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SDLRC - Scientific Articles all years by Author - Si-Sm
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
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International Geology Review, Vol. 58, 12, pp. 1461-1480.
India
Carbonatite
Abstract: The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [d13CVPDB = -6.7 to -5.8‰ and d18OVSMOW = 7.5-8.7‰ except a single 18O-enriched sample (d18O = 20.0‰)] represent unmodified mantle compositions. The eNd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive eNd values, close to CHUR (eNd = -0.35 to 2.94) and named high-eNd group while the low-eNd group samples show negative values (-5.69 to -8.86), corresponding to depleted and enriched source components, respectively. The 87Sr/86Sri ratios of the two groups also can be distinguished: the high-eNd ones have low 87Sr/86Sri ratios (0.70161-0.70244) while the low-eNd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.
International Geology Review, Vol. 58, 12, pp. 1461-1480.
India
carbonatites
Abstract: The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [d13CVPDB = -6.7 to -5.8‰ and d18OVSMOW = 7.5-8.7‰ except a single 18O-enriched sample (d18O = 20.0‰)] represent unmodified mantle compositions. The eNd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive eNd values, close to CHUR (eNd = -0.35 to 2.94) and named high-eNd group while the low-eNd group samples show negative values (-5.69 to -8.86), corresponding to depleted and enriched source components, respectively. The 87Sr/86Sri ratios of the two groups also can be distinguished: the high-eNd ones have low 87Sr/86Sri ratios (0.70161-0.70244) while the low-eNd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.
Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., Campos, T., Mougel, B., Brehme, I., Hemond, C., Motoki, A., Moura, D., Scalabrin, C., Pessanha, I., Alves, E., Ayres, A., Oliveira, P.
Abstract: Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults1, 2, 3, 4, and, along transforms, by transtension due to changes in ridge/transform geometry5, 6. Less common, exhumation by compressive stresses has been proposed for the large-offset transforms of the equatorial Atlantic7, 8. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ~10?million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ~11?million years ago5 initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly9 is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime.
Geophysical Research , Vol. 20, EGU2018-13349 1p. Abstract
Iceland
magmatism
Abstract: Porous rocks represent the products of all explosive volcanic eruptions. As magma ascends to the Earth’s surface, bubbles form as a consequence of the evolving saturation state of volatiles dissolved in the melt. The presence of pores (either filled with pressurized volatiles or not) strongly controls the rheological behaviour of magma and thus influences all volcanic processes (pre- syn- and post-eruptive). Nevertheless, the effects of porosity on the rheology of magma are not well characterised, and a general parameterization is not available yet. Here we present a new set of experiments designed to investigate the rheology of porous melts at high temperature (750-800
C), low strain rates (10^6-10^7 s^-1) and variable porosity. Experiments were performed at 1 atm using a Setaram Setsys vertical dilatometer. The starting materials are 5 x 5 mm cores of natural rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland (vesicle and crystal-free) initially containing 0.11(4) wt% dissolved H2O. The experimental procedure is composed by two steps: 1) synthesis of bubble-bearing materials by heating and expansion due to foaming; 2) deformation of the foamed samples. During the first step, the obsidian cores are heated above the glass transition temperature to 900- 1050
C and held for set amounts of time (2-24 h); the volume of the foamed samples increases because H2O vapour-filled bubbles nucleate and expand. The change in volume (measured by He-pycnometry) is linked to the change in porosity (10-50 vol%). For the second step, two different experimental strategies are employed, hereafter “single-stage” and “doublestage” measurements. Single-stage measurements involve deformation of the samples directly after foaming (without quenching). The sample is cooled down from the foaming T to different target T (750-800
C), a constant load (150 g) is applied by silica or alumina probes to the core, and the cores deform isothermally for 5-20 hours. Conversely, double-stage measurements involve deformation of previously synthesised and quenched pore-bearing cores. In this case the sample is heated up to the target T and deformed under an applied load for similar amount of time (5-20 hours). In both cases, the variation in length (displacement) and volume (porosity) is continuously recorded and used to calculate the viscosity of the foamed cores using Gent’s equations. Preliminary results suggest for single-stage measurements a lower effect of bubbles on the bulk viscosity, compared to double-stage measurements. We suggest that the different behaviour may be related to the different microstructure of the experimental materials. For single-stage measurements, closed and H2O vapour-filled bubbles contribute to the observed higher viscosity, whereas in double-stage measurements, possible gas leaking and melt micro-cracking during quenching are able to weaken the porous material and markedly lower suspension viscosity.
Abstract: The Arctic is warming 2-3 times faster than the global average. The rapid increase of near-surface air temperatures at high latitudes is driving a loss of ice in oceans, rivers, mountain glaciers, and soil. Permafrost, the perennially frozen ground found in frigid climates, is estimated to store approximately 1,500 gigatons of carbon, or about half of the world’s underground stores. This carbon is slowly escaping from the soil as permafrost thaws; this thawing could release as much carbon into the atmosphere as current emissions from global land use change over the next 80 years. Like many other models of future conditions, uncertainty plagues the estimates of permafrost carbon release. Salmon et al. explored how nitrogen, an important contributor to this uncertainty, interacts with carbon in thawing soils. Nitrogen is an essential nutrient for plants and soil microbes but occurs in limited supply in tundra soils. This limitation restricts plant growth and microbial decomposition, which are critical pieces of the carbon cycle. The researchers drilled soil cores at the Eight Mile Lake site in interior Alaska to depths of 85 centimeters to evaluate the annually thawed active layer (0-55 centimeters) as well as the upper permafrost (below 55 centimeters). They then incubated the soil cores at 15°C for about 8 months and measured the subsequent nitrogen levels and microbial biomass. The data collected in the incubation informed statistical models that were used to analyze the effects of depth, time, and growing season conditions on nitrogen and carbon dynamics. The findings revealed that both carbon loss and microbial biomass decreased significantly with soil depth. Models predicted that soil decomposition would release the largest amount of mineral nitrogen from soils located in the middle of the active layer. Permafrost soils at the bottom of the soil profile, however, released a large flush of mineral nitrogen during the initial thaw but a small flux of mineral nitrogen during subsequent decomposition. These patterns indicate that microbes near the soil surface are nitrogen limited, whereas deep microbial communities are more limited by carbon. The team’s calculations estimate that mineral nitrogen released from the soil profile would increase tenfold during the first 5 years of permafrost thaw. Should permafrost continue to thaw in the Arctic, these results suggest that tundra ecosystems may experience an increase in nitrogen availability that exceeds plant and microbial demands. Excess nitrogen, in turn, could precipitate increased decomposition of soil carbon and increased levels of nitrogen in streams draining from thawing permafrost landscapes. The study offers critical insights into how warming temperatures in the Arctic could dramatically increase permafrost thaw and initiate profound changes in carbon and nitrogen cycling in tundra ecosystems.
iron-copper-rare earth elements (REE) deposits in Middle Proterozoic rocks of the Midcontinent region of the United States..are they Olympic Dam-type deposits?
The Gangue, Geological Association of Canada (GAC)/Mineral Deposits Newsletter, No. 42, April pp. 1-4
Proceedinds of the National Academy of Sciences, Vol. 117, 19, 9p. Pdf
United States, Colorado
geothermometry
Abstract: The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of =676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.
Geochemistry International, Vol. 55, 12, pp. 1118-1130.
Russia
deposit - Lomonosov
Abstract: Thermodynamic numerical simulations were carried out to determine the principal simple and complex migration species of Ca, Mg, Na, K, Al, B, Mn, Mo, Sr, and U with Cl-, OH-, SO4-2, HCO3-, and CO32- in waters at the Lomonosov diamond deposit and to estimate the saturation indexes with respect to kaolinite, Na- and Mg-montmorillonite, Mg- and Na-saponite, muscovite and paragonite, biotite, phlogopite, chromite, pyrite, plagioclase (anorthite, labradorite, and andesine), olivine (forsterite and fayalite), diopside, pyrope, gypsum, anhydrite, barite, magnesite, calcite, dolomite, talc, chrysotile, chlorite, goethite, quartz, microcline, and albite. The waters are proved not to be saturated with respect to the primary (hydrothermal) minerals. The saturation of certain water samples with uranophane suggests that this mineral is of secondary genesis. The ascent of highly mineralized deep waters shall result in the dissolution of minerals whose concentrations are near the saturation ones. To maintain the ecological standards of the discharged waters, they should be diluted and/or purified by adsorbing dissolved U on a reducing reactive barrier.
Diamond & Related Materials, Vol. 89, pp. 101-107.
Mantle
boron
Abstract: Influence of growth medium composition on the efficiency of boron doping of carbonado-like diamond at 8-9 GPa was studied by diluting the C-B growth system with metallic solvents of carbon, Co and Ni. Addition of these metals to the original system leads to a decrease in the synthesis temperature, degree of doping with boron and suppression of superconductivity in diamond. According to XPS analysis, content of substitutional boron is equal to 0.07, 0.16 and 0.39 at.% in diamonds obtained in Co-C-B, Ni-C-B and C-B growth systems, respectively. Metallic behavior at normal temperatures and superconductivity below 5 K in diamond, synthesized in C-B system, change to semiconducting character of conductivity down to 2 K in diamonds obtained in the diluted systems; a faint hint of superconducting transition at 2 K was detected in the case of diamond grown in Ni-C-B system. By comparing phase composition of the inclusions and the doping efficiency of the diamonds, we are able to suggest that high chemical affinity of boron to boride-forming metals hinders the boron doping of diamond. The heavily boron-doped carbonado-like diamond compacts demonstrate high electrochemical activity in aqueous solutions and can be used as miniature electrodes in electrosynthesis and electroanalysis.
Flourine , yttrium and lanthaide rich cerianite (Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosive craters Gregory Rift Tanzania.
Mineralogical Magazine, Vol. 75, 6, pp. 2813-2822.
Shang, C.K., Satir, M., Siebel, W., Nsifa, E.N., Taubald, H., Ligeois, J.P., Tchoua, F.M.
TTG Magmatism in the Congo Craton: a view from major and trace element geochemistry, Rb Sr Sm Nd systematics: case of the Sangmelima region, Ntem Complex
Journal of African Earth Sciences, Vol. 39, 3-5, pp. 61-79.
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.
Contributions to Mineralogy and Petrology, Vol. 175, 18p. Pdf
China
lamproites
Abstract: Lamproites and kimberlites are natural probes of the subcontinental lithospheric mantle providing insights into the Earth’s continental lithosphere. Whole-rock major-, trace-element and Sr-Nd-Pb isotopic compositions of the Paleozoic (~?253 Ma) lamproite dikes from the Baifen zone of the Zhenyuan area in southeastern Guizhou Province (in the southern Yangtze Block, South China) are presented. The Baifen lamproites are characterized by high MgO (7.84-14.1 wt%), K2O (3.94-5.07 wt%) and TiO2 (2.69-3.23 wt%) contents, low SiO2 (41.3-45.7 wt%), Na2O (0.21-0.28 wt%) and Al2O3 (6.10-7.20 wt%) contents. All lamproites have elevated Cr (452-599 ppm) and Ni (485-549 ppm) abundances, as well as high Ba (1884-3589 ppm), La (160-186 ppm), Sr (898-1152 ppm) and Zr (532-632 ppm) concentrations. They show uniform REE distribution patterns that are strongly enriched in light REEs relative to heavy REEs [(La/Yb)N?=?71.1-87.6], and exhibit OIB-like geochemical features with obvious enrichment of both LILEs and HFSEs in the primitive mantle-normalized multi-element distribution diagram. Moderately radiogenic Sr (87Sr/86Sri?=?0.706336-0.707439), unradiogenic Nd (143Nd/144Ndi?=?0.511687-0.511704 and eNd(t)?=??-?12.2 to?-?11.9), and low initial Pb (206Pb/204Pbi?=?16.80-16.90, 207Pb/204Pbi?=?15.34-15.35 and 208Pb/204Pbi?=?37.43-37.70) isotopic compositions are obtained from the rocks. They yield old model ages of TDM(Nd)?=?1.48-1.54 Ga. These signatures suggest that the Baifen lamproite magmas are alkaline, ultrapotassic and ultramafic in character and mainly represent mantle-derived primary melts, which have undergone insignificant crustal contamination and negligible fractional crystallization. The Baifen lamproites originated from a veined metasomatized lithospheric mantle source. We envisage that they were derived by partial melting of old, mineralogically complex metasomatic vein assemblages in the subcontinental lithospheric mantle beneath the southern Yangtze Block. The source region experienced ancient mantle metasomatism with complex modification by enriched fluids and melts. The metasomatic agents are most likely to originate from pre-existing slab subduction beneath the southeastern margin of the Yangtze Block. Tectonically, the Baifen lamproites were emplaced at the southern margin of the Yangtze Block, and they formed in an intraplate extensional setting, showing an anorogenic affinity. In terms of time and space, the genesis of Baifen lamproites is presumably related to the Emeishan large igneous province. The Emeishan mantle plume is suggested as an effective mechanism for rapid extension and thinning of the lithosphere, followed by decompression melting of the subcontinental lithospheric mantle. Combined with the thermal perturbation from asthenospheric upwelling induced by the Emeishan mantle plume, the lamproite magmas, representing small volume and limited partial melts of ancient enriched mantle lithosphere, arose. We propose that the generation of the Baifen lamproite dikes probably was a consequence of the far-field effects of the Emeishan mantle plume.
Journal of Metamorphic Geology, Vol. 33, 5, pp. 463-494.
Africa, Ghana
Geochronology
Abstract: New petrological and geochronological data are presented on high-grade ortho- and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25-2.00 Ga) West African Craton. The study area is located in the interference zone between N-S and NE--SW-trending craton-scale shear zones, formed during the Eburnean orogeny (2.15-2.00 Ga). High-grade metamorphic domains are separated from low-grade greenstone belts by high-strain zones, including early thrusts, extensional detachments and late-stage strike-slip shear zones. Paragneisses sporadically preserve high-pressure, low-temperature (HP-LT) relicts, formed at the transition between the blueschist facies and the epidote-amphibolite sub-facies (10.0-14.0 kbar, 520-600 °C), and represent a low (~15 °C km-1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite-granulite facies transition. They reveal a clockwise pressure-temperature-time (P-T-t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0-8.0 kbar, which fit a 30 °C km-1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P-T-t path, characterized by peak metamorphic conditions of 7.0-10.0 kbar at 550-680 °C, which match a 20-25 °C km-1 apparent geotherm. These P-T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in-situ U-Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono-metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.
