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SDLRC - Scientific Articles all years by Author - Ao-Az
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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Abstract: The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8?Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U-Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85?Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89-1.85?Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0?Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570?Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate-Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.
Abstract: The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8?Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U-Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85?Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89-1.85?Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0?Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570?Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate-Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.
Abstract: Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies.
Contributions to Mineralogy and Petrology, Vol. 175, 97, 30p. Pdf
Africa, Tanzania
craton
Abstract: U-Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper-middle crust record thermal quiescence since the Archean (2.8-2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga,?>?900 °C zircon), the EAO (600-500 Ma rutile), and fluid influx during rifting (?650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1-1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting.
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 Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)-enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an “original” euhedral apatite, “turbid” apatite, and “overgrowths” of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REE-fluorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals.
Earth and Planetary Science Letters, Vol. 435, 1, pp. 31-35.
Russia
Deposit - Udachnaya
Abstract: The study of diamond and its solid inclusions is of paramount importance to acquire direct information on the deepest regions of the Earth. However, although diamond is one of the most studied materials in geology, the diamond-inclusion relationships are not yet understood: do they form simultaneously (syngenesis) or are inclusions pre-existing objects on which diamond nucleated (protogenesis)? Here we report, for the first time, adhesion energies between diamond (D) and forsterite (Fo) to provide a crucial contribution to the syngenesis/protogenesis debate. The following interfaces were investigated at quantum-mechanical level: (i) (001)D/(001)Fo, (ii) (001)D/(021)Fo, and (iii) (111)D/(001)Fo. Our data, along with the ones recently obtained on the (110)D/(101)Fo interface, revealed an unexpected thermodynamic behaviour, all interfaces showing almost equal and low adhesion energies: accordingly, diamond and olivine have an extremely low chemical affinity and cannot develop preferential orientations, even during an eventual epitaxial growth. Combining these results with those of our previous work concerning the morphology constraints of diamond on its inclusions, we can state that the two main arguments used so far in favour of diamond/inclusions syngenesis cannot be longer considered valid, at least for olivine.
Abstract: Stable isotope (?18O, ?13C) analyses were performed on well preserved belemnites, oysters, and rhynchonellid brachiopods from the Middle to Upper Jurassic of the Morondava Basin in southern Madagascar. Both brachiopods and oysters indicate similar average temperatures of 18.7 to 19.3?°C in the Early Callovian, followed by a temperature decrease towards the Middle Oxfordian (13.9?°C) and a minimum in the Early Kimmeridgian (12.3?°C). In contrast, belemnites from the Oxfordian show lower average temperatures of 10.0?°C, which is likely caused by specific conditions for these organisms (e.g., different fractionation or life habits). Additionally, three oysters from the Upper Oxfordian and Lower Kimmeridgian were used for high-resolution stable isotope analyses. The data show seasonal fluctuations of >6?°C around averages between 14.4 and 14.7?°C. Latitudinal temperature gradients for the Callovian and Kimmeridgian are similar to today at the examined low latitudes of the southern hemisphere. The observed cooling of around 5?°C from the Callovian to the Oxfordian/Kimmeridgian can be attributed to a concurrent southward drift of Madagascar during the break-up of Gondwana. Thus, the study underlines the importance of considering palaeogeography in interpreting stable isotope data as well as the potential of detecting and timing palaeogeographic events by using stable isotope analyses.
Majoritic garnet: a new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamonds.
Geochimica et Cosmochimica Acta, Vol. 74, 20, pp. 5939-5937.
Trace element heterogeneity in hydrothermal diopside: evidence for Ti depletion and Sr Eu LREE enrichment during hydrothermal metamorphism of mantle harzburgite.
Journal of Mineralogical and Petrological Sciences, Vol. 102, 2, pp. 143-149.
