming ages, ore forming conditions, and wallrock alteration are synthesized in this paper. REE mineralization within this belt is associated with carbonatite-syenite complexes, with syenites occurring as stocks intruded by carbonatitic sills or dikes. The mineralization is present as complex vein systems that contain veinlet, stringer, stockwork, and brecciated pipe type mineralization. Carbonatites in these carbonatite-related REE deposits (CARDs) are extremely rich in light REEs, Sr (>5000 ppm), and Ba (>1000 ppm), and have low Sr/Ba and high Ba/Th ratios, and radiogenic Sr-Nd isotopic compositions. These fertile magmas, which may lead to the formation of REE deposits, were generated by the partial melting of sub-continental lithospheric mantle (SCLM) that was metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. We suggest that this refertilization occurred along cratonic margins and, in particular, at a convergent margin where small-volume carbonatitic melts ascended along trans-lithospheric faults and transported REEs into the overlying crust, leading to the formation of the CARDs. The formation of fertile carbonatites requires a thick lithosphere and/or high pressures (>25 kbar), a metasomatized and enriched mantle source, and favorable pathways for magma to ascend into the overlying crust where REE-rich fluids exsolve from cooling magma. The optimal combination of these three factors only occurs along the margins of a craton with a continental root, rather than in modern subduction zones where the lithosphere is relatively thin. U-Pb zircon dating indicates that the Maoniuping, Lizhuang, and Muluozhai alkali igneous complexes in the northern part of the belt formed at 27-22 Ma, whereas the Dalucao complex in the southern part of the belt formed at 12-11 Ma. Biotite and arfvedsonite in Lizhuang and Maoniuping REE deposit have 40Ar/39Ar ages of 30.8 ± 0.4 Ma (MSWD = 0.98) and 27.6 ± 2.0 Ma (MSWD = 0.06), respectively. Biotitaion alteration in syenite and fenitization caused by the relatively amount of carbonatite on syenite and host rocks is the main alteration along the whole belt. Initial Sr (0.7059-0.7079), 143Nd/144Nd (0.5123-0.5127), and 207Pb/204Pb (15.601-15.628) and 208Pb/204Pb (38.422-38.604) isotopic compositions of fluorite, barite, celestite, and calcite in the MD belt are similar to those of the associated syenite and carbonatite. Given the relatively high contents of Cl, F, SO42-, and CO2 in the rocks of the complexes, it is likely that the REEs were transported by these ligands within hydrothermal fluids, and the presence of bastnäsite indicates that the REEs were precipitated as fluorocarbonates. Petrographic, fluid inclusion, and field studies of the ores indicate that bastnäsite and other REE minerals formed during the final stages (<300°C) of the evolution of magmatic-hydrothermal systems in the belt. The mineralization formed from magmatic and meteoric fluids containing CO2 derived from the decarbonation of carbonatite, as indicated by C-O isotopic values of hydrothermal calcite and bastnäsite (?13C= -4.8 to -8.7 and ?18O = 5.8 to 12.5 ‰) and O-H isotopic values of quartz (330°C) and arfvedsonite (260°C), which correspond to fluid isotope compositions of ?18O = 0.3 to 9.8‰ and ?D = -70.0 to -152.8‰ in the belt. This study indicates that formation the largest REE deposits are related to voluminous carbonatite-syenite complexes, compositionally similar ore-forming fluids, extensive alteration, multiple stages of REE mineralization, and tectonic setting.
Reviews in Economic Geology, Vol. 18, pp. 115-136.
China
REE deposits
Abstract: China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite-related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.
Journal of Asian Earth Sciences, Vol. 137, pp. 35-79.
China, Tibet
Carbonatite
Abstract: he Cenozoic Mianning-Dechang (MD) rare earth element (REE) belt in eastern Tibet is an important source of light REE in southwest China. The belt is 270 km long and 15 km wide. The total REE resources are >3 Mt of light rare earth oxides (REO), including 3.17 Mt of REO at Maoniuping (average grade = 2.95 wt.%), 81,556 t at Dalucao (average grade = 5.21 wt.%), 0.1 Mt at Muluozhai (average grade = 3.97 wt.%), and 5764 t of REO at Lizhuang (average grade = 2.38 wt.%). Recent results from detailed geological surveys, and studies of petrographic features, ore-forming ages, ore forming conditions, and wallrock alteration are synthesized in this paper. REE mineralization within this belt is associated with carbonatite-syenite complexes, with syenites occurring as stocks intruded by carbonatitic sills or dikes. The mineralization is present as complex vein systems that contain veinlet, stringer, stockwork, and brecciated pipe type mineralization. Carbonatites in these carbonatite-related REE deposits (CARDs) are extremely rich in light REEs, Sr (>5000 ppm), and Ba (>1000 ppm), and have low Sr/Ba and high Ba/Th ratios, and radiogenic Sr-Nd isotopic compositions. These fertile magmas, which may lead to the formation of REE deposits, were generated by the partial melting of sub-continental lithospheric mantle (SCLM) that was metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. We suggest that this refertilization occurred along cratonic margins and, in particular, at a convergent margin where small-volume carbonatitic melts ascended along trans-lithospheric faults and transported REEs into the overlying crust, leading to the formation of the CARDs. The formation of fertile carbonatites requires a thick lithosphere and/or high pressures (>25 kbar), a metasomatized and enriched mantle source, and favorable pathways for magma to ascend into the overlying crust where REE-rich fluids exsolve from cooling magma. The optimal combination of these three factors only occurs along the margins of a craton with a continental root, rather than in modern subduction zones where the lithosphere is relatively thin.
Abstract: Rare-earth deposits associated with intrusive carbonatite complexes are the world’s most important source of these elements (REE). One of the largest deposits of this type is Maoniuping in the Mianning-Dechang metallogenic belt of eastern Tibet (Sichuan, China). In the currently mined central part of the deposit (Dagudao section), REE mineralization is hosted by a structurally and mineralogically complex Late Oligocene (26.4 ±?1.2 Ma, 40Ar/39Ar age of fluorphlogopite associated with bastnäsite) hydrothermal vein system developed in a coeval syenite intrusion. Low-grade stockworks of multiple veinlets and breccias in the lower part of the orebody grade upwards into progressively thicker veins (up to 12 m in width) that are typically zoned and comprise ferromagnesian micas (biotite to fluorphlogopite), sodium clinopyroxenes (aegirine to aegirine-augite), sodium amphiboles (magnesio-arfvedsonite to fluororichterite), K-feldspar, fluorite, barite, calcite, and bastnäsite. The latter four minerals are most common in the uppermost 80 m of the Dagudao section and represent the climax of hydrothermal activity. Systematic variations in the fluid inclusion data indicate a continuous hydrothermal evolution from about 230-400 °C (fluid inclusions in feldspar, clinopyroxene, and amphibole) to 140-240 °C (fluid inclusions in bastnäsite, fluorite, calcite). Hydrothermal REE transport was probably controlled by F?, (SO4)2?, Cl?, and (CO3)2? as complexing ligands. We propose that at Dagudao, silicate magmas produced orthomagmatic fluids that explored and expanded a fissure system generated by strike-slip faulting. Initially, the fluids had appreciable capacity to transport REE and, consequently, no major mineralization developed. The earliest minerals to precipitate were alkali- and Fe-rich silicates containing low levels of F, which caused progressive enrichment of the fluid in Ca, Mg, F, Cl, REE, (SO4)2?, and (CO3)2?, leading to the crystallization of aegirine-augite, fluororichterite, fluorphlogopite, fluorite, barite, calcite, and bastnäsite gradually. Barite, fluorite, calcite, and bastnäsite are the most common minerals in typical ores, and bastnäsite generally postdates these gangue minerals. Thus, it is very probable that fluid cooling and formation of large amount of fluorite, barite, and calcite triggered bastnäsite precipitation in the waning stage of hydrothermal activity.
Geostandards and Geoanalytical Research, doi:10.111/ggr.12240
Global
peridotite
Abstract: A rapid sample preparation procedure is described to determine trace element compositions of peridotites using LA?ICP?MS. Peridotite powders were fused with albite in a molybdenum?graphite assembly to obtain homogeneous glasses. Best conditions for the fusion procedure (heating at 1500 to 1550 °C for 10 to 15 min with a sample?to?flux ratio of 1:2) were constrained with melting experiments on two USGS reference materials, PCC?1 and DTS?2B. Mass fractions of first series transition elements, Ba and Pb in quenched glasses of PCC?1 and DTS?2B are consistent with published data within 10% RSD. Three spinel peridotite xenoliths from eastern China were analysed following both our method and conventional solution ICP?MS. Compared with solution ICP?MS, the relative deviations of our method for most elements were within 10%, while for the REE, Ta, Pb, Th and U, were within 20%. In particular, volatile elements (e.g., Pb and Zn) are retained in the glass. Compared with conventional wet chemistry digestion, our method is faster. Additional advantages are complete sample fusion, especially useful for samples with acid?resistant minerals (spinel, rutile), and long?term conservation of glasses allowing unlimited repeated measurements with micro?beam techniques. The same approach can be used for analyses of other mantle rocks, such as eclogites and pyroxenites.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatismcould provide a unique windowinto deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646-0.70925, ?Nd(t) = ?2.1 to ?4.9, 206Pb/204Pb(t) = 17.14-18.12, 207Pb/204Pb(t) = 15.28-15.61, 208Pb/204Pb(t) = 37.82-38.67, and zircon ?Hf(t) = ?17 to ?21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts fromthe Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithosphericmantle of the eastern NCCwere reduced in viscosity and intensity, and finally promoted partialmelting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and refertilized in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon UPb dating yielded an average age of 174?±?14?Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t)?=?0.70646-0.70925, ?Nd(t)?=??2.1 to ?4.9, 206Pb/204Pb(t)?=?17.14-18.12, 207Pb/204Pb(t)?=?15.28-15.61, 208Pb/204Pb(t)?=?37.82-38.67, and zircon ?Hf(t)?=??17 to ?21) are slightly enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
SEG Reviews In Economic Geology Chapter 6, Vol. 18, pp. 115-136.
China
REE
Abstract: China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite- related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.
Abstract: The rare earth elements (REEs) consist of the 15 lantha-nide elements (La to Lu). Because of the increasing application of REEs and yttrium (REY) in high-and green-tech industries, the demand for the REY is projected to increase in the future. Rare earth elements are relatively abundant in the Earth's crust, but discovered, minable concentrations are less common than for most other ore types. Bastnaesite and monazite are the main mineral source of REEs in the world. Bastnaesite-hosted deposits in China and the United States Abstract China has been the world's leading rare earth element (REE) and yttrium producer for more than 20 years and hosts a variety of deposit types. Carbonatite-related REE deposits are the most significant REE deposit type, with REY (REE and yttrium)-bearing clay deposits, or ion adsorption-type deposits, being the primary source of the world's heavy REEs. Other REY resources in China include those hosted in placers, alkaline granites, pegmatites, and hydrothermal veins, as well as in additional deposit types in which REEs may be recovered as by-product commodities. Carbonatite-related REE deposits in China provide nearly all the light REE production in the world. Two giant deposits are currently being mined in China: Bayan Obo and Maoniuping. The carbonatite-related REE deposits in China occur along the margins of Archean-Paleoproterozoic blocks, including the northern , southern, and eastern margins of the North China craton, and the western margin of the Yangtze craton. The carbonatites were emplaced in continental rifts (e.g., Bayan Obo) or translithospheric strike-slip faults (e.g., Maoniuping) along reactivated craton margins. The craton margins provide the first-order control for carbonatite-related REE resources. Four REE metallogenic belts, including the Proterozoic Langshan-Bayan Obo, late Paleozoic-early Mesozoic eastern Qinling-Dabie, late Mesozoic Chishan-Laiwu-Zibo, and Cenozoic Mianning-Dechang belts, occur along cratonic margins. Geologic and geochemical data demonstrate that the carbonatites in these belts originated from mantle sources that had been previously enriched, most likely by recycled marine sediments through subduction zones during the assembly of continental blocks. Although the generation of carbonatite magma is debated, a plausible mechanism is by liquid immiscibility between silicate and carbonate melts. This process would further enrich REEs in the carbonatite end member during the evolution of mantle-derived magma. The emplacement of carbonatite magma in the upper crust, channeled by translithospheric faults in extensional environments, leads to a rapid decompression of the magma and consequently exsolution of a hydrothermal fluid phase. The fluid is characterized by high temperature (600°-850°C), high pressure (up to 350 MPa), and enrichment in sulfate, CO2, K, Na, Ca, Sr, Ba, and REEs. Immiscibility of sulfate melts from the aqueous fluid, and phase separation between CO2 and water may take place upon fluid cooling. Although both sulfate and chloride have been called upon as important ligands in hydrothermal REE transport, results of our studies suggest that sulfate is more important. The exsolution of a sulfate melt from the primary carbonatite fluid would lead to a significant decrease of the sulfate activity in the fluid and trigger REE precipitation. The subsequent unmixing between CO2 and water may also play an important role in REE precipitation. Because of the substantial ability of the primary carbonatite fluid to contain REEs, a large-volume magma chamber or huge fluid flux are not necessary for the formation of a giant REE deposit. A dense carbonatite fluid and rapid evolution hinder long distance fluid transportation and distal mineralization. Thus, carbonatite-related alteration and mineralization occur in or proximal to carbonatite dikes and sills, and this is observed in all carbonatite-related REE deposits in China. Ion adsorption-type REE deposits are primarily located in the South China block and are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits. Hydro-thermal alteration by fluids exsolved from late Mesozoic granites or related alkaline rocks (e.g., syenite) may have enriched the parent rocks in REEs, particularly the heavy REEs. Furthermore, this alteration process led to the transformation of some primary REE minerals to secondary REE minerals that are more readily broken down during subsequent weathering. During the weathering process, the REEs are released from parent rocks and adsorbed onto kaolinite and halloysite in the weathering profile, and further enriched by the loss of other material to form the ion adsorption-type REE deposits. A warm and humid climate and a low-relief landscape are important characteristics for development of ion adsorption REE deposits.
Abstract: China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite- related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.
Abstract: Nitrogen availability is a pivotal control on terrestrial carbon sequestration and global climate change. Historical and contemporary views assume that nitrogen enters Earth’s land-surface ecosystems from the atmosphere. Here we demonstrate that bedrock is a nitrogen source that rivals atmospheric nitrogen inputs across major sectors of the global terrestrial environment. Evidence drawn from the planet’s nitrogen balance, geochemical proxies, and our spatial weathering model reveal that ~19 to 31 teragrams of nitrogen are mobilized from near-surface rocks annually. About 11 to 18 teragrams of this nitrogen are chemically weathered in situ, thereby increasing the unmanaged (preindustrial) terrestrial nitrogen balance from 8 to 26%. These findings provide a global perspective to reconcile Earth’s nitrogen budget, with implications for nutrient-driven controls over the terrestrial carbon sink.
Abstract: The composition of the lower mantle—comprising 56% of Earth’s volume—remains poorly constrained. Among the major elements, Mg/Si ratios ranging from ~0.9-1.1, such as in rocky Solar-System building blocks (or chondrites), to ~1.2-1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing, and thus homogenous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity associated with a 20-fold change in viscosity, such as due to the dominance of intrinsically strong (Mg, Fe)SiO3-bridgmanite in low-Mg/Si domains, is sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize mantle convection patterns, and coherently persist at depths of about 1,000-2,200?km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such bridgmanite-enriched ancient mantle structures (BEAMS) may reconcile the geographical fixity of deep-rooted mantle upwelling centres, and geophysical changes in seismic-tomography patterns, radial viscosity, rising plumes and sinking slabs near 1,000?km depth. Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.
Physics of the Earth and Planetary Interiors, Vol. 308, 106552, 14p. Pdf
Mantle
geophysics - seismics
Abstract: Constraining Earth's bulk composition is fundamental to understanding our planet's formation and evolution. While the lower mantle accounts for a majority of the bulk silicate Earth, it is also the least accessible. As experimental and theoretical mineral physics constraints on mineral elasticity at lower mantle temperatures and pressures have improved, comparisons between predicted seismic velocity and density profiles for hypothesized bulk compositions and 1D seismic models have become commonplace. However, the degree to which a given composition is a better or worse fit than another composition is not always reported, nor are the influences of the assumed temperature profile and other uncertainties discussed. Here we compare seismic velocities and densities for perovskitite, pyrolite, and harzburgite bulk compositions calculated using advanced ab initio techniques to explore the extent to which the associated uncertainties affect our ability to distinguish between candidate compositions. We find that predicted differences between model compositions are often smaller than the influence of temperature uncertainties and therefore these comparisons lack discriminatory power. The inability to distinguish between compositions is largely due to the high sensitivity of seismic properties to temperature accompanied by uncertainties in the mantle geotherm, coupled with diminished sensitivity of seismic velocity to composition toward the base of the mantle. An important exception is the spin transition in (Mg,Fe)O-ferropericlase, which is predicted to give a distinct variation in compressional wave velocity that should distinguish between relatively ferro-magnesian and silica-rich compositions. However, the absence of an apparent spin transition signature in global 1D seismic profiles is a significant unresolved issue in geophysics, and it has important geochemical implications. The approach we present here for establishing discriminatory power for such comparisons can be applied to any estimate of seismic velocities and associated uncertainties, and offers a straightforward tool to evaluate the robustness of model comparisons.
Physics of the Earth and Planetary Interiors, Vol. 308, di.org/10.1016 /jpepi.2020. 106552 14p. Pdf
Mantle
geophysics - seismics
Abstract: Constraining Earth's bulk composition is fundamental to understanding our planet's formation and evolution. While the lower mantle accounts for a majority of the bulk silicate Earth, it is also the least accessible. As experimental and theoretical mineral physics constraints on mineral elasticity at lower mantle temperatures and pressures have improved, comparisons between predicted seismic velocity and density profiles for hypothesized bulk compositions and 1D seismic models have become commonplace. However, the degree to which a given composition is a better or worse fit than another composition is not always reported, nor are the influences of the assumed temperature profile and other uncertainties discussed. Here we compare seismic velocities and densities for perovskitite, pyrolite, and harzburgite bulk compositions calculated using advanced ab initio techniques to explore the extent to which the associated uncertainties affect our ability to distinguish between candidate compositions. We find that predicted differences between model compositions are often smaller than the influence of temperature uncertainties and therefore these comparisons lack discriminatory power. The inability to distinguish between compositions is largely due to the high sensitivity of seismic properties to temperature accompanied by uncertainties in the mantle geotherm, coupled with diminished sensitivity of seismic velocity to composition toward the base of the mantle. An important exception is the spin transition in (Mg,Fe)O-ferropericlase, which is predicted to give a distinct variation in compressional wave velocity that should distinguish between relatively ferro-magnesian and silica-rich compositions. However, the absence of an apparent spin transition signature in global 1D seismic profiles is a significant unresolved issue in geophysics, and it has important geochemical implications. The approach we present here for establishing discriminatory power for such comparisons can be applied to any estimate of seismic velocities and associated uncertainties, and offers a straightforward tool to evaluate the robustness of model comparisons.
Abstract: The two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the?~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below?~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.
Abstract: The two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the?~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below?~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.
New stratigraphic subdivisions and redefinition of late Archean and early Proterzoic metasedimentary and metavolcanic rocks of the Sierra Madre and Medicine Bow Moun
United States Geological Survey (USGS) Paper, No. P 1520, 50p. $ 4.00
Intrusion of horizontal dikes -tectonic significance of middle Proterozoic diabase sheets Wide spread in the Upper crust of the southwestern United States
Journal of Geophysical Research, Vol. 96, No. B7, July 10, pp. 12, 461-12, 478
Abstract: Permian dikes, sills, and diatremes in southern Illinois and northwestern Kentucky (the Omaha, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, Grant, and Clay Lick intrusions) share similar geochemistry and are classified as ultramafic lamprophyres. Major element compositions are 30-35 wt% SiO2, 6-7% Al2O3, 12-14% FeOt, 16-19% MgO, 3-5% TiO2, 11-16% CaO, 0.1-0.7% Na2O, 1.2-2.7% K2O, and 0.4-1.3% P2O5. The Grant Intrusive Breccia is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO. Typically, these rocks are fine grained, with phlogopite, serpentinized olivine ( Fo88), diopside, perovskite, Fe-Ti-spinel, apatite, and calcite. Blocky and lath-shaped pseudomorphs in some samples probably represent melilite, which would make the rocks alnöites. The Grant and Williams diatremes contain sedimentary and igneous clasts (including amphibole megacrysts) within a carbonate-rich matrix. The Grant exhibits pelletal lapilli and is characterized as a lamprophyre?carbonatite tuffisite. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant Intrusive exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. Geochemical characteristics, including distributions of 143Nd/144Nd and 87Sr/86Sr, are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. Derivation of the lamprophyres from carbonate-rich parental melts similar to the Grant Intrusive could be achieved by separation of carbonatite. A narrow range of initial 87Sr/86Sr (0.70301-0.70449), and initial ?Nd (3.7-5.1), suggests a uniform mantle source close to Bulk Earth. T-depleted mantle model ages range from 540 to 625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia.
Abstract: Hundreds of millions of years ago something crashed into the planet Mars with enough force to eject pieces of Martian rock into space. Some of these pieces of rock made their way to Earth where they entered our atmosphere as meteors. A precious few landed on the surface of our planet as meteorites. Thanks to scientists like Geoffrey Howarth, a geologist based at the University of Cape Town (UCT), these Martian meteorites are now being studied to better understand the structure and geological history of the red planet. Here’s what we know so far.
Petrology of the hypapbyssal kimberlite of the Kroonstad group II kimberlite (orangeite) cluster, South Africa: evolution of the magma within the cluster
Abstract: Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3 He/ 4 He) and concentra-tion data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kim-berlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3 He/ 4 He ratios span a range from 0.1 to 9.8 R A (where R A = air 3 He/ 4 He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3 He/ 4 He = 0.41 ± 0.30 R A (1?); n = 7 excluding 1 outlier), and 2) 'mantle-like' post plume Obnazhennaya samples (mean clinopyroxene 3 He/ 4 He = 4.20 ± 0.90 R A (1?); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3 He/ 4 He than clinopyroxenes (or garnet). Helium con-tents in Udachnaya samples ([He] = 0.13–1.35 ?cm 3 STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 ?cm 3 STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49– 349 ?cm 3 STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3 He/ 4 He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre-and post-plume 3 He/ 4 He ra-tios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Mantle
NAMs Nominally Anhydrous Minerals
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Abstract: Olivine is the dominant mineral present in kimberlite magmas; however, due to the volatile-rich nature of most kimberlites, they rarely survive late-stage serpentinisation. Here we present major and trace element data for a rare example of ultra-fresh olivine in a macrocrystic calcite kimberlite from the Benfontein kimberlite sill complex. Olivines are characterised by xenocrystic cores surrounded by multiple growth zones representing melt crystallisation and late-stage equilibration. Two distinct core populations are distinguished: Type 1) low Fo (88-89), Ni-rich, Ca- and Na-rich cores, interpreted here to be the result of carbonate-silicate metasomatism potentially as part of the earliest stages of kimberlite magmatism, and Type 2) high Fo (91-93), Ni-rich, low-Ca cores derived from a typical garnet peridotite mantle source. In both cases, the cores have transitional margins (Fo89-90) representing equilibration with a proto-kimberlite melt. Trace element concentrations, in particular Cr, of these transition zones suggest formation of the proto-kimberlite melt through assimilation of orthopyroxene from the surrounding garnet peridotite lithology. Trace element trends in the surrounding melt-zone olivine (Fo87-90) suggest evolution of the kimberlite through progressive olivine crystallisation. The final stages of olivine growth are represented by Fe-rich (Fo85) and P-rich olivine indicating kimberlite evolution to mafic compositions. Fine (< 60 ?m), Mg-rich olivine rims (Fo94-98) represent equilibration with the final stages of kimberlite evolution back to Fe-poor carbonatitic melts. We present a step-by-step model for kimberlite magma genesis and evolution from mantle to crust tracked by the chemistry of olivines in the Benfontein kimberlite. These steps include early stages of metasomatism and mantle assimilation followed by direct crystallisation of the kimberlite melt and late-stage equilibration with the evolved carbonatitic residual liquids. The Ca contents of the Type 1 xenocrystic olivines are the highest yet measured for mantle olivines, and do not overlap with any known mantle xenolith lithologies. These olivines likely represent an important stage of metasomatism directly related to the early stages of kimberlite melt ponding at the base of the lithospheric mantle.
Earth and Planetary Science Letters, Vol. 475, pp. 143-151.
Africa
peridotites
Abstract: Continental Flood Basalts (CFB) result from voluminous outpourings of magma that often precede continental break-up. Notwithstanding the petrogenetic importance of CFBs, the nature of the mantle source for such magmas is contentious, particularly with regard to picrites with Ni-rich olivine phenocrysts. Previous studies have suggested that Ni-rich olivines associated with plume volcanism in regions of thickened (>90 km) lithosphere are related to either source mineralogy differences (peridotite versus pyroxenite) or change in olivine-melt partitioning due to pressure increase. In order to evaluate these two hypotheses, we present trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa. Karoo picrite olivines are Ni-rich, Ca- and Mn-poor, and have low (<1.4) 100*Mn/Fe. These compositions are consistent with a pyroxenitic source. Etendeka Horingbaai/LTZ-L picrite olivines do not show Ni-enrichment, but are characterized by high Al and Cr, and high (>1.4) 100*Mn/Fe, which is more consistent with high temperature melting of a dominantly peridotitic source. We also show that the Karoo and Etendeka olivines are characterized by distinct Mn/Zn ratios of <13 and >15, respectively.In addition, bulk rock geochemical data compilations and previously reported olivine for Karoo and Etendeka CFBs are discussed in order to further constrain source components based on previously described pyroxenite melt geochemical indices such as MgO-CaO systematics, FeO/MnO, Zn/Fe, and FC3MS (FeO/CaO-3*MgO/SiO2). These geochemical indices suggest a pyroxenite-dominated source for Karoo CFBs as well as for Etendeka ferropicrites whereas a peridotite-dominated source is indicated for Etendeka Horingbaai/LTZ-L type picrites analyzed in this study. Based on our data, Ni-enrichment of olivine in plume-related magmas in regions of thickened lithosphere in southern Africa is not ubiquitous. We therefore suggest that mineralogical variation of the source is a more likely major control of olivine chemistry and parent melt variations for Karoo and Etendeka CFBs. We also show that olivine Mn-Zn correlations are a useful discriminator for source variation and recommend the use of olivine for a pyroxenite-dominated source relative to olivine for a peridotite-dominated source.
Abstract: Olivine phenocryst chemistry is a useful tracer of mantle source lithology as olivine is typically the first mineral to crystallize in a range of mafic to alkaline magma types and contains geochemical information about the primary parent magma composition. It is particularly useful in discriminating between pyroxenite (i.e.,recycled crustal component) and peridotite mantle source lithologies. Radiogenic isotope studies of HIMU basalts have shown the important role of a recycled crustal component in the source. However, olivine chemistry of HIMU basalts suggests a dominantly peridotitic mantle source with a subduction-derived metasomatic carbonate component. To further investigate how olivine chemistry can be used to understand the source of HIMU magmas, I present major and trace element data for olivine megacrysts related to the 89?Ma Cr-poor megacryst suite from the Monastery kimberlite (South Africa), which have previously been interpreted to crystallize at high P-T conditions of ~1400?°C and ~5?GPa from a magma sourced from a HIMU reservoir. Olivine megacrysts have high-Ni concentrations at a given Fo (forsterite) content, overlapping ocean island basalts (OIB) interpreted to have formed from pyroxenite-dominated sources but are distinct from typical olivine in HIMU basalts. However, they have low 100*Mn/Fe (0.8-1.1) and no correlation is observed between Ni and trace elements indicative of recycled components such as Co, Li, or Zn. The olivine megacryst chemistry is similar to that of aillikite olivine interpreted to be controlled by phlogopite in the source rather than pyroxenite. Comparison with olivine chemistry from orangeites/Group II kimberlites (sourced from phlogopite-rich mantle lithologies) supports a phlogopite controlon low 100*Mn/Fe in olivine. Further comparison with olivine phenocryst chemistry of HIMU melilitites (76-58?Ma) in southern Africa suggests a mineralogically heterogeneous HIMU reservoir formed by metasomatic modification of the lithospheric mantle in southern Africa. Thus, olivine megacryst chemistry supports interpretations for the recycled component of the HIMU reservoir as mineralogically complex metasomatic lithologies formed by the infiltration of subduction-derived melts into the base of the lithospheric mantle. In addition to carbonate-richHIMU mantle lithologies in southern Africa and worldwide, Monastery kimberlite olivine megacrysts and olivine in melilitites suggest that a phlogopite-richHIMU lithology is present in southern Africa. An important question arising from this study is in reconciling the homogenous isotopic ratios of HIMU basalts worldwide with an apparent heterogeneous lithological source indicated by olivine chemistry.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel - magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Abstract: The evaluation of primary kimberlite compositions is hindered by significant melt modifications during ascent through the lithosphere by entrainment of xenolithic material, volatile degassing, and near surface alteration. Consequently, hypabyssal kimberlite emplaced in the upper crust may not provide a true reflection of the primary kimberlite magma. This contribution places new constraints on kimberlite melt composition by providing an assessment of quenched glass-rich polymineralic melt inclusions hosted in olivine megacrysts from the Monastery kimberlite, South Africa. Melt inclusions predominantly contain variable proportions of euhedral or skeletal grains of calcite, phlogopite, spinel, perovskite, serpentine, and fresh to devitrified glass. Estimates of the bulk compositions of melt inclusions, and the compositions of crystalline phases present therein, are compatible to those of hypabyssal kimberlites worldwide and show a volatile-rich (CO2?+?H2O ~10-17?wt%) carbonated silicate (SiO2 ~27-41?wt%) composition. The glass component has a Si-Mg-Fe-rich and largely CaO-, K2O- and TiO2-free major element composition and is REE-depleted. It also contains approximately 10?wt% H2O but is CO2?free. The glass represents a residual melt that existed after crystallization of the observed mineral assemblage. From some, but not all melt inclusions, apophyses radiate outwards. These fractures typically contain partially devitrified glass that is compositionally identical to the fresh residual glass within the melt inclusions, indicating fracture formation during decompression of the hosting megacryst and at a stage after the melt had evolved. These features are consistent with a trapping of the melt inclusions at high pressure, prior to kimberlite ascent to the surface, in the SCLM at a depth corresponding to 4.5-6?GPa. Textures and compositions of phases within the melt inclusions represent stages of the kimberlite melt and magma evolution. They provide evidence in support of high-pressure experimental studies suggesting a carbonated silicate primary melt rather than a carbonatite. Furthermore, the composition of fresh glass in the melt inclusions, which is compositionally similar to serpentine, suggests that much groundmass serpentine in hypabyssal kimberlites may have formed from similar silicate melt or devitrified glass.