Abstract: Recent palaeomagnetic observations1 report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth2 is the primary driver of Earth’s present-day geodynamo3, 4, 5, but the inner core is too young6 to explain the existence of a magnetic field before about one billion years ago. Theoretical models7 propose that the exsolution of magnesium oxide—the major constituent of Earth’s mantle—from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core8, 9, 10 and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models11, we show that extreme events during Earth’s accretion (such as the Moon-forming giant impact12) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution7 of buoyant magnesium oxide would have taken place at the core-mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.
Abstract: Recent palaeomagnetic observations report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth is the primary driver of Earth's present-day geodynamo, but the inner core is too young to explain the existence of a magnetic field before about one billion years ago. Theoretical models propose that the exsolution of magnesium oxide--the major constituent of Earth's mantle--from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models, we show that extreme events during Earth's accretion (such as the Moon-forming giant impact) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution of buoyant magnesium oxide would have taken place at the core-mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.
Journal of Geophysical Research: Solid Earth, doi:10.1029/2018JB015991
Mantle
core mantle boundary
Abstract: Experiments wherein molten metal and silicate (rock-building) phases un-mix themselves due to their physical properties, i.e. metal-silicate partitioning, can be conducted at the high temperatures and pressures (HP-HT) that characterized Earth's differentiation into a core and mantle. The redistribution of elements between the metal and silicate phases - their partitioning - during this process can be measured and mathematically described, then placed into numerical models to better understand Earth's formation history. Here, we have mathematically characterized the HP-HT partitioning of copper, combined this with results for sulfur from literature, and input these characterizations into numerical models that track their distribution between Earth's core and mantle as it grows to its present mass. Copper and sulfur were chosen because they display different sensitivities to the physical mechanisms that govern planetary formation, and we can leverage this to better understand Earth's formation and differentiation history. Our results indicate that ~75% of Earth's precursor materials grew incrementally from relatively small bits of material - on average ~0.1% of Earth's mass or less - that is most compositionally similar to meteorite classes that are made up of iron-rich metal and silicate solids (chondrules) that are depleted in easily vaporized (volatile) elements, especially sulfur.
Geophysical Research Letters, Vol. 45, 24, pp. 13,240-13,248.
Mantle
geodynamics
Abstract: We measure the incorporation of magnesium oxide (one of the main components of Earth's mantle) into iron (the main constituent Earth's core), using extremely high pressure and temperature experiments that mimic the conditions of Earth's mantle and core. We find that magnesium oxide dissolution depends on temperature but not on pressure, and on metal (i.e., core) composition but not silicate (i.e., mantle) composition. Our findings support the idea that magnesium oxide dissolved in the core during its formation will precipitate out during subsequent core cooling. The precipitation should stir the entire core to produce a magnetic field in Earth's distant past, at least as intense as the present-day field.
Abstract: Zircon- and bulk-rock Zr-based thermometric parameters have become fundamental to petrogenetic models of magmatism, from which broader geochronological and tectonic implications are being made. In particular, petrogenetic models have become increasingly reliant on Ti concentration in zircon geothermometry (TZircTi) and zircon saturation temperature (TZircsat). A feature of many of these studies is an implicit assumption that all zircons present in the host igneous rock are autocrystic, that is, crystallised from the surrounding melt. However, it has long been recognised that zircons present in an igneous rock can be inherited either from the surrounding country rock or source region (xenocrysts), or from earlier phases of magmatism or the magmatic plumbing system (antecrysts). Distinguishing these different origins for zircon crystals or domains within crystals is not straightforward. Here, we first review the utility and reliability of zircon-based thermometers for petrogenetic studies and show that TZircsat is a theoretical temperature and cannot be used to constrain magmatic or partial melting temperatures. It is a dynamic variable that changes during magma crystallisation, and essentially increases as fractional crystallisation proceeds, whereas true magmatic temperatures (TMagma) decrease. Generally, in Temperature-SiO2 space, the cross-over point of these two temperatures is magmatic system dependent, and also affected by the type of calibration used for the TZircsat calculations. Consequently, each magmatic system needs to be evaluated independently to assess the validity and usefulness of TZircsat. A fundamental conclusion of TZircsat and TMagma relationships assessed here is that new zircon generally only crystallises in silicic (granitic/rhyolitic) melt compositions, and thus autocrystic zircons should not be assumed to be present in igneous rocks with bulk compositions < 64 wt% SiO2, although inherited and minor zircons crystallising from late-stage differentiated melt pockets can be present. This highlights the importance of discriminating autocrystic from inherited zircons in igneous rocks. We then review techniques available to discriminate autocrystic from inherited zircons, and propose a new methodology to assist in the identification of autocrystic zircons for emplacement age determination and separate evaluation of inherited zircon components. The approach uses two strands of data: 1) zircon data such as zircon morphologies, textures, compositions and U-Pb ages, and 2) whole-rock data, in particular SiO2 and coupled geothermometry (TZircsat and TMagma) to estimate whether the magma was zircon-saturated or undersaturated. To test this new protocol, we use as examples, several Phanerozoic granitic rocks intersected by drilling in Queensland where contextual information is limited, and show how antecrystic and xenocrystic zircons and monazites can be distinguished. In contrast, where zircons are metamict (for example, high U and Th-rich zircons), much of the ability to discriminate is impacted because such zircons have suffered Pb loss and have modified compositions (e.g., higher TZircTi). We recommend an integrated approach incorporating whole-rock chemistry, independent geothermometric constraints, zircon composition, textures and ages obtained by routine cathodoluminescence and LA-ICP-MS or ion microprobe analysis to provide increased confidence for the discrimination of inherited zircons from autocrystic zircons and determination of the emplacement age.
Abstract: Zircon- and bulk-rock Zr-based thermometric parameters have become fundamental to petrogenetic models of magmatism, from which broader geochronological and tectonic implications are being made. In particular, petrogenetic models have become increasingly reliant on Ti concentration in zircon geothermometry (TZircTi) and zircon saturation temperature (TZircsat). A feature of many of these studies is an implicit assumption that all zircons present in the host igneous rock are autocrystic, that is, crystallised from the surrounding melt. However, it has long been recognised that zircons present in an igneous rock can be inherited either from the surrounding country rock or source region (xenocrysts), or from earlier phases of magmatism or the magmatic plumbing system (antecrysts). Distinguishing these different origins for zircon crystals or domains within crystals is not straightforward. Here, we first review the utility and reliability of zircon-based thermometers for petrogenetic studies and show that TZircsat is a theoretical temperature and cannot be used to constrain magmatic or partial melting temperatures. It is a dynamic variable that changes during magma crystallisation, and essentially increases as fractional crystallisation proceeds, whereas true magmatic temperatures (TMagma) decrease. Generally, in Temperature-SiO2 space, the cross-over point of these two temperatures is magmatic system dependent, and also affected by the type of calibration used for the TZircsat calculations. Consequently, each magmatic system needs to be evaluated independently to assess the validity and usefulness of TZircsat. A fundamental conclusion of TZircsat and TMagma relationships assessed here is that new zircon generally only crystallises in silicic (granitic/rhyolitic) melt compositions, and thus autocrystic zircons should not be assumed to be present in igneous rocks with bulk compositions < 64 wt% SiO2, although inherited and minor zircons crystallising from late-stage differentiated melt pockets can be present. This highlights the importance of discriminating autocrystic from inherited zircons in igneous rocks. We then review techniques available to discriminate autocrystic from inherited zircons, and propose a new methodology to assist in the identification of autocrystic zircons for emplacement age determination and separate evaluation of inherited zircon components. The approach uses two strands of data: 1) zircon data such as zircon morphologies, textures, compositions and U-Pb ages, and 2) whole-rock data, in particular SiO2 and coupled geothermometry (TZircsat and TMagma) to estimate whether the magma was zircon-saturated or undersaturated. To test this new protocol, we use as examples, several Phanerozoic granitic rocks intersected by drilling in Queensland where contextual information is limited, and show how antecrystic and xenocrystic zircons and monazites can be distinguished. In contrast, where zircons are metamict (for example, high U and Th-rich zircons), much of the ability to discriminate is impacted because such zircons have suffered Pb loss and have modified compositions (e.g., higher TZircTi). We recommend an integrated approach incorporating whole-rock chemistry, independent geothermometric constraints, zircon composition, textures and ages obtained by routine cathodoluminescence and LA-ICP-MS or ion microprobe analysis to provide increased confidence for the discrimination of inherited zircons from autocrystic zircons and determination of the emplacement age.
Geophysical Research Letters, Vol. 46, 15, pp. 8731-8740.
Mantle
bridgmanite
Abstract: The lower mantle encompasses the largest region of the Earth's interior and is mainly composed of the perovskite-structured mineral (Mg,Fe,Al)(Al,Si)O3 bridgmanite. Its properties, therefore, control both the diffusive transport of elements and solid state flow in the lower mantle, which will be strongly influenced by point defects. We have identified and quantified defects in bridgmanite that arise from the replacement of silicon by aluminum and result in the creation of a vacant oxygen site. These oxygen defects are also found to form clusters in the structure, which in other perovskite structured minerals have been shown to strongly affect physical properties. As defect formation and ordering is dependent on composition and pressure, strong variations in physical properties may be expected within the upper 300 km of the lower mantle.
Abstract: Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-Săo Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.
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.
Abstract: Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800-700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-Săo Francisco, West African and Amazonian cratons collided at ca. 630-600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610-600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran-Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.
Abstract: New U-Pb and first Hf data were obtained from the Nico Pérez and Piedra Alta Terranes as well as from the Congo Craton. Results indicate that the Nico Pérez Terrane was mostly built during Archean episodic crustal growth and this crust underwent significant Paleo- and Neoproterozoic crustal reworking at ca. 2.2-2.0, 1.7 and 0.6 Ga. The Piedra Alta Terrane of the Río de la Plata Craton, in contrast, records only Paleoproteorozoic crustal growth. These evidences together with available geological, geochronological and isotopic data indicate the allochthony of the Nico Pérez Terrane. Furthermore, data point to an African origin of the Nico Pérez Terrane, particularly related to the southwestern Congo Craton. After Cryogenian rifting from the latter during Rodinia break-up, the Nico Pérez Terrane was accreted to the eastern Río de la Plata Craton along the Sarandí del Yí Shear Zone and underwent further crustal reworking during the evolution of the Dom Feliciano Belt.
Abstract: Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Abstract: Security of supply of “hi-tech” raw materials (including the rare earth elements (REE) and some high-field-strength elements (HFSEs)) is a concern for the European Union. Exploration and research projects mostly focus on deposit- to outcrop-scale description of carbonatite- and alkaline igneous-associated REE-HFSE mineralization. The REE-HFSE mineral system concept and approach are at a nascent stage, so developed further here. However, before applying the mineral system approach to a chosen REE-HFSE metallogenic province its mineral system extent first needs defining and mapping. This shifts a mineral system project’s foundation from the mineral system concept to a province’s mineral system extent. The mapped extent is required to investigate systematically the pathways and potential trap locations along which the REE-HFSE mass may be distributed. A workflow is presented to standardize the 4-D definition of a REE-HFSE mineral system at province-scale: (a) Identify and hierarchically organize a mineral system’s genetically related sub-divisions and deposits, (b) map its known and possible maximum extents, (c) name it, (d) discern its size (known mineral endowment), and (e) assess the favorability of the critical components to prioritize further investigations. The workflow is designed to generate process-based perspective and improve predictive targeting effectiveness along under-evaluated plays of any mineral system, for the future risking, comparing and ranking of REE-HFSE provinces and plays.
Abstract: The Southern Irumide Belt (SIB) of Zambia consists of predominantly Mesoproterozoic terranes that record a pervasive tectono-metamorphic overprint from collision between the Congo and Kalahari cratons in the final stages of Gondwana amalgamation. This study applies multi-method thermochronology to samples throughout southern Zambia to constrain the post-collisional, Phanerozoic thermo-tectonic evolution of the region. U-Pb apatite and 40Ar/39Ar muscovite data are used to constrain the cooling history of the region following Congo-Kalahari collision, and reveal ages of c. 550-450?Ma. Variations in the recorded cooling ages are interpreted to relate to localised post-tectonic magmatism and the proximity of analysed samples to the Congo-Kalahari suture. Apatite fission track data are used to constrain the low-temperature thermo-tectonic evolution of the region and identify mean central ages of c. 320-300, 210-200 and 120-110?Ma. Thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the Phanerozoic. Carboniferous to Permian-Triassic heating is suggested to relate to the development of Karoo rift basins found throughout central Africa and constrain the timing of sedimentation in the basin. Permian to Jurassic cooling is identified in a number of samples, reflecting exhumation as a result of the Mauritanian-Variscan and Gondwanide orogenies. Subsequent cooling of the majority of samples occurs from the Cretaceous and persists until present, reflecting exhumation in response to larger scale rifting associated with the break-up of Gondwana. Each model reveals a later phase of enhanced cooling beginning at c. 30?Ma that, if not an artefact of modelling, corresponds to the development of the East African Rift System. The obtained thermochronological data elucidate the previously unconstrained thermal evolution of the SIB, and provides a refined regional framework for constraining the tectonic history of central Africa throughout the Phanerozoic.
Geochemistry, Geophysics, Geosystems, Vol. 20, 9, pp. 4426-4456.
Russia
xenoliths
Abstract: This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500-700 °C. These fluids are derived from seawater that was transported to 30- to 50-km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka-Pacific Plate boundary that led to an increase in temperature and the formation of small quantities of melt that crystallized to a distinctive group of secondary minerals that are present in the samples and that postdate (overprint) the initial effects of fluid addition. The final step in the evolution of the samples was infiltration by an Fe- and Mg-rich magma that crystallized principally amphibole-group minerals.
Galloway, M., Nowicki, T., Van Coller, B., Mukodzani, B., Siemens, K., Hetman, C., Webb, K., Gurney, J.