Abstract: Chromitites (aggregates of chromite or chromian spinel) inform us of various mantle processes, including magmatism, magma/peridotite reaction and mantle dynamics [1]. They typically form as magmatic cumulates from chromiteoversaturated melt within conduits in the mantle peridotite [2]. They are usually enveloped by replacive dunite [1]. In Oman, both concordant and discordant chromitites are of low-P (upper mantle) magmatic origin [3, 4]. Their chromite grains contain inclusions of pargasite, aspidolite and pyroxenes, which suggest low P. Mineral chemistry suggests involvement of MORB for the concordant chromitite, and of arc-related magma for the discrodant one. This is consistent with the switch of tectonic setting, from MOR to SSZ, for the Oman ophiolite magmatism. Only the concordant chromitite shows metamorphic characters, i.e. exsolution of diopside in chromite and outward diffusion of Ni (< 30 cm) in the dunite envelope [5], indicating its longer residence in the mantle. Ultra-high pressure (UHP) chromitites have been reported from the Tibetan and Polar Ural ophiolites [6, 7]. Most of their petrographic characteristics can be explained by UHP "metamorphism" of low-P magmatic chromitites above [8]. This may suggest recycling of low-P chromitite as deep as the transion-zone mantle [9]. The UHP chromitite is, however, still highly enigmatic: some characteristics, e.g., the amount and origin of carbon as diamond, are difficult to explain. High-T aqueous fluids containing Cl, S and C, can mobilize Cr and precipitate chromite in the mantle [10]. Chromite was dissolved and precipitated in/from high-T fluids which formed diopsidites in Oman. Chromite was concentrated to form thin "hydrothermal chromitite". Sub-arc metasomatized peridotites contain secondary chromite closely associated with fluid inclusions, indicating Cr mobility via fluids within the mantle wedge. Hydrothermal chromitites are expected in the mantle where fluid circulation is available.
Journal of African Earth Sciences, Vol. 127, pp. 3-15.
Africa, Morocco
Craton - magmatism
Abstract: Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760-700 Ma), syn-collisional Bou-Azzer magmatism (680-640 Ma), followed by widespread Ediacaran magmatism (620-555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma. Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330-300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins. Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in the Internal Maghrebian flysch nappes as well as in the external Mesorif. This event consists of Middle-Upper Jurassic MORB tholeiites emplaced during opening of the Alpine Tethys ocean. The Central High Atlas also records Early Cretaceous alpine Tethys magmatism associated with the aborted Atlas rift, or perhaps linked to plume activity on the edge of the WAC. Cenozoic magmatism is associated with Tertiary and Quaternary circum-Mediterranean alkaline provinces, and is characterized by an intermittent activity over 50 Ma from the Anti-Atlas to the Rif Mountain along a SW-NE volcanic lineament which underlines a thinned continental lithosphere.
Abstract: Boninites are widely distributed along the western margin of the Pacific Plate extruded during the incipient stage of the subduction zone development in the early Paleogene period. This paper discusses the genetic relationships of boninite and antecedent protoarc basalt magmas and demonstrates their recycled ancient slab origin based on the T-P conditions and Pb-Hf-Nd-Os isotopic modeling. Primitive melt inclusions in chrome spinel from Ogasawara and Guam islands show severely depleted high-SiO2, MgO (high-silica) and less depleted low-SiO2, MgO (low-silica and ultralow-silica) boninitic compositions. The genetic conditions of 1?346?°C at 0.58?GPa and 1?292?°C at 0.69?GPa for the low- and ultralow-silica boninite magmas lie on adiabatic melting paths of depleted mid-ocean ridge basalt mantle with a potential temperature of 1?430?°C in Ogasawara and of 1?370?°C in Guam, respectively. This is consistent with the model that the low- and ultralow-silica boninites were produced by remelting of the residue of the protoarc basalt during the forearc spreading immediately following the subduction initiation. In contrast, the genetic conditions of 1?428?°C and 0.96?GPa for the high-silica boninite magma is reconciled with the ascent of more depleted harzburgitic source which pre-existed below the Izu-Ogasawara-Mariana forearc region before the subduction started. Mixing calculations based on the Pb-Nd-Hf isotopic data for the Mariana protoarc basalt and boninites support the above remelting model for the (ultra)low-silica boninite and the discrete harzburgite source for the high-silica boninite. Yb-Os isotopic modeling of the high-Si boninite source indicates 18-30?wt% melting of the primitive upper mantle at 1.5-1.7?Ga, whereas the source mantle of the protoarc basalt, the residue of which became the source of the (ultra)low-Si boninite, experienced only 3.5-4.0?wt% melt depletion at 3.6-3.1?Ga, much earlier than the average depleted mid-ocean ridge basalt mantle with similar degrees of melt depletion at 2.6-2.2?Ga.