Geochimica et Cosmochimica Acta, in press available, 16p.
Africa, South Africa
deposit - Kimberly region
Abstract: Carbonate-bearing assemblages in the mantle have been interpreted to be the source for Si-undersaturated, CO2-rich magmas, including kimberlites. However, direct evidence for carbonate in the mantle is rare in the contemporary literature. Here we present petrography, trace element, and C-O-Sr-Nd-Pb isotope composition for a suite of carbonate xenoliths from the Kimberley region kimberlites to ascertain their mantle or crustal origin and gain insight to the potential for the occurrence of carbonate in the mantle. Carbonate xenoliths were found in large kimberlite blocks from the Bultfontein kimberlite and Big Hole region. The xenoliths are characterised by pale green alteration margins made of fine-grained microlites of an unknown mineral as well as spherules surrounded by glassy material. They are generally 1–4?cm in size, coarse-grained (1–2?mm), and comprised entirely of calcite. Carbonate xenoliths from the Bultfontein kimberlite have low total REE concentrations (0.2–4.9?ppm), constant 87Sr/86Sri (0.7047–0.7049) combined with variable ?Ndi (?0.1 to ?26.2) and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 16.7–18.8, 15.3–15.6, 36.5–38.4, respectively. Xenoliths from the Big Hole sample have higher 87Sr/86Sri (0.7088–0.7095), lower ?Ndi (?24.5 to ?3.8), and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 18.9–19.9, 15.7–15.8, 38.4–38.8, respectively. The ?13C values for both Bultfontein (?5.7 to ?6.6‰) and Big Hole (?4.7 to ?5.4‰) carbonates are within the typical range expected for mantle-derived carbonate. The ?18O values (15.5–17.5‰) are higher than those of mantle silicate rocks, indicative of late-stage low-temperature interaction with fluids; a common feature of groundmass calcite in the Kimberley kimberlites. The Sr- and C- isotope composition of the Bultfontein xenoliths indicates a mantle origin whereas the Big Hole xenolith Sr- and C-isotopes are more ambiguous. Isotope mixing models are inconsistent with interaction between the host kimberlite and carbonate xenoliths. Correlation between ?Ndi and ?18O values for the Bultfontein xenoliths indicates late-stage interaction with low-temperature fluids, which may also be responsible for the large range in ?Ndi. This in turn indicates that the highest ?Ndi of ?0.1 represents the primary carbonate xenolith signature, and this value overlaps typical Group I kimberlites. We discuss two possible origins for the carbonate xenoliths. (1) Carbonate xenoliths from the sub-continental lithospheric mantle (SCLM), where quenched margins and the large range of ?Ndi are related to formation in the mantle. (2) Carbonate xenoliths from an earlier phase of carbonatite magmatism. The similarity of isotope signatures of the Bultfontein carbonates to Group I kimberlite may further suggest a link between kimberlite and carbonatite volcanism such as observed elsewhere in the world.
Geochimica et Cosmochimica Acta, in press available 49p. Pdf
Africa, South Africa
deposit - Benfontein
Abstract: Olivine chemistry has been widely used to track the petrogenesis of mafic and ultramafic magmas from their mantle source to eruption at the surface. A major challenge in these studies is deciphering crystal growth versus diffusion controlled zoning. Here we report a multi-element approach using high-precision electron microprobe techniques to evaluate crystal growth versus diffusion in kimberlitic olivine from the Benfontein kimberlite, South Africa. These results have implications for both the petrogenesis of kimberlite magmas and the understanding of crystal growth and diffusion-based zoning in igneous olivine in general. The Benfontein olivine contain multiple phosphorous (P)-rich and P-poor zones. Core zones are characterized by homogenous low-P (<78?ppm) concentrations, consistent with xenocrystic origins. Gradational changes in Fo, Ni, Cr and other minor/trace elements at core-margins are similarly characterized by constant low-P concentrations that are indistinguishable from the central regions of the core. Olivine P-maps effectively outline the original xenocryst core, whereas gradational margins are interpreted as diffusion controlled zones related to early-stage equilibration of xenocrystic olivine with proto-/kimberlite melt. Multiple P-poor (100-150?ppm) and P-rich (200-450?ppm) concentric, oscillatory zones with inclusions of kimberlitic oxide phases are observed surrounding the low-P xenocrystic cores. Oxide phases change from chromite in the inner zones to ilmenite in the intermediate zones to magnetite-rich spinel in the outer zones of the olivine. The P-zoning corresponds with changes in Fo content implying that stages of crystal growth was preserved by both fast and slow diffusing elements rather than diffusion processes. Elements compatible with olivine (±chromite) crystallization (i.e., Ni and Cr) display a constant decrease across all zones, suggesting that magma mixing is unlikely a controlling process for P-zoning. We interpret P-rich zones to result from stages of solute trapping related of rapid disequilibrium growth driven by extrinsic factors such as changes in pressure-temperature during kimberlite evolution. In contrast, P-poor zones represent stages of equilibrium crystal growth. The outer olivine zones are characterized by an increase in Fo contents up to Fo96, and in conjunction with a change to more Fe3+-rich oxides, suggest late stage increase in fO2. Correlated Fo and P changes in the Benfontein olivine suggest that major element zonation represents an example where crystal growth-induced Fo zoning has been preserved in olivine. Furthermore, P-rich olivine zones preserve evidence for concentric growth rather than common dendritic structures seen in other occurrences. These results have implications for understanding the effect of magma dynamics and changes in pressure-temperature-fO2 conditions on olivine growth in igneous rocks.
Abstract: Diamondiferous rock types worldwide are broadly divided into kimberlite and lamproite, the latter of which have unique characteristics in different regions and include carbonate-rich varieties (formerly orangeites/Group II kimberlites). Diamondiferous rocks in West Africa are typically micaceous and share petrographic, mineralogical, and geochemical characteristics with both kimberlites and lamproites. To further constrain the classification and petrogenesis of diamondiferous rocks worldwide and their variability between different cratonic regions, in this study we combine detailed petrographic observations with olivine, phlogopite, and spinel chemistry for hypabyssal samples from the Jurassic Tongo dike (Sierra Leone) and the Neoproterozoic Weasua cluster (Liberia). The Tongo dike contains macrocrysts of olivine and phlogopite in a groundmass of olivine, abundant phlogopite, spinel, perovskite, and apatite with a base of calcite, dolomite, and lesser serpentine. The phlogopite is characterised by concurrent FeO and Al2O3 enrichment, which is typical of kimberlites and unlike lamproites. These features and the kimberlite-like spinel compositions allow us to classify the Tongo samples as micaceous kimberlites. The Weasua rocks comprise macrocrysts of olivine in a groundmass of olivine, phlogopite, diopside (zoned towards aegirine-rich rims), spinel, perovskite, and apatite with a base of serpentine and less common calcite. The composition of Weasua phlogopite trends to significant FeO enrichment and Al2O3 depletion, i.e. towards tetraferriphlogopite. The enrichment in mica, phlogopite chemistry and presence of magmatic diopside indicates that these rocks are olivine lamproites. The populations of olivine macrocrysts and microcrysts at Tongo and Weasua are similar and characterised by distinct core and rim zones. Two distinct olivine core populations are observed. 1) forsterite-rich (Fo?>?90) olivine interpreted to reflect xenocrysts from typical mantle peridotites. Al-in-olivine thermometry suggests that these cores have P-T equilibration within diamond stability at Weasua and Tongo. 2) Al-, Ca- and Na- rich cores with P-T formation conditions extending beyond the mantle adiabat. These cores are interpreted to reflect metasomatic and thermal perturbation linked with the infiltration of kimberlite/lamproite melts in the deep lithosphere shortly before entrainment in the ascending magma. The olivine rims at Tongo and Weasua show limited variations in Fo contents at similar values of 88.9?±?0.8 for Tongo and 89.6?±?1.2 for Weasua, as well as similar minor and trace element concentrations. Thus, whereas the Tongo and Weasua rock types are classified as kimberlite and olivine lamproite, respectively, the olivine chemistry suggests a similar petrogenetic evolution.
Abstract: Kaapvaal lamproites, also known as orangeites, are H2O-rich, diamondiferous, highly micaceous, ultrapotassic rocks. Olivines in kimberlites have been shown to be extremely useful in tracking melt evolution, highlighting the importance of the chemical effects of SCLM assimilation on asthenosphere-derived melts. Kaapvaal lamproites are derived from melting metasomatised SCLM and may be expected to form an endmember to the asthenosphere melt-SCLM trend defined by kimberlites. In this contribution, we use olivine composition in Kaapvaal lamproites to further understand melt evolution in the SCLM and assess the similarity between Kaapvaal lamproite, other diamondiferous lamproites, and kimberlite petrogenesis in cratonic regions. We present olivine composition for representative on- and off-craton Kaapvaal lamproites from Finsch and Melton Wold, respectively. Olivines from these Kaapvaal lamproites are characterized by distinct core and rim zones, regardless of the size of individual grains. Polycrystalline grains are abundant at Finsch but relatively rare at Melton Wold. The olivine cores from both occurrences are predominantly Mg-rich (Fo>89) whereas Fe-rich cores (Fo<89) are rare. Mg-rich cores are interpreted to be derived from the disaggregation of mantle peridotites, including sheared peridotites, whereas Fe-rich cores are derived from olivines of the Cr-poor megacryst suite. The average Fo and NiO concentrations of the Melton Wold cores are lower than Finsch cores, likely related to less refractory off-craton mantle. The olivine rims at Finsch and Melton Wold are characterized by reverse zoning with ranges of Fo89-92 and Fo90-91, respectively. The rims are interpreted to represent crystallisation related to a complex interplay between increasing oxidation, assimilation of orthopyroxene, and increasing alkali content of the melt during evolution. The average core and rim compositions of Finsch and Melton Wold, in conjunction with data from diamondiferous lamproites of other cratonic regions, define a broad positive correlation. Kaapvaal lamproites have Mg-rich core and rim compositions, similar to that of Lac de Gras kimberlites, and interpreted to reflect sampling and equilibration of low volume Kaapvaal lamproite melt with refractory mantle. In contrast to Lac de Gras kimberlites, Kaapvaal lamproites have high abundances of groundmass phlogopite that reflect metasomatic material in the SCLM source, likely present as veins within refractory peridotite. This suggests that increasing proportions of melt-metasomatised SCLM interactions are not always linked with increasing Fe content of melts. We show that the petrogenesis of Kaapvaal lamproites is similar to that of kimberlites and lamproites from other cratonic regions, however, the high abundance of phlogopite and Fo-rich olivine rims suggest a distinct metasomatic lithology in the source and that olivine composition; i.e., a proxy for melt composition, may be strongly controlled by melt volume during melt-SCLM interactions.
Howell, D., Piazolo, S., Dobson, D.P., Wood, I.G., Jones, A.P., Watte, N., Frost, D.J., Fisher, D., Griffin, W.L.
Quantitative characterization of plastic deformation of single diamond crystals: a high pressure high temperature (HPHT) experimental deformation study combines with electron backscatter diffraction.
Diamond and Related Materials, Vol. 30, pp. 20-30.
Howell, D., Stern, R.A., Griffin, W.L., Southworth, R., Mikhail, S., Stachel, T., Verchovsky, A.B., O'Reilly, S.Y., Pearson, N.J.
New thermodynamic models and calculated phase equilibration temperatures in NCFMAS for basic and ultrabasic compositions through the transition zone into the uppermost lower mantle.
Abstract: Large peridotite massifs are scattered along the 1500?km length of the Yarlung-Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low fO2 (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high-T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high-P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr-Al spinel ± cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to?>?13?GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375?Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low-P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low-fO2 phases. A proposed P-T-t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c. 375?Ma, residence in the upper Transition Zone for >200 Myr, and rapid exhumation at c. 170-150?Ma or 130-120?Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in?
Earth and Planetary Science Letters, Vol. 430, pp. 284-295.
Asia, Tibet
Luobusa Massif Type Iib
Abstract: For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal-alloy inclusions and highly negative View the MathML source?C13 values are present in both sets of diamonds. The Tibetan diamonds (n=3n=3) show an exceptionally large range in View the MathML source?N15 (?5.6 to +28.7‰+28.7‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with View the MathML source?N15=0‰ in {111} sectors and +30‰+30‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.
Geochimica et Cosmochimica Acta, Vol. 157, pp. 1-12.
Technology
Diamond morphology
Abstract: Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {1 1 1} and {1 0 0} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (?15N) measurements, along with carbon isotope (?13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the ?15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {1 0 0} sectors by ?+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4-1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different ?15N values. Both the ?13C (average of ??8.7‰) and ?15N (average of ?0‰) data show only minor offsets from the typical mantle values (?13C = ?5 ± 3‰, ?15N = ?5 ± 4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering ?13C values and elevating ?15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.
Abstract: Microinclusions of high-density fluids (HDF's) occur in cloudy diamonds from the Mir and Internatsionalnaya kimberlite pipes (Malobotuobia kimberlite field, Siberian platform). These HDFs are of typical high-Mg carbonatitic composition; a few diamonds contain microinclusions that define a low-Mg carbonatitic to silicic trend. The observed variations are interpreted as resulted from mixing of two contrasting fluids derived from the partial melting mainly of carbonated peridotite (the high-Mg carbonatitic HDFs) and eclogite (silica-rich HDFs and HDFs with high Ca/(Ca + Mg + Fe)). Immiscibility of carbonatitic and silica-rich fluids provides a possible mechanism for the co-existence of the observed HDFs but needs further proof. The uniform carbon isotope composition of cloudy diamonds with high-Mg carbonatitic microinclusions from both kimberlite pipes implies a single peridotitic source.
DiaMap: new applications for processing IR spectra of fluid rich diamonds and mapping diamonds containing isolated nitrogen ( type Ib) and boron ( type II b)
Mineralogy and Petrology, doi.org/10.1007/ s00710-018- 0587-6 16p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Zimmi diamonds (Sierra Leone) have 500 million year mantle residency times whose origin is best explained by rapid tectonic exhumation to shallower depths in the mantle, associated with continental collision but prior to kimberlite eruption. Here we present spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the link between their spectroscopic features and their unusual geological history. Cathodoluminesence (CL) imaging of these diamonds revealed irregular patterns with abundant deformation lamellae, associated with the diamonds' tectonic exhumation. Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/- centres and H3 (NVN0). The brownish-yellow to greenish-yellow colours observed in Zimmi Ib-IaA diamonds result from visible absorption by a combination of isolated substitutional nitrogen ( {N}S^0 ) and deformation-related vacancy clusters. Colour-forming centres and other spectroscopic features can all be attributed to the unique geological history of Zimmi Ib-IaA diamonds and their rapid exhumation after formation.
deposit - Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya, Venetia, Wawa, Diavik
Abstract: Earth’s mantle is by far the largest silicate-hosted reservoir of carbon. Diamonds are unrivalled in their ability to record the cycle of mantle carbon and other volatiles over a vast portion of the Earth’s history. They are the product of ascending, cooling, carbon-saturated, metasomatic fluidsmelts and/or redox reactions, predominantly within peridotitic and eclogitic domains in the mantle lithosphere. This paper reports the results of a major secondary ion mass spectrometry (SIMS) carbon isotope study, carried out on 127 diamond samples, spanning a large range of geological time. Detailed transects across the incremental growth zones within each diamond were measured for C isotopes, N abundances and, for samples with N >~200 at.ppm, N isotopes. Given that all of the samples are fragments, recovered when the original crystals were broken to liberate their inclusions, 81 of the analytical traverses have confirmed growth direction context. 98 samples are from studies that have confirmed the dates of the individual diamonds through analysis of their silicate or sulphide inclusions, from source localities including Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya & Venetia. Additional samples come from Wawa (a minimum age) and Diavik where the samples are tied via inclusion paragenesis to published ages. The peridotitic dataset covers the age range of ~3.3 - 2.0 Ga, with the eclogitic data from 2.9 - 1.0 Ga. In total, 751 carbon isotope and nitrogen concentration measurements have been obtained (425 on peridotitic diamonds, and 326 on eclogitic diamonds) with 470 nitrogen isotope measurements (190 P, 280 E). We attempt to constrain the diamond carbon isotope record through time and its implications for (i) the mantle carbon reservoir, (ii) its oxygen fugacity, (iii) the fluid / melt growth environment of diamonds, (iv) fractionation trends recorded in individual diamonds, and (v) diamond population studies using bulk combustion carbon isotope analysis.
Geochemical Perspectives Letters, Vol. 7, pp. 1-2.
Mantle
moissanite
Abstract: Ballhaus et al. (2017) use electric-discharge experiments to argue that lightning strikes could produce ultra-high pressure (UHP) and super-reduced (SuR) phases "identical to those found in 'high-pressure' ophiolites" and that thus there is "not sufficient evidence to challenge long-established models of ophiolite genesis", specifically for the UHP processing of Tibetan ophiolites. However, the authors produced no evidence for UHP phases in their experiments. There are pertinent observations, relevant to the authors’ assertions, in the literature regarding the relationship between the UHP and SuR assemblages in the Tibetan peridotites. Their conclusions are not consistent with this evidence.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 253-266.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 169-190.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 201-222.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 223-236.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 191-200.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307.
Africa, Sierra Leone
deposit - zimmi
Abstract: Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/? centers, and H3 (NVN0). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.
Abstract: At the core of many Earth-scale processes is the question of what the deep mantle is made of. The only direct samples from such extreme depths are diamonds and their inclusions. It is commonly assumed that these inclusions reflect ambient mantle or are syngenetic with diamond, but these assumptions are rarely tested. We have studied inclusion-host growth relationships in two potentially superdeep diamonds from Juina (Brazil) containing nine inclusions of Fe-rich (XFe ?0.33 to ?0.64) ferropericlase-magnesiowüstite (FM) by X-ray diffractometry, X-ray tomography, cathodoluminescence, electron backscatter diffraction, and electron microprobe analysis. The inclusions share a common [112] zone axis with their diamonds and have their major crystallographic axes within 3°-8° of those of their hosts. This suggests a specific crystallographic orientation relationship (COR) resulting from interfacial energy minimization, disturbed by minor post-entrapment rotation around [112] due to plastic deformation. The observed COR and the relationships between inclusions and diamond growth zones imply that FM nucleated during the growth history of the diamond. Therefore, these inclusions may not provide direct information on the ambient mantle prior to diamond formation. Consequently, a “non-pyrolitic” composition of the lower mantle is not required to explain the occurrence of Fe-rich FM inclusions in diamonds. By identifying examples of mineral inclusions that reflect the local environment of diamond formation and not ambient mantle, we provide both a cautionary tale and a means to test diamond-inclusion time relationships for proper application of inclusion studies to whole-mantle questions.
Abstract: The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, ?-implantation, ?-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71?Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.
Diamond and Related Materials, in press available 33p.
Global
DiaMap
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 22. Abstract
Mantle
diamond inclusions
Abstract: Much of the temporal record of Earth’s evolution, including its trace of plate tectonics, is blurred due to the dynamic nature of the crust-mantle system. While zircon provides the highest fidelity crustal record, diamond takes over in the mantle as the go-to mineral, capable of retaining critical information for a variety of geochemical proxies, over billion year timescales. Here we use diamond and its inclusions to tell the story of the recycling of C, N, O, H and B from the crust to various depths in Earth’s mantle. In this story, altered oceanic crust (AOC) and lithospheric mantle will play a prominent role. The carbon isotope record of diamond has long been thought to reflect the mixing of primitive mantle carbon with carbon recycled from isotopically light organic material originating from the crust. A major difficulty has been reconciling this view with the highly varied nitrogen and carbon isotope signatures in diamonds of eclogitic paragenesis, which cannot be interpreted by the same mechanism. Recent work on AOC of igneous origin (Li et al., EPSL in press) shows how isotopically varied carbon and nitrogen can be subducted to great depth and retained in spatial juxtaposition with the mafic silicate component of AOC to form the complex C-N isotope systematics observed in diamonds and the varied O isotope compositions of their inclusions. In this model a large portion of the 13C depleted carbon originated from biogenic carbonate within the AOC rather than from overlying sediments. Metamorphosed and partially devolatilized AOC will have very variable C/N ratios and highly variable nitrogen isotopes, explaining why simple two component mixing between organic matter and convecting upper mantle cannot explain the complexity of C-N isotope systematics in diamonds. Igneous AOC and its underlying altered mantle are considerably more efficient than subducted sediment at retaining their volatile inventory when recycled to transition zone and even lower mantle depths. Hence, this combination of mixing between AOC-derived volatiles and those from the convecting mantle produces the isotopic fingerprints of superdeep diamonds and their inclusions. These amazing diamonds, some worth millions of dollars, can contain pristine ultra-high pressure mineral phases never before seen in terrestrial samples. The first hydrous ringwoodite found in Earth provides evidence in support of a locally water-saturated transition zone that may result from altered oceanic lithospheric mantle foundering at that depth in the mantle. The O isotope composition of deep asthenosphere and transition zone phases document clearly crustal precursors that have interacted with the hydrosphere before residing hundreds of km deep within the Earth. Finally, spectacular blue diamonds contain boron, an element of strong crustal affinities, transported into the deep Earth along with crustal carbon, by the plate tectonic conveyor system. Diamond - such a simple mineral - and its inclusions, will continue to provide a unique, brightly illuminating light into the darkest recesses of Earth’s mantle for many years to come.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 99-122.
Mantle
carbon
Abstract: Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ?3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ?3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total ?¹³C data range from ?17.1 to ?1.9 ‰ (P = ?8.4 to ?1.9 ‰; E = ?17.1 to ?2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in ?¹³C of peridotitic diamonds has been at ?5 (±2) ‰ since the earliest diamond growth ?3.5 Ga, and this mode is also observed in the eclogitic diamond record since ?3 Ga. The skewness of eclogitic diamonds’ ?¹³C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal ?¹³C mode of ?5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Diamonds & Related Materials, In press available, 30p. Pdf
Global
synthetics
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Abstract: Wave?dominated deltas and strandplains make up the majority of the world’s depositional coastlines, provide an important record of sea?level change and serve as hydrocarbon reservoirs worldwide. Satellite imagery forms a great source of data on the recent depositional history of modern deltaic systems. In the subsurface, three?dimensional seismic and well data make the three?dimensional assessment of large?scale deltaic reservoir bodies possible but struggle to resolve internal heterogeneities away from wells. To bridge this gap in characterizing deltaic sedimentation, this study combines measurements from both the shallow, high?resolution section of three?dimensional seismic data of the Eocene Halibut Delta in the Outer Moray Firth, offshore Scotland, with information from Google Earth’s satellite imagery and digital elevation model on south?east Brazilian river deltas (São Francisco, Jequitinhonha, Doce and Paraíba do Sul) to present a means of predicting the location of fluvial sediment input points with respect to clinoform geometry. The key measurement for this study is the delta front and clinoform dip which has been measured at multiple locations along strike of the coastline of the examined deltas. Dip decreases away from the inferred river mouth for all deltas by 50% within 7.2 km. The river mouth location was inferred from the position of palaeo?channels visible on the delta top and coarse sediment recorded in grab samples offshore for the south?east Brazilian deltas, and from imprints of palaeo?channels on attribute maps for the Eocene Halibut Delta. In summary, this study found that delta front dip is steepest at the location of the river mouth and decreases, along with grain size, away from it. This suggests that high dip values correlate with the proximity to the channel mouth and can be used to predict fluvial channel facies in modern deltaic systems and subsurface reservoirs.
Earth Science Reviews , Vol. 219, 103616 231p. Pdf
Africa, Namibia
Craton - Congo
Abstract: Otavi Group is a 1.5-3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran?Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S?N crustal stretching (770-655?Ma), followed by post-rift thermal subsidence (655-600?Ma). Rifting under southern Swakop Group continued until 650-635?Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S?N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746?Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770-717?Ma; (2) glacial early Cryogenian/Sturtian, 717-661?Ma; (3) non-glacial middle Cryogenian, 661-646?±?5?Ma; (4) glacial late Cryogenian/Marinoan, 646?±?5-635?Ma; and (5) non-glacial early Ediacaran, 635-600?±?5?Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585?measured sections (totaling >190?km of strata) and?>?8764 pairs of ?13C/?18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5-9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3-8.8?m Myr?1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ?1.6?km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9?m regional average thickness, n?=?140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ?90?m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ?0.6?km of paleobathymetric relief. Late Tonian shallow-neritic ?13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770-760?Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737-717?Ma) in Swakop Group. Devede Fm is isotopically heavy, +4-8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746?Ma volcanics and shows two negative excursions bridged by heavy ?13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic ?13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0-3.5‰. Early Ediacaran neritic ?13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (??+?11‰) with extreme point-to-point variability (?10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform ?13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9?±?1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60-0.58?Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ?1.85?km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
The Mount Grace carbonatite- an niobium and light rare earth element enriched marble of probable pyroclastic origin in the Shuswapcomplex, southeastern British Columbi
Economic Geology, Vol. 81, No. 6, Sept-Oct. pp. 1374-1386
Abstract: How is hydrogen distributed among minerals and how is it bonded in their crystal structures? These are important questions, because the amount of hydrogen and the bonding configuration of hydrogen in crystalline materials governs many of that material’s properties: its thermal and compressional behavior, P-T phase stability, rheology, and electrical conductivity. A reliable reconstruction of the Earth’s interior, or the prediction of mineral transformations in complex industrial processes, must account for these parameters. Neutron diffraction can locate hydrogen sites in mineral structures, reveal any static or dynamic hydrogen disorder, help define the libration regime of hydrogen, and elucidate hydrogen-bonding configurations. Thus, that most elusive element for X-ray probes is perfectly detectable using neutrons.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 109.
Canada, Northwest Territories
Deposit - Kennady North
Abstract: The Kennady North Property, wholly owned by Kennady Diamonds Inc. (KDI) is located 300 km northeast of Yellowknife adjacent to the DeBeers/Mountain Province Gahcho Kué mine site. Exploration on the property dates back to the early 90’s, during which time several kimberlites were discovered. Since 2012 Kennady Diamonds has completed a number of geophysical, hand and RC till sampling and diamond drill programs. In 2015, KDI completed a large diameter reverse circulation drill program to bulk sample the southern lobe of the Kelvin kimberlite. Following the RC program, diamond drilling and ground geophysical surveys continued in the Kelvin-Faraday Corridor (KFC) and at various exploration targets on the property including the MZ Dyke and Doyle Sill. The field season started in January with the completion of the Kelvin camp and the construction of the RC drill icepad on Kelvin Lake. The pad and a seasonal spur road off the Gahcho Kue seasonal road were completed to coincide with the opening of the Tibbit-Contwoyto winter road and facilitated the mobilization of two large diameter RC rigs operated by Midnight Sun Drilling Inc. to the property. A total of 446 tonnes of the Kelvin kimberlite were obtained via RC drilling between February 19 and April 2. The bulk sample was processed via DMS at the Saskatchewan Research Council in Saskatoon. One diamond drill commenced drilling prior to the RC program and two other drills commenced on the heels of the RC program. A total of 31,000 meters of NQ and HQ core have been drilled during 2015 to the end of October. Drilling at Kelvin has focused on geotechnical and related environmental baseline work as well as further delineation of the pipe-like body with the aim of generating a NI43-101 compliant resource in early 2016. Diamond drilling at the Faraday group of kimberlites delineated the Faraday 1 and Faraday 2 kimberlites. These pipe-like bodies share a similar pipe-like structure and internal geology to the Kelvin kimberlite. Aurora conducted 8848 stations of ground gravity and 521.32 line-kilometers of OhmmapperTM capacitively coupled resistivity in the KFC, MZ dyke, and Doyle Sill during March and April. A 87 line-kilometer bubble seismic survey over the Kelvin, Faraday and MZ complexes was conducted in September. Kennady Diamond Inc. is very encouraged by the exploration results to date and anticipates a successful and exciting 2016.
45th. Annual Yellowknife Geoscience Forum, p. 4 abstract
Canada, Northwest Territories
deposit - Kelvin, Faraday
Abstract: The Kennady North Project kimberlites are located approximately 280 kilometers east-northeast of Yellowknife, in the Northwest Territories of Canada. The unusual geometry and extent of the kimberlite magmatic system is revealed by renewed exploration drilling activities by Kennady Diamonds since 2012. It has become clear that the system comprises multiple intrusive dykes within which several volcaniclastic bodies have developed, all within 11 kilometres of the Gahcho Kué kimberlite cluster and diamond mine. The detailed exploration of the entire system provides unique evidence for subterranean volcanic conduit growth processes that may have scientific and practical exploration benefits. The identified Kennady North Project volcaniclastic bodies are named Kelvin, Faraday 1, Faraday 2 and Faraday 3, and have complex geometries atypical of the more common subvertical kimberlite pipes. Rather, these pipe-like bodies are inclined between 12 and 30 degrees towards the northwest. Kelvin has sharp angular change in trend towards the north. On-going detailed petrographic studies have shown that the pipes contain layers of complex volcaniclastic units with variable volumes of xenolithic fragments, as well as coherent magmatic layers. The pipe textures include evidence for high energy magma and country rock fragmentation processes typically observed in open volcanic systems. The pipes have developed within a shallow 20 degree northwest dipping kimberlite dyke system. Detailed structural geology studies, using fault observations in oriented and unoriented drill core, have identified at least two important fault-fracture trends. The first fault-fracture system is parallel to the dyke segments, and likely related to the intrusion of the dykes and the regional stress tensor during emplacement. The second fault system is subvertical and north-south striking, parallel to the lithological layering within the metasedimentary country rock. The north-south faults match the contact geometry of the Kelvin pipe’s north-south limb exactly. The dykes have been 3-D modelled along with the pipes. Three possible renditions of the dykes have been created, based on different interpretations of dyke segment continuity. The renditions have been labelled “Optimistic”, “Realistic” and “Pessimistic”. The assumptions made have important implications for developing dyke-type mineral resources. The realistic dyke model defines dyke segments that intersect the Kelvin pipe, and those intersections match geometric trends and irregularities in the pipe shape. The coincidental geometries strongly imply that the pipe development interacted with a penecontemporaneous dyke system. The north-south faults also controlled the local trend of Kelvin pipe development, possibly by enhancing fluid permeability, alteration and brecciation along the faults, connecting from one shallow dipping dyke to the next above. Breccia bodies have been observed on similar dipping dykes at Snap Lake mine that intersect fault structures. We conclude that the pipe development geometry and process is governed by a combination of stress, structure and magmatic fluids, and speculate on the nature of the energy required for fragmentation and development of the pipe at some still unknown depth in the crust.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0621-8 16p.