Constraining kimberlite geology through integration of geophysical, geological and geochemical methods: a case study of the Mothae kimberlite, northern Lesotho.
Bonadiman, C., Beccaluva, L., Coltort, M., Siena, F.
Kimberlite like metasomatism and garnet signature in spinel peridotite xenoliths from Sal, Cape Verde Archipelago: relics of subcontinental mantle domain.
Abstract: A comprehensive study including new field, petrological and geochemical data is reported on the Jacupiranga alkaline-carbonatite complex (133-131 Ma) which, together with other alkaline complexes, occurs in southern Brazil and is coeval with the Paraná CFB province. It consists of a shallow intrusion (ca. 65 km2) in the Precambrian crystalline basement, and can be subdivided in two main diachronous plutonic bodies: an older dunite-gabbro-syenite in the NW and a younger clinopyroxenite-ijolite (s.l.) in the SE, later injected by a carbonatitic core (ca. 1 km2). An integrated petrogenetic model, based on bulk rock major and trace element analyses, mineral chemistry and Sr-Nd-Pb-C isotopic data, suggests that the two silicate intrusions generated from different mantle-derived magmas that evolved at shallow level (2-3 km depth) in two zoned cup-shaped plutonic bodies growing incrementally from independent feeding systems. The first intrusion was generated by OIB-like alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, discontinuously surrounded by gabbroic cumulates, in turn injected by subanular syenite intrusive and phonolite dykes. Nephelinitic (± melilite) melts - likely generated deep in the lithosphere at = 3 GPa - were the parental magmas of the second intrusion and gave rise to large coarse-grained clinopyroxenite ad- to meso-cumulates, in turn surrounded, and partially cut, by semi-annular fine-layered melteigite-ijolite-urtite ortho-cumulates. The available isotopic data do not evidence genetic links between carbonatites and the associated silicate intrusions, thus favouring an independent source from the mantle. Moreover, it may be suggested that, unlike gabbro-syenites and carbonatites, mostly generated from lithospheric mantle sources, the parental magmas of the ijolite-clinopyroxenite intrusion also record the influence of sublithospheric (plume-related?) geochemical components.
Natali, C., Beccaluva, L., Bianchini, G., Siena, F.
Coexistence of alkaline carbonatite complexes and high MgO CFB in the Parana-Etendeka province: insights on plume lithosphere interactions in the Gondwana realm.
Abstract: The effect of aluminum (Al) on the elasticity of majorite-pyrope garnets was investigated by means of ultrasonic interferometry measurements on well-fabricated polycrystalline specimens. Both velocities and elastic moduli increase almost linearly with increasing Al content within analytical uncertainty. No significant variation of the velocities and elastic moduli is observed across the tetragonal-to-cubic phase transition at majorite with the pyrope content up to 26 mol% along the majorite-pyrope system. The elasticity variation of majorite-pyrope garnets is largely dominated by the Al content, while the phase transition as a result of cation ordering/disordering of Mg and Si via substitution of Al on octahedral sites cannot significantly affect elastic properties. Seismic velocity variations of a garnet-bearing mantle transition zone are therefore dominated by garnet composition (e.g., Al, Fe, Ca, and Na) rather than the tetragonal-to-cubic phase transition because of cation ordering/disordering.
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.
Abstract: Hydrogen (H) and carbon (C) have probably been delivered to the Earth mainly during accretion processes at High Temperature (HT) and High Pressure (HP) and at variable redox conditions. We performed HP (1-15?GPa) and HT (1600-2300°C) experiments, combined with state-of-the-art analytical techniques to better understand the behavior of H and C during planetary differentiation processes. We show that increasing pressure makes H slightly siderophile and slightly decreases the highly siderophile nature of C. This implies that the capacity of a growing core to retain significant amounts of H or C is mainly controlled by the size of the planet: small planetary bodies may retain C in their cores while H may have rather been lost in space; larger bodies may store both H and C in their cores. During the Earth's differentiation, both C and H might be sequestrated in the core. However, the H content of the core would remain one or two orders of magnitude lower than that of C since the (H/C)core ratio might range between 0.04 and 0.27.
Passarelli, C.R., Basei, M.A.S., Wemmer, K., Siga, O., Oyhantcabal, P.
Major shear zones of southern Brazil and Uruguay: escape tectonics in the eastern border of Rio de la Plat a and Parananpanema cratons during West Gondwana
International Journal of Earth Sciences, in press available,
Passarelli, C.R., Basei, M.A.S., Wemmer,K., Siga, O., Oyhantcabal, P.
Major shear zones of southern Brazil and Uruguay: escape tectonics in the eastern border of Rio de la Plat a and Paranapanema cratons during w. Gondwana amal.
International Journal of Earth Sciences, Vol. 100, 2, pp. 391-414.
Abstract: Gravity anomalies across the Indian region depict most of the geological and tectonic domains of the Indian continental lithosphere, which evolved through Archean cratonic nucleation, Proterozoic accretion, Phanerozoic India-Eurasia plate convergence, and modification through many thermal perturbations and rifting. Integrated analysis of gravity and geoid anomalies together with topographic and heat flow data led to deciphering the mechanism of isostatic compensation of topographic and geological loads, lithospheric structure, and composition. This study discusses the nature of gravity (free-air, Bouguer and Isostatic) and geoid anomalies in relation to the topography, geology, and tectonics, and presents a lithospheric density model across the peninsular India and Himalaya. Southern peninsular Indian region shows relatively low Bouguer gravity anomalies compared to the northern region. The mobile belts are generally observed to have relatively higher Bouguer gravity anomalies, e.g., Eastern Ghats Mobile Belt compared to the shield regions. The gravity lows are observed over topographic features like the Western Ghats and Himalaya, while some of the topographic highs like Aravalli show positive gravity anomaly. The Indian Ocean Geoid Low varies from -82 m over Dharwar Craton to -98 m over the Southern Granulite Terrain and finally reaches a significant low of -106 m in the Indian Ocean. Flexural isostatic compensation with variable Effective Elastic Thickness (EET) ~10 km to 50 km prevails over the stable continental region. The lithospheric thickness varies from 80 km along the coastal region to 120-130 km beneath the Saurashtra Plateau, the Southern Granulite Terrain, and the Eastern Indian Shield, and reaches to more than 200 km under the Himalayan orogenic belt in the north. From Dharwar Craton to Bundelkhand Craton in central India, the lithospheric thickness varies between 160 and 180 km.
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).
Abstract: The Re-Os isotope system is well suited to constraining the timing of melt depletion of Archean mantle peridotites. In contrast, the variability inherent in post-Archean mantle Os isotope evolution leads to increasing uncertainty in Re-Os model ages. The Lu-Hf isotopic system has shown some potential for dating peridotite formation ages, providing valuable ages that are complementary to the Re-Os system. For post-Archean mantle peridotites, the key target in the Lu-Hf isotopic work is clinopyroxene (Cpx), because of its high Lu and Hf concentrations and the typical absence of garnet in these rocks. However, orthopyroxene (Opx) can contrain 20% or more of the Hf budget of spinel peridotites and somethimes over 40% of the Lu budget, with Lu/Hf ratios 3-4 times those of Cpx. Thus, Opx Lu-Hf isotopic compositions cannot be ignored or simply calculated, as the equilibrium temperatures of mantle peridotites prior to eruption could be lower or higher than the Hf closure temperature (Tc(Hf)~900şC). Here we explore Lu-Hf partitioning in spinel peridotite xenoliths from the Cameroon Volcanic Line in additin to WR Re-Os analyses. The Hf isotopic composition of Opx in these rocks is equal to or higher than that of Cpx, consistent with some samples having equilibrium temperatures close to Tc(Hf). Combining Cpx and Opx, the constructed WR Lu-Hf isochron yields an age of 2.01±0.36 Ga (2s; MSWD = 11.4; ?Hfi = -0.8±19.2), which is in accordance with the oldest of the variable Re-Os model ages. The continental sector of the Cameroon Line runs close to the edge of the Congo craton. The Hf-Os data indicate that the lithosphere underpinning this region formed in the Paleoproterozoic (~2Ga) most likely during the Paleoproterzoic assembly between the Congo and West African Cratons. We emphasize that Opx and Cpx should be combined together to construct the WR isochron in order to obtain the precise age and initial Hf isotope compositions of post-Archean spinel peridotites.
Geochimica et Cosmochimica Acta, in press available. 13p.
Africa, Cameroon
peridotite
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1s), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1s). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ~2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ~2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 177-198.
Africa, Cameroon
peridotites
Abstract: Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9-91.2) and low spinel Cr# (8.4-19.3), together with moderate to high whole-rock Al2O3 contents (2.0-3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01?±?0.18?Ga (1s), and when olivine and spinel are considered, is 1.82?±?0.14?Ga (1s). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2-2.4?Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0?Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ~2.0?Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ~2.0?Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.
Earth and Planetary Science Letters, Vol. 494, 1, pp. 92-98.
Mantle
water
Abstract: In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, d-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that d-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D? layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of d-AlOOH by the decomposition of phase Egg is associated with a ~2.1-2.5% increase in density (?) and a ~19.7-20.4% increase in bulk sound velocity (VF). The increase in ? across the phase Egg to d-AlOOH phase transition can facilitate the subduction of d-AlOOH to the lower mantle. Compared to major lower-mantle phases, d-AlOOH has the lowest ? but greatest VF, leading to an anomalous low ? /VF ratio which can help to identify the potential presence of d-AlOOH in the region. More importantly, water released from the breakdown of d-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. d-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone.
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 709-714.
Africa, Botswana
deposit - Karowe
Abstract: Historically, the recovery of large diamonds in conventional treatment plant flow sheets has been associated with dense media separation (DMS). This is attributed mainly to DMS's highly efficient and proven track record in the concentration and separation of ores with variable solids densities. In most instances, DMS has been utilized as a pre-concentration step ahead of any recovery plant, due to its ability and versatility in reducing feed within a specific size range to manageable volumes for downstream X-ray processing and subsequent diamond recovery. The benefit of using carbon-signature-based detection equipment for retrieving large stones upfront in the flow sheet not only equates to earlier recovery of diamonds from the system, but also lessens the exposure of diamond-bearing ore to additional materials handling, pumping, and/or crushing, which has been known to damage or even break diamonds and decrease revenue.
Geophysical Research Letters, Vol. 46, 20, pp. 11065-110670.
Mantle
geothermometry
Abstract: Earth's magnetic field is produced by a dynamo in the core that requires motion of the fluid Fe alloy. Both thermal convection, arising from the transport of heat in excess of conducted heat, and compositional convection, arising from light element exsolution at the freezing inner core boundary, are suggested as energy sources. The contribution of thermal convection (possibly ranging from nothing to significant) depends on thermal conductivity of the outer core. Our experimental measurements of electrical resistivity of solid and liquid Fe at high pressures show that resistivity is constant along the pressure-dependent melting boundary of Fe. Using our derived thermal conductivity value at the inner core (freezing) boundary, we calculate the heat conducted in the liquid outer core and find that thermal convection is needed to carry additional heat through the outer core to match the heat extracted through the core-mantle boundary.
Diamonds & Related Materials, Vol. 101, 107640, 13p. Pdf
Russia
Popigai
Abstract: The special features of impact diamonds are the orientation of the nanosized grains relative to each other, the presence of hexagonal diamond (lonsdaleite, L) in a large part of the samples and the increased wear resistance. Using Raman spectroscopy and XRD, two groups of translucent samples of Popigai impact diamonds (PIDs) were selected: with and without lonsdaleite and the effect of lonsdaleite on the optical properties of the samples was studied. In all L-containing PIDs there is a strong absorption band of about 1230 cm-1 in the one-phonon region, in the mid-IR. The absorption edge is blurred and described by the Urbach rule. The estimated value of Eg ~4 eV for L is consistent with the first principles calculations. Impurity nitrogen is found only in L-free PIDs: There are signals from nitrogen-vacancy complexes in the photoluminescence (PL) spectra. Variations in the number of nitrogen atoms (N = 1 to 4) in the structure of these centers indicate significant variations in the parameters of PID annealing. L-containing PIDs are characterized by large strains in the lattice and, as a consequence, there are problems with the defect diffusion. The narrow lines in PL spectra, uncommon for diamond, can be the result of several orders of magnitude higher concentrations of impurities in PIDs formed during the solid-phase transition. The broadened peaks of 180, 278 and 383 K are distinguishable in the curves of thermostimulated luminescence (TSL) for L-free PIDs, but in the presence of L the TSL glow becomes continuous as in natural IaA-type diamonds with platelets. In general, lonsdaleite deteriorates the optical properties of impact diamonds and makes it difficult to create certain types of impurity-vacancy complexes for different applications.
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: Geometallurgy aims to develop and deploy predictive spatial models based on tangible and quantitative resource characteristics that are used to optimize the efficiency of minerals beneficiation and extractive metallurgy operations. Whilst most current applications of geometallurgy are focused on the major commodity to be recovered from a mineral deposit, this contribution delineates the opportunity to use a geometallurgical approach to provide an early assessment of the economic potential of by-product recovery from an ongoing mining operation. As a case study for this methodology possible REE-recovery as a by-product of Nb-production at the Catalăo I carbonatite complex, the Chapadăo mine is used. Catalăo I is part of the Alto Paranaíba Igneous Province in the Goias Province of Brazil. Nowadays, niobium is produced in the complex as a by-product of the Chapadăo phosphates mine. This production is performed on the Tailings plant, the focus of this study. Rare earth elements, albeit present in significant concentrations, are currently not recovered as by-products. Nine samples from different stages of the Nb beneficiation process in the Tailings plant were taken and characterized by Mineral Liberation Analyzer, X-ray powder diffraction, and bulk rock chemistry. The recovery of rare earth elements in each of the tailing streams was quantified by mass balance. The quantitative mineralogical and microstructural data are used to identify the most suitable approach to recover REE as a by-product-without placing limitations on niobium production. Monazite, the most common rare earth mineral identified in the feed, occurs as Ce-rich and La-rich varieties that can be easily distinguished by SEM-based image analysis. Quartz, FeTi-oxides and several phosphate minerals are the main gangue minerals. The highest rare earth oxide content concentrations (1.75 wt.% TREO) and the greatest potential for REE processing are reported for the final flotation tailings stream. To place tentative economic constraints on REE recovery from the tailings material, an analogy to the Browns Range deposit in Australia is drawn. Its technical flow sheet was used to estimate the cost for a hypothetical REE-production at Chapadăo. Parameters derived from SEM-based image analysis were used to model possible monazite recovery and concentrate grades. This exercise illustrates that a marketable REE concentrate could be obtained at Chapadăo if the process recovers at least 53 % of the particles with no less than 60% of monazite on their surface. Applying CAPEX and OPEX values similar to that of Browns Range suggest that such an operation would be profitable at current REE prices.