Abstract: The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E-W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al2O3, CaO and clinopyroxene (< 3 vol%) in bulk peridotites. The primary olivine is Fo89.3-Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1 wt% Al2O3, 0.7 wt% Cr2O3, and 0.2 wt% Na2O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41-0.80, with low TiO2 (0.03 wt%), MnO (0. 3 wt%) and YFe [(Fe3 +/(Cr + Al + Fe3 +) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25-40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.
Abstract: Highly carbonated mantle xenoliths have been found in rift-related alkaline basalts at the Wadi Natash area in the southern part of the Eastern Desert of Egypt. Although all olivine and most orthopyroxene was replaced by carbonate and/or quartz, textural and mineral chemical features show that they are plagioclase-free spinel peridotites (lherzolite to harzburgite). Cr and Mg numbers (Cr#, Mg#) of Cr-spinel vary from 0.06 to 0.45 and 0.73 to 0.81, respectively. The correlation between Cr# and Mg# of the Cr-spinel in the studied xenoliths is weakly negative and its TiO2 content is slightly higher than in abyssal peridotite that was not affected by melt injection. The chemistry of ortho- and clinopyroxene suggests enstatite and chromian diopside compositions, respectively, with distinct signatures of a sub-continental mantle source. In particular, the Na2O contents (>1.0?wt%) and AlVI/AlIV ratios (1.2-2.6) of chromian diopside suggest such an origin. Two-pyroxene geothermometry indicates a temperature of about 900?°C, which is slightly lower than that of ordinary spinel peridotite xenoliths from other rift zones. It is evident that the studied peridotite xenoliths had experienced mantle processes (e.g. decompression melting, magma upwelling and metasomatism) at higher pressure than abyssal peridotites. The trace-element chemistry of clinopyroxene, e.g. high LREE/HREE ratios {(Ce/Yb)n?=?7}, high LREE contents (>3.6?ppm and up to 30.0?ppm Ce) and high Sr between >85.6?ppm and 466?ppm, indicates metasomatic alteration of the peridotite. Clinopyroxene in one sample has very low Ti/Eu and high LREE/HREE ratios. Clinopyroxene with (Ce/Yb)n higher than 3-4 and Ti/Eu ratio lower than 1500 may have experienced carbonatite or carbonate-rich melt metasomatism prior to their incorporation into the host basalt. The basalt itself is almost devoid of any carbonatization and hence the studied mantle peridotites were carbonatized before the generation of the basaltic magma but following an earlier event of K-metasomatism as indicated by the presence of phlogopite. The studied peridotites from the Wadi Natash area were altered by a carbonate-rich melt during a rifting stage. The results of the present paper indicate that the Natash basalts with their peridotite xenoliths extruded along transversal fractures of the NW-trending Nuqra-Kom Ombo-Kharit continental rift on its western shoulder in the south Eastern Desert of Egypt.
Nishiyama, T., Ohfuji, H., Fukuba, K., Terauchi, M., Nishi, U., Harada, K., Unoki, K., Moribe, Y., Yoshiasa, A., Ishimaru, S., Mori, Y., Shigeno, M., Arai, S.