Canada, Northwest Territories
deposit - Kelvin, Faraday
Abstract: The Kennady North Project kimberlites (Northwest Territories of Canada) comprises multiple shallow dipping dykes and several volcaniclastic bodies that have an unusual shallow plunging geometry and complex "pipe" shapes that are termed chonoliths. The detailed exploration of the entire system provides exceptional evidence for subterranean volcanic conduit growth processes. The possible processes leading to the development of the kimberlite bodies are discussed, with emphasis on the importance of the subsurface intrusive system geometry and the local stress tensor. Emplacement into a locally compressive stress regime (i.e. ?1 and ?2 inclined at a low angle to surface) could change the kimberlite emplacement geometries to that observed at Kennady North. Models are proposed for the development of the chonoliths, to emphasize aspects of the growth of kimberlite systems that are not well understood. The conclusions challenge or evolve current emplacement models and should influence kimberlite exploration and resource definition assumptions.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
The case for Greenfields Renaissance.... not specific to diamonds but overview ' increases in commodity prices alone are not enough for poor quality deposits'
Abstract: Two Neoproterozoic carbonatite suites of spatially related carbonatites and associated silicate alkaline rocks from Sevattur and Samalpatti, south India, have been investigated in terms of petrography, chemistry and radiogenic–stable isotopic compositions in order to provide further constraints on their genesis. The cumulative evidence indicates that the Sevattur suite is derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint. The stable (C, O) isotopic compositions confirm mantle origin of Sevattur carbonatites with only a modest difference to Paleoproterozoic Hogenakal carbonatite, emplaced in the same tectonic setting. On the contrary, multiple processes have shaped the petrography, chemistry and isotopic systematics of the Samalpatti suite. These include pre-emplacement interaction with the ambient crustal materials with more pronounced signatures of such a process in silicocarbonatites. Calc-silicate marbles present in the Samalpatti area could represent a possible evolved end member due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. In addition, Samalpatti carbonatites show a range of C–O isotopic compositions, and ?13CV-PDB values between + 1.8 and + 4.1‰ found for a sub-suite of Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates. These heavy C–O isotopic signatures in Samalpatti carbonatites could be indicative of massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites, discovered at Samalpatti, seek their origin in the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O isotopic compositions. Field and petrographic observations as well as isotopic constraints must, however, be combined with the complex chemistry of incompatible trace elements as indicated from their non-uniform systematics in carbonatites and their individual fractions. We emphasise that, beside common carriers of REE like apatite, other phases may be important for incompatible element budgets, such as mckelveyite–(Nd) and kosmochlor, found in these carbonatites. Future targeted studies, including in-situ techniques, could help further constrain temporal and petrologic conditions of formation of Sevattur and Samalpatti carbonatite bodies.
Ackerman, L., Magna, T., Rapprich, V., Upadhyay, D., Kratky, O., Cejkova, B., Erban, V., Kochergina, Y.V., Hrstka, T.
Contrasting petrogenesis of spatially related carbonatites from Samalpatti and Sevattur, Tamil Nadu, India: insights from trace element and isotopic geochemistry.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 31-33.
India
deposit - Samalpatti, Sevattur
Abstract: The Tamil Nadu region in southern India hosts several carbonatite bodies (e.g., Hogenakal, Samalpatti, Sevattur, Pakkanadu-Mulakkadu) which are closely associated with alkaline silicate rocks such as syenites, pyroxenites or dunites (e.g, Kumar et al., 1998; Schleicher et al., 1998; Srivastava, 1998). This is in contrast to the carbonatite occurrences in north-western India associated with the Deccan Trap basalts (e.g., Amba Dongar) or Proterozoic Newania dolomitic carbonatites. We have studied two, spatially related, Neoproterozoic carbonatite-silico(carbonatite) suites in association with alkaline silicate rocks (e.g., pyroxenite, gabbro) from Sevattur and Samalpatti in terms of petrography, chemistry and radiogenic-stable isotopic compositions in order to provide constraints on their genesis and evolution. In these two bodies, several different carbonatite types have been reported previously with striking differences in their trace element and isotopic compositions (Srivastava, 1998; Viladkar and Subramanian, 1995; Schleicher et al., 1998; Pandit et al., 2002). Collected data for previously poorly studied calcite carbonatites from the Sevattur representing the first carbonatite magmas on this locality, indicate similar geochemical characteristics to those of dolomitic carbonatites, such as high LREE/HREE ratios, very high Sr and Ba contents, large amounts of apatite and magnetite, identical Sr-Nd-C-O isotopic compositions. Thus, they were derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint, in agreement with previous studies (e.g., Schleicher et al., 1998). Detailed microprobe analyses revealed that high levels of some incompatible elements (e.g., REE, Y, Sr, Ba) cannot be accounted by matrix calcite hosting only significant amounts of SrO (~0.6-1.2 wt.%). On the other hand, abundant micro- to nano-scale exsolution lamellae and/or inclusions of mckelveyite-(Nd) appear to host a significant fraction of LREE in parallel with apatite. Distribution of Sr is most likely influenced also by common but heterogeneously dispersed barite and strontianite. Newly acquired as well as detailed inspection of available geochemical data permits distinguish two different types of carbonatites in Samalpatti: (1) Type I similar to Sevattur carbonatites in terms of mineralogy, trace element and radiogenic-stable isotopic compositions and (2) Type II with remarkably low concentrations of REE and other incompatible trace elements, more radiogenic Sr isotopic compositions and extremely variable C–O isotopic values. The petrogenesis of the Type II seems to be intimately associated with the presence of silicocarbonatites and abundant silicate mineral domains. Instead of liquid immiscible separation from a silicate magma, elevated SiO2 contents observed in silico-carbonatites may have resulted from the interaction of primary carbonatitic melts and crustal rocks prior to and/or during magma emplacement. Arguments for such hypothesis include variable, but radiogenic Sr isotopic compositions correlated with SiO2 and other lithophile elements (e.g., Ti, Y, Zr, REE). Calc-silicate marbles present in the Samalpatti area could represent a possible evolved crustal end member for such process due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. The wide range of C-O isotopic compositions found in Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates and can be best explained by massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites were discovered at Samalpatti forming centimetre to decimetre-sized enclaves enclosed in pyroxenites with sharp contacts at hand specimen scale. Detailed microprobe analyses revealed peculiar chemical compositions of the Mgamphibole with predominantly sodic composition embaying and replacing Na-Cr-rich pyroxene (kosmochlor), accompanied by the common presence of Cr-spinel and titanite. Such association have been reported for hydrous metasomatism by Na-rich carbonatitic melts at upper mantle conditions (Ali and Arai, 2013). However, the mineralogy and the mode of occurrence of Samalpatti Mg–-r-rich silicocarbonatites argue against such origin. We explain the petrogenesis of these rocks through the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O and Hf isotopic compositions.
2015 IEEE 10th International Conference on Nan/Micro Engineered and Molecular Systems, NEMS 2015, pp. 372-374.
Technology
X-ray dectector
Abstract: Development synthesizing diamond film on silicon substrate was processed by Hot Filament Chemical Vapor Deposition (HFCVD). The gas processes using of H2 and CH4 to produce intrinsic diamond and MSM device constructed on the diaphragm of diamond film. Schottky junctions on the top and the lower diaphragm were produced by aluminum metal. After that, the result of detecting a direct and indirect X-ray of MSM diamond was satisfactory because it could respond along with increasing of X-ray intensity. The X-ray expose time of indirect expose by BaF2 scintillator faster more than direct expose.
Chemical Geology, doi: 10.1016/ j.chemgeo .2019.119290 46p. Pdf
Europe, Czech Republic, Germany, Poland, Austria
lamproites
Abstract: Orogenic lamproites represent a group of peralkaline, ultrapotassic and perpotassic mantle-derived igneous rocks that hold the potential to sample components with extreme compositions from highly heterogeneous orogenic mantle. In our pilot study, we present highly siderophile element (HSE) and ReOs isotope systematics of Variscan orogenic lamproites sampled in the territories of the Czech Republic, Austria and Poland, i.e., from the termination of the Moldanubian and Saxo-Thuringian zones of the Bohemian Massif. Orogenic lamproites of the Bohemian Massif are distinguished by variably high contents of SiO2, high Mg# and predominant mineral associations of K-rich amphibole and Fe-rich microcline. The HSE show (i) consistently very low contents in all investigated orogenic lamproites compared to the estimated concentrations in majority of mid-ocean ridge basalts, hotspot-related volcanic rocks (e.g., ocean island basalts, continental flood basalts, komatiites, some intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites) and arc lavas, and (ii) marked differences in relative and absolute HSE abundances between the samples from the Moldanubian and Saxo-Thuringian Zone. Such a regional dependence in HSE from mantle-derived melts is exceptional. Orogenic lamproites have highly variable and high initial suprachondritic 187Os/188Os values (up to 0.631) compared with rather chondritic to subchondritic Os isotope values of the young lithospheric mantle below the Bohemian Massif. The highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised vein assemblages sitting in depleted peridotite mantle. This process appears to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area. As a specific feature of the orogenic lamproites from the Bohemian Massif, originally ultra-depleted mantle component correlative with remnants of the Rheic Ocean lithosphere in the Moldanubian Zone was metasomatised by a mixture of evolved and juvenile material, whereas the lithospheric mantle in the Saxo-Thuringian Zone was enriched through the subduction of evolved crustal material with highly radiogenic Sr isotope signature. As a result, this led to observed unique regionally dependent coupled HSE, RbSr and ReOs isotope systematics.
Abstract: Orogenic lamproites represent a group of peralkaline, ultrapotassic and perpotassic mantle-derived igneous rocks that hold the potential to sample components with extreme compositions from highly heterogeneous orogenic mantle. In our pilot study, we present highly siderophile element (HSE) and ReOs isotope systematics of Variscan orogenic lamproites sampled in the territories of the Czech Republic, Austria and Poland, i.e., from the termination of the Moldanubian and Saxo-Thuringian zones of the Bohemian Massif. Orogenic lamproites of the Bohemian Massif are distinguished by variably high contents of SiO2, high Mg# and predominant mineral associations of K-rich amphibole and Fe-rich microcline. The HSE show (i) consistently very low contents in all investigated orogenic lamproites compared to the estimated concentrations in majority of mid-ocean ridge basalts, hotspot-related volcanic rocks (e.g., ocean island basalts, continental flood basalts, komatiites, some intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites) and arc lavas, and (ii) marked differences in relative and absolute HSE abundances between the samples from the Moldanubian and Saxo-Thuringian Zone. Such a regional dependence in HSE from mantle-derived melts is exceptional. Orogenic lamproites have highly variable and high initial suprachondritic 187Os/188Os values (up to 0.631) compared with rather chondritic to subchondritic Os isotope values of the young lithospheric mantle below the Bohemian Massif. The highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised vein assemblages sitting in depleted peridotite mantle. This process appears to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area. As a specific feature of the orogenic lamproites from the Bohemian Massif, originally ultra-depleted mantle component correlative with remnants of the Rheic Ocean lithosphere in the Moldanubian Zone was metasomatised by a mixture of evolved and juvenile material, whereas the lithospheric mantle in the Saxo-Thuringian Zone was enriched through the subduction of evolved crustal material with highly radiogenic Sr isotope signature. As a result, this led to observed unique regionally dependent coupled HSE, RbSr and ReOs isotope systematics.
Partition of oxygen isotopes and trace elements between carbonate and silicate melts at 1 kilobar, 800 degrees C and its bearing on the origin ofcarbonatite
Abstract: In 2015, a field team from GIA visited the Indian city of Jaipur to capture the full scope of its gem and jewelry industry: colored stone cutting, wholesale trading, jewelry design, manufacturing, and retail. The authors documented the current state of the industry from a manufacturing as well as a business perspective. The results substantiated many of the team's prior assessments but also brought to light recent developments with far-reaching effects. The impact of vertical integration, consolidation, globalization, and jewelry television retail far exceeded expectations. Once known as a colored stone manufacturing center, Jaipur has rapidly climbed the value chain into jewelry manufacturing and retail by successfully incorporating experience and tradition with technology and innovation.
Abstract: Colombia is synonymous with fine emerald, and production is believed to date back well over a thousand years. Over the centuries the beautiful verdant gemstone, which emerges from areas that are also a lush green, has been linked to violence and human exploitation. Nevertheless, the desire of the Colombian people to mine for this treasure and strike it rich has endured, with enough dreams coming true to drive their passion. In recent years, industry changes have accelerated, perhaps more profoundly than ever before. While government ownership and regulation, criminal activity, and violence have affected production over the years, the industry’s greatest opportunities may still be ahead. Multinational companies are investing heavily in Colombian emerald mining, which has led to modernization. The government’s position on emerald mining has also improved dramatically in this period. Calls for transparency and traceability have led to branding and a revamping of the industry’s image. The loose system of independent miners (figure 1) is seeing efforts at formalization. These landmark changes are occurring at a time when most of the country’s emerald reserves have yet to be mined. In October 2015, a joint GIA and Colombian team met at the First International Emerald Symposium in Bogotá to interview industry leaders and government officials. Many topics involving industry change were discussed at the symposium. Afterward, the team traveled to Colombia’s major mines and visited dealers and cutters in Bogotá to document the current state of the mine-to-market industry. We were also able to collect rough emerald samples for the GIA laboratory’s country-of-origin reference collection.
Abstract: Precise measurement of variations in the local gravitational acceleration is valuable for natural hazard forecasting, prospecting, and geophysical studies. Common issues of the present gravimetry technologies include their high cost, high mass, and large volume, which can potentially be solved by micro-electromechanical-system (MEMS) technology. However, the reported MEMS gravimeter does not have a high sensitivity and a large dynamic range comparable with those of the present commercial gravimeters, lowering its practicability and ruling out worldwide deployment. In this paper, we introduce a more practical MEMS gravimeter that has a higher sensitivity of 8??Gal/?Hz and a larger dynamic range of 8000 mGal by using an advanced suspension design and a customized optical displacement transducer. The proposed MEMS gravimeter has performed the co-site earth tides measurement with a commercial superconducting gravimeter GWR iGrav with the results showing a correlation coefficient of 0.91.
Lan, T-G., Fan, H-R., Santosh, M., Hu, F-F., Yang, Y-H, Liu, Y.
Geochemistry and Sr Nd Pb Hf isotopes of the Mesozoic Dadian alkaline intrusive complex in the Sulu orogenic belt, eastern China: implications for crust mantle interaction.
Yang, K-F, Fan, H-R., Santosh, M., Hu, F-F., Wang, K-Y.
Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: constraints for the mechanism of super accumulation of rare earth elements.
Yang, K-F., Fan, H-R., Santosh, M., Hu, F-F., Wang, K-Y.
Mesoproterozoic mafic and carbonatitic dykes from the northern margin of the North Chin a craton: implications for the fin al breakup of Columbia supercontinent.
Abstract: The widespread mafic to intermediate dykes in the northern Sulu orogen provide important constrains on mantle source characteristics and geodynamic setting. Here we present LA-ICPMS zircon U-Pb ages which indicate that the dykes were emplaced during Early Cretaceous (~ 113-108 Ma). The rocks show SiO2 in the range of 46.2 to 59.5 wt.% and alkalic and shoshonitic affinity with high concentrations of MgO (up to 7.6 wt.%), Cr (up to 422 ppm) and Ni (up to 307 ppm). They are enriched in light rare earth elements LREE (La, Ce, Pr, Nd, Sm and Eu) and large ion lithophile elements (LILE, Rb, Sr, Ba, U and Th) and show strong depletion in high field strength elements (HFSE, Nb, Ta, Ti and P). The dykes possess uniformly high (87Sr/86Sr)i (0.70824-0.70983), low ?Nd(t) (? 14.0 to ? 17.4) and (206Pb/204Pb)i (16.66-17.02) and negative ?Hf(t) (? 23.5 to ? 13.7). Our results suggest that the source magma did not undergo any significant crustal contamination during ascent. The systematic variation trends between MgO and major and trace elements suggest fractionation of olivine and clinopyroxene. The highly enriched mantle source for these rocks might have involved melts derived from the subducted lower crust of Yangtze Craton that metasomatized the ancient lithospheric mantle of the North China Craton.
Earth and Planetary Science Letters, Vol. 500, pp. 156-167.
United States
geodynamics
Abstract: The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Journal of Asian Earth Sciences, Vol. 172, pp. 56-65.
China, Mongolia
deposit - Bayan Obo
Abstract: In the Bayan Obo REE deposit in Inner Mongolia, Northern China, three major orebodies are hosted in dolomite marble of the Bayan Obo Group. There are carbonatite dikes in the ore district. Apatite is a common accessary mineral in the ore-hosting dolomite marble (DM apatite) and in carbonatite dikes (IC apatite). These two types of apatite are both fluorapatite, and have low SiO2, uniform P2O5, and variable CaO contents. Total REY (REEs?+?Y) contents are correlated with Na2O contents, indicating that REY of both types of apatite enter lattice via the substitution reaction: Na+ + (REY)3+ = 2Ca2+. These features, combined with high REY (6230-18,906?ppm) and Sr (9653-17,200?ppm) contents of DM apatite, indicate that DM apatite likely had a carbonatite origin. Some DM apatite grains are partially replaced by albite and quartz. Fluid inclusions crosscutting both apatite and albite or quartz indicate that they formed later than quartz and albite replacement. The back-scattered electron images show that DM apatite grains contain many micro-pores (fluid inclusions), and monazite inclusions formed from the fluid inclusions. However, no monazite inclusions are observed within quartz and albite, excluding the possibility that the monazite inclusions were precipitated directly from the fluids. The monazite inclusions were therefore formed during fluid-induced dissolution-reprecipitation processes, where DM apatite served as the source of LREEs. This also explains the depletion of some LREEs in DM apatite. The formation of monazite inclusions in apatite requires fluids with relatively low Na and Si concentrations, different from the fluids responsible for quartz and albite replacement. DM apatite was affected by two stages of fluid activities: the first stage of metasomatism by alkaline fluids that were likely derived from carbonatite magmas when the deposit first formed (represented by quartz and albite replacement), followed by a second stage of modification that caused LREEs depletion and the formation of new REE minerals. Thus, the Bayan Obo REE ore deposit was modified by a significant thermal event after the formation, which provided negligible or only small amounts of REEs.
Minerals MDPI, Vol. 10, 1117, doi:10.3390/ min10121117 20p. Pdf
China
deposit - Bayan Obo
Abstract: The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407-4184 ppm and 63-152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35-4.20 wt.% and SrO at 1.75-2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10-8.21 wt.%) and SrO (2.83-3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17-2.35 wt.%) and SrO (1.51-1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
Geochimica et Cosmochimica Acta, Vol. 294, pp. 215-231. pdf
Mantle
bridgmanite
Abstract: The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (?-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its ?-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller ?-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.
Abstract: How water cycles through the Earth's interior, presumably is of fundamental importance for understanding the evolution of our planet. The presence of even trace amounts of water (or hydrogen) can dramatically affect many physical and chemical properties of Earth materials, such as phase stability conditions, viscosity, thermal conductivity, etc. Here, we report that the reaction between water and iron to form a pyrite-structured hydrogen-bearing iron peroxide, FeO2Hx (with x = 0 to 1), under the pressure-temperature conditions relevant to the Earth’s deep lower mantle. Combined with theoretical calculations and high-pressure experiments using laser-heated diamond anvil cells coupled with a suite of insitu and characterization techniques (e.g. nuclear resonant inelastic X-ray scattering spectroscopy, X-ray absorption spectroscopy, and X-ray diffraction), we find that this extremely oxygen-rich form of iron peroxide has properties consistent with ultralow velocity zones that are seismically observed at the core-mantle boundary. This phase may also have implications for deep volatile cycling and mantle redox.
Abstract: A drastically altered chemistry was recently discovered in the Fe-O-H system under deep Earth conditions, involving the formation of iron superoxide (FeO2Hx with x?=?0 to 1), but the puzzling crystal chemistry of this system at high pressures is largely unknown. Here we present evidence that despite the high O/Fe ratio in FeO2Hx, iron remains in the ferrous, spin-paired and non-magnetic state at 60-133?GPa, while the presence of hydrogen has minimal effects on the valence of iron. The reduced iron is accompanied by oxidized oxygen due to oxygen-oxygen interactions. The valence of oxygen is not -2 as in all other major mantle minerals, instead it varies around -1. This result indicates that like iron, oxygen may have multiple valence states in our planet’s interior. Our study suggests a possible change in the chemical paradigm of how oxygen, iron, and hydrogen behave under deep Earth conditions.
Abstract: Hydrogen (H) is the most abundant element in the known universe, and on the Earth’s surface it bonds with oxygen to form water, which is a distinguishing feature of this planet. In the Earth’s deep mantle, H is stored hydroxyl (OH?) in hydrous or nominally anhydrous minerals. Despite its ubiquity on the surface, the abundance of H in the Earth’s deep interior is uncertain. Estimates of the total H budget in the Earth’s interior have ranged from less than one hydrosphere, which assumes an H-depleted interior, to hundreds of hydrospheres, which assumes that H is siderophile (iron-loving) in the core. This discrepancy raises the questions of how H is stored and transported in the Earth’s deep interior, the answers to which will constrain its behavior in the deep lower mantle, which is defined as the layer between 1700 km depth and the core-mantle boundary. Hydrogen is the lightest element and exhibits superior mobility under high pressure-temperature (P-T) conditions. Hydrogen, once it has lost its only electron, is electronically equivalent to the proton and can substitute at cation sites in minerals, which in turn enhances their ionic conductivity through the Grotthuss mechanism, or “hydrogen hopping,” in which a proton or proton defect diffuses through the crystal lattice by the formation and concomitant breaking of hydroxyl bonding. Grotthuss-type diffusion is dominant for H-incorporated silicate in the asthenosphere, particularly in regions under relatively high-temperature and low-pressure conditions.1 With increasing depth, H may be liberated from hydroxyl bonding and diffuse freely in the host crystalline lattice, entering an exotic superionic phase.2,3 The concept of a superionic phase is borrowed from the electric battery industry, and the existence of such a phase in ice is widely recognized. The recent discovery of superionic ice-silica in the interiors of giant planets suggests that superionic phases may be common in planetary deep interiors.4 The electrical and seismic features of superionic phases are of great importance, and they have been the subjects of recent studies.2,4 However, what is more challenging is the nature of H in these exotic “semi-fluid” like phases. Will superionicity induce distinct behavior in the distribution of H in major mineral phases? What is the role of proton transportation in the convection of materials? Will it have a large-scale impact? We take superionic FeOOH as an illustrative example, since this is one of the few superionic phases that have been experimentally confirmed. From a survey of data in the literature, we have found that the pressure and amount of escaped H (nH) are correlated (Fig. 1). Here, we define nH as the fraction of H that has escaped from FeOOH, with nH = 0 for fully hydrous FeOOH, and nH = 1 for complete H depletion. Escape of H is inhibited by increasing pressure, and nH converges to a minimum value of ?0.2 at the pressure of the core-mantle boundary (Fig. 1), which is consistent with a recent kinetic experiment involving the heating of FeOOH and the prediction of FeOOH0.75 as a stable stoichiometry.5 The regression of P-nH indicates that cross-boundary diffusion is more intense at relatively low pressures.
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104310, 50p. Pdf
China
carbonatite, REE
Abstract: The Huayangchuan deposit in the North Qinling alkaline province of Central China is a unique carbonatite-hosted giant U-Nb-REE polymetallic deposit. The mineralization is characterized by the presence of betafite, monazite, and allanite as the main ore minerals, but also exhibit relatively high budgets of heavy rare earth elements (HREE = Gd-Lu and Y). The origin of carbonatites has long been controversial, thus hindering our understanding of the genesis of the deposit. Here, we conducted an in-situ trace elemental, Sr-Nd isotopic, and bulk C-O isotopic analyses of multi-type calcites in the deposit. Two principal types (Cal-I and Cal-II), including three sub-types (Cal-I-1, Cal-I-2 and Cal-I-3) of calcites were identified based on crosscutting relationships and calcite textures. Texturally, Cal-I calcites in carbonatites display cumulates with the grain size decreasing from early coarse- (Cal-I-1) to medium- (Cal-I-2) and late fine-grained (Cal-I-3), whereas Cal-II calcites coexist with zeolite displaying zeolite-calcite veinlets. Geochemically, Cal-I calcites contain relatively high REE(Y) (151-2296 ppm), Sr (4947-9566 ppm) and Na (28.6-390 ppm) contents, characterized by right- to left-inclined flat distribution patterns [(La/Yb)N=0.2-4.2] with enrichment of HREE(Y) (136-774 ppm), whereas Cal-II calcites display low REE, Sr and undetectable Na contents, characterized by a right-inclined distribution pattern [(La/Yb)N=13.5, n=16]. The U-Nb-REE mineralization, accompanied with intense and extensive fenitization and biotitization, is mainly associated with the Cal-I-3 calcites which show flat to relatively left-inclined flat REE distribution patterns [(La/Yb)N=0.2-1.0]. Isotopic results show that Cal-I calcites with mantle signatures are primarily igneous in origin, whereas Cal-II are hydrothermal, postdating the U-Nb-REE mineralization. Cal-I calcites (Cal-I-1, Cal-I-2 and Cal-I-3) from mineralized and unmineralized carbonatites, displayed regular changes in REE, Na and Sr contents, but similar trace element distribution patterns and Sr-Nd-C-O isotopic signatures, indicating that these carbonatites originated from the same enriched mantle (EM1) source by low-degree partial melting of HREE-rich carbonated eclogites related to recycled marine sediments. The combination of trace elements and Sr-Nd isotopic composition of calcites further revealed that these carbonatites have undergone highly differentiated evolution. Such differentiation is conducive to the enrichment of ore-forming elements (U-Nb-REE) in the late magmatic-hydrothermal stages owing to extensive ore-forming fluids exsolved from carbonatitic melts. The massive precipitation of the U-Nb-REE minerals from ore-forming hydrothermal fluids may have been triggered by intense fluid-rock reactions indicated by extensive and intense fenitization and biotitization. Therefore, the Huayangchuan carbonatite-related U-Nb-REE deposit may have formed by a combination of processes involving recycled U-Nb-REE-rich marine sediments in the source, differentiation of the produced carbonatitic magmas, and subsequent exsolution of U-Nb-REE-rich fluids that precipitated ore minerals through reactions with wall rocks under the transitional tectonic regime from compression to extension at the end of Late Triassic.
Hu, S., Raza, A., Min, K., Kohn, B.P., Reiners, Ketcham, Wang, Gleadow
Late Mesozoic and Cenozoic thermotectonic evolution along a transect from the north Chin a craton through the Qinling orogen into the Yangtze craton, central.
Mineralium Deposita, 10.1007/s00126 -021-01055-2 18p. Pdf
China
carbonatites
Abstract: The Huangshui'an deposit located in East Qinling (China) is an unusual case of a Si-rich carbonatite hosting economic Mo and minor Pb and REE mineralization. The role of mantle-sourced carbonatite melts and fluids in the formation of the Mo mineralization remains poorly understood. Our integrated study based on field geology, petrography, microthermometry, and LA-ICP-MS analysis of single fluid inclusions, and noble gas isotopes of pyrite permits to reconstruct the source characteristics, the magmatic-hydrothermal evolution of the carbonatitic fluids, and their controls on Mo mineralization. Fluid inclusions hosted in calcite in the carbonatite dikes have the highest concentrations of Mo (9.9-62 ppm), Ce (820-9700 ppm), Pb (1800-19500 ppm), and Zn (570-5800 ppm) and represent the least modified hydrothermal fluid derived from the carbonatite melt. Fluid inclusions hosted in calcite (Cal) and quartz (Qz2 and Qz3) of the stage I carbonatite dikes have different metal concentrations, suggesting that they formed from two distinct end member fluids. The FIA in calcite represent fluid A evolved from carbonatite melt with relatively high-ore metal concentrations, and those in quartz characterize fluid B having a crustal signature due to metasomatic reactions with the wall rocks. The FIA in quartz (Qz1) within the altered wall rock have overlapping elemental concentrations with those of massive quartz (Qz2) and vuggy quartz (Qz3) in carbonatite. This suggests that the volumetrically significant quartz in the Huangshui'an carbonatite has been formed by the introduction of Si by fluid B. The positive correlations between Rb, B, Al, Cl, and Sr in stage II fluid inclusions in late fluorite + quartz + calcite veins indicate that this late mineralization formed from the mixing of primary hydrothermal fluid B with meteoric water. The He-Ar isotope data, in combination with available C-O-Sr-Nd-Pb isotope data, constrain the carbonatite source as an enriched mantle source modified by contributions from crustal material which was probably the fertile lower crust in the region. This distinct source facilitated the enrichment in Mo, REE, and Pb in the primary carbonatite magma. The carbonatite magmatism and Mo mineralization at 209.5-207 Ma occurred in the regional-scale extensional setting at the postcollision stage of the Qinling Orogenic Belt.
Abstract: Carbon has been suggested as one of the light elements existing in the Earth's core. Under core conditions, iron carbide Fe7C3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe7C3, however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe7C3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V0 = 745.2(2) Å3, bulk modulus K0 = 167(4) GPa, its first pressure derivative K0? = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation ?T = 4.7(9) × 10-5 + 3(5) × 10-8 × (T - 300) K-1. We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth's inner core.