Oliveira, E.P., Talavera, C., Windley, B.F., Zhao, L., Semprich, J.J., McNaughton, N.J., Amaral, W.S., Sombini, G., Navarro, M., Silva, D.
Mesoarchean ( 2820 Ma )high pressure mafic granulite at Uaus, Sao Francisco craton, Brazil, and its potential significance for the assembly of Archean supercraton.
Society for Mining, Metallurgy and Exploration (SME) stock exchange - it could happen... comments on letter from previous column on concerns as to status .
Mining Engineering, Vol. 48, No. 7, July p. 10, 17
Riding the Wilson cycle: the theory of plate tectonics continues to evolve. Now new research is answering some of the most exciting lingering questions ....
45th. Annual Yellowknife Geoscience Forum, p. 65 abstract
Canada, Northwest Territories
deposit - Giraffe
Abstract: Several Lac de Gras kimberlite pipes host thick accumulations of stratified post-eruptive lacustrine sediment and peat. Given the range of Lac de Gras kimberlite emplacement ages, these fills - though rare - provide a unique sedimentary archive of paleoenvironments during the sustained Early Cenozoic “greenhouse” interval, in a high-latitude region otherwise devoid of Phanerozoic sediment cover. Extensive exploration drilling has provided a valuable window into this unique sedimentary record, which would have otherwise remained covered by Quaternary glacial deposits. Our focus to date has been multidisciplinary study of the Giraffe pipe sediment fill: an ~80 m-thick sequence of post-eruptive lacustrine silt overlain by peat, which paints a remarkable picture of a humid-temperate Middle Eocene forest ecosystem on the Canadian Shield. Post-eruptive chronology is provided by interbedded distal tephra horizons, likely sourced from Alaska, that have been dated by glass fission-track and zircon U-Pb techniques. Paleoclimate proxies derived from pollen, wood cellulose oxygen isotopes, and biomarkers converge on reconstructed mean annual temperatures >17 °C warmer than present, with mean winter temperatures above freezing, and mean annual precipitation ~4x present. Two independent reconstructions of CO2 from well preserved conifer foliage suggest that this warming occurred under relatively modest atmospheric CO2 concentrations of 430-630 ppm. These findings provide direct field-based evidence for dramatic past arctic warming at CO2 concentrations that were well within the range of projections under “business-as-usual” emissions scenarios, underscoring the capacity for exceptional polar amplification of climate change under modest CO2 concentrations once both fast and slow feedbacks processes become expressed. Our studies at Giraffe pipe also highlight the scientific value of archived exploration drill core in the Lac de Gras kimberlite field, particularly with respect to pipes that are unremarkable for the purpose of diamond exploration.
Silversmit, G., Vekemans, B., Appel, K., Schmitz, S., Schoonjans, T., Brenker, F.E., Kaminsky, F., Vincze, L.
Three dimensional Fe speciation of an inclusion cloud within an ultradeep diamond by confocal u-x-ray absortion near edge structure: evidence for late stage
Abstract: The paper discusses questions related to the generation of increasing crustal horizontal compressive stresses compared to the idea of the standard gravitational state at the elastic stage or even from the prevalence of horizontal compression over vertical stress equal to the lithostatic pressure. We consider a variant of superfluous horizontal compression related to internal lithospheric processes occurrin in the crust of orogens, shields, and plates. The vertical ascending movements caused by these motions at the sole of the crust or the lithosphere pertain to these and the concomitant exogenic processes giving rise to denudation and, in particular, to erosion of the surfaces of forming rises. The residual stresses of the gravitational stressed state at the upper crust of the Kola Peninsula have been estimated for the first time. These calculations are based on the volume of sediments that have been deposited in Arctic seas beginning from the Mesozoic. The data speak to the possible level of residual horizontal compressive stresses up to 90 MPa in near-surface crustal units. This estimate is consistent with the results of in situ measurements that have been carried out at the Mining Institute of the Kola Science Center, Russian Academy of Sciences (RAS), for over 40 years. It is possible to forecast the horizontal stress gradient based on depth using our concept on the genesis of horizontal overpressure, and this forecasting is important for studying the formation of endogenic deposits.
Evaluation of the Diamond Content of Deep Seated Rocks (kimberlites) Based on the Calculation of Free Energy of the Diamond Dissolution Iron Containing Melt.
Doklady Academy of Sciences AKAD. NAUK SSSR., Vol. 271, No. 2, PP. 443-446.
Abstract: There are four main types of natural diamonds and related formation processes. The first type comprises the interstellar nanodiamond particles. The second group includes crustal nano- and micron-scale diamonds associated with coals, sediments and metamorphic rocks. The third one includes nanodiamonds and microndiamonds associated with secondary alteration and replacing of mafic and ultramafic rocks. The fourth one includes macro-, micron- and nano-sized mantle diamonds which are associated with kimberlites, mantle peridotites and eclogites. Each diamond type has its specific characteristics. Nano-sized diamond particles of lowest nanometers in size crystallize from abiotic organic matter at lower pressures and temperatures in space during the stages of protoplanetary disk formation. Nano-sized diamonds are formed from organic matter at P-T exceeding conditions of catagenesis stage of lithogenesis. Micron-sized diamonds are formed from fluids at P-T exceeding supercritical water stability. Macrosized diamonds are formed from metal-carbon and silicate-carbonate melts and fluids at P-T exceeding 1150 °C and 4.5 GPa. Nitrogen and hydrocarbons play an important role in diamond formation. Their role in the formation processes increases from macro-sized to nano-sized diamond particles. Introduction of nitrogen atoms into the diamond structure leads to the stabilization of micron- and nano-sized diamonds in the field of graphite stability.
Abstract: Samples of gilsonite from Adzharia, anthraxolite and graphite of coal from Taimyr, shungite from Karelia, and anthracite from Donbass are studied using Raman spectroscopy. Peaks at 1600 cm-1, indicating the presence of nanographite, are recorded in all samples. The anthracite sample from Donbass, 1330 cm-1, corresponds to the sp3-line of carbon hybridization conforming to a nanodiamond. It is concluded that in nature diamonds can be formed at late stages of lithogenesis (catagensis, metagenesis), and for coals, it can occur at the zeolite stage of regional metamorphism of rocks, before the green schist stage.
Abstract: The processes of formation of some diamond types still raise contentious issues, mainly on the origin of the largest diamond crystals recovered from kimberlites. These diamonds constitute less than 2% of worldwide resources and correspond to rare type IIa. They possess some peculiar features: (i) silicate and oxide inclusions are extremely rare, (ii) their d13C ranges from -17 to -21‰. The detailed estimation of the Premier pressure-temperature-oxygen fugacity parameters and the physic-chemical modeling of diamond growth-dissolution processes suggest that extra-large diamonds have multiple origins. Their formation may occur from lower mantle to crustal depths. Their main building-up takes place from fluids in the pegmatitic veins solidified along the contacts of kimberlite magma at a crustal depth. The model explains the main features of the largest kimberlitic diamonds, i.e. their great sizes, light d13C signatures, low nitrogen contents, high degree of resorption, absence of mantle-derived mineral inclusions and their occurrence in the form of rare isolated crystals in the host kimberlite.
Doklady Earth Sciences, Vol. 482, 2, pp. 1317-1319.
Mantle
geothermometry
Abstract: It is known that the ?-? parameters of diamond-bearing kimberlite xenoliths correspond to subductive paleogeotherms lying between the 36 and 41 mW/m2 conductive models. There are some studies showing the correlation of diamond ability with oxygen fugacity and the fluid composition of mantle xenoliths.The most diamondiferous samples correspond to the water compositions of the calculated O-H-C fluid with a minimum atomic carbon content in it. From the calculations it follows that the fluid carbon atomic content increases with a temperature increase and with the pressure decreasing. The most minor C contents have the 35 mW/m2 conductive model in comparison with the 40 and 45 mW/m2 models. As a result, it is possible to conclude that the low temperature fields (less than 1100°C) of the “cold” geotherms have the highest diamondiferous ability.
Simandl, G.J., Paradis, S., Stone, R.S., Fajber, R., Kressall, R.D., Grattan, K., Crozier, J., Simandl, L.J.
Applicablity of handheld X-ray fluroescence spectrometry in the exploration and development of carbonatite related niobium deposits: a case study of the Aley carbonatite, British Columbia, Canada.
Geochemistry: Exploration, Environment, Analysis, Vol. 14, 3, pp. 211-221.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 241-244.
Canada, British Columbia
Carbonatite
Abstract: Carbonatites host economic deposits of niobium (Nb), rare earth elements (REE), phosphate, baddeleyite (natural zirconia), vermiculite, and fl uorspar, and historically, supplied copper, uranium, carbonate (for cement industries) and sodalite (Pell, 1994 and Simandl, this volume). The Upper Fir carbonatite is in southeastern British Columbia, approximately 200 km north of Kamloops (Fig. 1). It is one ofmany known carbonatite occurrences in the British Columbia alkaline province, which follows the Rocky Mountain Trench and extends from the southeastern tip of British Columbia to its northern boundaries with the Yukon and Northwest Territories (Pell, 1994). The Upper Fir is a strongly deformed carbonatite with an indicated mineral resource of 48.4 million tonnes at 197 ppm of Ta2O5 and 1,610 ppm of Nb2O5, and an inferred resource of 5.4 million tonnes at 191 ppm of Ta2O5 and 1760 ppm of Nb2O5 (Kulla et al. 2013). The Nb, Ta, and vermiculite mineralization is described by Simandl et al. (2002, 2010), Chong, et al, (2012), and Chudy (2014). In this document we present the results of an orientation survey designed to determine the biogechemical signature of a typical carbonatite in the Canadian Cordillera. This survey suggests that needles and twigs of White Spruce (Picea glauca) and Subalpine Fir (Abies lasio carpa) are suitable sampling media to explore for carbonatites and carbonatite-related rare earth elements (REE), niobium (Nb), and tantalum (Ta) deposits.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 13-22.
Global
Niobium, tantalum
Abstract: Until 2014, niobium (Nb) and tantalum (Ta) were on the critical metals list of the European Union (European Commission, 2011; 2014). Both Ta and Nb have high levels of supply chain risk and even temporary disruptions in supply could be difficult to cope with. The Ta market is subject to infl ux of ‘conflict’ columbite-tantalite concentrate, or ’Coltan,’ into the supply chain, displacing production in Australia and Canada. The growing consumer appetite for goods made of ethically sourced or ‘confl ict-free’ minerals and metals has put pressure on manufacturers of components for consumer electronics, such as smart phones, laptop computers, computer hard drives, digital cameras, GPS navigation systems, and airbag triggers to stop using Ta from ‘confl ict’ areas. Other uses of Ta include medical implants, super alloys used in jet turbine and rocket nozzle production, corrosion prevention in chemical and nuclear plants, as a sputtering target, and in optical lenses (Tantalum-Niobium International Study Center, 2015a, b). These applications make Ta economically and strategically important to industrialised countries (European Commission, 2011, 2014; Brown et al., 2012; Papp, 2012). Niobium (Nb) is primarily used in high-strength low-alloy (HSLA) steel used extensively in the oil and gas and automotive industries. Niobium is also a major component in vacuum-grade alloys used in rocket components and other aeronautic applications (Tantalum-Niobium International Study Center, 2015a, c). Demand for Nb is increasing due to greater use of Nb in steel making in China, India, and Russia (Roskill, 2013b; Mackay and Simandl, 2014). Because most primary Nb production is restricted to a single country (Brazil), security of supply is considered at risk (European Commission, 2014). New sources of supply may be developed to diversify geographic location of supply for strategic reasons (Mackay and Simandl, 2014). Herein we summarize the geology, market, and supply chains of Niobium and Tantalum metals.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 211-218.
Technology
Rare earths
Abstract: Quantitative Evaluation of Materials by Scanning electron microscopy (QEMSCAN®) was used to assess carbonatite indicator minerals in fl uvial sediments from the drainage area of the Aley carbonatite, in north-central British Columbia. QEMSCAN® is a viable method for rapid detection and characterization of carbonatite indicator minerals with minimal processing other than dry sieving. Stream sediments from directly above, and up to 11 km downstream, of the carbonatite deposit were selected for this indicator mineral study. The geology of the Aley carbonatite is described by Mäder (1986), Kressal et al. (2010), McLeish (2013), Mackay and Simandl (2014), and Chakhmouradian et al. (2015). Traditional indicator mineral exploration methods use the 0.25-2.0 mm size fraction of unconsolidated sediments (Averill, 2001, 2014; McCurdy, 2006, 2009; McClenaghan, 2011, 2014). Indicator minerals are detectable by QEMSCAN® at particle sizes smaller than those used for hand picking (<0.25 mm). Pre-concentration (typically by shaker table) is used before heavy liquid separation, isodynamic magnetic separation, optical identifi cation using a binocular microscope, and hand picking (McClenaghan, 2011). Following additional sieving, the 0.5-1 and 1-2 mm fractions are hand picked for indicator minerals while the 0.25-0.5 mm fraction is subjected to paramagnetic separation before hand picking (Averill, 2001; McClenaghan, 2011). Hand picking indicator minerals focuses on monomineralic grains, and composite grains may be lost during processing. Composite grains are diffi cult and time consuming to hand pick and characterize using optical and Scanning Electron Microscopy (SEM) methods. A single grain mount can take 6-12 hours to chemically analyse (Layton- Matthews et al., 2014). Detailed sample analysis using the QEMSCAN® Particle Mineral Analysis routine allows for 5-6 samples to be analyzed per day. When only mineral identifi cation and mineral concentrations and counts are required, the use of a Bulk Mineral Analysis routine reduces the analysis time from ~4 hours to ~30 minutes per sample.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 251-264.
Technology
Rare earths
Abstract: Fluorite (CaF2) belongs to the isometric system, with a cubic, face-centred lattice. Fluorite commonly forms cubes or octahedrons, less commonly dodecahedrons and, rarely, tetrahexahedrons, trapezohedrons, trisoctahedrons, hexoctahedrons, and botyroidal forms. Fluorite is transparent to translucent, and has vitreous luster. It occurs in a variety of colours including purple, green, blue, or yellow, however it can also be colourless, and can exhibit colour zoning, (Staebler et al., 2006). Fluorite from many localities is fl uorescent (Verbeek, 2006). Fluorite density varies from 3.0-3.6 g/cm3, depending to a large extent on inclusions and impurities in the crystal lattice (Staebler et al., 2006), and its hardness is 4 on Mohs scale (Berry et al., 1983). Many single fl uorite crystals display sector zoning, refl ecting preferential substitution and incorporation of trace elements along successive crystal surfaces (Bosce and Rakovan, 2001). The Ca2+ ion in the fl uorite crystal structure can be substituted by Li+, Na+, K+, Mg2+, Mn2+, Fe2+,3+, Zn2+, Sr2+, Y3+, Zr4+, Ba2+, lanthanides ions, Pb2+, Th4+, and U4+ ions (Bailey et al., 1974; Bill and Calas, 1978, Gagnon et al., 2003; Schwinn and Markl, 2005; Xu et al., 2012; Deng et al., 2014). Concentrations of these impurities do not exceed 1% (Deer, 1965) except in yttrofl uorite (Ca,Y)F2-2.33 and cerfl uorite (Ca,Ce)F2-2.33 (Sverdrup, 1968). Fluorite occurs in a variety of rocks, as an accessory and as a gangue mineral in many metalliferous deposits and, in exceptional cases, as the main ore constituent of economic deposits (Simandl, 2009). Good examples of fl uorite mines are Las Cuevas, Encantada-Buenavista (Mexico); St. Lawrence pluton-related veins and the Rock Candy Mine (Canada); El Hamman veins (Morocco) and LeBurc Montroc -Le Moulinal and Trebas deposits (France) as documented by Ruiz et al. (1980), Grogan and Montgomery (1975), González-Partida et al. (2003), Munoz et al. (2005), and Fulton III and Miller (2006). Fluorite also commonly occurs adjacent to or within carbonatites and alkaline complexes (Kogut et al., 1998; Hagni,1999; Alvin et al., 2004; Xu et al., 2004; Salvi and Williams-Jones, 2006); Mississippi Valley-type (MVT) Pb- Zn-F-Ba deposits; F-Ba-(Pb-Zn) veins (Grogan and Bradbury, 1967 and 1968; Baxter et al., 1973; Kesler et al., 1989; Cardellach et al., 2002; Levresse et al., 2006); hydrothermal Fe (±Au, ±Cu) and rare earth element (REE) deposits (Borrok et al., 1998; Andrade et al., 1999; Fourie, 2000); precious metal concentrations (Hill et al., 2000); fl uorite/metal-bearing skarns (Lu et al., 2003); Sn-polymetallic greissen-type deposits (Bettencourt et al., 2005); and zeolitic rocks and uranium deposits (Sheppard and Mumpton, 1984; Cunningham et al., 1998; Min et al., 2005). Ore deposit studies that document the trace element distribution in fl uorite are provided by Möller et al. (1976), Bau et al. (2003), Gagnon et al. (2003), Schwinn and Markl (2005), and Deng et al. (2014). The benchmark paper by Möller et al. (1976) identifi ed variations in the chemical composition of fl uorites according their origin (sedimentary, hydrothermal, or pegmatitic). Recently, Makin et al. (2014) compiled trace-element compositions of fl uorite from MVT, fl uorite-barite veins, peralkaline-related, and carbonatite-related deposits. They showed that fl uorite from MVT and carbonatite deposits can be distinguished through trace element concentrations, and that the REE concentration of fl uorite from veins is largely independent of the composition of the host rock. Based on the physical and chemical properties of fl uorite, its association with a variety of deposit types, and previous studies, it is possible that fl uorite can be used as a proximal indicator mineral to explore for a variety of deposit types. Unfortunately, the compilation by Makin et al. (2014) contained chemical analyses performed at different laboratories using different analytical techniques (including laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), electron microprobe, neutron activation, and ICP-MS), and precision and accuracy varied accordingly. As an orientation survey, herein we present data from fi ve deposits, with two samples from the Rock Candy deposit (British Columbia), and one sample from each of Kootenay Florence (British Columbia), Eaglet (British Columbia), Eldor (Quebec), and Hastie quarry (Illinois) deposits (Table 1). The main objectives of this study are to: 1) assess variations in chemical composition of fl uorite in the samples and deposit types; 2) evaluate relations between analyses made using laser ablation-inductively coupled plasma mass spectrometry on individual grains [LA-ICP-MS(IG)], and those made using laser ablation-inductively coupled plasma mass spectrometry on fused beads [LA-ICP-MS(FB)] and X-ray fl uorescence (XRF); 3) test the use of stoichiometric Ca content as an internal fl uorite standard, such has been done by Gagnon et al. (2003) and Schwinn and Markl, (2005); 4) select the elements that are commonly present in concentrations above the lower limit of detection of LA-ICP-MS and available for constructing discrimination diagrams; 5) consider if our results agree with the preliminary discrimination diagrams of Makin et al. (2014).
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 31-38.
Global
Carbonatite
Abstract: Mineralized carbonatite systems are multi-commodity, highly sought after, but poorly understood exploration targets (Mariano, 1989a, b; Pell, 1996; Birkett and Simandl, 1999). They are the main sources of niobium and rare earth elements (REE), which are considered critical metals for some key economic sectors (European Commission, 2014), and have become popular exploration targets for junior mining companies worldwide. Carbonatites also contribute to our understanding of the Earth’s mantle (e.g., Bell and Tilton, 2001, 2002). Herein, we discuss the defi nition and classifi cation of carbonatites; summarize information pertinent for carbonatite exploration such as tectonic setting, shape, geophysical signature, associated rocks, alteration, and temporal distribution; and highlight the multi-commodity aspect of carbonatiterelated exploration targets and mineral prospectivity. 2. Defi nition and classifi cation Carbonatites are defi ned by the International Union of Geological Sciences (IUGS) as igneous rocks containing more than 50% modal primary carbonates (Le Maitre, 2002). Depending on the predominant carbonate mineral, a carbonatite is referred to as a ‘calcite carbonatite’ (sövite), ‘dolomite carbonatite’ (beforsite) or ‘ankerite carbonatite’. If more than one carbonate mineral is present, the carbonates are named in order of increasing modal concentrations, for example a ‘calcite-dolomite carbonatite’ is composed predominately of dolomite. If non-essential minerals (e.g., biotite) are present, this can be refl ected in the name as ‘biotite-calcite carbonatite’. Where the modal classifi cation cannot be applied, the IUGS chemical classifi cation is used (Fig. 1). This classifi cation subdivides carbonatites into calciocarbonatites, magnesiocarbonatites, and ferrocarbonatites. For calciocarbonatites, the ratio of CaO/(CaO+MgO+FeO +Fe2O3+MnO) is greater than 0.8. The remaining carbonatites are subdivided (based on wt.% ratios) into magnesiocarbonatite [MgO > (FeO+Fe2O3+MnO)] and ferrocarbonatite [MgO < (FeO+Fe2O3+MnO)] (Woolley and Kempe, 1989; Le Maitre, 2002). If the SiO2 content of the rock exceeds 20%, the rock is referred to as silicocarbonatite. When the IUGS chemical classifi cation is used, care should be taken to ensure that magnetite and hematite-rich calciocarbonates or magnesiocarbonatites are not erroneously classifi ed as ferrocarbonatites (Gittins and Harmer, 1997). A refi nement to the IUGS chemical classifi cation based on molar proportions (Gittins and Harmer, 1997), introduced ‘ferrugineous’ carbonatites (Fig. 2). The boundary separating calciocarbonatites from magnesiocarbonatites and ‘ferrugineous’ carbonatites is set at 0.75, above which carbonatites contain more than 50% calcite on a molar basis. Although not universally accepted, Gittins and Harmer’s classifi cation is commonly used in studies of carbonatitehosted ore deposits. A mineralogical-genetic classifi cation of carbonatites was proposed by Mitchell (2005). His paper points to pitfalls of the IUGS classifi cation and subdivides carbonatites into ‘primary carbonatites’ and ‘carbothermal residua’. The introduction of the term ‘carbothermal residua’ is signifi cant as it alerts mantle specialists to fundamental processes involved in the formation of many carbonatite-related deposits, and reduces rifts between camps of ore deposit geologists, petrologists, and mantle specialists. From the exploration
Abstract: In response to rising demand of the rare earth elements (REE), recent exploration of the British Columbia alkaline province has identified the Wicheeda Carbonatite, which contains an estimated 11.3 million tons of light REE-enriched ore grading 1.95 wt.% TREO, to be the highest-grade prospect known in British Columbia. However, research of the deposit is restricted to one paper describing mineralization in carbonatite dikes adjacent to the main plug. This study describes the nature and origin of REEmineralization in the Wicheeda plug. The carbonatite was emplaced in metasedimentary limestone and argillaceous limestone belonging to the Kechika Group, which has been altered to potassic fenite immediately adjacent to the carbonatite and to sodic fenite at greater distances from it. The carbonatite comprises a ferroan dolomite core, which passes outwards gradationally into calcite carbonatite. Three texturally distinct varieties of dolomite have been recognized. Dolomite 1 constitutes most of the carbonatite; Dolomite 2 replaced Dolomite 1 near veins and vugs; Dolomite 3 occurs as a fracture and vug-lining phase with the REE mineralization. Stable carbon and oxygen isotopic ratios indicate that the calcite carbonatite is of mantle origin, that Dolomite 1 is of primary igneous origin, that Dolomite 2 is largely primary igneous with minor hydrothermal signature contamination, and that Dolomite 3 is of hydrothermal origin. Rare-metal mineralization in the deposit is, with the exception of pyrochlore, which occurs in the calcite carbonatite, restricted to veins and vugs in the dolomite carbonatite. There it occurs as hydrothermal veins and in vugs infilled by REE-fluorocarbonates, i.e., bastnäsite-(Ce), ancylite-(Ce), and monazite- (Ce) together with accessory pyrite, barite, molybdenite, and thorite. A model is proposed in which calcite carbonatite was the earliest magmatic phase to crystallize. The calcite carbonatite magma saturated with niobium relatively early, precipitating pyrochlore. The magma later evolved to a dolomite carbonatite composition which, upon cooling exsolved an aqueous carbonic fluid, which altered the Kechika metasediments to potassic fenite and mixed with formational waters further from the carbonatite to produce sodic fenite. This fluid mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite. Upon progressive fluid-rock interaction, the REE precipitated largely in response to cooling and pH. Hydrothermal concentration led to remarkable grade consistency, with virtually all of the dolomite carbonatite containing >1 wt.% TREO, making the Wicheeda Carbonatite a very attractive exploration target.
Journal of Geochemical Exploration, Vol. 165, pp. 159-173.
Canada, British Columbia
Geochemistry - carbonatites
Abstract: This orientation survey indicates that Quantitative Evaluation of Materials by Scanning electron microscopy (QEMSCAN®) is a viable alternative to traditional indicator mineral exploration approaches which involve complex processing followed by visual indicator mineral hand-picking with a binocular microscope. Representative polished smear sections of the 125-250 µm fraction (dry sieved and otherwise unprocessed) and corresponding Mozley C800 table concentrates from the drainages of three carbonatites (Aley, Lonnie, and Wicheeda) in the British Columbia Alkaline Province of the Canadian Cordillera were studied. Polished smear sections (26 × 46 mm slide size) contained an average of 20,000 exposed particles. A single section can be analyzed in detail using the Particle Mineral Analysis routine in approximately 3.5-4.5 h. If only mineral identification and mineral concentrations are required, the Bulk Mineral Analysis routine reduces the analytical time to 30 min. The most useful carbonatite indicator minerals are niobates (pyrochlore and columbite), REE-fluorocarbonates, monazite, and apatite. Niobate minerals were identified in the 125-250 µm fraction of stream sediment samples more than 11 km downstream from the Aley carbonatite (their source) without the need for pre-concentration. With minimal processing by Mozley C800, carbonatite indicator minerals were detected downstream of the Lonnie and Wicheeda carbonatites. The main advantages of QEMSCAN® over the traditional indicator mineral exploration techniques are its ability to: 1) analyze very small minerals, 2) quickly determine quantitative sediment composition and mineralogy by both weight percent and mineral count, 3) establish mineral size distribution within the analyzed size fraction, and 4) determine the proportions of monomineralic (liberated) grains to compound grains and statistically assess mineral associations in compound grains. One of the key advantages is that this method permits the use of indicator minerals based on their chemical properties. This is impossible to accomplish using visual identification.
in: Ferbey, T. Plouffe, A., Hickein, A.S. eds. Indicator minerals in tills and stream sediments of the Canadian Cordillera. Geological Association of Canada Special Paper,, Vol. 50, pp. 175-190.
Canada, British Columbia
carbonatite - Aley, Lonnie, Wicheeda
Abstract: This volume consists of a series of papers of importance to indicator minerals in the Canadian Cordillera. Topics include the glacial history of the Cordilleran Ice Sheet, drift prospecting methods, the evolution of survey sampling strategies, new analytical methods, and recent advances in applying indicators minerals to mineral exploration. This volume fills a notable knowledge gap on the use of indicator minerals in the Canadian Cordillera. We hope that the volume serves as a user guide, encouraging the wider application of indicator minerals by the exploration community.