Abstract: Carbonatite represents a major host rock for niobium (Nb) resources worldwide. Both magmatic and post-magmatic metasomatic processes are crucial for Nb mineralization in carbonatites. However, the roles of these metasomatic processes are difficult to be evaluated due to their multiple origins and complexity of the physico-chemical conditions. In this study, we present detailed mineralogical investigations of pyrochlore group minerals and chemical U-Th-Pb geochronology of uraninite within the Miaoya carbonatite complex, aiming to better characterize the role of post-magmatic metasomatic events. The Miaoya complex (ca. 420-440?Ma) hosts the second largest carbonatite-related Nb deposit in China, mainly in the form of pyrochlore group minerals, ferrocolumbite and Nb-bearing rutile. Primary pyrochlore group minerals evolved from pyrochlore to uranpyrochlore, and ultimately reaching the betafite end-member during the magmatic stage. They have then experienced an episode of metasomatic events at 235.4?±?4.1?Ma, as determined by U-Th-Pb chemical ages of secondary uraninite. Fluids activity for uranpyrochlore alteration was concomitant with the hydrothermal reworking of REE mineralization, which was probably related to tectono-thermal events that occurred during the Triassic closure of the ancient Mianlue Ocean. During this process, hydration and decomposition of uranpyrochlore were characterized by the leaching of Na, Ca and F from its structure, the incorporation of Fe, Si, Sr and Ba from the fluids, and the final in situ replacement by secondary ferrocolumbite, uraninite and Nb-bearing rutile. In addition, parts of Nb and U liberated from uranpyrochlore by metamictization were then transported over distances of several hundreds of microns in relatively reducing (Fe, Si, S, CO2)-bearing fluids under high temperature, and were ultimately re-precipitated in amorphous Fe-Si-U-Nb-bearing oxide veins and poorly crystallized Nb-Ti-Ca-Fe-rich oxides. The relatively weak fluids activity failed to efficiently promote the Nb re-enrichment.
Trace element composition of continentally subducted slab-derived melt: insight from multiphase solid inclusions in ultrahigh pressure eclogite in the Dabie Orogen.
Journal of Metamorphic Geology, Vol. 31, 4, pp. 453-468.
Multiphase solid inclusions in zoisite bearing eclogite: evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision.
Multiphase solid inclusions in zoisite bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision.
Abstract: The widespread mafic to intermediate dykes in the northern Sulu orogen provide important constrains on mantle source characteristics and geodynamic setting. Here we present LA-ICPMS zircon U-Pb ages which indicate that the dykes were emplaced during Early Cretaceous (~ 113-108 Ma). The rocks show SiO2 in the range of 46.2 to 59.5 wt.% and alkalic and shoshonitic affinity with high concentrations of MgO (up to 7.6 wt.%), Cr (up to 422 ppm) and Ni (up to 307 ppm). They are enriched in light rare earth elements LREE (La, Ce, Pr, Nd, Sm and Eu) and large ion lithophile elements (LILE, Rb, Sr, Ba, U and Th) and show strong depletion in high field strength elements (HFSE, Nb, Ta, Ti and P). The dykes possess uniformly high (87Sr/86Sr)i (0.70824-0.70983), low ?Nd(t) (? 14.0 to ? 17.4) and (206Pb/204Pb)i (16.66-17.02) and negative ?Hf(t) (? 23.5 to ? 13.7). Our results suggest that the source magma did not undergo any significant crustal contamination during ascent. The systematic variation trends between MgO and major and trace elements suggest fractionation of olivine and clinopyroxene. The highly enriched mantle source for these rocks might have involved melts derived from the subducted lower crust of Yangtze Craton that metasomatized the ancient lithospheric mantle of the North China Craton.
Geochimica et Cosmochimica Acta, in press available 78p.
Mantle
Bulk chemistry
Abstract: The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflect the secular evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and less so komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite-trondhjemite-granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties.
Abstract: SiC and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a SiC-dominated ultra-reduced mineral assemblage, including SiC, TiC, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. SiC is 20-50 ?m in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of SiC polytypes were identified, which are dominated by ?-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These SiC are featured by 13C-depleted isotopic compositions (?13C = ?13.2 to ?22.8‰, average = ?17.7‰) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low ?13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
Abstract: Mineral water contents, together with the major and trace element compositions of minerals and whole-rock, were determined for garnet pyroxenites enclosed by ultrahigh-pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The garnet pyroxenites have low SiO2 contents of 40.25 to 46.68 wt% and MgO contents of 10.99 to 14.79 wt%. They are characterized by enrichment in LREE and LILE (Ba, Sr, Pb) but depletion in HFSE (Nb, Zr) and HREE. They were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424-660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water-rich clinopyroxene. Garnet also has the high water contents, suggesting its importance in hosting water at mantle depths. Calculated whole-rock H2O/Ce ratios are 63-145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene-rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the presence of ultramafic metasomatites in the mantle wedge would enhance its water storage and thus reduce the water transport into deeper mantle by subduction.
Abstract: Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7-41) and low Ti/Eu (873-2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.
Abstract: Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of orogenic lithospheric mantle in collisional orogens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 ± 3 to 217 ± 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78-5.23 wt%) and K2O/Na2O (0.71-1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial 87Sr/86Sr isotopic ratios of 0.7058-0.7061 and negative ?Nd(t) values of ?13.0 to ?15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.
Geochimica et Cosmochimica Acta, in press available, 53p.
China
eclogite
Abstract: Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and ?Nd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.
Hua, C., Zhili, Q., Taijin, L., Stern, R., Stachel, T., Yuan, S., Jian, Z., Jie, K., Shyu, P., Shecai, Q.
Variations in carbon isotopic composition in the subcontinental lithospheric mantle beneath the Yangtze and North Chin a cratons; evidence from in-situ analysis of diamonds using SIMS.
Abstract: Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr# [100Cr/(Cr+Al)] of about 47-53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.
Journal of Analytical At. Spectrometry, Vol. 33, pp. 231-239.
United States, Arkansas, China, Hebei, Russia, Kola Peninsula, Europe, Sweden, Canada, Ontario
geochronology
Abstract: We report the first U-Pb geochronological investigation of schorlomite garnet from carbonatite and alkaline complexes and demonstrate its applicability for U-Pb age determination using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) due to its relatively high U and Th abundances and negligible common Pb content. The comparative matrix effects of laser ablation of zircon and schorlomite are investigated and demonstrate the necessity of a suitable matrix-matched reference material for schorlomite geochronology. Laser-induced elemental fractional and instrumental mass discrimination were externally-corrected using an in house schorlomite reference material (WS20) for U-Pb geochronology. In order to validate the effectiveness and robustness of our analytical protocol, we demonstrate the veracity of U-Pb age determination for five schorlomite samples from: the Magnet Cove complex, Arkansas (USA); the Fanshan ultrapotassic complex, Hebei (China); the Ozernaya alkaline ultramafic complex, Kola Peninsula (Russia); the Alnö alkaline-rock carbonatite complex (Sweden); and the Prairie Lake carbonatite complex, Ontario (Canada). The schorlomite U-Pb ages range from 96 Ma to 1160 Ma, and are almost identical to ages determined from other accessory minerals in these complexes and support the reliability of our analytical protocol. Schorlomite garnet U-Pb geochronology is considered to be a promising new technique for understanding the genesis of carbonatites, alkaline rocks, and related rare-metal deposits.
Geostandards and Geoanalltical Research, Vol. 43, 3, pp. 543-565.
China, Europe, Sweden, Asia, Mongolia, United States, Africa, Malawi, Madagascar
REE
Abstract: Bastnäsite is the end member of a large group of carbonate-fluoride minerals with the common formula (REE) CO3F•CaCO3. This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnäsite is easily altered and commonly contains inclusions of earlier?crystallised minerals, in situ analysis is considered the most suitable method to measure its U?Th?Pb and Sr?Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi?collector) inductively coupled plasma?mass spectrometry of forty?six bastnäsite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in?house bastnäsite reference material (K?9) demonstrated that precise and accurate U?Th?Pb ages could be obtained after common Pb correction. Moreover, the Th?Pb age with its high precision is preferable to the U?Pb age because most bastnäsites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.
Geochemistry, Geophysics, Geosystems, in press available pdf 20p.
Global
geodynamics
Abstract: The relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume?push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross?lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20-50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80?km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5-50 MPa. In both homogeneous and orogeny?embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1-2 order of magnitude smaller (0-5 MPa). In our two end?member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents.
Abstract: Understanding the dominant force responsible for supercontinent breakup is crucial for establishing Earth's geodynamic evolution that includes supercontinent cycles and plate tectonics. Conventionally, two forces have been considered: the push by mantle plumes from the sub-continental mantle which is called the active force for breakup, and the dragging force from oceanic subduction retreat which is called the passive force for breakup. However, the relative importance of these two forces is unclear. Here we model the supercontinent breakup coupled with global mantle convection in order to address this question. Our global model features a spherical harmonic degree-2 structure, which includes a major subduction girdle and two large upwelling (superplume) systems. Based on this global mantle structure, we examine the distribution of extensional stress applied to the supercontinent by both sub-supercontinent mantle upwellings and subduction retreat at the supercontinent peripheral. Our results show that: (1) at the center half of the supercontinent, plume push stress is ?3 times larger than the stress induced by subduction retreat; (2) an average hot anomaly of no higher than 50 K beneath the supercontinent can produce a push force strong enough to cause the initialization of supercontinent breakup; (3) the extensional stress induced by subduction retreat concentrates on a ?600 km wide zone on the boundary of the supercontinent, but has far less impact to the interior of the supercontinent. We therefore conclude that although circum-supercontinent subduction retreat assists supercontinent breakup, sub-supercontinent mantle upwelling is the essential force.
Abstract: Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV0). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV0. SiV0 exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near -transform-limited optical linewidths. These combined properties make SiV0 a promising defect for quantum network applications.
Geophysical Research Letters, Vol. 46, 2, pp. 678-688.
Global
craton
Abstract: Low??18O magma has received great attention and it has profound implications on geological and climate evolution. Neoproterozoic era is a unique period to breed low??18O magmas and snowball Earth. This manuscript first report Neoproterozoic moderately 18O?depleted zircons from the central part of the Cathaysia Block in South China, and it builds a four end?member Hf?O isotopic mixing model to explain the global low??18O magmas at Neoproterozoic era. Our compilation of low??18O zircon data and our new data confirms that globally Neoproterozoic 18O?depleted magmatic activities generally began after 800 Ma and reached a peak at 780-760 Ma. This provides new information on the rifting of Rodinia supercontinent and suggests close connections between northwest India, Madagascar, and South China in the Rodinia supercontinent. This manuscript deals with the hot?debated topics on oxygen isotopes and supercontinent cycle. We believe that this manuscript will attract international readers from a wide scope of geosciences.
Abstract: Redox-sensitive transition group elements are involved in almost all fundamental geochemical processes. Of these elements, vanadium (V) contributes a particularly powerful tool to decipher the Earth's history and its link to extraterrestrial bodies. A comprehensive view of V includes the formation and interaction between the Earth's interior layers, the evolution of the Earth's surface to a habitable zone, biogeochemical cycling, and anthropogenic impacts on the environment. Tracing the geochemical behavior of V through the Earth's compartments reveals critical connections between almost all disciplines of Earth sciences. Vanadium has a history of application as a redox tracer to address the early accretion history of the Earth, to identify connections between the mantle and crust by subduction and melting, and to interpret past surface environments. The geochemical cycling of V from the deep Earth to the surface occurs through magmatism, weathering and digenesis, reflecting variations of fO2 and V species in different Earth compartments. Minerals form a link between deep Earth reservoirs of vanadium and surface environments, and the study of V in minerals has increased the understanding of V cycling. Finally, the exploitation of V has been increasing since the Industrial Revolution, and significant amounts of V have been released as a consequence into natural systems. Environmental concerns are promoting new areas of research to focus on V cycling between water, air, soil and sediment compartments. An increased understanding of V in all compartments, and knowledge of the processes that connect the compartments, is vital to tracing the fate of this intriguing element in natural systems.
Abstract: Diamond formation has typically been attributed to redox reactions during precipitation from fluids or magmas. Either the oxidation of methane or the reduction of carbon dioxide has been suggested, based on simplistic models of deep fluids consisting of mixtures of dissolved neutral gas molecules without consideration of aqueous ions. The role of pH changes associated with water–silicate rock interactions during diamond formation is unknown. Here we show that diamonds could form due to a drop in pH during water–rock interactions. We use a recent theoretical model of deep fluids that includes ions, to show that fluid can react irreversibly with eclogite at 900?°C and 5.0?GPa, generating diamond and secondary minerals due to a decrease in pH at almost constant oxygen fugacity. Overall, our results constitute a new quantitative theory of diamond formation as a consequence of the reaction of deep fluids with the rock types that they encounter during migration. Diamond can form in the deep Earth during water–rock interactions without changes in oxidation state.
Geochimica et Cosmochimica Acta, in press available 45p.
Africa, Russia
Geochronology
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ to 0.060‰ with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Metasomatism
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243 to ?0.204 with an average of ?0.225 ± 0.037 (2?, n = 19), and ?56Fe from ?0.038‰ o 0.060 with an average of ?0.003 ± 0.068 (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (±0.05 for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Craton, Peridotite
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ 0.060 with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Barium isotopic composition of the mantle constrained by carbonatites.
Goldschmidt Conference, 1p. Abstract
Africa, Tanzania, east Africa, Canada, Europe, Germany, Greenland
carbonatite
Abstract: Deep mantle origin and ultra-reducing conditions in podiform chromitite: diamonds, moissanite, and other unusual minerals in podiform chromitites from the Pozanti-Karsanti ophiolite, southern Turkey
Earth and Planetary Science Letters, Vol. 474, pp. 128-137.
Mantle
peridotites
Abstract: To better constrain the Ca isotopic composition of the Bulk Silicate Earth (BSE) and explore the Ca isotope fractionation in the mantle, we determined the Ca isotopic composition of 28 peridotite xenoliths from Mongolia, southern Siberia and the Siberian craton. The samples are divided in three chemical groups: (1) fertile, unmetasomatized lherzolites (3.7-4.7 wt.% Al2O3); (2) moderately melt-depleted peridotites (1.3-3.0 wt.% Al2O3) with no or very limited metasomatism (LREE-depleted cpx); (3) strongly metasomatized peridotites (LREE-enriched cpx and bulk rock) further divided in subgroups 3a (harzburgites, 0.1-1.0% Al2O3) and 3b (fertile lherzolites, 3.9-4.3% Al2O3). In Group 1, ?44/40Ca of fertile spinel and garnet peridotites, which experienced little or no melting and metasomatism, show a limited variation from 0.90 to 0.99‰ (relative to SRM 915a) and an average of 0.94 ± 0.05‰ (2SD, ), which defines the Ca isotopic composition of the BSE. In Group 2, the ?44/40Ca is the highest for three rocks with the lowest Al2O3, i.e. the greatest melt extraction degrees (average ‰, i.e. ?0.1‰ heavier than the BSE estimate). Simple modeling of modal melting shows that partial melting of the BSE with ranging from 0.10 to 0.25 can explain the Group 2 data. By contrast, ?44/40Ca in eight out of nine metasomatized Group 3 peridotites are lower than the BSE estimate. The Group 3a harzburgites show the greatest ?44/40Ca variation range (0.25-0.96‰), with ?44/40Ca positively correlated with CaO and negatively correlated with Ce/Eu. Chemical evidence suggests that the residual, melt-depleted, low-Ca protoliths of the Group 3a harzburgites were metasomatized, likely by carbonate-rich melts/fluids. We argue that such fluids may have low (?0.25‰) ?44/40Ca either because they contain recycled crustal components or because Ca isotopes, similar to trace elements and their ratios, may be fractionated by kinetic and/or chromatographic effects of melt percolation in the mantle. The ?44/40Ca in Group 3b lherzolites (0.83-0.89‰) are lower than in the BSE as well, but the effects of metasomatism on ?44/40Ca are smaller, possibly because of the high Ca contents in their protoliths and/or smaller ?44/40Ca differences between the protoliths and metasomatic agents. The BSE estimates based on fertile peridotites in this study fall in the ?44/40Ca ranges for oceanic and continental basalts, various meteorites (achondrites; carbonaceous, ordinary and enstatite chondrites), Mars, and the Moon. These results provide benchmarks for the application of Ca isotopes to planet formation, mantle evolution, and crustal recycling.
Abstract: Diamonds containing fluid inclusions provide the most direct samples of upper mantle fluids. In eclogites, diamond can form by pH drop during fluid-rock interaction. However, in peridotites, the cause of the chemical evolution of the fluids and minerals, including the wide range of observed salinities involved, are still unclear. Here we used new experimental calibrations of the Deep Earth Water model involving organic and inorganic complexes of the major rockforming elements to show that fluid mixing can cause diamond formation in the peridotitic environment. Models of the saline and carbonatitic fluid inclusion compositions consistent with the chemistry of measured solid inclusions in Panda diamonds were used to simulate the irreversible, chemical mass transfer when a carbonatitic fluid infiltrates harzburgite containing a saline fluid at 950°C and 4.5 GPa. Simultaneous oxidation of aqueous hydrocarbons in the peridotitic fluid and reduction of the organic acid anion formate as well as bicarbonate in the carbonatitic fluid during mixing and reaction with harzburgite resulted in the formation of diamond, olivine, and garnet, and increases in the logfo2 and pH. Olivine was predicted to become more Ferich and garnet more Ca and Fe-rich with reaction progress, in agreement with reported temporal trends in the composition of mineral inclusions from octahedral cores to coated rims on Panda diamonds. Aqueous phase concentrations of all elements changed consistent with measured trends in fluid inclusion compositions from saline to less saline. For comparison, we also simulated a saline fluid infiltrating a harzburgite containing a carbonatitic fluid. Diamond again formed, but the compositional trends of the silicate minerals and the trend of salinity with reaction progress were all in the opposite direction to data from the Panda diamonds. Overall, our study strongly suggests that mixing of fluids containing carbon from both reduced and oxidized sources, and simultaneous reaction with harzburgite can cause precipitation of diamond, without the need for triggering by temperature or pressure changes, while adding Ca and Fe to the sub-lithospheric mantle.
Geostandards and Geoanalytical Research, doi:10.111/ggr.12240
Global
peridotite
Abstract: A rapid sample preparation procedure is described to determine trace element compositions of peridotites using LA?ICP?MS. Peridotite powders were fused with albite in a molybdenum?graphite assembly to obtain homogeneous glasses. Best conditions for the fusion procedure (heating at 1500 to 1550 °C for 10 to 15 min with a sample?to?flux ratio of 1:2) were constrained with melting experiments on two USGS reference materials, PCC?1 and DTS?2B. Mass fractions of first series transition elements, Ba and Pb in quenched glasses of PCC?1 and DTS?2B are consistent with published data within 10% RSD. Three spinel peridotite xenoliths from eastern China were analysed following both our method and conventional solution ICP?MS. Compared with solution ICP?MS, the relative deviations of our method for most elements were within 10%, while for the REE, Ta, Pb, Th and U, were within 20%. In particular, volatile elements (e.g., Pb and Zn) are retained in the glass. Compared with conventional wet chemistry digestion, our method is faster. Additional advantages are complete sample fusion, especially useful for samples with acid?resistant minerals (spinel, rutile), and long?term conservation of glasses allowing unlimited repeated measurements with micro?beam techniques. The same approach can be used for analyses of other mantle rocks, such as eclogites and pyroxenites.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Russia, Siberia
carbonatite
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the ?44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. ?44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than ?44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk ?44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease ?44/40Ca values of the mantle, but its effects are usually limited (?0.3‰); (b) Ca isotopes cannot distinguish "carbonatite" and "silicate" types of mantle metasomatism. The lowest ?44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high ?44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Mantle
carbonatite
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the ?44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. ?44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than ?44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk ?44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease ?44/40Ca values of the mantle, but its effects are usually limited (?0.3‰); (b) Ca isotopes cannot distinguish "carbonatite" and "silicate" types of mantle metasomatism. The lowest ?44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high ?44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Geochimica et Cosmochimica Acta, Vol. 248, pp. 1-13.
Mantle, Asia, Mongolia, Russia, Siberia
metasomatism
Abstract: Ca isotopes can be strongly fractionated at the Earth’s surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the ?44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. ?44/40Ca in 15 Obn xenoliths shows limited variation (0.74-0.97‰) that overlaps the value (0.94?±?0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than ?44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10?±?0.03‰). Bulk ?44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81?±?0.08‰ to 0.83?±?0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease ?44/40Ca values of the mantle, but its effects are usually limited (?0.3‰); (b) Ca isotopes cannot distinguish “carbonatite” and “silicate” types of mantle metasomatism. The lowest ?44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high ?44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale.
Geochimica et Cosmochimica Acta, in press available, 93p.
Mantle
metasomatism
Abstract: Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of. Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.
Geostandards and Geoanalytical Research, Vol. 43, 2, pp. 291-300.
Global
carbonatites
Abstract: This study presents a high?precision method to measure barium (Ba) isotope compositions of international carbonate reference materials and natural carbonates. Barium was purified using chromatographic columns filled with cation exchange resin (AG50W?X12, 200-400 mesh). Barium isotopes were measured by MC?ICP?MS, using a 135Ba-136Ba double?spike to correct mass?dependent fractionation during purification and instrumental measurement. The precision and accuracy were monitored by measuring Ba isotope compositions of the reference material JCp?1 (coral) and a synthetic solution obtained by mixing NIST SRM 3104a with other matrix elements. The mean ?137/134Ba values of JCp?1 and the synthetic solution relative to NIST SRM 3104a were 0.21 ± 0.03‰ (2s, n = 16) and 0.02 ± 0.03‰ (2s, n = 6), respectively. Replicate measurements of NIST SRM 915b, COQ?1, natural coral and stalagmite samples gave average ?137/134Ba values of 0.10 ± 0.04‰ (2s, n = 18), 0.08 ± 0.04‰ (2s, n = 20), 0.27 ± 0.04‰ (2s, n = 16) and 0.04 ± 0.03‰ (2s, n = 20), respectively. Barium mass fractions and Ba isotopes of subsamples drilled from one stalagmite profile were also measured. Although Ba mass fractions varied significantly along the profile, Ba isotope signatures were homogeneous, indicating that Ba isotope compositions of stalagmites could be a potential tool (in addition to Ba mass fractions) to constrain the source of Ba in carbonate rocks and minerals.
Geochimica et Cosmochimica Acta, in press available doi.org/10.1016 / j.gca.2019.06.041 36p.
Africa, Tanzania, Canada, East Africa, Europe, Germany, Greenland
deposit - Oldoinyo Lengai
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Geochimica et Cosmochimica Acta, in press available 59p. Pdf
Canada, Northwest Territories
deposit - Panda
Abstract: Diamonds containing fluid inclusions provide invaluable samples of upper mantle fluids, the study of which illuminates not only diamond formation but also the long-term evolution of the subcratonic, lithospheric mantle. The very large range of inclusion compositions worldwide has been interpreted to represent four end-member fluids: saline (rich in Na+K+Cl); silicic (rich in Si+Al); and carbonatitic (rich in Ca+Mg+Fe, with low-Mg and high-Mg end members). However, the sources and evolution of these fluids and the processes involved in diamond formation are still unclear. We used an unusual study of diamonds from the Panda kimberlite (Ekati Mine, Northwest Territories, Canada) in which both mineral and fluid inclusions in the diamonds were analyzed (Tomlinson et al., 2006) to develop models of the saline, silicic, and low-Mg carbonatitic fluids present in the Panda fluid inclusions. The models used aqueous speciation and solubility calculations to link the solid and fluid inclusion chemistry with model upper mantle rock types. We used the extended Deep Earth Water model to calculate equilibrium constants previously calibrated with experimental rock solubilities referring to upper mantle temperatures and pressures (Huang and Sverjensky, 2019). Our results at 950 °C and 4.5 GPa suggest that the saline fluid could originate from peridotite, the silicic fluid from eclogite, and the low-Mg carbonatitic fluid from carbonated dunite. The fluid models were then used to predict the irreversible, chemical mass transfer when the carbonatitic fluid infiltrated a harzburgite containing a saline fluid. Simultaneous reduction of formate and bicarbonate in the carbonatitic fluid and oxidation of aqueous hydrocarbons from the peridotitic fluid during mixing and reaction with harzburgite resulted in the formation of diamond, olivine, garnet, and clinopyroxene, and increases in the and . Olivine was predicted to become more Fe-rich and garnet more Ca and Fe-rich with reaction progress, in agreement with reported temporal trends (core-to-rim) in the Panda mineral inclusions. The fluid at the site of diamond formation became more saline with reaction progress and the predicted aqueous phase concentrations of all elements changed consistent with trends in Panda fluid inclusions. In contrast, a prediction for a saline fluid infiltrating a harzburgite containing a carbonatitic fluid resulted in trends of the silicate minerals and the salinity with reaction progress that were in the opposite direction to data from the Panda diamonds. Overall, our study strongly supports the notion that fluids from subducting slabs could mix and precipitate diamonds containing carbon from both oxidized and reduced sources, while adding Ca and Fe to the sub-lithospheric cratonic mantle through metasomatic reactions.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 235-243.
Mantle
carbonatite
Abstract: To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar ?137/134Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in ?137/134Ba values from ?0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average ?137/134Ba value of their mantle sources is estimated to be +0.04?±?0.06‰ (2SD, n?=?16), which is similar to the average value of +0.05?±?0.06‰ for mid-ocean ridge basalts. The lower ?137/134Ba value of ?0.08‰ in a Canadian sample and higher ?137/134Ba values of +0.14‰ and?+?0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = -1.1‰ to -0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = ?1.1‰ to ?0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Fullerenes, nanotubes and carbon nanostructures, Vol. 28, 11, pp. 877-885.
global
synthetics
Abstract: In this paper, the phase transformation mechanism of various carbon sources in the synthesis of diamond by direct detonation method was studied. Through designing comparison experiment and the X-ray diffraction (XRD) characterization technique, an experimental study was conducted on the detonation process with the external of the combined carbon source and free carbon source, and without the participation of the external carbon source. The laws of phase transformation of the various carbon sources are obtained, in which the surplus carbon in the explosives participates in the formation of diamonds through the collision growth of droplet-like carbon, the added bonded carbon does not participate in the synthesis of diamond, and the added free carbon forms diamonds through the Martensitic transformation.
Earth Planetary Science Letters, Vol. 492, pp. 197-205.
United States, Colorado
mantle - discontinuity
Abstract: We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010). The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at and respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine-wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4-10 km) (Smyth and Frost, 2002). The findings of a wide olivine-wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350-450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region (Schmandt and Humphreys, 2010), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains.
Earth and Planetary Science Letters, Vol. 492, pp. 197-205.
United States, Colorado
geophysics - seismic
Abstract: We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010). The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at and respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine-wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4-10 km) (Smyth and Frost, 2002). The findings of a wide olivine-wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350-450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region (Schmandt and Humphreys, 2010), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains.
Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North Chin a Craton: implications for the interleaving of the Dabie Sulu orogenic crust.
International Geology Review, Vol. 57, 9-10, pp. 1305-1325.
Melting of subducted continent: element and isotopic evidence for a genetic relationship between Neoproterozoic and Mesozoic granitoids in the Sulu orogen.
Chemical Geology, Vol. 229, 4, May 30, pp. 227-256.
Abstract: Regolith-hosted rare earth element (REE) deposits predominate global resources of heavy REEs. Regoliths are underlain by various types of igneous rocks and do not always host economically valuable deposits. Thus a feasible and convenient method is desired to identify REE mineralization in a particular regolith. This study presents a detailed visible short-wave infrared reflectance (VSWIR) spectroscopic study of the Renju regolith-hosted REE deposit, South China, to provide diagnostic parameters for targeting REE orebodies in regoliths. The results show that the spectral parameters, M794_2nd and M800_2nd, derived from the VSWIR absorption of Nd3+ at approximately 800 nm, can be effectively used to estimate the total REE concentrations in regolith profiles. M1396_2nd/M1910_2nd ratios can serve as proxies to evaluate weathering intensities in a regolith. Abrupt changes of specific spectral features related to mineral abundances, chemical compositions, and weathering intensities can be correlated with variations of protolith that formed a regolith. These VSWIR proxies are robust and can be used for exploration of regolith-hosted REE deposits.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = -1.1‰ to -0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = ?1.1‰ to ?0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Abstract: Xenocrystic zircons from Cretaceous pyroclastic vents on Mt. Carmel, N. Israel, document two major periods of earlier mafic magmatism: Permo-Triassic (285-220?Ma) and Jurassic (200-160?Ma). Related alluvial deposits also contain these zircon populations. However, most alluvial zircons are Cretaceous (118-80?Ma) or younger, derived from Miocene to Pliocene volcanic episodes. The Permo-Triassic-Jurassic zircons are typically large and glassy; they have irregular shapes and a wide variety of internal zoning patterns. They appear to have grown in the interstitial spaces of coarse-grained rocks; many show evidence of recrystallization, including brecciation and rehealing by chemically similar zircon. Grains with relict igneous zoning have mantle-like ?18O (5.5?±?1.0‰), but brecciation leads to lower values (mean 4.8‰, down to 3.1‰). Hf-isotope compositions lie midway between the Chondritic Uniform Reservoir (CHUR) and Depleted Mantle (DM) reservoirs; Hf model ages suggest that the source region separated from DM in Neoproterozoic time (1500-1000?Ma). Most Cretaceous zircons have 176Hf/177Hf similar to those of the older zircons, suggesting recrystallization and/or Pb loss from older zircons in the Cretaceous thermal event. The Permo-Jurassic zircons show trace-element characteristics similar to those crystallized from plume-related magmas (Iceland, Hawaii). Calculated melts in equilibrium with them are characterized by strong depletion in LREE and P, large positive Ce anomalies, variable Ti anomalies, and high and variable Nb, Ta, Th and U, consistent with the fractionation of monazite, zircon, apatite and Ti-bearing phases. We suggest that these coarse-grained zircons crystallized from late differentiates of mafic magmas, ponded near the crust-mantle boundary (ca 30?km depth), and were reworked repeatedly by successively younger igneous/metasomatic fluids. The zircon data support a published model that locates a fossil Neoproterozoic plume head beneath much of the Arabia-Levant region, which has been intermittently melted to generate the volcanic rocks of the region. The Cretaceous magmas carry mantle xenoliths derived from depths up to 90?km, providing a minimum depth for the possible plume head. Post-Cretaceous magmatism, as recorded in detrital zircons, shows distinct peaks at 30?Ma, 13?Ma, 11.4?±?0.1?Ma (a major peak; n?=?15), 9-10?Ma and 4?Ma, representing the Lower and Cover Basalts in the area. Some of these younger magmas tapped the same mantle source as the Permian-Jurassic magmatism, but many young zircons have Hf-isotope compositions extending up to DM values, suggesting derivation of magmas from deeper, more juvenile sources.
Abstract: Redox-sensitive transition group elements are involved in almost all fundamental geochemical processes. Of these elements, vanadium (V) contributes a particularly powerful tool to decipher the Earth's history and its link to extraterrestrial bodies. A comprehensive view of V includes the formation and interaction between the Earth's interior layers, the evolution of the Earth's surface to a habitable zone, biogeochemical cycling, and anthropogenic impacts on the environment. Tracing the geochemical behavior of V through the Earth's compartments reveals critical connections between almost all disciplines of Earth sciences. Vanadium has a history of application as a redox tracer to address the early accretion history of the Earth, to identify connections between the mantle and crust by subduction and melting, and to interpret past surface environments. The geochemical cycling of V from the deep Earth to the surface occurs through magmatism, weathering and digenesis, reflecting variations of fO2 and V species in different Earth compartments. Minerals form a link between deep Earth reservoirs of vanadium and surface environments, and the study of V in minerals has increased the understanding of V cycling. Finally, the exploitation of V has been increasing since the Industrial Revolution, and significant amounts of V have been released as a consequence into natural systems. Environmental concerns are promoting new areas of research to focus on V cycling between water, air, soil and sediment compartments. An increased understanding of V in all compartments, and knowledge of the processes that connect the compartments, is vital to tracing the fate of this intriguing element in natural systems.
Abstract: This ending talk, focused on the ongoing cooperative research of Prof. Griffin and his team at Macquarie University and Shefa Yamim, since January 2014, highlighting unique corundum species characteristics. Preliminary results of this research were presented in the IGS Annual Meeting of 2015, whereas this year Prof. Griffin has shared innovative findings only microscopically tracked within titanium-rich corundum aggregates. One of the more abundant minerals is Tistarite (Ti2O3), previously known only as a single grain in a primitive type of meteorite (!). An article has been submitted to a scientific journal detailing this first terrestrial occurrence. Several other minerals are common in meteorites, but unknown or extremely rare on Earth. About half of these minerals are unknown to science, and will be described as new minerals in the scientific literature. The first of these is a Titanium-Aluminium-Zirconium oxide, informally known as TAZ; it will be submitted to the International Mineralogical Association for recognition as a new mineral, ShefaTAZite. Using state of the art technologies such as Thermal Ionisation Mass Spectrometry (TIMS) and Electron Microscopy Facility (EMF) that has three scanning electron microscopes, all with EBSD capability, and a transmission electron microscope - Prof. Griffin revealed spectacular imagery of minerals and rare compounds associated with titanium rich corundum aggregates.
Abstract: Large peridotite massifs are scattered along the 1500?km length of the Yarlung-Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low fO2 (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high-T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high-P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr-Al spinel ± cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to?>?13?GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375?Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low-P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low-fO2 phases. A proposed P-T-t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c. 375?Ma, residence in the upper Transition Zone for >200 Myr, and rapid exhumation at c. 170-150?Ma or 130-120?Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in?
Earth and Planetary Science Letters, Vol. 430, pp. 284-295.
Asia, Tibet
Luobusa Massif Type Iib
Abstract: For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal-alloy inclusions and highly negative View the MathML source?C13 values are present in both sets of diamonds. The Tibetan diamonds (n=3n=3) show an exceptionally large range in View the MathML source?N15 (?5.6 to +28.7‰+28.7‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with View the MathML source?N15=0‰ in {111} sectors and +30‰+30‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.
Abstract: The minimum oxygen fugacity (fO2) of Earth's upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO ? Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (fO2 = 6-8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low fO2 (IW < -10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth's upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.
Abstract: Magnesium and oxygen are critical elements in the solid Earth and hydrosphere. A better understanding of the combined behavior of Mg and O isotopes will refine their use as a tracer of geochemical processes and Earth evolution. In this study, the Mg-isotope compositions of garnet and omphacite separated from well-characterized xenolithic eclogites from the Roberts Victor kimberlite pipe (South Africa) have been measured by solution multi-collector ICP-MS. The reconstructed whole-rock ?26Mg values of Type I (metasomatized) eclogites range from ? 0.61‰ to ? 0.20‰ (Type IA) and from ? 0.60‰ to ? 0.30‰ (Type IB) (mean ? 0.43‰ ± 0.12‰), while ?26Mg of Type IIA (fresh, least metasomatized) eclogites ranges from ? 1.09‰ to ? 0.17‰ (mean ? 0.69‰ ± 0.41‰); a Type IIB (fresh, least metasomatized) has ?26Mg of ? 0.37‰. Oxygen-isotope compositions of garnet were analyzed in situ by SIMS (CAMECA 1280) and cross-checked by laser fluorination. Garnets have ?18O of 6.53‰ to 9.08‰ in Type IA, 6.14‰ to 6.65‰ in Type IB, and 2.34‰ to 2.91‰ in Type IIB. The variation of ?26Mg and ?18O in Type IA and IB eclogites is consistent with the previously proposed model for the evolution of these samples, based on major and trace elements and radiogenic isotopes. In this model, the protoliths (Type II eclogites) were metasomatized by carbonatitic to kimberlitic melts/fluids to produce first Type IA eclogites and then Type IB. Metasomatism has changed the O-isotope compositions, but the Mg-isotope compositions of Type IA are mainly controlled by the protoliths; those of Type IB eclogites reflect mixing between the protoliths and the kimberlitic melt/fluid. The combination of a large range of ?26Mg and low ?18O in Type II eclogites cannot be explained easily by seawater alteration of oceanic crust, interaction of carbonate/silicate sediments with oceanic crust, or partial melting of mafic rocks.
Geochimica et Cosmochimica Acta, in press available 45p.
Africa, Russia
Geochronology
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ to 0.060‰ with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Metasomatism
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243 to ?0.204 with an average of ?0.225 ± 0.037 (2?, n = 19), and ?56Fe from ?0.038‰ o 0.060 with an average of ?0.003 ± 0.068 (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (±0.05 for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Craton, Peridotite
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ 0.060 with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53-13.63 wt%) and crystallized at ~1300-1500 °C and 16-30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950-1100 °C and 12-18 kbar before entrainment in the basanites. Rare earth element (REE) thermobarometry of garnets and coexisting clinopyroxenes preserves evidence of an intermediate stage (1032 °C and 21 kbar). These results imply that the protoliths of the garnet pyroxenite formed at a range of depths from ~50 to 100 km, and then during or shortly after cooling, they were tectonically emplaced to higher levels (~40-60 km; i.e., uplifted by at least 10-20 km) along the prevailing geotherm. This uplift may have been connected with lithosphere-scale faulting during the Paleozoic orogeny, or during Mesozoic-Cenozoic rifting of eastern Australia.
Geological Society of London, Chapter 2, pp. 27-48.
Australia, Victoria
xenoliths
Abstract: Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53–13.63 wt%) and crystallized at ~1300–1500 °C and 16–30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950–1100 °C and 12–18 kba
Abstract: Moissanite (SiC) occurs in mantle and mantle-generated rocks from different tectonic settings. SiC is stable only at low oxygen fugacity (ƒO2) ?IW. Israeli SiC is assiociated with corundum, Fe globules, native V and other phases in Cretaceous pyroclastic rocks from Mt Carmel and associated alluvial deposits[1]. The SiC grains contain inclusions of Si metal, FeSi2, FeTiSi2, FeAlSi2 and CaSi2+xSi2-x, which were liquids before being trapped during SiC crystallization. SiC has been found included in corundum, associated with Fe-Ti silicides, connecting the formation of SiC, reduced melts in corundum and conrundum itself. All grains are of the 6H polytype. ?13C ranges from - 32.1 to -24.5‰ and ?30Si from -0.68 to +1.42‰. These SiC grains are one product of the interaction of basaltic magma and mantle methane in a volcanic plumbing system. SiC crystallized from metallic melts that became immiscible during the reduction of the magma. Its low ?13C may reflect Rayleigh fractionation under reduced conditions; the variation in Si isotopes may reflect fractionation between SiC and immiscible metallic melts. SiC samples from the Udachnaya and Mir kimberlite pipes contain inclusions of Si metal, FeSi2, FeSi, FeTiSi2, Si(N,O). The SiC has ?13C ranging from -28.5 to -24.8‰, and ?30Si from -1.72 to +1.42‰. SiC from harzburgites, chromitites and pyroxenites of the Tibetan Zedang ophiolites have inclusions of Si metal and unmixed Fe-Ni-Ti-Si alloy. Their ?13C ranges from -30.6 to -24.7‰ and ?30Si from -0.85 to +1.26‰. SiC samples from these different settings show very similar characteristics, implying that they may be formed in similar mantle conditions, where the flux of mantle methane gradually reduces magmas and interacts with them to produce different reduced phases at different stages.
European Journal of Mineralogy, Vol. 29, 4, pp. 557-570.
Europe, Israel
mineralogy
Abstract: Ultrahigh-pressure (UHP) materials (e.g., diamond, high-pressure polymorph of chromite) and super-reduced (SuR) phases (e.g., carbides, nitrides, silicides and native metals) have been identified in chromitites and peridotites of the Tibetan and Polar-Urals ophiolites. These unusual assemblages suggest previously unrecognized fluid- or melt-related processes in the Earth’s mantle. However, the origin of the SuR phases, and in particular their relationships with the UHP materials in the ophiolites, are still enigmatic. Studies of a recently recognized SuR mineral system from Cretaceous volcanics on Mt Carmel, Israel, suggest an alternative genesis for the ophiolitic SuR phases. The Mt Carmel SuR mineral system (associated with Ti-rich corundum xenocrysts) appears to reflect the local interaction of mantle-derived CH4 ± H2 fluids with basaltic magmas in the shallow lithosphere (depths of ?30-100 km). These interactions produced desilication of the magma, supersaturation in Al2O3 leading to rapid growth of corundum, and phase assemblages requiring local oxygen fugacity (fO2) gradually dropping to ?11 log units below the iron-wüstite (IW) buffer. The strong similarities between this system and the SuR phases and associated Ti-rich corundum in the Tibetan and Polar-Urals ophiolites suggest that the ophiolitic SuR suite probably formed by local influx of CH4 ± H2 fluids within previously subducted peridotites (and included chromitites) during their rapid exhumation from the deep upper mantle to lithospheric levels. In the final stages of their ascent, the recycled peridotites and chromitites were overprinted by a shallow magmatic system similar to that observed at Mt Carmel, producing most of the SuR phases and eventually preserving them within the Tibetan and Polar-Urals ophiolites.
Mineralogy and Petrology, in press available, 14p.
Europe, Israel
metasomatism
Abstract: Oxygen fugacity (fO2) is a key parameter of Earth’s mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping fO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low fO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of ?13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing fO2 with depth (e.g. Yaxley et al. in Lithos 140:142-151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.
Griffin, W.L., Huang, J-X., Thomassot, E., Gain, S.E.M., Toledo, V., O'Reilly, S.Y.
Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle ( Mt. Carmel).
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0575-x 14p.
Abstract: We report preliminary results of a systematic search for fluid/melt microinclusions in mantle minerals. “Dusty” garnets from xenolith XRV6 [1], a heavily metasomatised Type I eclogite from Roberts Victor mine, SA, carry many microinclusions (<1 ?m). FTIR analyses of "dusty" zones indicate the presence of molecular water in the inclusions and hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions constrains the bulk composition of the microinclusions. Compared to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in Al2O3 and MgO. The silica contents seem to be similar to that of the host garnet. Figure 1: a. Backscatter image of the microinclusions in XRV6 garnet. b. K2O vs. MgO of the clear garnet (red) and the microinclusions (+ their surrounding garnet, blue). Most of the elements form compositional mixing arrays of microinclusion+garnet (Fig. 1b). The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point towards the array of silicic to low-Mg carbonatitic high density fluids (HDFs) trapped in diamonds, indicating the role of such fluids in mantle metasomatism.
Abstract: Hibonite (CaAl12O19) is a constituent of some refractory calcium-aluminum inclusions (CAIs) in carbonaceous meteorites, commonly accompanied by grossite (CaAl4O7) and spinel. These phases are usually interpreted as having condensed, or crystallized from silicate melts, early in the evolution of the solar nebula. Both Ca-Al oxides are commonly found on Earth, but as products of high-temperature metamorphism of pelitic carbonate rocks. We report here a unique occurrence of magmatic hibonitegrossite-spinel assemblages, crystallized from Ca-Al-rich silicate melts under conditions [high-temperature, very low oxygen fugacity (fO2)] comparable to those of their meteoritic counterparts. Ejecta from Cretaceous pyroclastic deposits on Mt Carmel, N. Israel, include aggregates of hopper/skeletal Ti-rich corundum, which have trapped melts that crystallized at fO2 extending from 7 log units below the iron-wustite buffer (?IW = -7; SiC, Ti2O3, Fe-Ti silicide melts) to ?IW ? -9 (native V, TiC, and TiN). The assemblage hibonite + grossite + spinel + TiN first crystallized late in the evolution of the melt pockets; this hibonite contains percentage levels of Zr, Ti, and REE that reflect the concentration of incompatible elements in the residual melts as corundum continued to crystallize. A still later stage appears to be represented by coarse-grained (centimeter-size crystals) ejecta that show the crystallization sequence: corundum + Liq ? (low-REE) hibonite ? grossite + spinel ± krotite ? Ca4Al6F2O12 + fluorite. V0 appears as spheroidal droplets, with balls up to millimeter size and spectacular dendritic intergrowths, included in hibonite, grossite, and spinel. Texturally late V0 averages 12 wt% Al and 2 wt% Mn. Spinels contain 10-16 wt% V in V0-free samples, and <0.5 wt% V in samples with abundant V 0. Ongoing paragenetic studies suggest that the fO2 evolution of the Mt Carmel magmatic system reflects the interaction between OIB-type mafic magmas and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Temperatures estimated by comparison with 1 atm phase-equilibrium studies range from ca. 1500 °C down to 1200-1150 °C. When fO2 reached ca. ?IW = -7, the immiscible segregation of Fe,Ti-silicide melts and the crystallization of SiC and TiC effectively desilicated the magma, leading to supersaturation in Al2O3 and the rapid crystallization of corundum, preceding the development of the hibonite-bearing assemblages. Reports of Ti-rich corundum and SiC from other areas of explosive volcanism suggest that these phenomena may be more widespread than presently realized, and the hibonite-grossite assemblage may serve as another indicator to track such activity. This is the first reported terrestrial occurrence of krotite (CaAl2O4), and of at least two unknown Zr-Ti oxides.
Abstract: Although hydrogen is the most abundant element in the solar system, the mechanisms of exchange of this element between the deep interior and surface of Earth are still uncertain. Hydrogen has profound effects on properties and processes on microscopic-to-global scales. Here we report the discovery of the first hydride (VH2) ever reported in nature. This phase has been found in the ejecta of Cretaceous pyroclastic volcanoes on Mt Carmel, N. Israel, which include abundant xenoliths containing highly reduced mineral assemblages. These xenoliths were sampled by their host magmas at different stages of their evolution but are not genetically related to them. The xenoliths are interpreted as the products of extended interaction between originally mafic magmas and CH4+H2 fluids, derived from a deeper, metal-saturated mantle. The last stages of melt evolution are recorded by coarse-grained aggregates of hibonite (CaAl12O19) + grossite (CaAl4O7) + V-rich spinels ± spheroidal to dendritic inclusions of metallic vanadium (V0), apparently trapped as immiscible metallic melts. The presence of V0 implies low oxygen fugacities and suggests crystallization of the aggregates in a hydrogen-rich atmosphere. The presence of such reducing conditions in the upper mantle has major implications for the transport of carbon, hydrogen and other volatile species from the deep mantle to the surface.
Minerals MDPI, Vol. 10, 1118, doi:10.3390/ min10121118 10p. Pdf
Europe, Israel
deposit - Mt. Carmel
Abstract: Here, we describe two new minerals, kishonite (VH2) and oreillyite (Cr2N), found in xenoliths occurring in pyroclastic ejecta of small Cretaceous basaltic volcanoes exposed on Mount Carmel, Northern Israel. Kishonite was studied by single-crystal X-ray diffraction and was found to be cubic, space group Fm3¯m, with a = 4.2680(10) Å, V = 77.75(3) Å3, and Z = 4. Oreillyite was studied by both single-crystal X-ray diffraction and transmission electron microscopy and was found to be trigonal, space group P3¯1m, with a = 4.7853(5) Å, c = 4.4630(6) Å, V = 88.51 Å3, and Z = 3. The presence of such a mineralization in these xenoliths supports the idea of the presence of reduced fluids in the sublithospheric mantle influencing the transport of volatile species (e.g., C, H) from the deep Earth to the surface. The minerals and their names have been approved by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association (No. 2020-023 and 2020-030a).
Abstract: Xenolithic corundum aggregates in Cretaceous mafic pyroclastics from Mount Carmel contain pockets of silicate melts with mineral assemblages [SiC (moissanite), TiC, Ti2O3 (tistarite), Fe-Ti-Zr silicides/phosphides] indicative of magmatic temperatures and oxygen fugacity (fO2) at least 6 log units below the iron-wüstite buffer (?IW ? -6). Microstructural evidence indicates that immiscible, carbon-rich metallic (Fe-Ti-Zr-Si-P) melts separated during the crystallization of the silicate melts. The further evolution of these metallic melts was driven by the crystallization of two main ternary phases (FeTiSi and FeTiSi2) and several near-binary phases, as well as the separation of more evolved immiscible melts. Reconstructed melt compositions fall close to cotectic curves in the Fe-Ti-Si system, consistent with trapping as metallic liquids. Temperatures estimated from comparisons with experimental work range from ?1500 °C to ca. 1150 °C; these probably are maximum values due to the solution of C, H, P, and Zr. With decreasing temperature (T), the Si, Fe, and P contents of the Fe-Ti-Si melts increased, while contents of Ti and C decreased. The increase in Si with declining T implies a corresponding decrease in fO2, probably to ca. ?IW-9. The solubility of P in the metallic melts declined with T and fO2, leading to immiscibility between Fe-Ti-Si melts and (Ti,Zr)-(P,Si) melts. Decreasing T and fO2 also reduced the solubility of C in the liquid metal, driving the continuous crystallization of TiC and SiC during cooling. The lower-T metallic melts are richer in Cr, and to some extent V, as predicted by experimental studies showing that Cr and V become more siderophile with decreasing fO2. These observations emphasize the importance of melt-melt immiscibility for the evolution of magmas under reducing conditions. The low fO2 and the abundance of carbon in the Mt. Carmel system are consistent with a model in which differentiating melts were fluxed by fluids that were dominated by CH4+H2, probably derived from a metal-saturated sublithospheric mantle. A compilation of other occur-rences suggests that these phenomena may commonly accompany several types of explosive volcanism.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = -1.1‰ to -0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = ?1.1‰ to ?0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Abstract: Many carbonatites are associated both spatially and temporally with large igneous provinces (LIPs), and considered to originate from a mantle plume source lacking any contribution from recycled crustal materials. Here, we report an occurrence of carbonatite enriched in rare-earth elements (REE) and associated with the Tarim LIP in northwestern China. The Tarim LIP comprises intrusive and volcanic products of mantle plume activity spanning from ~ 300 to 280 Ma. The carbonatites at Wajilitage in the northwestern part of Tarim are dominated by calcite and dolomite varieties, and contain abundant REE minerals (principally, monazite and REE-fluorcarbonates). Th-Pb age determination of monazite yielded an emplacement age of 266 ± 5.3 Ma, i.e. appreciably younger than the eruption age of flood basalts at ~ 290 Ma. The carbonatites show low initial 87Sr/86Sr (0.7037-0.7041) and high ?Nd(t) (1.2-4) values, which depart from the isotopic characteristics of plume-derived basalts and high-Mg picrites from the same area. This indicates that the Wajilitage carbonatites derived from a mantle source isotopically distinct from the one responsible for the voluminous (ultra)mafic volcanism at Tarim. The carbonatites show ?26MgDSM3 values (? 0.99 to ? 0.65‰) that are significantly lower than those in typical mantle-derived rocks and rift carbonatites, but close to marine sediments and orogenic carbonatites. We propose that the carbonatites in the Tarim LIP formed by decompressional melting of recycled sediments mixed with the ambient mantle peridotite. The enriched components in the Tarim plume could be accounted for by the presence of recycled sedimentary components in the subcontinental mantle.
Abstract: Many carbonatites are associated both spatially and temporally with large igneous provinces (LIPs), and considered to originate from a mantle plume source lacking any contribution from recycled crustal materials. Here, we report an occurrence of carbonatite enriched in rare-earth elements (REE) and associated with the Tarim LIP in northwestern China. The Tarim LIP comprises intrusive and volcanic products of mantle plume activity spanning from ~ 300 to 280 Ma. The carbonatites at Wajilitage in the northwestern part of Tarim are dominated by calcite and dolomite varieties, and contain abundant REE minerals (principally, monazite and REE-fluorcarbonates). Th–Pb age determination of monazite yielded an emplacement age of 266 ± 5.3 Ma, i.e. appreciably younger than the eruption age of flood basalts at ~ 290 Ma. The carbonatites show low initial 87Sr/86Sr (0.7037–0.7041) and high ?Nd(t) (1.2–4) values, which depart from the isotopic characteristics of plume-derived basalts and high-Mg picrites from the same area. This indicates that the Wajilitage carbonatites derived from a mantle source isotopically distinct from the one responsible for the voluminous (ultra)mafic volcanism at Tarim. The carbonatites show ?26MgDSM3 values (? 0.99 to ? 0.65‰) that are significantly lower than those in typical mantle-derived rocks and rift carbonatites, but close to marine sediments and orogenic carbonatites. We propose that the carbonatites in the Tarim LIP formed by decompressional melting of recycled sediments mixed with the ambient mantle peridotite. The enriched components in the Tarim plume could be accounted for by the presence of recycled sedimentary components in the subcontinental mantle.
Earth and planetary Science Letters, Vol. 579, 117359, 11p. Pdf
Mantle
ringwoodite
Abstract: The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances the Vp (P-wave velocity) jump, yet decreases the Vs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases both Vp and Vs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ?1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence of Vs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.
Abstract: The Earth’s mantle is characterized by a sharp seismic discontinuity at a depth of 660?km that can provide insights into deep mantle processes. The discontinuity occurs over only 2?km—or a pressure difference of 0.1?GPa—and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure, high-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg-Fe binary post-spinel transition by applying advanced multi-anvil techniques with in situ X-ray diffraction with the help of Mg-Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01?GPa, corresponding to 250?m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole-mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped on the basis of discontinuity sharpness.
Geochimica et Cosmochimica Acta, Vol. 30, pp. 110-136.
Mantle
redox
Abstract: The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth’s lower mantle. To investigate the controls on the chemistry of Brg, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo2) at 25 GPa. The Fe3+/?Fe ratio of Brg increases with Brg Al content and fo2 and decreases with increasing total Fe content and with temperature. The dependence of the Fe3+/?Fe ratio on fo2 becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al-Mg-Si-O, Fe-Mg-Si-O and Fe-Al-Mg-Si-O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of fo2 with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of fo2 and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly fo2 dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the fo2 at the top of the lower mantle is ?0.86 log units below the iron-wüstite buffer (IW) with a Brg Fe3+/?Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7?wt.% Fe-Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe3+/?Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the fo2 recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Abstract:
Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit. The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive ?18OSMOW (7.29 to 15.53‰) and negative ?13CPDB (?5.26 to ?10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.
Africa, South Africa, China, United States, Canada, South America
diamond inclusions
Abstract: We present the first evidence for inclusions of ice-VII in diamonds from southern Africa, China, North- and South-America [1]. Combining synchrotron X-ray diffraction, - X-ray fluorescence and IR spectroscopy, we show the presence of ice-VII as inclusions in diamonds that have formed at depth > 410 km to about 800 km in the Earth's mantle. What is now crystalline ice-VII, a high pressure polymorph of water-ice, was component of an aqueous fluid entrapped in the diamonds that were growing in the deep mantle. Because of the confinement by the host diamonds, the inclusions retain high pressures. The same holds for inclusions of magnesian calcite, halite, and ilmenite found in the same diamond specimens. These inclusions reflect the presence of aqueous and carbonaceous fluids in the mantle transition zone and the shallow lower mantle. Using their current residual pressures and the equations of state, we can reconstruct their recovery paths [2,3]. Further, we can use the intersection of modelled recovery paths to better constrain the encapsulation pressure and temperature of these inclusions in diamonds.
Abstract: Mantle plumes sample the deep mantle. A limited number of geochemical endmember components can describe the isotopic and compositional variations in the ocean island basalts (OIBs), which are produced by plume volcanism. The endmembers are correlated to compositions in the OIB source regions or represent incorporation of material upon ascent. However, their actual nature and origins are still highly debated. The depths of plume sources have been proposed to be anywhere between the core-mantle boundary and the upper mantle, and need not be the same for all plume-related volcanic activities. Using a combination of synchrotron micro-X ray fluorescence and -diffraction mapping, and in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we show that the elemental features of HIMU-rich OIBs, such as Bermuda, St Helena, and Cook-Austral, exactly match the geochemical signature of a multiphase inclusion in a diamond. The geochemical signature in our studied diamond inclusion is markedly different from that of inclusions in lithospheric diamonds. The phases identified in the inclusion are majorite-rich garnet, ilmenite, the sodic 10Å-phase (TAP), and liebermannite. Furthermore, we show that this inclusion was entrapped at 14.5 ± 0.5 GPa (420-440 km) and 1450 ± 50 K. At the conditions of entrapment, the diamond inclusion phase assembly was garnet + ilmenite + liebermannite + clinopyroxene + stishovite + fluid. Sodic TAP is a retrograde product of reaction between clinopyroxene, stishovite, and fluid upon ascent. Its presence shows that the HIMU source is water-saturated. Entrapment in diamond indicates that the fluid also contained carbonate. The conditions of 14.5 ± 0.5 GPa and 1450 ± 50 K plot right on top of the alkaline carbonatite solidus, and match the formation of carbonatitic melt from subducted slabs plus diamond formation from reaction of carbonate with iron. In summary, our data show that the transition zone source accounts for the global HIMU endmember.
Abstract: Calcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.
Abstract: n recent years, nominal type IaAB and IIa diamonds with transient 2800 cm-1 FTIR absorption peaks arising from uncompensated boron produced under UV radiation have been reported (J. Li et al., A diamond with a transient 2804 cm-1 absorption peak, Journal of Gemmology, Vol. 35, 2016, pp. 248-252; Winter 2016 Lab Notes, pp. 412-413). The National Center of Supervision and Inspection on Quality of Gold and Silver Products recently examined a type IaB diamond that exhibited instantaneous 2803 cm-1 FTIR absorption shortly after exposure to an ultra-short-wave (< 230 nm) UV source.
Abstract: A cut diamond of intense yellowish green color has been characterized using microscopy and spectroscopic techniques. The diamond has been unambiguously identified as color-treated. The simultaneous presence of multiple centers related to irradiation and annealing—including H1a, H1b, NV0, NV-, H3, H4, GR1, and H2—was revealed. UV-Vis-NIR absorption spectroscopy showed that the diamond owes its color to the two major bands related to H3 and GR1. The combination of these spectroscopic features in one diamond has not been reported in the gemological literature, suggesting that this diamond was subjected to a complex treatment procedure that is not frequently applied. Taking into account the thermal stability of the defects involved and the defect transformations at high temperatures, two possible treatment procedures explaining the observed combination of spectroscopic features are proposed.
Geochimica et Cosmochimica Acta, Vol. 303, pp. 92-109, pdf
Mantle
melting
Abstract: The role of water on melting in the mantle wedge is still debated due to large uncertainty on the estimates of H2O flux beneath arcs. B has been proven as an effective proxy for water flux because B and H2O show similar chemical behaviors during subduction. The Habahe mafic dikes from the Chinese Altai were emplaced within a narrow area (<20?km from south to north) during the northward subduction of the Junggar Ocean in the middle Paleozoic. These dikes have been classified into four types with distinct geochemical and Sr-Nd-Hf-Pb isotopic compositions, which originated from mantle sources metasomatized by different subduction components, including melts from subducted sediments (Type-I, Type-IV), fluids from subducted sediments (Type-II), and melts from subducted oceanic crust (Type-III). We present B content and isotope data for the Habahe mafic dikes to investigate the influence of subduction components on melting in the mantle wedge. Type-I and -III mafic dikes all have negative ?11B values (?7.7‰ to ?5.0‰) with variable B contents (3.65-13.4?ppm) and B/Nb ratios (2.10-7.39), indicating B isotopically light features for the subducted sediments and oceanic crust. Type-II mafic dikes have lower B contents (3.97-9.90?ppm) and higher B/Nb ratios (7.07-14.4) than Type-I mafic dikes, with a wide range of ?11B values from ?7.8‰ to ?2.7‰. This suggests that their mantle source may have been metasomatized by fluids from subducted serpentinite besides fluids from subducted sediments. Type-IV mafic dikes have higher B contents (17.0-27.5?ppm) and B/Nb ratios (25.0-40.8), and heavier B isotopic compositions (?11B?=??2.9‰ to +3.5‰) than Type-I mafic dikes. This indicates involvement of fluids from the slab serpentinite in metasomatism of their mantle source in addition to melts from the subducted sediments. The Habahe mafic dikes show wide range of B/Nb ratios, suggesting that different amounts of water were added into their mantle sources. These dikes exhibit variable Zr/Yb and Nb/Yb ratios, and constantly low TiO2/Yb, indicating their formation through different degrees melting of depleted mantle sources. Their Zr/Yb and Nb/Yb ratios are negatively correlated with B/Nb, which reflects elevation of the melting degree of their mantle sources as increasing water input. Similar trends are also observed in basalts from global arcs and their major and trace elements correlate well with B/Nb ratios. Thus, water flux should play an important role on melting in the mantle wedge and control magma compositions of the arcs.