Applied Earth Science ( Trans. Inst. Min. Metall B), 31p. Doi.org/10.1080/25726388.2018.1516935 31p. Open access
Global
carbonatite - review
Abstract: Most carbonatites were emplaced in continental extensional settings and range in age from Archean to recent. They commonly coexist with alkaline silicate igneous rocks, forming alkaline-carbonatite complexes, but some occur as isolated pipes, sills, dikes, plugs, lava flows, and pyroclastic blankets. Incorporating cone sheets, ring dikes, radial dikes, and fenitisation-type halos into an exploration model and recognising associated alkaline silicate igneous rocks increases the footprint of the target. Undeformed complexes have circular, ring, or crescent-shaped aeromagnetic and radiometric signatures. Carbonatites can be effectively detected by soil, till, and stream-sediment geochemical surveys, as well as biogeochemical and indicator mineral surveys Carbonatites and alkaline-carbonatite complexes are the main sources of rare earth elements (REE) and Nb, and host significant deposits of apatite, vermiculite, Cu, Ti, fluorite, Th, U, natural zirconia, and Fe. Nine per cent of carbonatites and alkaline-carbonatite complexes contain active or historic mines, making them outstanding multi-commodity exploration targets.
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.
Applied Earth Science Transactions of the Institute of Mining and Metallurgy, doi.org/10.1080/ 25726838.2018.1516935 32p. Pdf
Global
carbonatite
Abstract: Most carbonatites were emplaced in continental extensional settings and range in age from Archean to recent. They commonly coexist with alkaline silicate igneous rocks, forming alkaline-carbonatite complexes, but some occur as isolated pipes, sills, dikes, plugs, lava flows, and pyroclastic blankets. Incorporating cone sheets, ring dikes, radial dikes, and fenitisation-type halos into an exploration model and recognising associated alkaline silicate igneous rocks increases the footprint of the target. Undeformed complexes have circular, ring, or crescent-shaped aeromagnetic and radiometric signatures. Carbonatites can be effectively detected by soil, till, and stream-sediment geochemical surveys, as well as biogeochemical and indicator mineral surveys Carbonatites and alkaline-carbonatite complexes are the main sources of rare earth elements (REE) and Nb, and host significant deposits of apatite, vermiculite, Cu, Ti, fluorite, Th, U, natural zirconia, and Fe. Nine per cent of carbonatites and alkaline-carbonatite complexes contain active or historic mines, making them outstanding multi-commodity exploration targets.
Simandl, G.J., Paradis, S., Stone, R.S., Fajber, R., Kressall, R.D., Grattan, K., Crozier, J., Simandl, L.J.
Applicablity of handheld X-ray fluroescence spectrometry in the exploration and development of carbonatite related niobium deposits: a case study of the Aley carbonatite, British Columbia, Canada.
Geochemistry: Exploration, Environment, Analysis, Vol. 14, 3, pp. 211-221.
Geochemistry, Geophysics, Geosystems, Vol. 20, pp. 3268-3288.
Mantle
subduction
Abstract: Molnar and England (1990, https://doi.org/10.1029/JB095iB04p04833) introduced equations using a semianalytical approach that approximate the thermal structure of the forearc regions in subduction zones. A detailed new comparison with high-resolution finite element models shows that the original equations provide robust predictions and can be improved by a few modifications that follow from the theoretical derivation. The updated approximate equations are shown to be quite accurate for a straight-dipping slab that is warmed by heat flowing from its base and by shear heating at its top. The approximation of radiogenic heating in the crust of the overriding plate is less accurate but the overall effect of this heating mode is small. It is shown that the previous and updated approximate equations become increasingly inaccurate with decreasing thermal parameter and increasing variability of slab dip. It is also shown that the approximate equations cannot be extrapolated accurately past the brittle-ductile transition. Conclusions in a recent paper (Kohn et al., 2018, https://doi.org/10.1073/pnas.1809962115) that modest amount of shear heating can explain the thermal conditions of past subduction from the exhumed metamorphic rock record are invalid due to a number of compounding errors in the application of the Molnar and England (1990, https://doi.org/10.1029/JB095iB04p04833) equations past the brittle-ductile transition. The use of the improved approximate equations is highly recommended provided their limitations are taken into account. For subduction zones with variable dip and/or low thermal parameter finite element modeling is recommended.
Earth and Planetary Science Letters, Vol. 536, 116161 7p. Pdf
Mantle
geothermometry
Abstract: Knowledge of thermal conductivity of mantle minerals is crucial for understanding heat transport from the Earth's core to mantle. At the pressure-temperature conditions of the Earth's core-mantle boundary, calculations of lattice thermal conductivity based on atomistic models have determined values ranging from 1 to 14 W/m/K for bridgmanite and bridgmanite-rich mineral assemblages. Previous studies have been performed at room temperature up to the pressures of the core-mantle boundary, but correcting these to geotherm temperatures may introduce large errors. Here we present the first measurements of lattice thermal conductivity of mantle minerals up to pressures and temperatures near the base of the mantle, 120 GPa and 2500 K. We use a combination of continuous and pulsed laser heating in a diamond anvil cell to measure the lattice thermal conductivity of pyrolite, the assemblage of minerals expected to make up the lower mantle. We find a value of W/m/K at 80 GPa and 2000 to 2500 K and 5.9 W/m/K at 124 GPa and 2000 to 3000 K. These values rule out the highest calculations of thermal conductivity of the Earth's mid-lower mantle (i.e. W/m/K at 80 GPa), and are consistent with both the high and low calculations of thermal conductivity near the base of the lower mantle.
Earth and Planetary Science Letters, Vol. 536, 116161, 11p. Pdf
Mantle
geothermometry
Abstract: Knowledge of thermal conductivity of mantle minerals is crucial for understanding heat transport from the Earth's core to mantle. At the pressure-temperature conditions of the Earth's core-mantle boundary, calculations of lattice thermal conductivity based on atomistic models have determined values ranging from 1 to 14 W/m/K for bridgmanite and bridgmanite-rich mineral assemblages. Previous studies have been performed at room temperature up to the pressures of the core-mantle boundary, but correcting these to geotherm temperatures may introduce large errors. Here we present the first measurements of lattice thermal conductivity of mantle minerals up to pressures and temperatures near the base of the mantle, 120 GPa and 2500 K. We use a combination of continuous and pulsed laser heating in a diamond anvil cell to measure the lattice thermal conductivity of pyrolite, the assemblage of minerals expected to make up the lower mantle. We find a value of W/m/K at 80 GPa and 2000 to 2500 K and 5.9 W/m/K at 124 GPa and 2000 to 3000 K. These values rule out the highest calculations of thermal conductivity of the Earth's mid-lower mantle (i.e. W/m/K at 80 GPa), and are consistent with both the high and low calculations of thermal conductivity near the base of the lower mantle.
Deijanin, B., Simic, D., Zaitsev, A., Chapman, J., Dobrinets, I., Widemann, A., Del Re, N., Middleton, T., Dijanin, E., Se Stefano, A.
Characterization of pink diamonds of different origin: natural ( Argyle, non-Argyle), irradiated and annealed, treated with multi-process, coated and synthetic.
Diamond and Related Materials, Vol. 17, 7-10, pp. 1169-1178.
Direct discovery of concealed kimberlites with microbial community fingerprinting.
2018 Yellowknife Geoscience Forum , p. 36. abstract
Canada, Northwest Territories
mineral chemistry
Abstract: Mineral exploration in Canada is becoming increasingly complex as the majority of undiscovered commodities are likely deeply buried beneath significant glacial overburden and bedrock, reducing the effectiveness of many existing tools. The development of innovative exploration protocols and techniques is imperative to the continuation of discovery success. Preliminary experimentation has demonstrated the potential viability of microbial fingerprinting through genetic sequencing to directly identify the projected subcrop of mineralization in addition to the more distal entrained geochemical signatures in till. With the advent of inexpensive modern sequencing technology and big-data techniques, microbiological approaches to exploration are becoming more quantitative, cost effective, and efficient. The integration of microbial community information with soil chemistry, mineralogy and landscape development coupled with geology and geophysics propagates the development of an improved decision process in mineral exploration. Soils over porphyry, kimberlite, and VMS deposits have undergone microbial community profiling. These community-genome derived datasets have been integrated with trace metal chemistry, mineralogy, surface geology and other environmental variables including Eh and pH. Analyses of two kimberlites in the Northwest Territories show significant microbial community shifts that are correlated with subsurface mineralization, with distinctive microbial community profiles present directly above the kimberlite. The relationship between microbial profiles and mineralization leads to the use of microbial fingerprinting as a method for more accurately delineating ore deposits in glacially covered terrain. As databases are developed, there is potential for application as a field based technique, as sequencing technology is progressively developed into portable platforms.
Yellowknife Forum NWTgeoscience.ca, abstract volume p. 42-43.
Canada, Northwest Territories
kimberlite
Abstract: Mineral exploration in northern latitudes is challenging in that undiscovered deposits are likely buried beneath significant glacial overburden. The development of innovative exploration strategies and robust techniques to see through cover is imperative to future discovery success. Microbial communities are sensitive to subtle environmental fluctuations, reflecting these changes on very short timescales. Shifts in microbial community profiles, induced by chemical differences related to geology, are detectable in the surficial environment, and can be used to vector toward discrete geological features. The modernization of genetic sequencing and big-data evaluation allows for efficient and cost-effective microbial characterization of soil profiles, with the potential to see through glacial cover. Results to date have demonstrated the viability of microbial fingerprinting to directly identify the surface projection of kimberlites in addition to entrained geochemical signatures in till. Soils above two kimberlites in the Northwest Territories, have undergone microbial community profiling. These community-genome derived datasets have been integrated with chemistry, mineralogy, surface geology, vegetation type and other environmental variables including Eh and pH. Analyses show significant microbial community shifts, correlated with the presence of kimberlites, with a distinct community response at the species level directly over known deposits. Diversity of soil bacteria is also depressed in the same regions of the microbial community response. The relationship between microbial profiles and buried kimberlites has led to the application of microbial fingerprinting as a method to accurately delineate potential ore deposits in covered terrain. The integration of microbial community information with soil chemistry and landscape development coupled with geology and geophysics significantly improves the drill / no-drill decision process and has proven to be far more accurate than traditional surficial exploration methods. There is high potential for application as a field-based technique as microbial databases for kimberlites in northern regions are refined, and as sequencing technology is progressively developed into portable platforms.
Joint seismic geodynamic mineral physical modeling of African geodynamics: a reconciliation of deep mantle convection with surface geophysical constraints.
Earth and Planetary Science Letters, Vol. 295, 3-4, pp. 329-341.
Journal of the Geological Society, Vol. 76, pp. 817-829.
Europe, Scotland
impact crater
Abstract: The Lairg Gravity Low may represent a buried impact crater c. 40 km across that was the source of the 1.2 Ga Stac Fada Member ejecta deposit but the gravity anomaly is too large to represent a simple crater and there is no evidence of a central peak. Reanalysis of the point Bouguer gravity data reveals a ring of positive anomalies around the central low, suggesting that it might represent the eroded central part of a larger complex crater. The inner or peak rings of complex craters show a broadly consistent 2:1 relationship between ring diameter and total crater diameter, implying that the putative Lairg crater may be as much as 100 km across. This would place the crater rim within a few kilometres of the Stac Fada Member outcrop, a location inconsistent with the thickness and clast size of the ejecta deposit. We propose that the putative impact crater originally lay further east, substantially further from the Stac Fada Member than today, and was translocated westwards to its present location beneath Lairg during the Caledonian Orogeny. This model requires that a deep-seated thrust fault, analogous to the Flannan and Outer Isles thrusts, exists beneath the Moine Thrust in north-central Scotland.
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.
Abstract: There are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively. Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa. (i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift. (ii) Central Africa is an extensive etchplain-type weathering surface - called the African Surface - from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift. (iii) The present-day topography of Central Africa is younger than 40 -30 Ma and records very long wavelength deformations (1000 -2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15 -12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10 -3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20 -16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10 -3 Ma.
Abstract: Field, petrographic and geochemical data combined with in situ zircon U-Pb LA-ICP-MS ages are documented for the Săo Tiago Batholith (southernmost portion of the Săo Francisco Craton) to understand its origin and magmatic evolution. The geologic relations indicate that the batholith is composed of granitic to granodioritic orthogneisses (L2) with tonalitic xenoliths (L1) intruded by pegmatite (L3) and metagranite (L4). L1 consists of two facies of tonalitic orthogneiss, one biotite-rich, and the other biotite-poor. The geochemical evidence, including high K2O with mantle-like chemical signature, suggests that the Bt-rich tonalitic gneiss (2816?±?30?Ma) was derived from contamination of mafic magmas by crustal-derived components. The Bt-poor tonalitic gneiss, of TTG affinity, was generated by partial melting of LILE-enriched mafic rocks, possibly from oceanic plateus in a subduction environment. L2 includes two distinct types of rocks: (i) granodioritic orthogneiss, chemically ranging from medium-pressure TTGs to potassic granitoids originated via partial melting of previous TTG crust, including L1 Bt-poor; and (ii) granitic gneiss (2664?±?4?Ma), geochemically similar to crustal-derived granites, produced by melting of the L1 Bt-rich tonalitic gneiss or mixed TTG/metasedimentary sources. L3 pegmatite (2657?±?23?Ma) results from melting of L2, whereas L4 metagranite (dikes and stocks) shows petrogenesis similar to that of the L2 granitic gneiss. Related orthogneisses occur near the Săo Tiago Batholith: (i) a hornblende-bearing tonalitic gneiss, and (ii) a hybrid hornblende-bearing granitic gneiss (2614?±?13?Ma), whose genesis is linked with interaction of sanukitoid and felsic potassic melts, representing the last Archean magmatic pulse of the region. The Minas strata along the Jeceaba-Bom Sucesso lineament near our study region encircle the Săo Tiago Archean crust, representing an irregular paleo-coastline or a micro-terrane amalgamation with the Săo Francisco Proto-craton, with possible subsequent dome-and-keel deformational processes. Our petrological and geochronological data reevaluate nebulous concepts in the literature about the SFC, revealing (i) a chemically and compositionally diverse crustal segment generated at the Late Archean in diverse geodynamic scenarios, and (ii) a more complex lineament than previously thought in terms of the paleogeography of the southern Săo Francisco Craton.
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.
Simon, S.J., Wei, C.T., Viladkar, S.G., Ellmies, R., Soh, Tamech, L.S., Yang, H., Vatuva, A.
Metamitic U rich pyrochlore from Epembe sovitic carbonatite dyke, NW Namibia.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 12.