Zhao, D., Smith, D.G.W. Smith, Zhou, M., Jang, J., Deng, C., Huang, Y.
Yinniugou lamproites in Datong, northern Shanxi Province, Chin a: first occurrence in the North Chin a craton.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 133-140.
Abstract: In recent years we have confirmed the existence of ophiolite-hosted diamonds on Earth, which occur in mantle peridotites and podiform chromitites of many ophiolites. These diamonds differ significantly from most kimberlite varieties, particularly in their inclusions. The typical inclusions in the diamonds are Mn-rich phases, i.e., NiMnCo alloy, native Mn, MnO, galaxite, Mn olivine and Mn garnet. Ca-silicate perovskite, a typical lower mantle mineral, was identified as mineral inclusions in diamond. One occurs as a 60-nanometer, euhedral grain associated with NiMnCo alloy and graphite, while another one occurs as a 50-nanometer grain within a large inclusion containing both NiMnCo alloy and Nd-Se-Cu-S phase. By EDS the perovskite has Ca 48.3%, Si 37.7% and Mn 14.1% with oxygen. TEM diffraction data show that the inclusion has d-spacings and angles between adjacent lattice planes are consistent to the Ca-silicate perovskite with an orthorhombic structure. The only known source of such light carbon is organic material in surface sediments and the best known sources of abundant manganese are Fe-Mn-rich sediments and Mn nodules, both of which are common on the seafloor. Many parts of the modern seafloor are also covered by sediments with a continental provenance. Phases such as SiO2 and Al2O3 are not expected in mantle peridotites and must have been introduced from shallow levels. We propose that subduction of oceanic lithosphere carries C, Mn, Si, Al and REE to the transition zone or lower mantle where the material is mixed with highly reduced material, perhaps derived from greater depths. Crystallization of diamond from a C-rich fluid encapsulates the observed inclusions. The diamonds and associated minerals are incorporated into chromite grains during chromite crystallization at depth of mantle transition zone, and are carried to shallower levels by mantle convection. Accumulation of chromite grains produces podiform chromitites containing a range of exotic minerals. However, the presence of diamonds and other UHP minerals in ophiolitic peridotites indicates that such phases can persist far outside their normal stability fields.
Abstract: The transition from breakup of Nuna (or Columbia, 2.0-1.6 Ga) to assembly of Rodinia (1.0-0.9 Ga) is investigated by means of U?Pb and Lu?Hf data of detrital zircons from three Neoproterozoic metasedimentary rocks in the Central Tianshan Block (CTB), NW China. These data yield six age peaks around 1.0, 1.13, 1.34, 1.4-1.6, 1.75, and 2.6 Ga. Few zircons are detected between 2.0 and 2.5 Ga. The Paleoproterozoic to Neoproterozoic detrital zircons have Hf isotopic compositions (?22.1 to +13.0) similar to those of coeval magmatic rocks in the CTB, indicating a proximal provenance. These results, together with the geological evidence and the presence of 1.4 Ga orogenic granitoids in the CTB, rule out most cratons as the CTB sources but support a Fennoscandia ancestry. Zircon U?Pb ages and Hf isotopic compositions from the CTB and Fennoscandia suggest that from 1.8 to 1.4 Ga, the ?Hf(t) values increased toward more positive values, consistent with an exterior orogen characteristic that the lower crust was replaced by a juvenile arc crust. In contrast, from 1.4 to 0.9 Ga, zircon ?Hf(t) values decreased to more negative values, reflecting an interior orogen, characterized by enhanced contribution of recycled crustal material from collided continental fragments. This marked shift most likely reflected a transition from breakup of Nuna to assembly of Rodinia, accomplished by a transformation from an exterior orogen to an interior one.
Abstract: Numerical experiments are used to investigate the thermo-mechanical controls for inducing flat subduction and why flat subduction is rare relative to normal/steep subduction. Our modeling results demonstrate that flat subduction is an end-member of a steady state subduction geometry and is characterized by a curved slab with a nearly-horizontal slab section. Intermediate cases between normal/steep and flat subduction appear to be transient in origin and evolve toward one of the stable end-members. Physical parameters inducing flat subduction can be classified into four categories: buoyancy of the subducting oceanic lithosphere (e.g., slab age, oceanic crustal thickness), viscous coupling between the overriding and downgoing plates (e.g., initial subduction angle), external kinematic conditions, and rheological properties of the subduction zone. On the basis of parameter sensitivity tests and the main characteristics of present-day flat subduction zones, positive buoyancy from either the young slab or the thickened oceanic crust is considered as the primary controlling parameter. Our results show that the possibility of flat subduction is directly proportional to oceanic crustal thickness and inversely proportional to the slab age. Furthermore, oceanic crust must be thicker than 8 km to induce flat subduction, when the slab is older than 30 Ma with an initial subduction angle of ? 20° and without absolute trenchward motion of the overriding plate. The lower the initial subduction angle or the thicker the overriding continental lithosphere, the more likelihood for flat subduction. The initial subduction angle is more influential for the development of flat subduction than the overriding lithospheric thickness, and a thick overriding lithosphere induces flat subduction only under the condition of an initial subduction angle of ? 25°, with a slab age of ? 30 Ma and without absolute trenchward motion of the overriding plate. However, when the initial subduction angle is increased to > 25°, no flat subduction is predicted. All the parameters are evaluated within the constraints of a mechanical framework in which the slab geometry is regarded as a result of a balance between the gravitational and hydrodynamic torques. Any factor that can sufficiently reduce gravitational torque or increase hydrodynamic torque will exert a strong effect on flat subduction development. Our results are consistent with the observations of modern flat subduction zones on Earth.
Pure and Applied Physics, Vol. 175, 6, pp. 2141-2151.
Canada, Northwest Territories
mining
Abstract: Kimberlite is an igneous rock which sometimes bears diamonds. Most of the diamonds mined in the world today are found in kimberlite ores. Burst potential in kimberlite has not been investigated, because kimberlite is mostly mined using open-pit mining, which poses very little threat of rock bursting. However, as the mining depth keeps increasing, the mines convert to underground mining methods, which can pose a threat of rock bursting in kimberlite. This paper focuses on the burst potential of kimberlite at a diamond mine in northern Canada. A combined model with the methods of principal component analysis (PCA) and fuzzy comprehensive evaluation (FCE) is developed to process data from 12 different locations in kimberlite pipes. Based on calculated 12 fuzzy evaluation vectors, 8 locations show a moderate burst potential, 2 locations show no burst potential, and 2 locations show strong and violent burst potential, respectively. Using statistical principles, a Mahalanobis distance is adopted to build a comprehensive fuzzy evaluation vector for the whole mine and the final evaluation for burst potential is moderate, which is verified by a practical rockbursting situation at mine site.
Abstract: Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle-ductile transition around a magma chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2?±?0.5?kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures ?1.5?kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures >rsim2.5?kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only within a relatively narrow range of pressures around 2?±?0.5?kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.
Cosuna, Correlation Of Stratigraphic Units Of North America Project, American Association of Petroleum Geologists, Cat. No. 710, COSUNA No. 10, 1, 41 X 55 $10
Hudgins, T.R., Mukasa, S.B., Simon, A.C., Moore, G., Barifaijo, E.
Melt inclusion evidence for CO2 rich melts beneath the western branch of the East African Rift: implications for long term storage of volatiles in the deep lithospheric mantle.
Contributions to Mineralogy and Petrology, Vol. 169, 5p.
Mineral associations in diamonds from the lowermost upper mantle and uppermost lower mantle.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 235-254.
Journal of the Geological Society, Vol. 174, pp. 609-626.
South America, Uruguay
magmatism
Abstract: The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T–t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape.
Van Kranendonk, M.J., Hugh Smithies, R., Hickman, A.H., Champion, D.C.
Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia.
Abstract: Madagascar has become one of the world’s top sources of fine blue sapphire in recent times. In addition to beautiful untreated material, increasing numbers of treated stones have appeared in the market. Some have been heated to relatively low temperatures, below 1350°C, to lighten their color. To help separate unheated and heated Madagascar sapphire, the authors performed experiments to document the changes they undergo with low-temperature heat treatment in air, which is an oxidizing atmosphere.
Geochimica et Cosmochimica Acta, available in press 23p.
Africa, Botswana
deposit - Orapa
Abstract: Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth’s crust and subsequent surface weathering. However, at temperatures below 1100 °C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, ‘flaking’ of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187Os/188Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100 °C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (µCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby widening the scope for multiple methods for quantitative analysis, even on ‘flakes’ of single BMS inclusions. Finally we show that the trace elements present in peridotite (P-type) and eclogitic (E-type) BMS are distinct, with P-type diamonds having systematically higher total platinum-group element (particularly Os, Ir, Ru) and Te and As concentrations. These distinctions suggest that the PGE and semi-metal budgets of mantle-derived partial melts will be significantly dependent upon the type(s) and proportions of sulphides present in the mantle source.
Geochimica et Cosmochimica Acta, in press available, 23p.
Technology
diamond inclusions
Abstract: Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth’s crust and subsequent surface weathering. However, at temperatures below 1100 °C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, ‘flaking’ of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187Os/188Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100 °C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (µCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby widening the scope for multiple methods for quantitative analysis, even on ‘flakes’ of single BMS inclusions. Finally we show that the trace elements present in peridotite (P-type) and eclogitic (E-type) BMS are distinct, with P-type diamonds having systematically higher total platinum-group element (particularly Os, Ir, Ru) and Te and As concentrations. These distinctions suggest that the PGE and semi-metal budgets of mantle-derived partial melts will be significantly dependent upon the type(s) and proportions of sulphides present in the mantle source.
Geochimica et Cosmochimica Acta, Vol. 216, pp. 335-357.
Technology
diamond inclusions - microtomography
Abstract: Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth’s crust and subsequent surface weathering. However, at temperatures below 1100 °C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, ‘flaking’ of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187Os/188Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100 °C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (µCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby widening the scope for multiple methods for quantitative analysis, even on ‘flakes’ of single BMS inclusions. Finally we show that the trace elements present in peridotite (P-type) and eclogitic (E-type) BMS are distinct, with P-type diamonds having systematically higher total platinum-group element (particularly Os, Ir, Ru) and Te and As concentrations. These distinctions suggest that the PGE and semi-metal budgets of mantle-derived partial melts will be significantly dependent upon the type(s) and proportions of sulphides present in the mantle source.
Lithos, doi.org/10.1016/ j.lithos.2020.105918 67p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se this editing is incorrect. This should read "(with 87% of analyses having S/Se < 10,000)" Please note the 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0610-y 18p.
Europe, Scotland
aillikite
Abstract: The Glen Gollaidh aillikite dyke (58.36741°N 4.69751°W), N.W. Scotland, occurs within the Neoproterozoic sedimentary rocks of the Moine Supergroup ~4 km east of the Moine Thrust. Phlogopite 40Ar/36Ar measurements give a late Devonian maximum emplacement age of 360.3?±?4.9 (2?) Ma. This age occurs in a quiet period of Scottish magmatic history c. 30 Ma after the closure of the Iapetus and before the start of intra-plate alkali magmatism which affected southern Scotland for ~60 My from c. 350 Ma. Abundant chromites and Cr-diopsides and a few unaltered olivines, reflecting a mantle provenance, were recovered from heavy mineral concentrates. The North Atlantic Craton, exposed in Lewisian gneisses west of the Moine thrust, is therefore inferred to extend east at depth under Glen Gollaidh, presenting an opportunity to investigate the thickness and composition of the cratonic margin in the Devonian. The aillikite was found to be barren of diamond and no picro-ilmenites or garnets were definitively identified. However, mineral chemistry suggests that a proportion of Glen Gollaidh xenocrysts crystallised in equilibrium with garnet. Most spinels are Mg, Al chromites, with some Mg chromite present. All fall within the garnet peridotite field based on Ti and Cr but with insufficient Cr2O3 (up to 47.2 wt%) to be consistent with the diamond stability field. Amongst Cr-diopsides 30% of grains have Cr and Al contents consistent with derivation from garnet peridotite. The majority of clinopyroxenes also show a marked depletion in heavy compared to light rare-earth elements, again consistent with equilibration with garnet. The opx-cpx solvus thermometer demonstrates that average Cr-diopside compositions require at least 37 kbar to give a temperature (979 °C) lying even on a relatively warm 40 mWm?2 geotherm (Hasterok and Chapman Earth Planet Sc Lett 307:59-70, 2011). Large variations in the chemistry of mantle minerals reflect a complex history of metasomatism akin to constituents of alkali igneous rocks elsewhere in the Hebridean and Northern Highlands Terranes. Fertilised mantle provided the conditions for generation of aillikite melts, probably triggered by break-off of the advancing Avalonia slab. The cratonic root underlying the Glen Gollaidh aillikite during the late Devonian was apparently too thin to lie within the diamond stability field, consistent with xenoliths from alkali basalts further south. Nonetheless, sufficient geophysical and mineral chemical evidence supports Glen Gollaidh aillikite sitting close to the edge of diamond-prospective mantle therefore suggesting diamond potential a short distance to the west within the Lewisian and what is now East Greenland.
Abstract: The late Mesoproterozoic Ngualla carbonatite complex in southwest Tanzania comprises a central magnesiocarbonatite plug surrounded sequentially by an annular calcite carbonatite intrusion and fenitised felsic igneous country rocks. The calcite carbonatite contains phlogopite-rich (glimmerite) enclaves interpreted as fenitised wallrock xenoliths that have contributed silicate minerals, apatite and magnetite through dispersal and interaction, mainly within the calcite carbonatite magma. Ultramafic magmas were emplaced into the magnesiocarbonatite magma chamber before complete solidification of the magnesiocarbonatite. Contemporaneity allowed the two magmas to mingle. Rounded enclaves of hematite-barite in the magnesiocarbonatite are tentatively attributed to magma immiscibility. Following complete solidification of the calcite carbonatite, and overlapping late crystallization of the magnesiocarbonatite plug, late magnesiocarbonatite dikes and ultramafic dikes were emplaced, some of the latter as diatremes. Crystallization of ferroan dolomite in the magnesiocarbonatite plug resulted in residual magmatic concentration of Si, Ba, F and rare earth elements (REE), and crystallization of barite, quartz, calcite, fluorite and REE fluorocarbonates in miarolitic cavities. Concentrations of (total) rare earth oxides (TREO) in the unweathered magnesiocarbonatite are 1 to 2%. REE ore with 3 to 6% TREO resulted from weathering, during which CaCO3 and MgCO3 were leached from ferroan dolomite leaving a porous goethite-rich residue containing barite and bastnaesite, the latter having replaced primary synchesite. Other commodities with potential economic significance include phosphate and niobium, both of which were enriched by residual accumulation over the calcite carbonatite as a result of karstic weathering. Although weathering was a critical factor in the formation of REE ore at Ngualla, the primary proto-ore resulted mainly from in situ igneous processes. This genetic model is different from that used to account for many carbonatite-hosted ore bodies, which result from late-stage hydrothermal processes. Examples of hydrothermal rare earth deposits include those of the late Jurassic to early Cretaceous Chilwa Province, located 800?km south of Ngualla. The differences in ore-forming processes may reflect the relative ages of the carbonatites and a deeper level of erosion at Ngualla.
Pleistocene stratigraphy, paleopedology and paleoecology of a multiple till sequence exposed on the Little Bear River, western district of Mackenzie, northwest Territories.
Canadian Journal of Earth Sciences, Vol. 30, No. 4, April pp. 851-866
Northwest Territories
Geomorphology, Physiography western district of Mackenzie
Earth and Planteray Science Letters, Vol. 487, pp. 1-17.
Mantle
Core
Abstract: The conventional view of Earth's inner core is that it began to crystallize at Earth's center when the temperature dropped below the melting point of the iron alloy and has grown steadily since that time as the core continued to cool. However, this model neglects the energy barrier to the formation of the first stable crystal nucleus, which is commonly represented in terms of the critical supercooling required to overcome the barrier. Using constraints from experiments, simulations, and theory, we show that spontaneous crystallization in a homogeneous liquid iron alloy at Earth's core pressures requires a critical supercooling of order 1000 K, which is too large to be a plausible mechanism for the origin of Earth's inner core. We consider mechanisms that can lower the nucleation barrier substantially. Each has caveats, yet the inner core exists: this is the nucleation paradox. Heterogeneous nucleation on a solid metallic substrate tends to have a low energy barrier and offers the most straightforward solution to the paradox, but solid metal would probably have to be delivered from the mantle and such events are unlikely to have been common. A delay in nucleation, whether due to a substantial nucleation energy barrier, or late introduction of a low energy substrate, would lead to an initial phase of rapid inner core growth from a supercooled state. Such rapid growth may lead to distinctive crystallization texturing that might be observable seismically. It would also generate a spike in chemical and thermal buoyancy that could affect the geomagnetic field significantly. Solid metal introduced to Earth's center before it reached saturation could also provide a nucleation substrate, if large enough to escape complete dissolution. Inner core growth, in this case, could begin earlier and start more slowly than standard thermal models predict.
Neodynmium and strontium isotopic and trace element composition of aMesozoic CFB suite from Dronning Maud Land: lithosphere and asthenosphere ...Karoo magmatism
Geochimica et Cosmochimica Acta, Vol. 62, No. 15, pp. 2701-14.
Mertanen, S., Vuollo, J.I., Huhma, H., Arestova, N.A., Kovalenko, A.
Early Paleoproterozoic Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield - new paleomagnetic, isotope age, geochemical investigations.
Precambrian Research, Vol. 144, 3-4, Feb. 10, pp. 239-260.
Peltonen, P., Manttari, I., Huhma, H., Whitehouse, M.J.
Multi stage origin of the lower crust of the Karelian craton from 3.5 to 1.7 Ga based on isotopic ages of kimberlite derived mafic granulite xenoliths.
Precambrian Research, Vol. 147, 1-2, June 10, pp. 107-123.
Abstract: Bolide impact is a ubiquitous geological process in the Solar System, which produced craters and basins filled with impact melt sheets on the terrestrial planets. However, it remains controversial whether these sheets were able to undergo large-scale igneous differentiation, or not. Here, we report on the discovery of large discrete bodies of melanorites that occur throughout almost the entire stratigraphy of the 1.85-billion-year-old Sudbury Igneous Complex (SIC) - the best exposed impact melt sheet on Earth - and use them to reaffirm that conspicuous norite-gabbro-granophyre stratigraphy of the SIC is produced by fractional crystallization of an originally homogeneous impact melt of granodioritic composition. This implies that more ancient and compositionally primitive Hadean impact melt sheets on the Earth and other terrestrial planets also underwent large-volume igneous differentiation. The near-surface differentiation of these giant impact melt sheets may therefore have contributed to the evolution and lithological diversity of the proto-crust on terrestrial planets.
Abstract: The Paleoproterozoic Lapland-Kola Orogen in Fennoscandia has been studied for decades and several plate tectonic models have been proposed including one-sided subduction zone, either towards SW or NW, or two opposite-verging subduction zones before the collision. Based on new structural and isotope data from Finland and recently published data from Russia, we propose a revised tectonic model for the Paleoproterozoic Lapland-Kola Orogen. The main components are foreland in the NE followed by cryptic suture, Inari arc, retro-arc basin and retro-arc foreland in the SW. The latter three constitute the Inari Orocline.
Subduction towards present SW and subsequent arc magmatism (Inari arc) started at ca. 1.98?Ga followed by voluminous sedimentation in the deepening retro-arc basin. Underplating of a mid-ocean ridge caused flat subduction and magmatic flare at 1.92?Ga over a broad distance in the retro-arc basin. Rapid heating led to melting of the retro-arc basin sediments and voluminous amounts of granulite-facies diatexites formed. During collision (D1) at 1915-1910?Ma, large thrust nappes formed on the foreland. Deformation in the retro-arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite-diatexite packages in the upper parts. A post-collisional stage is seen as 1904?Ma appinites and decompression derived granites at 1.90-1.89?Ga. Renewed shortening (D2), due to far-field effects in SW at 1.88-1.87?Ga, led to thick-skin shortening of the Archean middle crust, large-scale crustal duplexing of already cooled granulites towards the retro-arc foreland and inclined upright folding of granulites in the opposite direction towards the Inari arc. A switch in the stress field from NE-SW to NW-SE led to orogen-parallel contraction and buckling started along a dextral strike-slip fault zone to form the Inari Orocline. Buckling is seen in the bending of pre-orocline fabrics and formation of syn-orocline fabrics: radial conical folds (D3), radial fractures, a strike-slip fault zone and thrusting at the hinge zone. The end-result is a mega-scale parallel multi-layer fold composed of the Inari arc, retro-arc basin and possibly also the heated retro-arc foreland.
Kara, J., Vaisanen, M., Heinonen, J.S., Lahaye, Y., O'Brien, H., Huhma, H.
Tracing arcologites in the Paleoproteroic era - a shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian shield.
Geochimica et Cosmochimica Acta, Vol. 267, pp. 179-195.
Africa, Tanzania
peridotite
Abstract: Hydrogen concentration profiles through olivine and pyroxene in peridotite xenoliths carried in rift basalts from northern Tanzania (Lashaine, Eledoi, and Kisite localities) show bell-shaped distributions, indicating that diffusive hydrogen loss has occurred in all minerals. Homogeneous major element concentrations and equilibration of hydrogen between the cores of olivine and coexisting pyroxene suggest that hydrogen loss resulted from diffusive degassing during host magma emplacement. For these samples, hydrogen diffusivities in olivine and coexisting pyroxene must be within the same order of magnitude, similar to experimentally determined diffusivities, but in contrast to previous observations made on xenolithic peridotites. We demonstrate here, for the first time using natural samples, that significant differences in activation energy is likely the primary parameter that causes the discrepancy between hydrogen diffusion in olivine and pyroxene observed in different suites of mantle xenoliths. Because hydrogen diffuses faster in olivine than in pyroxene as temperature increases, hydrogen loss in the Tanzanian mantle xenoliths must have occurred at relatively low temperatures (?750 - ?900 °C), whereas hydrogen loss observed in previous xenolith studies likely occurred at higher temperatures (?950 to > 1200 °C). Thus, the diffusive loss of hydrogen in the Tanzanian mantle xenoliths may have occurred at shallow depths or at the Earth’s surface.
Abstract:
Water and iron are believed to be key constituents controlling the strength and density of the lithosphere and, therefore, play a crucial role in the long-term stability of cratons. On the other hand, metasomatism can modify the water and iron abundances in the mantle and possibly triggers thermo-mechanical erosion of cratonic keels. Whether local or large scale processes control water distribution in cratonic mantle remains unclear, calling for further investigation. Spinel peridotite xenoliths in alkali basalts of the Cenozoic Tok volcanic field sampled the lithospheric mantle beneath the southeastern margin of the Siberian Craton. The absence of garnet-bearing peridotite among the xenoliths, together with voluminous eruptions of basaltic magma, suggests that the craton margin, in contrast to the central part, lost its deep keel. The Tok peridotites experienced extensive and complex metasomatic reworking by evolved, Ca-Fe-rich liquids that transformed refractory harzburgite to lherzolite and wehrlite. We used polarized Fourier transform infrared spectroscopy (FTIR) to obtain water content in olivine, orthopyroxene (Opx), and clinopyroxene (Cpx) of 14 Tok xenoliths. Olivine, with a water content of 0-3 ppm H2O, was severely degassed, probably during emplacement and cooling of the host lava flow. Orthopyroxene (49-106 ppm H2O) and clinopyroxene (97-300 ppm H2O) are in equilibrium. The cores of the pyroxene grains, unlike olivine, experienced no water loss due to dehydration or addition attributable to interaction with the host magma. The water contents of Opx and Cpx are similar to those from the Kaapvaal, Tanzania, and North China cratons, but the Tok Opx has less water than previously studied Opx from the central Siberian craton (Udachnaya, 28-301 ppm; average 138 ppm). Melting models suggest that the water contents of Tok peridotites are higher than in melting residues, and argue for a post-melting (metasomatic) origin. Moreover, the water contents in Opx and Cpx of Tok peridotites are decoupled from iron enrichments or other indicators of melt metasomatism (e.g., CaO and P2O5). Such decoupling is not seen in the Udachnaya and Kaapvaal peridotites but is similar to observations on Tanzanian peridotites. Our data suggest that iron enrichments in the southeastern Siberian craton mantle preceded water enrichment. Pervasive and large-scale, iron enrichment in the lithospheric mantle may strongly increase its density and initiate a thermo-magmatic erosion. By contrast, the distribution of water in xenoliths is relatively “recent” and was controlled by local metasomatic processes that operate shortly before the volcanic eruption. Hence, water abundances in minerals of Tok mantle xenoliths appear to represent a snapshot of water in the vicinity of the xenolith source regions.
Abstract: White diamonds color grading is one of the basic diamond evaluations. The color value based on a scale that ranges from D to Z, with D being the more colorless and more valuable, among other qualifications. As the diamond grade moves on this scale, its color appears more yellow progressively. This yellowish color, present only in Type I diamonds, is mainly due to the nitrogen related defects such as N3 center and C-center. The current color grading system is based on a visual method, where gemologist compares the sample with a Master Color set. However, this method is very subjective. Several defects responsible for light absorption in diamond are carrying electron spin and appear in Electron Paramagnetic Resonance (EPR) spectrum. In this study, we developed a new EPR based technique for a quantitative measurement of N3 center and C-center in diamond through quantitative EPR spectroscopy. The correlation between EPR spectra and color grades of diamond was established.
Abstract: Observing and controlling macroscopic quantum systems has long been a driving force in quantum physics research. In particular, strong coupling between individual quantum systems and mechanical oscillators is being actively studied. Whereas both read-out of mechanical motion using coherent control of spin systems and single-spin read-out using pristine oscillators have been demonstrated, temperature control of the motion of a macroscopic object using long-lived electronic spins has not been reported. Here we observe a spin-dependent torque and spin-cooling of the motion of a trapped microdiamond. Using a combination of microwave and laser excitation enables the spins of nitrogen-vacancy centres to act on the diamond orientation and to cool the diamond libration via a dynamical back-action. Furthermore, by driving the system in the nonlinear regime, we demonstrate bistability and self-sustained coherent oscillations stimulated by spin-mechanical coupling, which offers the prospect of spin-driven generation of non-classical states of motion. Such a levitating diamond-held in position by electric field gradients under vacuum-can operate as a ‘compass’ with controlled dissipation and has potential use in high-precision torque sensing, emulation of the spin-boson problem15 and probing of quantum phase transitions. In the single-spin limit and using ultrapure nanoscale diamonds, it could allow quantum non-demolition read-out of the spin of nitrogen-vacancy centres at ambient conditions, deterministic entanglement between distant individual spins and matter-wave interferometry.
Jianxin, Z., Jingsui, Y., Fabcong, M.,Yusheng, W., Huimin, Li., Cailai, W.
U Pb isotopic studies of eclogites and their host gneisses in the Xitishan area of the North Qaidam mountains, western China: new evidence HP-UHP belt.
Journal of Asian Earth Sciences, Vol. 28, 2-3, Nov. 15, pp. 143-150.
Abstract: The parallelism between older collisional belts and younger rift systems is widely known and particularly well portrayed along the Atlantic Ocean. How tectonic inherited and new?formed shear zones control rift nucleation and the final architecture of rifted conjugate passive margins is still poorly understood. Here we present lithospheric?scale thermo?mechanical numerical models that self?consistently create extensional and contractional tectonic inheritance, where prior extension and contraction are systematically varied. Our results show that (1) initial reactivation occurs along the former lithospheric suture zones; (2) upper crustal thick?skinned basement thrusts are partially or fully reactivated depending on the amount of prior contraction and size of the orogen; (3) with a small amount of contraction, thick?skinned thrusts are efficiently reactivated in extension and provide the template for rifted margin formation; (4) with larger amounts of contraction, thick?skinned thrusts distal to the lithospheric suture zone do not reactivate in extension; and (5) reactivation of prior contractional shear zones dominates during the early stages of rifting, while during the final stage of margin formation new?formed extensional shear zones dominate. Force balance analysis predicts an inverse relation between midcrustal viscosity and the maximum offset for reactivation of weak upper crustal structures. Force balance also predicts that the degree of weakening or healing of the weak suture and the thermal thinning of the necking area control at which stage suture reactivation is deactivated and extension proceeds by mantle lithosphere thermal necking. Two rifted conjugate margins with orogenic inheritance in the North and South Atlantic are used for comparison.
Abstract: The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (?11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ~40 and ~2,600?Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ~2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between ?8.6 and +5.5, with all of the young (<300?Ma) carbonatites characterized by more positive ?11B values (>?4.0‰ whereas most of the older carbonatite samples record lower B isotope values. Given the ?11B value for asthenospheric mantle of ?7 ± 1‰ the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.
Abstract: The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (?11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ~40 and ~2,600?Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ~2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between ?8.6‰ and +5.5‰, with all of the young (<300?Ma) carbonatites characterized by more positive ?11B values (>?4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the ?11B value for asthenospheric mantle of ?7 ± 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.
Abstract: Mid-oceanic ridge basalts (MORBs) are depleted in incompatible elements, but ridge segments far from mantle plumes frequently erupt chemically enriched MORBs (E-MORBs). Two major explanations of E-MORBs are that these basalts are generated by the melting of entrained recycled crust (pyroxenite) beneath ridges or by the melting of refertilized peridotites. These two hypotheses can be discriminated with compatible element abundances from Sc to Ge, here termed the ScGe elements. Here, we demonstrate that E-MORBs have systematically lower Ge/Si and Sc contents and slightly higher Fe/Mn and Nb/Ta ratios than depleted MORBs (D-MORBs) due to the mixing of low-degree pyroxenite melts. The Ge/Si ratio is a new tracer that effectively discriminates between melts derived from peridotite sources and melts derived from mixed pyroxenite-peridotite sources. These new data are used to estimate the distribution of pyroxenite in the mantle sources of global MORB segments.
Geochemical Perspectives Letters, Vol. 11, pp. 6-11.