Africa, Namibia
deposit - Epembe
Abstract: The Epembe carbonatite dyke is located about 80 km north of Opuwo, NW Namibia. The 10 km long dyke is dominated by massive and banded sövitic carbonatite intrusions. Two distinct type of sövite have been recognized: (1) coarse-grained light grey Sövite I which is predominant in brecciated areas and (2) medium- to fine-grained Sövite II which hosts notable concentrations of pyrochlore and apatite. The contact between the carbonatite and basement gneisses is marked by K-feldspar fenite. The pyrochlore chemistry at Epembe shows a compositional trend from primary magmatic Ca-rich pyrochlore toward late hydrothermal fluid enriched carbonatite phase, giving rise to a remarkable shift in chemical composition and invasion of elements such as Si, U, Sr, Ba, Th and Fe. Enrichment in elements like U, Sr and Th lead to metamictization, alteration and A-site vacancy. It is therefore suggested that the carbonatite successive intrusive phases assimilated primary pyrochlore leading to extreme compositional variation especially around the rims of the pyrochlore. The genesis of the Epembe niobium deposit is linked to the carbonatite magmatism but the mechanism that manifested such niobium rich rock remains unclear and might be formed as a result of cumulate process and/or liquid immiscibility of a carbonate-silicate pair.
Abstract: The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440?Ma), and was subsequently impacted by a late-stage hydrothermal event (~232?Ma) involving REE-rich fluids/melt. This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by d11B values that vary between -7 (typical asthenospheric mantle) and?+?4‰; d11B values and B abundances (~0.2 to ~1?ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300?Ma old) carbonatites worldwide.
neodymium, lead, and Strontium isotopic dat a Napak carbonatite -nephelinite centre, eastern Uganda: implications for crustal assimilation and fractional crystalization
Eos Transactions, Vol. 73, No. 14, April 7, supplement abstracts p.329
Doornbos, C., Heaman, L.M., Doupe, J.P., England, J., Simonetti, A., Lejeunesse, P.
The first integrated use of in situ U Pb geochronology and geochemical analyses to determine long distance transport of glacial erratics from maIn land Canada into western Arctic Archipelgo.
Canadian Journal of Earth Sciences, Vol. 46, 2, pp. 101-122.
Tappe, S., Heaman, L.M., Smart, K.A., Muehlenbachs, K., Simonetti, A.
First results from Greenland eclogite xenoliths: evidence for an ultra depleted peridotitic component within the North Atlantic craton mantle lithosphere.
GAC/MAC/AGU Meeting held May 23-27 Toronto, Abstract only
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.
Combined U-Pb geochronology and Sr-Nd isotope analysis of the Ice River perovskite standard, with implications for kimberlite and alkaline rock petrogenesis.
In situ determination of major and trace elements in calcite and apatite, and U-Pb ages of apatite from the Oka carbonatite complex: insights into a complex crystallization history.
Smart, K.A., Chacko, T., Simonetti, A., Sharp, Z.D., Heaman, L.M.
A record of Paleoproterozoic subduction preserved in the northern Slave cratonic mantle: Sr-Pb-O isotope and trace element investigations of eclogite xenoliths from the Jericho and Muskox kimberlites.
Abstract: Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (< 2 mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.
Tectonic significance and redox state of Paleoproterozoic eclogite and pyroxenite components in the Slave cratonic mantle lithosphere, Voyageur kimberlite, Arctic Canada.
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 (d11B) 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 d11B values (>-4.0‰ whereas most of the older carbonatite samples record lower B isotope values. Given the d11B 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 (d11B) 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 d11B values (>-4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the d11B 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: Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (< 2 mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.
Abstract: Mantle-derived eclogite and pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3 +/SFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere. The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1 Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1 Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8 Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1 Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and pyroxenites (mantle) as part of the same subducting oceanic slab. Oxygen fugacity determinations for one pyroxenite and ten eclogite xenoliths show a range of 3 log units, from - 4.6 to - 1.6 ?FMQ, similar to the range observed for nearby Jericho and Muskox eclogites (?FMQ - 4.2 to - 1.5). Importantly, the northern Slave eclogite and pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths. Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.
Abstract: Mantle-derived eclogite xenoliths are key for studying the evolution of the cratonic lithosphere, because geochemical evidence suggests that they typically represent fragments of Archean and Proterozoic oceanic lithosphere [1]. Recently, it has been suggested that eclogite xenoliths can serve as redox sensors of the Precambrian upper mantle using V/Sc as a redox proxy [2]. However, metasomatism can change the original oxidation state of the cratonic mantle [3], thereby limiting its use for monitoring mantle redox evolution. Circa 1.8–2.2 Ga eclogite xenoliths erupted with Jurassic kimberlites of the northern Slave craton have geochemical features that indicate oceanic crust protoliths [4, 5]. Such Paleoproterozoic ages are common for Slave craton mantle eclogites [6], linking eclogite formation with 1.9 Ga subduction-collision events at the western craton margin. The eclogites studied here have highly variable Fe3+/SFe (0.019 – 0.076 ±0.01), with logfO2 (?FMQ-4 to +2 ±0.5) that are both relatively oxidized and reduced compared to Slave mantle peridotite xenoliths [3]. Also, eclogite fO2 positively correlates with some indicies of metasomatism, such as elevated TiO2 in garnet. In addition to considering the time gap between eclogite formation and kimberlite eruption, the highly variable fO2–depth systematics of the eclogites studied here illustrate the drawbacks of using averaged eclogite fO2 to define the redox evolution of the upper mantle. Despite this, the ca. 2 Ga northern Slave craton eclogites have an average depth-corrected logfO2 of ?FMQ-0.5±1.3 (1s) that overlaps with modern MORB, and complies with the upper mantle redox evolution trend predicted using V/Sc ratios of mantlederived melts [2]. However, given the debate around the secuarity of mantle redox [7], further research into the suitability of mantle eclogites as redox sensors is warranted.
Nd, Sr, Pb and B isotopic investigation of carbonatite/alkaline centers in west central India: insights into plume driven vs lithospheric controlled magmatism.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 17.
India
carbonatites
Abstract: The exact origin of carbonatite magmas remains debatable as there are two main hypothesis proposed; one relates magmatism to asthenospheric upwellings and/or mantle plumes, whereas the other argues for generation from metasomatized lithosphere. However, proponents of the latter rarely describe in detail the origin of the metasomatic agents required to generate the high concentrations of rare earth and highly incompatible elements present in carbonatite magmas. In a recent study, Boron isotopic signatures of carbonatite complexes worldwide, ranging in age from ~2600 to ~65 million years old, indicate greater input of recycled (subducted), crustal material and plume activity with increasing geologic age of the Earth. More positive Boron isotopic values with increasing geologic time were attributed to the change of Earth’s geodynamics to a modern style of plate tectonics. In this study, the radiogenic (Sr, Nd, Pb) and B isotope systematics of carbonatites and alkaline rocks from west-central India are reported and discussed with reference to the plume-lithosphere interaction model previously proposed for the generation of Deccan-related alkaline centers in this region of the Indian sub-continent
Abstract: The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (>?0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnäsite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr?=?0.70292-0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr?=?0.70295-0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first d11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and d13CPDB (-5.37 to -4.85‰) and d18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The d11B values for carbonates from Felix, Gum, and Howard Creek vary between -8.67 and -6.36‰, and overlap the range for asthenospheric mantle (-7.1?±?0.9‰), whereas two samples from Fir yield heavier values of -3.98 and -2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like d11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Chemical Geology, doi.org/10.1016/j.chemgeo.2019.07.015 59p.
Canada, British Columbia
carbonatite - Blue River
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first d11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and d13CPDB (-5.37 to -4.85‰) and d18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The d11B values for carbonates from Felix, Gum, and Howard Creek vary between -8.67 and -6.36‰, and overlap the range for asthenospheric mantle (-7.1?±?0.9‰), whereas two samples from Fir yield heavier values of -3.98 and -2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like d11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Geochimica et Cosmochimica Acta, in press available 45p. Pdf
China
deposit - Miaoya
Abstract: A majority of carbonatite-related rare earth element (REE) deposits are found in cratonic margins and orogenic belts, and metasomatic/hydrothermal reworking is common in these deposits; however, the role of metasomatic processes involved in their formation remains unclear. Here, we present a comprehensive in situ chemical and isotopic (C-Sr) investigation of calcite and fluorapatite within the Miaoya carbonatite complex located in the South Qinling orogenic belt, with the aim to better define the role of late-stage metasomatic processes. Carbonatite at Miaoya commonly occurs as stocks and dykes intruded into associated syenite, and can be subdivided into equigranular (Type I) and inequigranular (Type II) calcite carbonatites. Calcite in Type I carbonatite is characterized by the highest Sr (up to ~22,000?ppm) and REE (195-542?ppm) concentrations with slight LREE-enriched chondrite normalized patterns [(La/Yb)N?=?2.1-5.2]. In situ C and Sr isotopic compositions of calcite in Type I carbonatite define a limited range (87Sr/86Sr?=?0.70344-0.70365; d13C?=?-7.1 to -4.2 ‰) that are consistent with a mantle origin. Calcite in Type II carbonatite has lower Sr (1708-16322?ppm) and REEs (67-311?ppm) and displays variable LREE-depleted chondrite normalized REE patterns [(La/Yb)N?=?0.2-3.3; (La/Sm)N?=?0.2-2.0]. In situ 87Sr/86Sr and d13C isotopic compositions of Type II calcite are highly variable and range from 0.70350 to 0.70524 and -7.0 to -2.2 ‰, respectively. Fluorapatite in Type I and Type II carbonatites is characterized by similar trace-element and isotopic compositions. Both types of fluorapatite display variable trace element concentrations, especially LREE contents, whereas they exhibit relatively consistent near-chondritic Y/Ho ratios. Fluorapatite is characterized by consistent Sr isotopic compositions with a corresponding average 87Sr/86Sr ratio of 0.70359, which suggests that fluorapatite remained relatively closed in relation to contamination. The combined geochemical and isotopic data for calcite and fluorapatite from the Miaoya complex suggest that carbonatite-exsolved fluids together with possible syenite assimilation during the Mesozoic metasomatism overprinted the original trace-element and isotopic signatures acquired in the early Paleozoic magmatism. Hydrothermal reworking resulted in dissolution-reprecipitation of calcite and fluorapatite, which served as the dominant source of REE mineralization during the much younger metasomatic activity. The results from this study also suggest that carbonatites located in orogenic belts and cratonic edges possess a great potential for forming economic REE deposits, especially those that have undergone late-stage metasomatic reworking.
Cimen, O., Corcoran, L., Kuebler, C., Simonetti, S.S., Simonetti, A.
Geochemical, stable ( O, C, and B) and radiogenic ( Sr, Nd, Pb) isotopic data from the Eskisehir-Kizulxaoren ( NW-Anatolia) and the Malatya-Kuluncak ( E- central Anatolia) F-REE-Th deposits, Turkey: implications for nature of carbonate-hosted mineralizati
Turkish Journal of Earth Sciences, Vol. 29, doe:10.3906/yer-2001-7 18p. Pdf
Abstract: Mantle-derived eclogite xenoliths are key for studying the evolution of the cratonic lithosphere, because geochemical evidence suggests that they typically represent fragments of Archean and Proterozoic oceanic lithosphere [1]. Recently, it has been suggested that eclogite xenoliths can serve as redox sensors of the Precambrian upper mantle using V/Sc as a redox proxy [2]. However, metasomatism can change the original oxidation state of the cratonic mantle [3], thereby limiting its use for monitoring mantle redox evolution. Circa 1.8–2.2 Ga eclogite xenoliths erupted with Jurassic kimberlites of the northern Slave craton have geochemical features that indicate oceanic crust protoliths [4, 5]. Such Paleoproterozoic ages are common for Slave craton mantle eclogites [6], linking eclogite formation with 1.9 Ga subduction-collision events at the western craton margin. The eclogites studied here have highly variable Fe3+/SFe (0.019 – 0.076 ±0.01), with logfO2 (?FMQ-4 to +2 ±0.5) that are both relatively oxidized and reduced compared to Slave mantle peridotite xenoliths [3]. Also, eclogite fO2 positively correlates with some indicies of metasomatism, such as elevated TiO2 in garnet. In addition to considering the time gap between eclogite formation and kimberlite eruption, the highly variable fO2–depth systematics of the eclogites studied here illustrate the drawbacks of using averaged eclogite fO2 to define the redox evolution of the upper mantle. Despite this, the ca. 2 Ga northern Slave craton eclogites have an average depth-corrected logfO2 of ?FMQ-0.5±1.3 (1s) that overlaps with modern MORB, and complies with the upper mantle redox evolution trend predicted using V/Sc ratios of mantlederived melts [2]. However, given the debate around the secuarity of mantle redox [7], further research into the suitability of mantle eclogites as redox sensors is warranted.
Abstract: Mantle-derived eclogite and pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3 +/SFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere. The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1 Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1 Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8 Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1 Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and pyroxenites (mantle) as part of the same subducting oceanic slab. Oxygen fugacity determinations for one pyroxenite and ten eclogite xenoliths show a range of 3 log units, from - 4.6 to - 1.6 ?FMQ, similar to the range observed for nearby Jericho and Muskox eclogites (?FMQ - 4.2 to - 1.5). Importantly, the northern Slave eclogite and pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths. Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.
Geochimica et Cosmochimica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and d18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy d18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu*, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like d18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ~3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and d18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy d18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu*, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like d18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ~3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Geochimica et Cosmochimica Acta, Vol. 213, 1, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and d18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy d18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu*, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like d18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ~3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Abstract: The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440?Ma), and was subsequently impacted by a late-stage hydrothermal event (~232?Ma) involving REE-rich fluids/melt. This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by d11B values that vary between -7 (typical asthenospheric mantle) and?+?4‰; d11B values and B abundances (~0.2 to ~1?ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300?Ma old) carbonatites worldwide.
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first d11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and d13CPDB (-5.37 to -4.85‰) and d18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The d11B values for carbonates from Felix, Gum, and Howard Creek vary between -8.67 and -6.36‰, and overlap the range for asthenospheric mantle (-7.1?±?0.9‰), whereas two samples from Fir yield heavier values of -3.98 and -2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like d11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Chemical Geology, doi.org/10.1016/j.chemgeo.2019.07.015 59p.