Mantle
mantle plumes, hotspots
Abstract: Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.
Abstract: Mantle plumes sample the deep mantle. A limited number of geochemical endmember components can describe the isotopic and compositional variations in the ocean island basalts (OIBs), which are produced by plume volcanism. The endmembers are correlated to compositions in the OIB source regions or represent incorporation of material upon ascent. However, their actual nature and origins are still highly debated. The depths of plume sources have been proposed to be anywhere between the core-mantle boundary and the upper mantle, and need not be the same for all plume-related volcanic activities. Using a combination of synchrotron micro-X ray fluorescence and -diffraction mapping, and in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we show that the elemental features of HIMU-rich OIBs, such as Bermuda, St Helena, and Cook-Austral, exactly match the geochemical signature of a multiphase inclusion in a diamond. The geochemical signature in our studied diamond inclusion is markedly different from that of inclusions in lithospheric diamonds. The phases identified in the inclusion are majorite-rich garnet, ilmenite, the sodic 10Å-phase (TAP), and liebermannite. Furthermore, we show that this inclusion was entrapped at 14.5 ± 0.5 GPa (420-440 km) and 1450 ± 50 K. At the conditions of entrapment, the diamond inclusion phase assembly was garnet + ilmenite + liebermannite + clinopyroxene + stishovite + fluid. Sodic TAP is a retrograde product of reaction between clinopyroxene, stishovite, and fluid upon ascent. Its presence shows that the HIMU source is water-saturated. Entrapment in diamond indicates that the fluid also contained carbonate. The conditions of 14.5 ± 0.5 GPa and 1450 ± 50 K plot right on top of the alkaline carbonatite solidus, and match the formation of carbonatitic melt from subducted slabs plus diamond formation from reaction of carbonate with iron. In summary, our data show that the transition zone source accounts for the global HIMU endmember.
Abstract: Calcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.
South African Journal of Geology, Vol. 123, 4, pp. 597-614. pdf
Africa, South Africa
alkaline rocks
Abstract: Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world’s largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2 200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1 400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative ?Nd and ?Hf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.
Abstract: A fundamental goal of mineralogy and petrology is the deep understanding of mineral phase relationships and the consequent spatial and temporal patterns of mineral coexistence in rocks, ore bodies, sediments, meteorites, and other natural polycrystalline materials. The multi-dimensional chemical complexity of such mineral assemblages has traditionally led to experimental and theoretical consideration of 2-, 3-, or n-component systems that represent simplified approximations of natural systems. Network analysis provides a dynamic, quantitative, and predictive visualization framework for employing “big data” to explore complex and otherwise hidden higher-dimensional patterns of diversity and distribution in such mineral systems. We introduce and explore applications of mineral network analysis, in which mineral species are represented by nodes, while coexistence of minerals is indicated by lines between nodes. This approach provides a dynamic visualization platform for higher-dimensional analysis of phase relationships, because topologies of equilibrium phase assemblages and pathways of mineral reaction series are embedded within the networks. Mineral networks also facilitate quantitative comparison of lithologies from different planets and moons, the analysis of coexistence patterns simultaneously among hundreds of mineral species and their localities, the exploration of varied paragenetic modes of mineral groups, and investigation of changing patterns of mineral occurrence through deep time. Mineral network analysis, furthermore, represents an effective visual approach to teaching and learning in mineralogy and petrology.
Barry, P.H., de Moor, J.M., Giovannelli, D., Schrenk, M., Hummer, D.R., Lopez, T., Pratt, C.A., Alpizar Segua, Y., Battaglia, A., Beaudry, A., Bini, G., Cascante, M., d'Errico, G., di Carlo, M., Fattorini, D., Fullerton, K., H+Gazel, E., Gonzalez, G., Hal
Abstract: Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.
Abstract: Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of its rarity, but metasomatic weakening is an essential precursor. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slab-like geometries similar to modern oceanic lithosphere in these construction histories. Underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities, emphasizing the role of lateral accretion. Archean continental building blocks may resemble the modern lithosphere of oceanic plateau, but they better match the sort of refractory crust expected to have formed at Archean ocean spreading centres. Radiometric dating of mantle xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences, and these ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons stabilized during a granitic bloom at 2.61-2.55 Ga. This stabilization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more ‘fertile’ and more conductive by introducing or concentrating sulfides or graphite at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. Late Cretaceous flattening of Farallon lithosphere that included the Shatsky Rise conjugate appears to have weakened, eroded and displaced the base of the Wyoming craton below 140-160 km. This process replaced the old re-fertilized continental mantle with relatively young depleted oceanic mantle.
Abstract: Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith's quartz cement, leading to increased permeability and enhancing chemical interaction with a mixed Ca-K-Na fenitizing fluid. We estimated the HFSE masses transferred from the syenite to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12% of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass comparable to the tonnages of some of the world's largest HFSE resources. We argue that fenite can yield crucial information about the tipping points in magma evolution because retention and/or loss of volatile-bonded alkali and HFSEs are key factors in the development of magmatic zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone deposits.
Abstract: Carbonatites have formed for at least the past three billion years. But over the past 700 My the incidence of carbonatites have significantly increased. We compile an updated list of 609 carbonatite occurrences and plot 387 of known age on plate tectonic reconstructions. Plate reconstructions from Devonian to present show that 75% of carbonatites are emplaced within 600 km of craton edges. Carbonatites are also associated with large igneous provinces, orogenies, and rift zones, suggesting that carbonatite magmatism is restricted to discrete geotectonic environments that can overlap in space and time. Temporal constraints indicate carbonatites and related magmas may form an ephemeral but significant flux of carbon between the mantle and atmosphere.
De Wit, M., Bhebhe, Z., Davidson, J., Haggerty, S.E., Hundt, P., Jacob, J., Lynn, M., Marshall, T.R., Skinner, C., Smithson, K., Stiefenhofer, J., Robert, M., Revitt, A., Spaggiari, R., Ward, J.
Abstract: From the discovery of diamonds in South Africa in 1866 until the end of 2013, Africa is estimated to have produced almost 3.2 Bct out of a total global production of 5.03 Bct, or 63.6% of all diamonds that have ever been mined. In 2013 African countries ranked 2nd (Botswana), 3rd (DRC), 6th (Zimbabwe), 7th (Angola), 8th (South Africa), and 9th (Namibia), in terms of carat production and 1st (Botswana), 4th (Namibia), 5th (Angola), 6th (South Africa), 7th (Zimbabwe), and 9th (DRC), in terms of value of the diamonds produced. In 2013 Africa produced 70.6 Mct out of a global total of 130.5 Mct or 54.1%, which was valued at US$ 8.7 billion representing 61.5% of the global value of US$ 14.1 billion.
Obrebski, M., Allen, R.M., Pollitz, F., Hung, S-H.
Lithosphere asthenosphere interaction beneath the western United States from the joint inversion of body-wave traveltimes and surface wave phase veolocities.
Geophysical Journal International, March 25, In press available
Abstract: The Western Canada Sedimentary Basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. Its crystalline basement has documented more than 3 billion years of evolution history of western Laurentia. Here we conduct a high?resolution survey of the mantle P and S wave velocities using finite?frequency tomography. Our models show pronounced eastward increases of 4% P and 6% S wave velocities beneath the foreland region, which define a sharp seismic Cordillera?Craton boundary. In the cratonic region, distinctive high? (>2%) velocity anomalies representing depleted mantle lithospheres are well correlated with major Precambrian crustal domains. The largest lithosphere thickness contrast coincides with the Snowbird Tectonic Zone, where the Hearne province extends down to ~300 km, nearly 100 km deeper than the Proterozoic terranes in northern Alberta. In the latter region, a pronounced cylindrical negative velocity anomaly extends subvertically from 75 to ~300?km depth, which potentially results from significant tectonothermal modifications during subduction and/or plume activities. At the basin scale, mantle velocities show no apparent correlations with surface heat flux, suggesting a minimum mantle contribution to the regional thermal variability. Furthermore, the long?wavelength isostatic gravity correlates negatively with the velocities, which confirms that the melt extraction from Precambrian cratons is responsible for the formation of highly depleted mantle lithospheres. Moreover, our model reveals the increased concentrations of kimberlites and lamproites near the zones of high horizontal velocity gradients. The distinct spatial pattern may reflect either preferential formation or eruption of potentially diamondiferous rocks at lithospheric weak zones near the western margin of Laurentia.
Abstract: The Late Cretaceous kimberlites in northern Alberta, Canada, intruded into the Paleoproterozoic crust and represent a nonconventional setting for the discovery of diamonds. Here, we examined the origin of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (-1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicate a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Taken together, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 346-374.
United States, Colorado
volatiles
Abstract: The Colorado Plateau hosts several large accumulations of naturally occurring, non-hydrocarbon gases, including CO2, N2, and the noble gases, making it a good field location to study the fluxes of these gases within the crust and to the atmosphere. In this study, we present a compilation of 1252 published gas-composition measurements. The data reveal at least three natural gas associations in the field area, which are dominated by hydrocarbons, CO2, and N2 + He + Ar, respectively. Most gas accumulations of the region exhibit compositions that are intermediate between the three end members. The first non-hydrocarbon gas association is characterized by very high-purity CO2, in excess of 75 mol% (hereafter, %). Many of these high-purity CO2 fields have recently been well described and interpreted as magmatic in origin. The second non-hydrocarbon gas association is less well described on the Colorado Plateau. It exhibits He concentrations on the order of 1-10%, and centered log ratio biplots show that He occurs proportionally to both N2 and Ar. Overall ratios of N2 to He to Ar are ?100:10:1 and correlation in concentrations of these gases suggests that they have been sourced from the same reservoir and/or by a common process. To complement the analysis of the gas-composition data, stable isotope and noble-gas isotope measurements are compiled or newly reported from 11 representative fields (previously published data from 4 fields and new data from 7 fields). Gas sampled from the Harley Dome gas field in Utah contains nearly pure N2 + He + Ar. The various compositional and stable and noble gas isotopic data for this gas indicate that noble gas molecule/isotope ratios are near crustal radiogenic production values and also suggest a crustal N2 source. Across the field area, most of the high-purity N2 + He + Ar gas accumulations are associated with the mapped surface trace of structures or sutures in the Precambrian basement and are often accumulated in lower parts of the overlying Phanerozoic sedimentary cover. The high-purity gas association mostly occurs in areas interior to the plateau that are characterized by a narrow range of elevated, moderate heat flow values (53-74 mW/m2) in the ancient (1.8-1.6 Ga) basement terranes of the region. Collectively, the geochemical and geological data suggest that (1) the N2 + He + Ar gas association is sourced from a crustal reservoir, (2) the gas association migrates preferentially along structures in the Precambrian basement, and (3) the sourcing process relates to heating of the crust. Prospecting for noble-gas accumulations may target areas with elevated Cenozoic heat flow, ancient crust, and deep crustal structures that focus gas migration. High-purity CO2 gas may also migrate through regional basement structures, however, there is not always a clear spatial association. Rather, CO2 accumulations are more clearly associated with zones of high heat flow (>63 mW/m2) that sit above hot upper mantle and are proximal to Cenozoic volcanic rocks near the plateau margins. These observations are consistent with previous interpretations of a magmatic gas source, which were based on geochemical measurements.
Variation of Mineral Assemblages and Textures; Studies in Ultramafic and Associated Rocks from Western and Eastern Kentucky Along or Near the 38th. Parallel Lineament.
Geological Society of America (GSA), Vol. 9, No. 5, PP. 608-609. (abstract.).
Hunt, L., Stachel, T., Grutter, H., Armstrong, J., McCandless, T.E., Simonetti, A., Tappe, S.
Small mantle fragments from the Renard kimberlites, Quebec: powerful recorders of mantle lithosphere formation and modification beneath the eastern Superior Craton.
Contributions to Mineralogy and Petrology, Vol. 171, 15p.
Canada, Northwest Territories
Deposit - Diavik
Abstract: Fibrous diamonds are often interpreted as direct precipitates of primary carbonate-bearing fluids in the lithospheric mantle, sourced directly from common reservoirs of “mantle” carbon and nitrogen. Here we have examined fibrous growth layers in five diamonds (as three rims or “coats” and two whole-crystal cuboids) from the Diavik Diamond Mine, Canada, using in situ C- and N-isotope and N-abundance measurements to investigate the origin and evolution of their parental fluids, and in particular, to test for isotopic variability within a suite of fibrous diamonds. High-resolution growth structure information was gleaned from cathodoluminescence (CL) imaging and, in combination with the isotopic data, was used to assess the nature of the transition from gem to fibrous growth in the coated diamonds. The two cuboids are characterized by fine concentric bands of fibrous and/or milky opaque diamond, with one sample (S1719) having intermittent gem-like growth layers that are transparent and colourless. The three coated diamonds comprise octahedral gem cores mantled by massive or weakly zoned fibrous rims, with sharp and well-defined gem-fibrous boundaries. For the two cuboid samples, ? 13C and ? 15N values were ?7.7 to ?3.2 ‰ (mean ?6.3 ± 1.3 ‰; 1 SD; n = 84) and ?5.6 to ?2.1 ‰ (mean ?4.0 ± 0.8 ‰; 1 SD; n = 48), respectively. The three fibrous rims have combined ? 13C values of ?8.3 to ?4.8 ‰ (mean ?6.9 ± 0.7 ‰; 1 SD; n = 113) and ? 15N values of ?3.8 to ?1.9 ‰ (mean ?2.7 ± 0.4 ‰; 1 SD; n = 43). N-abundances of the combined cuboid-fibrous rim dataset range from 339 to 1714 at. ppm. The gem cores have ? 13C and ? 15N values of ?5.4 to ?3.5 ‰ and ?17.7 to +4.5 ‰, respectively, and N-abundances of 480 to 1699 at. ppm. Broadly uniform C- and N-isotope compositions were observed in each of the gem cores (variations of ~<1 ‰ for carbon and ~<3 ‰ for nitrogen). This limited C- and N- isotope variability implies that the gem cores formed from separate pulses of fluid that remained isotopically uniform throughout the duration of growth. Significant isotopic and abundance differences were observed between the gem and fibrous growth zones, including in one detailed isotopic profile ? 13C and ? 15N offsets of ~?2.4 and ~?3.7 ‰, respectively, and a ~230 at. ppm increase in N-abundance. Combined with the well-defined gem-fibrous boundaries in plane light and CL, these sharp isotopic differences indicate separate parental fluid histories. Notably, in the combined fibrous diamond dataset prominent C- and N-isotope differences between the whole-crystal cuboid and fibrous rim data were observed, including a consistent ~1.3 ‰ offset in ? 15N values between the two growth types. This bimodal N-isotope distribution is interpreted as formation from separate parental fluids, associated with distinct nitrogen sources. The bimodal N-isotope distribution could also be explained by differences in N-speciation between the respective parental fluids, which would largely be controlled by the oxidation state of the fibrous rim and cuboid growth environments (i.e., N2 vs. NH4 + or NH3). We also note that this C- and N-isotope variability could indicate temporal changes to the source(s) of the respective parental fluids, such that each stage of fibrous diamond growth reflects the emplacement of separate pulses of proto-kimberlitic fluid—from distinct carbon and nitrogen sources, and/or with varying N-species—into the lithospheric mantle.
Abstract: Based on the mineral inclusion content, diamonds from the Argyle Mine, Western Australia, derive primarily (~90%) from eclogitic sources with a minor peridotitic contribution from both harzburgitic and lherzolitic lithologies. The eclogitic inclusions cover a large compositional range and show in part unusually high concentrations of mantle incompatible elements (P, Ti, Na and K). Coherent trends in major elements (e.g., of Ti or Na versus Mg-number) suggest that the eclogitic diamond source was created by a single process, namely igneous fractionation. Calculated bulk rock REEN patterns match a section of oceanic crust reaching from lavas and sheeted dykes to upper gabbros. Positive Eu anomalies for garnet and clinopyroxene, with calculated bulk rock REEN patterns similar to upper (non-layered) gabbros, are strong evidence for plagioclase accumulation, which is characteristic for the gabbroic portions of oceanic crust. Linking previously published oxygen isotope analyses of eclogitic garnet inclusions with their major element composition reveals a correlation between ?18O (mean of +7.2‰) and Na content, consistent with coupled 18O and Na enrichment during low temperature alteration of oceanic crust. The carbon isotopic composition of Argyle eclogitic diamonds forms a normal distribution around a ?13C value of -11‰, indicative of mixing and homogenization of mantle and crustal (organic matter) derived carbon prior to diamond precipitation. Previously published noble gas data on Argyle diamonds support this two component model. Inclusion and nitrogen-in-diamond based thermometry indicate an unusually hot origin of the eclogitic diamond suite, indicative of derivation from the lowermost 25 km (about 180-205 km depth) of the local lithospheric mantle. This is consistent with emplacement of an oceanic protolith during subduction along the Kimberley Craton margin, likely during the Halls Creek Orogeny (about 1.85 Ga). For Argyle eclogitic diamonds the relationship between the rate of platelet degradation and mantle residence temperature indicates that both temperature and strain play an important role in this process. Therefore, ubiquitous platelet degradation and plastic deformation of Argyle diamonds are consistent with derivation from a high temperature environment (softening the diamond lattice) close to the lithosphere-asthenosphere boundary (inducing strain). In combination, the Argyle data set represents a uniquely strong case for a subduction origin of an eclogitic diamond source followed by mixing of mantle and crustal components during diamond formation. Some lherzolitic inclusions show a similarity in incompatible element enrichments (elevated P, Na and K) to the eclogitic suite. The presence of a mildly majoritic lherzolitic garnet further supports a link to eclogitic diamond formation, as very similar majoritic components were found in two eclogitic garnet inclusions. The carbon isotopic composition of peridotitic diamonds shows a mode between -5 to -4 ‰ and a tail extending towards the eclogitic mode (-11 ‰). This suggests the presence of multiple generations of peridotitic diamonds, with indications for an origin linked to the eclogitic suite being restricted to diamonds of lherzolitic paragenesis.
Argyle diamonds – how subduction along the Kimberley Craton edge generated the world's biggest diamond deposit.
The utility of clinopyroxene in diamond exploration.
2018 Yellowknife Geoscience Forum , p. 13. abstract
Global
thermobarometry
Abstract: Clinopyroxene single-crystal thermo-barometry is an essential tool in the identification and evaluation of prospective kimberlites. The paleogeothermal gradient preserved by clinopyroxene xenocrysts elucidates the thermal structure of the underlying lithospheric mantle; indicates the depth to and thickness of the “diamond window”. The widely used clinopyroxene thermometer-barometer of Nimis and Taylor (2000) requires that clinopyroxene equilibrated with both garnet and orthopyroxene. With the rare exception of wehrlites, equilibration with orthopyroxene is nearly a given for the majority of chrome-diopside clinopyroxene xenocrysts. Demonstrating equilibration with garnet, however, is a major obstacle for clinopyroxene-based thermobarometry. The most commonly used method for clinopyroxene discrimination is an Al2O3-Cr2O3 diagram proposed by Ramsay and Thompkins in 1994 supplemented with an additional MgO-Al2O3 from Nimis (1998) and an additional 1-dimensional filter based on chemical composition. Despite the aggressiveness of the filtering method, single-clinopyroxene pressure-temperature results have large scatter that can obscure the true paleogeothermal gradient. This is especially true of areas where the lithospheric mantle has undergone chemical modification by melt/fluid influx. Using a database of clinopyroxenes derived from kimberlite-borne mantle-derived lherzolites, we have developed a simple and effective discrimination plot that identifies clinopyroxene from garnet lherzolites and simultaneously removes clinopyroxene from metaosomatised peridotites. Calculated paleogeothermal gradients from clinopyroxene xenocrysts cut across model conductive geotherms which can complicate the interpretation of thermobarometry data. Grütter (2009) presented a solution to the problem by way of relative reference geotherms. He used xenocryst data from three Canadian locations with different thermal structures as references in comparison to the dataset under investigation. Taking a cue from this earlier work, we have developed a new set of relative reference geotherms that are based on single-clinopyroxene thermobarometry data for xenoliths from well-characterized regions - Somerset Island, Kaapvaal on-craton, and the Central Slave. A simple linear fit through the data produces sub-parallel clinopyroxene reference geotherms that are simpler to use and easier to visualize compared to the xenocrysts reference geotherms. Using these two new and simple tools will greatly help maximize the utility of clinopyroxene data in large exploration databases.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 343-358.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 145-168.
Abstract: Small amounts of H2O, on the order of tens to hundreds of parts per million, can significantly influence the physical properties of mantle rocks. Determining the H2O contents of nominally anhydrous minerals (NAMs) is one relatively common technique that has been applied to estimate mantle H2O contents. However, for many mantle NAMs, the relation between H2O activity and H2O content is not well known. Furthermore, certain mantle minerals may be prone to H2O loss during emplacement on Earth’s surface. The goal of this study is to apply mineral equilibria to estimate values of aH2O in rocks that originated below the Moho. The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel ± garnet were used to estimate values of temperature (T), pressure (P), aH2O, hydrogen fugacity (fH2), and oxygen fugacity (fO2) in 11 amphibole-bearing mantle xenoliths from the southwestern U.S.A. Application of amphibole dehydration equilibria yields values of aH2O ranging from 0.05 to 0.26 for these 11 samples and the compositions of coexisting spinel + olivine + orthopyroxene yield ?logfO2 (FMQ) of -1 to +0.6. For nine of the samples, values of fH2 were estimated using amphibole dehydrogenation equilibria, and these values of fH2 ranged from 6 to 91 bars. Values of fH2 and fO2 were combined, using the relation 2H2O = 2H2 + O2, to estimate a second value of aH2O that ranged from 0.01 to 0.57 for these nine samples. Values of aH2O, estimated using these two methods on the same sample, generally agree to within 0.05. This agreement indicates that the amphibole in these samples has experienced little or no retrograde H-loss and that amphibole equilibria yields robust estimates of aH2O that, in these xenoliths, are generally <0.3, and are often 0.1 or less.
Abstract: Seismology records the presence of various heterogeneities throughout the lower mantle1,2, but the origins of these signals—whether thermal or chemical—remain uncertain, and therefore much of the information that they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed seismic velocities requires knowledge of the seismic properties of all of Earth’s possible mineral components. Calcium silicate (CaSiO3) perovskite is believed to be the third most abundant mineral throughout the lower mantle. Here we simultaneously measure the crystal structure and the shear-wave and compressional-wave velocities of samples of CaSiO3 perovskite, and provide direct constraints on the adiabatic bulk and shear moduli of this material. We observe that incorporation of titanium into CaSiO3 perovskite stabilizes the tetragonal structure at higher temperatures, and that the material’s shear modulus is substantially lower than is predicted by computations3,4,5 or thermodynamic datasets6. When combined with literature data and extrapolated, our results suggest that subducted oceanic crust will be visible as low-seismic-velocity anomalies throughout the lower mantle. In particular, we show that large low-shear-velocity provinces (LLSVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal-cubic phase transition in Ti-bearing CaSiO3 perovskite.
Emplacement, crystallization and alteration of spinifex textured komatiitic basalt flows in the Archean Nondweni greenstone belt, southern Kaapvaal craton, South Afr
Contributions to Mineralogy and Petrology, Vol. 101, pp. 301-317. Database # 17781
Abstract: Alluvial diamonds from the Kasai River, Democratic Republic of the Congo (DRC) are sourced from Cretaceous kimberlites of the Lucapa graben in Angola. Analysis of 40 inclusion-bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the Congo craton. Silicate inclusions permitted us to classify diamonds as peridotitic, containing Fo91-95 and En92-94, (23 diamonds, 70% of the suite), and eclogitic, containing Cr-poor pyrope and omphacite with 11-27% jadeite (6 diamonds, 18% of the suite). Fluid inclusion compositions of fibrous diamonds are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. In situ cathodoluminescence, secondary ion mass spectrometry and Fourier transform infrared spectroscopy reveal large heterogeneities in N, N aggregation into B-centers (NB), and ?13C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962 ppm N, show 0-88% NB, and have ?13C isotopic compositions from ? 12.5‰ to ? 1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14-1432 ppm N, NB spanning 29%-68%, and wider and lighter ?13C isotopic compositions of ? 17.8‰ to ? 3.4‰. Fibrous diamonds on average contain more N (up to 2976 ppm) and are restricted in ?13C from ? 4.1‰ to ? 9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350-1375 °C at 5.8 to 6.3 GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280 °C, if the assumed mantle residence time is 0.9-3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41-44 mW/m2 during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190-210 km. The hotter-than-average cratonic mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes.
Physics and Chemistry of Minerals, doi.org/10.1007/s00269-018-0995-5 16p.
Mantle
carbonatite
Abstract: Thermodynamic modeling offers a powerful framework for studying melting reactions of carbonated mantle systems across a wide range of compositions, pressures, and temperatures. Such modeling requires knowledge of the standard state thermodynamic properties of the pure alkaline earth carbonate liquid components, which are difficult to determine experimentally due to their instability at 1 bar. Atomistic simulations offer a solution to these experimental difficulties by providing access to metastable states and supplying constraints on thermodynamic properties. We developed an empirically-derived potential model for the simulation of alkaline earth carbonates (MgCO3, CaCO3, SrCO3 and BaCO3), emphasizing the accurate simulation of the standard state thermodynamic properties of carbonate liquids. Molecular dynamics (MD) simulations of liquids in the CaCO3-SrCO3-BaCO3 system are performed over a geologically relevant temperature-pressure range (1100-3400 K and 0-43 GPa). Simulation data for each of these three components (up to a maximum of 2300 K and 30 GPa) are fitted to a temperature-dependent third-order Birch-Murnaghan equation-of-state to estimate their standard state thermodynamic properties. With a few exceptions, calculated properties agree well with available estimates from experiments and/or first-principles MD simulations. Exploration of binary mixtures supports ideal mixing of volumes, heat capacities, and compressibilities, reflecting the common liquid structure and pressure-temperature evolution for these three components. The success of this new model for CaCO3-SrCO3-BaCO3 liquids suggests that it can accurately predict the properties of MgCO3-bearing liquids, where experimental data are unavailable.
Earth and Planetary Science Letters, Vol. 531, 10p. Pdf
Mantle
carbonatite
Abstract: MgCO3 is one of the most important components of mantle-derived carbonatite melts, and yet also one of the most difficult to study experimentally. Attempts to constrain its thermodynamic properties are hampered by decarbonation, which occurs at only ?500 °C, far below its metastable 1 bar melting temperature. Molecular dynamic simulations, however, can predict the thermodynamic properties of the MgCO3 liquid component in spite of experimental challenges. Using the recently developed empirical potential model for high-pressure alkaline-earth carbonate liquids (Hurt and Wolf, 2018), we simulate melts in the MgCO3-CaCO3-SrCO3-BaCO3 system from 773 to 2373 K up to 20 GPa. At 1 bar, MgCO3 liquid assumes a novel topology characterized by a 4-fold coordination of the metal cation (Mg) with both the carbonate molecule and oxygen ion; this is distinct from the other alkaline-earth carbonate liquids in which the metal cation is in ?6- and ?8-fold coordination with carbonate and oxygen. With increasing pressure, MgCO3 liquid structure becomes progressively more like that of (Ca, Sr, Ba)CO3 liquids with approaching 6-fold coordination with carbonate groups. The novel network topology of MgCO3 liquid results in a melt that is significantly more buoyant and compressible than other alkaline-earth carbonate liquids. Simulations of mixed MgCO3-bearing melts show that metal cation coordination with O and C is independent of bulk composition. Mixed simulation also reveal that molar volume, compressibility, enthalpy and heat capacity do not mix ideally with (Ca, Sr, Ba)CO3 liquids at 1 bar, a consequence of preferential metal-cation ordering in MgCO3-bearing mixtures. As pressure increases, however, mixing progressively approaches ideality with respect to molar volume, becoming nearly ideal by 12 GPa. The model is further applied to mantle-derived primary carbonatite melts with compositions, temperatures and pressures determined by published phase equilibrium experiments. The voluminous structure of liquid MgCO3 results in a buoyant melt that inhibits a density crossover with the surrounding mantle. Assuming FeCO3 liquid also adopts the same anomalous high-volume structure as MgCO3, we predict that even the most Fe-rich ferrocarbonatites would remain buoyant and be barred from sinking or stagnating in the mantle.
IN: Cycle Concepts in Plate Tectonics, editors Wilson and Houseman , Geological Society of London special publication 470, pp. 1-17. pdf
Mantle
plate tectonics
Abstract: It is now more than 50 years since Tuzo Wilson published his paper asking ‘Did the Atlantic close and then re-open?’. This led to the ‘Wilson Cycle’ concept in which the repeated opening and closing of ocean basins along old orogenic belts is a key process in the assembly and breakup of supercontinents. This implied that the processes of rifting and mountain building somehow pre-conditioned and weakened the lithosphere in these regions, making them susceptible to strain localization during future deformation episodes. Here we provide a retrospective look at the development of the concept, how it has evolved over the past five decades, current thinking and future focus areas. The Wilson Cycle has proved enormously important to the theory and practice of geology and underlies much of what we know about the geological evolution of the Earth and its lithosphere. The concept will no doubt continue to be developed as we gain more understanding of the physical processes that control mantle convection and plate tectonics, and as more data become available from currently less accessible regions.
Abstract: The hydrogen-isotope [deuterium/hydrogen (D/H)] ratio of Earth can be used to constrain the origin of its water. However, the most accessible reservoir, Earth’s oceans, may no longer represent the original (primordial) D/H ratio, owing to changes caused by water cycling between the surface and the interior. Thus, a reservoir completely isolated from surface processes is required to define Earth’s original D/H signature. Here we present data for Baffin Island and Icelandic lavas, which suggest that the deep mantle has a low D/H ratio (?D more negative than -218 per mil). Such strongly negative values indicate the existence of a component within Earth’s interior that inherited its D/H ratio directly from the protosolar nebula.
Abstract: The Great Unconformity, a profound gap in Earth’s stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a century. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terrestrial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of 3-5 vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard.