Canada, British Columbia
carbonatite - Blue River
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first d11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and d13CPDB (-5.37 to -4.85‰) and d18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The d11B values for carbonates from Felix, Gum, and Howard Creek vary between -8.67 and -6.36‰, and overlap the range for asthenospheric mantle (-7.1?±?0.9‰), whereas two samples from Fir yield heavier values of -3.98 and -2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like d11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Cimen, O., Corcoran, L., Kuebler, C., Simonetti, S.S., Simonetti, A.
Geochemical, stable ( O, C, and B) and radiogenic ( Sr, Nd, Pb) isotopic data from the Eskisehir-Kizulxaoren ( NW-Anatolia) and the Malatya-Kuluncak ( E- central Anatolia) F-REE-Th deposits, Turkey: implications for nature of carbonate-hosted mineralizati
Turkish Journal of Earth Sciences, Vol. 29, doe:10.3906/yer-2001-7 18p. Pdf
Russian Journal of Pacific Geology, Vol. 11, 6, pp. 447-468.
Russia
picrites
Abstract: A great volume of original information on the formation of the ultrabasic rocks of the Siberian Platform has been accumulated owing to the study of melt inclusions in Cr-spinels. The inclusions show the general tendencies in the behavior of the magmatic systems during the formation of the ultrabasic massifs of the Siberian Platform, tracing the main evolution trend of decreasing Mg number with SiO2 increase in the melts with subsequent transition from picrites through picrobasalts to basalts. The compositions of the melt inclusions indicate that the crystallization conditions of the rocks of the concentrically zoned massifs (Konder, Inagli, Chad) sharply differ from those of the Guli massif. Numerical modeling using the PETROLOG and PLUTON softwares and data on the composition of inclusions in Cr-spinels yielded maximum crystallization temperatures of the olivines from the dunites of the Konder (1545-1430°C), Inagli (1530-1430°C), Chad (1460-1420°C), and Guli (1520-1420°C) massifs, and those of Cr-spinels from the Konder (1420-1380°C), Inagli (up to 1430°C), Chad (1430-1330°C), and Guli (1410-1370°C) massifs. Modeling of the Guli massif with the PLUTON software using the compositions of the melt inclusions revealed the possible formation of the alkaline rocks at the final reverse stage of the evolution of the picritic magmas (with decrease of SiO2 and alkali accumulation) after termination of olivine crystallization with temperature decrease from 1240-1230°C to 1200-1090°C. Modeling with the PLUTON software showed that the dunites of the Guli massif coexisted with Fe-rich (with moderate TiO2 contents) melts, the crystallization of which led (beginning from 1210°C) to the formation of pyroxenes between cumulate olivine. Further temperature decrease (from 1125°C) with decreasing FeO and TiO2 contents provided the formation of clinopyroxenes of pyroxenites. For the Konder massif, modeling with the PLUTON software indicates the possible formation of kosvites from picrobasaltic magmas beginning from 1350°C and the formation of clinopyroxenites and olivine-diopside rocks from olivine basaltic melts from 1250°C.
Abstract: 40Ar/39Ar analysis showed a simultaneous (at about 490 Ma) formation of the Paleozoic picrite and basalt complexes of the West Siberian Plate basement. The petrochemistry, trace and REE geochemistry, and composition of clinopyroxene indicate the formation of the picrite of well no. 11 (Chkalov area) as a result of intraplate magmatism of the OIB type. Calculations based on the compositions of clinopyroxene allowed crystallization of minerals of porphyric picrite at 1215-1275°C and 4.5-8 kbar. In general, it has been found that the picrite basalt complexes considered were formed from enriched igneous plume systems under intraplate conditions near the active margin of the ancient ocean.
Abstract: Experimental studies of phase relations in the oxide-silicate system MgO-FeO-SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO-FeO-SiO2-(Mg-Fe-Ca-Na carbonate)-carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic-basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.
Geochemistry International, Vol. 57, 9, pp. 1000-1007.
Mantle
diamond genesis
Abstract: The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2, and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of phase relations in the multicomponent diamond-forming MgO-FeO-SiO2-Na2CO3-CaCO3-K2CO3 system of the Earth mantle transition zone. An interaction of CaCO3 and SiO2 with a formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite controls physicochemically the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of deep horizons of the transition zone up to its boundary with the Earth lower mantle.
GEM: International Journal on Geomathematics, open access 38p. Pdf
Mantle
geophysics - magnetics
Abstract: We discuss the resolving power of three geophysical imaging and inversion techniques, and their combination, for the reconstruction of material parameters in the Earth’s subsurface. The governing equations are those of Newton and Poisson for gravitational problems, the acoustic wave equation under Hookean elasticity for seismology, and the geodynamics equations of Stokes for incompressible steady-state flow in the mantle. The observables are the gravitational potential, the seismic displacement, and the surface velocity, all measured at the surface. The inversion parameters of interest are the mass density, the acoustic wave speed, and the viscosity. These systems of partial differential equations and their adjoints were implemented in a single Python code using the finite-element library FeNICS. To investigate the shape of the cost functions, we present a grid search in the parameter space for three end-member geological settings: a falling block, a subduction zone, and a mantle plume. The performance of a gradient-based inversion for each single observable separately, and in combination, is presented. We furthermore investigate the performance of a shape-optimizing inverse method, when the material is known, and an inversion that inverts for the material parameters of an anomaly with known shape.
Geological Society of America Special Paper, No. 514, pp. SPE514-08.
Mantle
Hotspots
Abstract: Thorne et al. (2004), Torsvik et al. (2010; 2006) and Burke et al. (2008) have suggested that the locations of melting anomalies ("hot spots") and the original locations of large igneous provinces ("LIPs") and kimberlite pipes, lie preferentially above the margins of two "large lower-mantle shear velocity provinces", or LLSVPs, near the bottom of the mantle, and that the geographical correlations have high confidence levels (> 99.9999%) (Burke et al., 2008, Fig. 5). They conclude that the LLSVP margins are "Plume-Generation Zones", and that deep-mantle plumes cause hot spots, LIPs, and kimberlites. This conclusion raises questions about what physical processes could be responsible, because, for example, the LLSVPs are apparently dense and not abnormally hot (Trampert et al., 2004). The supposed LIP-hot spot-LLSVP correlations probably are examples of the "Hindsight Heresy" (Acton, 1959), of performing a statistical test using the same data sample that led to the initial formulation of a hypothesis. In this process, an analyst will consider and reject many competing hypotheses, but will not adjust statistical assessments correspondingly. Furthermore, an analyst will test extreme deviations of the data, , but not take this fact into account. "Hindsight heresy" errors are particularly problematical in Earth science, where it often is impossible to conduct controlled experiments. For random locations on the globe, the number of points within a specified distance of a given curve follows a cumulative binomial distribution. We use this fact to test the statistical significance of the observed hot spot-LLSVP correlation using several hot-spot catalogs and mantle models. The results indicate that the actual confidence levels of the correlations are two or three orders of magnitude smaller than claimed. The tests also show that hot spots correlate well with presumably shallowly rooted features such as spreading plate boundaries. Nevertheless, the correlations are significant at confidence levels in excess of 99%. But this is confidence that the null hypothesis of random coincidence is wrong. It is not confidence about what hypothesis is correct. The correlations probably are symptoms of as-yet-unidentified processes.
Recovery Plant Practice at de Beers Consolidated Mines Limited, Kimberley with Particular Reference to Improvements Made for the Sorting of the Final Concentrates.
South African Institute of Mining and Metallurgy. Journal, Vol. 80, No. 9, PP. 317-328.
Short hold time parameters. Diavik mine water treatment plant.
2018 Yellowknife Geoscience Forum , p. 74-75. abstract
Canada, Northwest Territories
deposit - Diavik
Abstract: It is well known that it is very difficult to transport samples from remote locations to the laboratory and allow sufficient time to commence analysis within the prescribed short hold times for certain parameters. Also, the majority of published hold times are based on legacy as opposed to hard science. In an attempt to determine the validity of specific short hold times, a joint study between Diavik and Maxxam was undertaken. The purpose was to determine the stability of short hold time parameters over time using real samples from Diavik sites. Data from two sites will be presented. The first from the Diavik mine water treatment plant influent, which had relatively high levels of the target analytes. The second from a lake water sample with lower native levels of the target analytes. The parameters studied were ammonia (preserved and unpreserved), total nitrogen, nitrite, nitrate, phosphate, total phosphorus and turbidity. All target parameters have a prescribed 3-day hold time.1 pH was also monitored. Samples were collected by Diavik personnel in one-litre containers and extraordinary logistical measures were taken to get them to Maxxam's Burnaby laboratory as soon as possible. On receipt, they were immediately subsampled into appropriate containers. Each parameter (except pH and turbidity) was split into three containers: 1) as received; 2) low level spike added and 3) medium level spike added. All samples were analyzed within 3 at approximately 3-day intervals thereafter for a period of two weeks.
Kimberlite indicator minerals in southwestern Saskatchewan
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 53-58.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 359-378.
Short hold time parameters. Diavik mine water treatment plant.
2018 Yellowknife Geoscience Forum , p. 74-75. abstract
Canada, Northwest Territories
deposit - Diavik
Abstract: It is well known that it is very difficult to transport samples from remote locations to the laboratory and allow sufficient time to commence analysis within the prescribed short hold times for certain parameters. Also, the majority of published hold times are based on legacy as opposed to hard science. In an attempt to determine the validity of specific short hold times, a joint study between Diavik and Maxxam was undertaken. The purpose was to determine the stability of short hold time parameters over time using real samples from Diavik sites. Data from two sites will be presented. The first from the Diavik mine water treatment plant influent, which had relatively high levels of the target analytes. The second from a lake water sample with lower native levels of the target analytes. The parameters studied were ammonia (preserved and unpreserved), total nitrogen, nitrite, nitrate, phosphate, total phosphorus and turbidity. All target parameters have a prescribed 3-day hold time.1 pH was also monitored. Samples were collected by Diavik personnel in one-litre containers and extraordinary logistical measures were taken to get them to Maxxam's Burnaby laboratory as soon as possible. On receipt, they were immediately subsampled into appropriate containers. Each parameter (except pH and turbidity) was split into three containers: 1) as received; 2) low level spike added and 3) medium level spike added. All samples were analyzed within 3 at approximately 3-day intervals thereafter for a period of two weeks.
Abstract: The geochemical evolution of mine-waste rock often includes concurrent acid generation and neutralization processes. Deposition of mine-waste rock in large, oxygenated, and partially saturated piles can result in release of metals and decreased pH from weathering of sulfide minerals. Acid neutralization processes can often mitigate metals and pH impacts associated with sulfide oxidation. The Diavik Waste Rock Project included large field experiments (test piles built in 2006) conducted to characterize weathering of sulfide waste rock at a scale representative of full size waste-rock piles. Water samples from the unsaturated interior of one of the test piles, constructed of waste rock with ~0.05 wt.% S, were collected using soil water solution samplers and drains at the base of the pile. Field observations indicated pH decreased throughout the depth of the pile during 2008 and 2009 and that carbonate mineral buffering was entirely depleted by 2011 or 2012. Carbonate mineral exhaustion was accompanied by increased concentrations of dissolved Al and Fe in effluent samples collected at the basal drains. These results suggest that dissolution of Al and Fe(oxy)hydroxides occurred after the depletion of carbonate minerals following an acid neutralization sequence that is similar to observations made by previous researchers. A conceptual model of acid neutralization proceses within the pile, developed using physical and geochemical measurements conducted from 2008 to 2012, was used to inform reactive transport simulations conducted in 2017 to quantify the dominant acid neutralization processes within the test pile interior. Reactive transport simulations indicate that the conceptual model developed using the results of field samples provides a reasonable assessment of the evolution of the acid neutralization sequence.
International Journal of Earth Sciences, in press available 17p.
Africa, Namibia
Alkaline rocks
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242?±?6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial eSr-eNd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
International Journal of Earth Sciences, Vol. 106, pp. 2797-2813.
Africa, Namibia
carbonatites
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242?±?6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial eSr-eNd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
International Journal of Earth Sciences, Vol. 106, 8, pp. 2797-2813.
Africa, Namibia
carbonatite
Abstract: Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242 ± 6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Paraná-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial ?Sr-?Nd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.
Abstract: Magmatic pulses in intraplate sedimentary Basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe. Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.
Abstract: Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38?km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8-20?km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the "Fermor line", which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.
Abstract: Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38?km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8-20?km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the “Fermor line”, which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.
Abstract: We analyzed satellite gravity and geoid anomaly and topography data to determine the 3D lithospheric density structure of the Singhbhum Protocontinent. Our density model shows that distinct vertical density heterogeneities exist throughout the lithosphere beneath the Singhbhum Protocontinent. The crustal structure identified includes a lateral average crustal density variation from 2800 to 2890 kg/m3 as well as a relatively flat Moho at 35-40 km depth in Singhbhum Protocontinent and Bastar Craton. A similar Moho depth range is found for the Mahanadi, Damodar, and Bengal basins. In the northern part of the area, Moho undulates between more than 40 km under the confluence of Mahanadi-Damodar Gondwana basins and the Ganga foreland basin, and 36-32 km under the Eastern Ghats Mobile belt and finally reaches 24 km in the Bay of Bengal. The lithosphere-asthenosphere boundary (LAB) across the Singhbhum Protocontinent is at a depth of about 130-140 km. In the regions of Bastar Craton and Bengal Basin, the LAB dips to about 155 ± 5 km depth. The confluence of Mahanadi and Damodar Gondwana basins toward the north-west and the foreland Ganga Basin toward the north are characterized by a deeper LAB lying at a depth of over 170 and 200 km, respectively. In the Bay of Bengal, the LAB is at a shallower depth of about 100-130 km except over the 85 0E ridge (150 km), and off the Kolkata coast (155 km). Significant density variation as well as an almost flat crust-mantle boundary indicates the effect of significant crustal reworking. The thin (135-140 km) lithosphere provides compelling evidence of lithospheric modification in the Singhbhum Protocontinent. Similarities between the lithospheric structures of the Singhbhum Craton, Chhotanagpur Gneiss Complex, and Northern Singhbhum Mobile Belt confirm that the repeated thermal perturbation controlled continental lithospheric modification in the Singhbhum Protocontinent.