Abstract: Rock quantity and age are fundamental features of Earth's crust that pertain to many problems in geoscience. Here we combine new estimates of igneous rock area in continental crust from the Macrostrat database (https://macrostrat.org/) with a compilation of detrital zircon ages in order to investigate rock cycling and crustal growth. We find that there is little or no decrease in igneous rock area with increasing rock age. Instead, igneous rock area in North America exhibits four distinct Precambrian peaks, remains low through the Neoproterozoic, and then increases only modestly toward the recent. Peaks in Precambrian detrital zircon age frequency distributions align broadly with peaks in igneous rock area, regardless of grain depositional age. However, detrital zircon ages do underrepresent a Neoarchean peak in igneous rock area; young grains and ca. 1.1 Ga grains are also overrepresented relative to igneous area. Together, these results suggest that detrital zircon age distributions contain signatures of continental denudation and sedimentary cycling that are decoupled from the cycling of igneous source rocks. Models of continental crustal evolution that incorporate significant early increase in volume and increased sedimentation in the Phanerozoic are well supported by these data.
Earth and Planetary Science Letters, Vol. 502, pp. 96-103.
Mantle
subduction
Abstract: Since several decades, the processes allowing for the subduction of the continental lithosphere less dense than the mantle in a collision context have been widely explored, but models that are based upon the premise that slab pull is the prominent driver of plate tectonics fail. The India-Asia collision, where several episodes of continental subduction have been documented, constitute a case study for alternative views. One of these episodes occurred in the early collision time within the Asian plate where continental lithosphere not attached to any oceanic lithosphere subducted southward in front of the Indian lithosphere during its northward subduction that followed the oceanic subduction of the Tethys ocean. This process, known as collisional subduction, has a counter-intuitive behavior since the subduction is not driven by slab pull. It has been speculated that the mantle circulation can play an important role in triggering collisional subduction but a detailed, qualitative analysis of it is not available, yet. In this work we explore the southward subduction dynamics of the Asian lithosphere below Tibet by means of analogue experiments with the aim to highlight how the mantle circulation induces or responds to collisional subduction. We found that during the northward oceanic subduction (analogue of Tethys subduction) attached to the indenter (Indian analogue), the main component of slab motion is driven vertically by its negative buoyancy, while the trench rolls back. In the mantle the convective pattern consists in a pair of wide convective cells on both sides of the slab. But when the indenter starts to bend and plunge in the mantle, trench motion reverses. Its advance transmits the far field forces to two upper plates (Asian analogues). The more viscous frontal plate thickens, and the less viscous hinterland plate, which is attached to the back wall of the box, subducts. During this transition, a pair of sub-lithospheric convective cells is observed on both sides of the Asian analogue slab, driven by the shortening of the frontal plate. It favors the initiation of the backwall plate subduction. Such subduction is maintained during the entire collision by a wide cell with a mostly horizontal mantle flow below Tibet, passively advecting the Asian analogue slab. Experimental results suggest that once the tectonic far-field force related to the forward horizontal motion becomes dominant upon the buoyancy forces, trench advancing and the transmission of the tectonic force to the upper and backwall plates are promoted. This peculiar condition triggers the subduction of the backwall plate, despite it is light and buoyant.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 83-94.
United States, Canada
Supercontinents
Abstract: The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nuna's assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel-copper deposits peak during closure of Nuna's interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting.
Abstract: Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10?megatonnes of metal), occur within 200?kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two?billion?years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone.
Mineralogy and Petrology, DOI 10. 1007/s00710-015-0401-7
South America, Brazil, Mato Grosso
Juina area
Abstract: Two carbonatitic mineral assemblages, calcite + wollastonite and calcite + monticellite, which are encapsulated in two diamond grains from the Rio Soriso basin in the Juina area, Mato Grosso State, Brazil, were studied utilizing the NanoSIMS technique. The assemblages were formed as the result of the decomposition of the lower-mantle assemblage calcite + CaSi-perovskite + volatile during the course of the diamond ascent under pressure conditions from 15 to less than 0.8 GPa. The oxygen and carbon isotopic compositions of the studied minerals are inhomogeneous. They fractionated during the process of the decomposition of primary minerals to very varying values: ?18O from ?3.3 to +15.4?‰SMOW and ?13C from ?2.8 to +9.3?VPDB. These values significantly extend the mantle values for these elements in both isotopically-light and isotopically-heavy areas.
Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: constraints from broadband
Hutchinson, D.R., Lee, M.W., Behrendt, J., Cannon, W.F., Green
Variations in the reflectivity of the Moho transition zone beneath The midcontinent Rift System of North America. Results from true amplitude Glimpcedata
Journal of Geophysical Research, Vol. 97, No. B4, April 10, pp. 4721-4738
Abstract: Bull Hill is a carbonatite diatreme within the Paleogene Bear Lodge Carbonatite Complex in Wyoming, USA. Rare earth element (REE)-bearing carbonate, fluorocarbonate, phosphate, and oxide minerals occur within near-vertical carbonatite dikes on the western margin of Bull Hill. Changes in mineralogy and REE concentrations with depth are ascribed mainly to late-stage magmatic-hydrothermal and supergene alteration. Approximately 35 m of drill core from Bull Hill was analyzed and encompasses least altered, weakly weathered, and moderately weathered carbonatite. The least altered carbonatite contains magmatic burbankite, typically as inclusions within Mn-rich calcite (stage I). Secondary REE-bearing minerals, which pseudomorphically replaced unidentified hexagonal phenocrysts, include ancylite, bastnäsite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate (stage II). These replacive minerals generated small amounts of incipient porosity (~7-8%) and are largely stable in the lower portion of the weathering profile. Progressive weathering (stages III and IV) of the carbonatite involved the oxidation of pyrite to iron oxides and iron hydroxides, dissolution of calcite and strontianite, and the replacement of Mn-rich calcite by manganese oxides. These mineralogical changes resulted in an ~40% porosity gain in the core studied here. The volumetric concentration of weathering resistant REE-bearing minerals resulted in REE enrichment from an average of 5.4 wt % in the least weathered carbonatite to an average of 12.6 wt % in moderately weathered carbonatite, and to an overall increase in REE ore tenor of two to three times compared to the least altered carbonatite. Isocon plots confirm the increased concentration of REEs in the weathered carbonatite and demonstrate that REEs, along with TiO2, Ta, Nb, Zr, and Hf, were conserved in the lower weathered zone.
Integration of COCORP deep relfection and magnetic anomaly analysis in the southeast USA:implications for origin of the Brunswick and East Coast magneticanomalies
Geological Society of America (GSA) Bulletin, Vol. 102, No. 2, February pp. 271-279
Wittig, N., Webb, M., Pearson, D.G., Dale, C.W., Ottley, C.J., Hutchison, M., Jensen, S.M., Luget, A.
Formation of the North Atlantic craton: timing and mechanisms constrained from Re-Os isotope and PGE dat a of peridotite xenoliths from S.W. Greenland.
Wittig, N., Webb, M., Pearson, D.G., Dale, C.W., Ottley, C.J., Hutchison, M., Jensen, S.M., Luget, A.
Formation of the North Atlantic craton: timing and mechanisms constrained from Re-Os isotope and PGE dat a of peridotite xenoliths from S.W. Greenland.
Abstract: Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth's mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.
Abstract: Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth’s mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.
Diamond exploration and regional prospectivity of the Northern Territory of Australia.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 257-280.
Geological Survey of Western Australia, Report 179, 70p.
Australia
review - exploration
Abstract: Western Australia (WA) hosts 696 000 km2 of exposed, onshore, exclusively Archean rocks and 439 000 km2 of Paleoproterozoic rocks. In total, pre-1.6 Ga rocks comprise around 45% of the onshore area of the State, constituting the West Australian Craton (WAC; Yilgarn and the Pilbara Cratons) and the western part of the North Australian Craton (NAC). Seimic tomography demonstrates that considerable remaining portions of the State are also underlain by thick mantle lithosphere (Kennett et al., 2013), hosting the conditions under which diamonds form. Subsequently, most of the State is prospective for diamonds and numerous diamondiferous lamproite and kimberlite fields are known. Emplacement of diamond-bearing rocks spans much of geological time, from the c. 1868 Ma Brockman Creek kimberlite in the Pilbara (White, 2000) to the c. 17 Ma Walgidee Hills lamproite, Noonkanbah field, West Kimberley (Phillips et al, 2012). According to Kimberley Process statistics, Australia is estimated to have produced approximately 11% of global rough diamond production by weight in 2015, ranking it fourth in the world after the Russian Federation, Botswana and the Democratic Republic of Congo. These production figures are accounted for by two mines, both in WA. However, due to the closure of the Ellendale mine in 2015, responsible for a large proportion of the world’s fancy yellow production, only one currently producing mine remains in Australia (at the AK1 olivine lamproite at Argyle, NAC). In order to assess the effectiveness of prior exploration and draw attention to under-explored prospective areas, a thorough compilation and interpretation of WA diamond exploration data has been conducted.
Geological Survey of Western Australia, Report 2017/16, 24p.
Australia
review - exploration
Abstract: The Geological Survey of Western Australia’s Diamond exploration and prospectivity data package compiles over 40 years of diamond exploration data. In addition to samples derived from Western Australia’s established diamond mining areas at Ellendale and Argyle, a wide coverage of regional exploration data extending to the boundaries of the State is included. The database follows a similar methodology of attribution and has a compatible structure to the Diamond Exploration Database of the Northern Territory, allowing direct comparison of data throughout the North Australian Craton. The diamond exploration and prospectivity data package is the first of its kind to collate diamond exploration data statewide in a publicly accessible fashion. It incorporates the locations of over 88 000 diamond exploration samples. Associated with these samples are over 30 000 good-quality chemical analyses of mineral separate grains integrated into a standardized framework presented herein. In total, 524 discrete in situ bodies, which in principle have diamond potential (kimberlites, lamproites, ultramafic lamprophyres, and carbonatites) have also been compiled in the diamond exploration and prospectivity data package. With 114 confirmed to be diamondiferous, this part of the database considerably expands upon previous compilations of relevant Western Australian rocks, including the Geological Survey of Western Australia’s MINEDEX database and Bulletin 132. As a companion, 127 emplacement age determinations from 63 bodies are reported, encompassing most of the geographic extent of Western Australia’s known rocks with diamond potential. Analyses of the Western Australian data allow for an understanding of the exploration history in areas of known occurrences and identification of considerable gaps in the exploration coverage within areas of diamond potential. The Diamond exploration and prospectivity data package stands as a means to support and encourage future diamond exploration in the State in addition to providing a rigorous framework suitable for the establishment of diamond exploration databases elsewhere.
Mineralogy and Petrology, 10.1007/ s00710-018- 0579-6, 17p.
Australia
Indicator minerals
Abstract: Pre-1.6 Ga rocks comprise around 45% of the onshore area of Western Australia (WA), constituting the West Australian Craton (WAC) (including the Archean Yilgarn and Pilbara Cratons) and the western part of the North Australian Craton (NAC). These areas provide the conditions suitable for diamond formation at depth, and numerous diamondiferous lamproite and kimberlite fields are known. As emplacement ages span close to 2500 Ma, there are significant opportunities for diamond-affinity rocks being present near-surface in much of the State, including amongst Phanerozoic rocks. WA’s size, terrain, infrastructure and climate, mean that many areas remain underexplored. However, continuous diamond exploration since the 1970s has resulted in abundant data. In order to advance future exploration, a comprehensive database of results of diamond exploration sampling (Geological Survey of Western Australia 2018) has been assessed. The Yilgarn and Pilbara Cratons have spinel indicators almost exclusively dominated by chromite (>90% of grains), whereas (Mg,Fe,Ti)-bearing Al-chromites account for more of the indicator spinels in the NAC, up to 50% of grains at the Northern Territory (NT) border. Increasing dominance of Al in chromites is interpreted as a sign of weathering or a shallower source than Al-depleted Mg-chromites. Garnet compositions across the State also correlate with geological subdivisions, with lherzolitic garnets showing more prospective compositions (Ca-depleted) in WAC samples compared to the NAC. WAC samples also show a much broader scatter into strongly diamond-prospective G10 and G10D compositions. Ilmenites from the NAC show Mg-enriched compositions (consistent with kimberlites), over and above those present in NT data. However, ilmenites from the WAC again show the most diamond-prospective trends. Numerous indicator mineral concentrations throughout the State have unknown sources. Due in part to the presence of diamondiferous lamproites, it is cautioned that some accepted indicator mineral criteria do not apply in parts of WA. For example Ca-depleted garnets, Mg-depleted ilmenites and Cr-depleted and Al-absent clinopyroxenes are all sometimes associated with strongly diamondiferous localities. Quantitative prospectivity analysis has also been carried out based on the extent and results of sampling, age of surface rocks relative to ages of diamond-prospective rocks, and the underlying mantle structure. Results show that locations within the NAC and with proximity to WA’s diamond mines score well. However, results point to parts of the WAC being more prospective, consistent with mineral chemical data. Most notable are the Hamersley Basin, Eastern Goldfields Superterrane and the Goodin Inlier of the Yilgarn Craton. Despite prolific diamond exploration, WA is considerably underexplored and the ageing Argyle mine and recent closure of operations at Ellendale warrant a re-evaluation of diamond potential. Results of mineral chemistry and prospectivity analysis make a compelling case for renewed exploration.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0610-y 18p.
Europe, Scotland
aillikite
Abstract: The Glen Gollaidh aillikite dyke (58.36741°N 4.69751°W), N.W. Scotland, occurs within the Neoproterozoic sedimentary rocks of the Moine Supergroup ~4 km east of the Moine Thrust. Phlogopite 40Ar/36Ar measurements give a late Devonian maximum emplacement age of 360.3?±?4.9 (2?) Ma. This age occurs in a quiet period of Scottish magmatic history c. 30 Ma after the closure of the Iapetus and before the start of intra-plate alkali magmatism which affected southern Scotland for ~60 My from c. 350 Ma. Abundant chromites and Cr-diopsides and a few unaltered olivines, reflecting a mantle provenance, were recovered from heavy mineral concentrates. The North Atlantic Craton, exposed in Lewisian gneisses west of the Moine thrust, is therefore inferred to extend east at depth under Glen Gollaidh, presenting an opportunity to investigate the thickness and composition of the cratonic margin in the Devonian. The aillikite was found to be barren of diamond and no picro-ilmenites or garnets were definitively identified. However, mineral chemistry suggests that a proportion of Glen Gollaidh xenocrysts crystallised in equilibrium with garnet. Most spinels are Mg, Al chromites, with some Mg chromite present. All fall within the garnet peridotite field based on Ti and Cr but with insufficient Cr2O3 (up to 47.2 wt%) to be consistent with the diamond stability field. Amongst Cr-diopsides 30% of grains have Cr and Al contents consistent with derivation from garnet peridotite. The majority of clinopyroxenes also show a marked depletion in heavy compared to light rare-earth elements, again consistent with equilibration with garnet. The opx-cpx solvus thermometer demonstrates that average Cr-diopside compositions require at least 37 kbar to give a temperature (979 °C) lying even on a relatively warm 40 mWm?2 geotherm (Hasterok and Chapman Earth Planet Sc Lett 307:59-70, 2011). Large variations in the chemistry of mantle minerals reflect a complex history of metasomatism akin to constituents of alkali igneous rocks elsewhere in the Hebridean and Northern Highlands Terranes. Fertilised mantle provided the conditions for generation of aillikite melts, probably triggered by break-off of the advancing Avalonia slab. The cratonic root underlying the Glen Gollaidh aillikite during the late Devonian was apparently too thin to lie within the diamond stability field, consistent with xenoliths from alkali basalts further south. Nonetheless, sufficient geophysical and mineral chemical evidence supports Glen Gollaidh aillikite sitting close to the edge of diamond-prospective mantle therefore suggesting diamond potential a short distance to the west within the Lewisian and what is now East Greenland.
Abstract: At the core of many Earth-scale processes is the question of what the deep mantle is made of. The only direct samples from such extreme depths are diamonds and their inclusions. It is commonly assumed that these inclusions reflect ambient mantle or are syngenetic with diamond, but these assumptions are rarely tested. We have studied inclusion-host growth relationships in two potentially superdeep diamonds from Juina (Brazil) containing nine inclusions of Fe-rich (XFe ?0.33 to ?0.64) ferropericlase-magnesiowüstite (FM) by X-ray diffractometry, X-ray tomography, cathodoluminescence, electron backscatter diffraction, and electron microprobe analysis. The inclusions share a common [112] zone axis with their diamonds and have their major crystallographic axes within 3°-8° of those of their hosts. This suggests a specific crystallographic orientation relationship (COR) resulting from interfacial energy minimization, disturbed by minor post-entrapment rotation around [112] due to plastic deformation. The observed COR and the relationships between inclusions and diamond growth zones imply that FM nucleated during the growth history of the diamond. Therefore, these inclusions may not provide direct information on the ambient mantle prior to diamond formation. Consequently, a “non-pyrolitic” composition of the lower mantle is not required to explain the occurrence of Fe-rich FM inclusions in diamonds. By identifying examples of mineral inclusions that reflect the local environment of diamond formation and not ambient mantle, we provide both a cautionary tale and a means to test diamond-inclusion time relationships for proper application of inclusion studies to whole-mantle questions.
Abstract: Super-deep diamonds (SDDs) are those that form at depths between ?300 and ?1000 km in Earth’s mantle. They compose only 1% of the entire diamond population but play a pivotal role in geology, as they represent the deepest direct samples from the interior of our planet. Ferropericlase, (Mg,Fe)O, is the most abundant mineral found as inclusions in SDDs and, when associated with low-Ni enstatite, which is interpreted as retrogressed bridgmanite, is considered proof of a lower-mantle origin. As this mineral association in diamond is very rare, the depth of formation of most ferropericlase inclusions remains uncertain. Here we report geobarometric estimates based on both elasticity and elastoplasticity theories for two ferropericlase inclusions, not associated with enstatite, from a single Brazilian diamond. We obtained a minimum depth of entrapment of 15.7 (±2.5) GPa at 1830 (±45) K (?450 [±70] km depth), placing the origin of the diamond-inclusion pairs at least near the upper mantle-transition zone boundary and confirming their super-deep origin. Our analytical approach can be applied to any type of mineral inclusion in diamond and is expected to allow better insights into the depth distribution and origin of SDDs.
Agrosi, G., Tempesta, G., Mele, D., Caggiani, MC., Mangone, A., Della Ventura, G., Cestelli-Guidi, M., Allegretta, I., Hutchison, M.T., Nimis, P., Nestola, F.
Abstract: Super-deep diamonds and their mineral inclusions preserve very precious information about Earth’s deep mantle. In this study, we examined multiphase inclusions entrapped within a diamond from the Rio Vinte e um de Abril, São Luiz area (Juina, Brazil), using a combination of non-destructive methods. Micro-Computed X-ray Tomography (?-CXRT) was used to investigate the size, shape, distribution and X-Ray absorption of inclusions and mapping by micro X-ray Fluorescence (?-XRF), ?-Raman Spectroscopy and micro-Fourier Transform Infrared Spectroscopy (?-FTIR) were used to determine the chemical and mineralogical composition of the inclusions. Four large inclusions enclosed in the N-rich diamond core consist of dominant ferropericlase-magnesiowüstite and locally exsolved magnesioferrite. FTIR maps, obtained integrating the band at 1430 cm?1, show also the presence of carbonates. A fifth large inclusion (ca 100 ?m) was remarkable because it showed a very unusual flask shape, resembling a fluid/melt inclusion. Based on ?CXRT tomography and ?-Raman mapping, the flask-shaped inclusion is polyphase and consists of magnetite and hematite partly replacing a magnesiowüstite core and small-volume of gas/vacuum. ?-Raman spectra on the same inclusion revealed local features that are ascribed to post-spinel polymorphs, such as maohokite or xieite, which are stable at P ? 18 GPa, and to huntite, a carbonate with formula CaMg3(CO3)4. This represents the first finding of maohokite and huntite in diamond. We interpret the composition of the inclusions as evidence of formation of ferropericlase-magnesiowüstite and diamond in a carbonate-rich environment at depths corresponding at least to the Transition Zone, followed by oxidation of ferropericlase-magnesiowüstite by reaction with relatively large-volume entrapped melt during diamond ascent.
Ministry of Mineral Resources Report, Government of Greenland, 6,895KB pdf
Europe, Greenland
Data
Abstract: The Government of Greenland’s Diamond exploration data package compiles over 50 years of diamond exploration data. In addition to samples derived from Greenland’s established areas of diamondiferous rocks in central West Greenland, a wide coverage of regional exploration data extending throughout the country is included. The database follows a similar methodology of attribution and has a compatible structure to the Diamond exploration databases of the Northern Territory of Australia and Western Australia, and so meets international standards applied in areas of diamond mining. The Diamond exploration data package is the first of its kind to collate diamond exploration data country-wide in a publicly accessible fashion. It incorporates the locations of 25 000 diamond exploration samples. Associated with these samples are over 109 000 good-quality chemical analyses of mineral separate grains integrated into a standardised framework. In total, 100 discrete, named in-situ bodies, which in principle have diamond potential (kimberlites, lamproites, ultramafic lamprophyres, and carbonatites) have also been compiled in the diamond exploration data package. These occur among over 3 000 compiled in situ occurrences of dykes, pipes, sills and blows. With considerable data generated from bulk sampling of diamondiferous bodies, notably Garnet Lake, Qeqertaa and Majuagaa, this part of the database considerably expands upon previous compilations of relevant Greenland rocks, including the Geological Survey of Denmark and Greenland’s Report 2004-117. As a companion, 56 emplacement age determinations from 36 bodies are reported, encompassing most of the geographic extent of Greenland’s known rocks with diamond potential. Analyses of the exploration data allow for an understanding of exploration history in areas of known occurrences and identification of considerable gaps in the exploration coverage within areas of diamond potential. The Diamond exploration data package stands as a means to support and encourage future diamond exploration in Greenland in addition to further establishing a rigorous framework suitable for development of diamond exploration databases elsewhere.
www.trigon-gs.com/ publications_ms.html, external link free download
Europe, Greenland
Data
Abstract: The Department of Geology within the Mineral Resources Authority is pleased to announce the publication and release of a Greenland diamond exploration package. Greenland has seen significant diamond exploration, but remains heavily underexplored. The last diamond data package for Greenland was produced in 2004. However, considerable exploration has since been undertaken, generating abundant new exploration data. The new data package, covering the whole of Greenland, doubles the size of the previously available data. Furthermore, new discoveries of world-class significance were made in Greenland over the last fifteen years, which feature in the new product. The new diamond exploration data package collates publicly available, up-to-date information on Greenland’s diamond exploration history and sampling data from all across Greenland. It focuses on the locations of diamond-relevant rocks in-situ and as float, and the physical sampling and results of geochemical testing of these rocks. The package includes over 24,000 sample locations with over 10,000 being positive for diamond indicators. Accompanying these sample locations, there are over 121,000 mineral chemical analyses and detailed descriptions of over 1,000 diamonds. The database furthermore includes 3,000 in-situ locations of kimberlite, ultramafic lamprophyre, carbonatite and lamproites, in some cases with outcrop polygons and polylines, and geochronology data. Geophysical and remote sensing data are included by reference to other sources. Data are presented in raw formats and spatially as ArcMap and QGIS projects and as MapInfo files, as well as sample and analytical databases, and diamond-relevant exploration and survey reports. The package was commissioned by the Department of Geology, Mineral Resources Authority, and created by Mark T. Hutchison, Trigon GeoServices. To access the data package, contact us at [email protected], and you will receive the package through an ftp folder. The free data package aligns with the Mineral Resources Authority’s policy of publishing free, high quality, exploration-relevant geoscientific data. The data package highlights that Greenland has a major diamond potential. With this diamond data package, the Mineral Resources Authority aims to spark interest and support exploration for diamonds in Greenland.
Abstract: Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith's quartz cement, leading to increased permeability and enhancing chemical interaction with a mixed Ca-K-Na fenitizing fluid. We estimated the HFSE masses transferred from the syenite to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12% of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass comparable to the tonnages of some of the world's largest HFSE resources. We argue that fenite can yield crucial information about the tipping points in magma evolution because retention and/or loss of volatile-bonded alkali and HFSEs are key factors in the development of magmatic zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone deposits.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 47-81.
Australia
Supercontinents
Abstract: The Australian continent records c. 1860-1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860-1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790-1760 Ma terminated the rapid growth of the Australian continent.
Abstract: This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0•5 Ga and ca 1•0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2•0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by Fe-Ti-Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets.
Diamonds & Related Materials, in press available, 27p. Pdf
Global
nitrogen
Abstract: To understand the physical and chemical roles of catalytic metal Ni in the growth of diamond, ab-initio calculations of the structural, electronic, and kinetic properties of a Ni-covered C (111) surface were performed. Findings from this theoretical study highlight two important roles of Ni in addition to its carbon-solvency effect, widely known to play a catalytic role in the growth of diamond. The first role is to facilitate the formation of a thermodynamically stable Ni-C interface with a diamond bulk-like structure and the second is to induce surfactant-mediated growth enabling continuous layer-by-layer growth for diamond.
Abstract: The healed internal conjugated cleavages at the core of Aykhal octahedral diamond sample AH2 were decorated with {111}dia-facetted ultrapotassic fluid/melt inclusion pockets containing nanosized graphite, phlogopite and olivine (Fo92) inclusions. These olivines are either rounded in pockets with ample fluid, or facetted by the {111}dia mold in the pockets with a fluid film. Transmission electron microscopy revealed two distinct crystallographic characteristics of olivine inclusions: (1) pronounced crystallographic texture of olivines grouped in specific diamond domain, and (2) frequent parallelism or sub-parallelism of specific low-energy faces of the two phases, mainly (010)ol, {120}ol, (001)ol and {111}dia, {110}dia, {100}dia in the order of decreasing preference, forming prominent (010)ol/{111}dia, (010)ol/{110}dia, (001)ol/{110}dia, {120}ol/{111}dia, and {120}ol/{110}dia low-energy phase boundaries with thin liquid film of 1-2?nm in between. These findings not only testify to the extremely low adhesion energies of olivine-diamond boundary pairs, but also imply that, in the presence of a fluid phase, the interfacial energetics and the energetically favored crystallographic orientations of olivine inclusions in diamond can be controlled simply by the settlement/attachment of low-energy facets of olivine crystals precipitating from the parental fluid upon the low-energy {111}dia or {110}dia surfaces of diamond. Such interfacial energetics control and the resultant low-energy boundary pairs are characteristically distinct from the common topotaxy or epitaxy between oxide/silicate mineral pairs, but are in a sense like the Van der Waals heteroepitaxy in artificial systems.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and
Contributions to Mineralogy and Petrology, in press available, 14p.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole garnet
Contributions to Mineralogy and Petrology, Vol. 161, 6, pp. 901-920.
Abstract: Recent studies of mineral diversity and distribution lead to the prediction of >1563 mineral species on Earth today that have yet to be described-approximately one fourth of the 6394 estimated total mineralogical diversity. The distribution of these "missing" minerals is not uniform with respect to their essential chemical elements. Of 15 geochemically diverse elements (Al, B, C, Cr, Cu, Mg, Na, Ni, P, S, Si, Ta, Te, U, and V), we predict that approximately 25% of the minerals of Al, B, C, Cr, P, Si, and Ta remain to be described - a percentage similar to that predicted for all minerals. Almost 35% of the minerals of Na are predicted to be undiscovered, a situation resulting from more than 50% of Na minerals being white, poorly crystallized, and/or water soluble, and thus easily overlooked. In contrast, we predict that fewer than 20% of the minerals of Cu, Mg, Ni, S, Te, U, and V remain to be discovered. In addition to the economic value of most of these elements, their minerals tend to be brightly colored and/or well crystallized, and thus likely to draw attention and interest. These disparities in percentages of undiscovered minerals reflect not only natural processes, but also sociological factors in the search, discovery, and description of mineral species.
Abstract: Recently, statistical distributions have been explored to provide estimates of the mineralogical diversity of Earth, and Earth-like planets. In this paper, a Bayesian approach is introduced to estimate Earth’s undiscovered mineralogical diversity. Samples are generated from a posterior distribution of the model parameters using Markov chain Monte Carlo simulations such that estimates and inference are directly obtained. It was previously shown that the mineral species frequency distribution conforms to a generalized inverse Gauss-Poisson (GIGP) large number of rare events model. Even though the model fit was good, the population size estimate obtained by using this model was found to be unreasonably low by mineralogists. In this paper, several zero-truncated, mixed Poisson distributions are fitted and compared, where the Poisson-lognormal distribution is found to provide the best fit. Subsequently, the population size estimates obtained by Bayesian methods are compared to the empirical Bayes estimates. Species accumulation curves are constructed and employed to estimate the population size as a function of sampling size. Finally, the relative abundances, and hence the occurrence probabilities of species in a random sample, are calculated numerically for all mineral species in Earth’s crust using the Poisson-lognormal distribution. These calculations are connected and compared to the calculations obtained in a previous paper using the GIGP model for which mineralogical criteria of an Earth-like planet were given.
Proceedings of the National Academy of Sciences, Voll. 176, no. 26, 14738-44. pdf
Mantle
plate tectonics
Abstract: Early silicate differentiation events for the terrestrial planets can be traced with the short-lived 146Sm-142Nd system (?100-My half-life). Resulting early Earth-produced 142Nd/144Nd variations are an excellent tracer of the rate of mantle mixing and thus a potential tracer of plate tectonics through time. Evidence for early silicate differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by 142Nd/144Nd measurements of rocks that show both higher and lower (±20 ppm) values than the present-day mantle, demonstrating major silicate Earth differentiation within the first 100 My of solar system formation. We have obtained an external 2? uncertainty at 1.7 ppm for 142Nd/144Nd measurements to constrain its homogeneity/heterogeneity in the mantle for the last 2 Ga. We report that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 Ga are within 1.7 ppm of the average Earth 142Nd/144Nd value. Considering mafic and ultramafic compositions, we use a mantle-mixing model to show that this trend is consistent with a mantle stirring time of about 400 My since the early Hadean. Such a fast mantle stirring rate supports the notion that Earth’s thermal and chemical evolution is likely to have been largely regulated by plate tectonics for most of its history. Some young rocks have 142Nd/144Nd signatures marginally resolved (?3 ppm), suggesting that the entire mantle is not equally well homogenized and that some silicate mantle signatures from an early differentiated mantle (>4.1 Ga ago) are preserved in the modern mantle.