Nature Scientific Reports, Vol. 10, 11645 11p. Pdf
Asia, Japan
microdiamond
Abstract: Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote-blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C-O-H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.
Progress in Earth and Planetary Science, doi.org/10.1186 /s40645-019-0299-5
Mantle
geophysics - seismic
Abstract: The depth extent of the crustal seismogenic zone is closely related to the size of earthquakes. The mechanisms that control the depth of the lower transition of the seismogenic zone are important issues in seismology and disaster mitigation. Laboratory studies have shown that the mechanism of earthquake nucleation is controlled by the frictional properties of fault materials. We measured the velocity dependences of the steady-state friction of quartz and feldspar, two major components of crustal rocks, under dry and wet conditions at temperatures up to 600?°C. In the presence of water, the temperature range over which the velocity dependence of steady-state friction was negative was wider for feldspar than for quartz, thus indicating that the temperature range of earthquake nucleation is wider for feldspar than for quartz. Considering that temperature increases with depth, our findings indicate that the material properties of feldspar likely play a dominant role in limiting the depth extent of the seismogenic zone.
Juin a diamonds from kimberlites and alluvials: a omparison of morphology, spectral characteristics and carbon isotope composition.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 255-269.
Abstract: This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380-390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume-lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.
Korikovsky, S., Kotov, A., Salnikova, E., Aranovich, L., Korpechkov, D., Yakovleva, S., Tolmacheva, E., Anisimova, I.
The age of the protolith of metamorphic rocks in the southeastern Lapland granulite belt, southern Kola Peninsula: correlation with the Belomorian mobile belt in the context of the problem of Archean eclogites.
Abstract: The results of experimental studies are presented for reactions in the orthopyroxene-garnet-phlogopite system in the presence of H2O-KCl fluid at 3-5 GPa and 900-1000°C, which model the processes of phlogopite formation in garnet peridotites and pyroxenites during alkaline metasomatism of the upper mantle. The experiments demonstrated regular variations in the composition of garnet, pyroxenes, and phlogopite depending on the KCl content of the fluid. With increasing KCl content of the fluid, enstatite and garnet become unstable, the Al2O3 content of enstatite decreases, and the amount of grossular and knorringite components in garnet are maximum at a KCl content of ~10 mol %. Our results illustrate well the regular variations in the compositions of the coexisting minerals and their zoning in phlogopite-bearing peridotites of the lithospheric mantle.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 245-250.
Technology
Carbonatite
Abstract: We measured 10 major (SiO2, TiO2, Al2O3, total Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) and 32 trace (Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Pb, Th, and U) elements in 16 geochemical reference samples (AGV-1, AGV-2, BCR-1, BCR- 2, BHVO-2, BIR-1a, DNC-1a, G-2, GSP-1, GSP-2, MAG-1, QLO-1, RGM-1, RGM-2, SGR-1b, and STM-1) distributed by United States Geological Survey (USGS) and three reference rock samples (SY-2, SY-3, and MRG-1) provided by Canadian Certifi ed Reference Materials Project (CCRMP) using inductively coupled plasma -mass spectrometry coupled with the femtosecond laser ablation sample introduction technique (fsLA-ICP-MS). Before the elemental analysis, fused glassbeads were prepared from the mixture of sample powder and high-purity alkali fl ux with a mixing ratio of 1:10. The abundances of the major and trace elements were externally calibrated by using glass beads containing the major and trace elements prepared from 17 Geological Survey of Japan (GSJ) geochemical reference samples (JB-1, JB-1a, JB-2, JB-3, JA-1, JA-2, JA-3, JR-1, JR-2, JR-3, JP-1, JGb-1, JGb-2, JG-1a, JG- 2, JG-3, and JSy-1). Typical analysis repeatabilities for these geochemical reference samples were better than 3% for Al2O3 and Na2O; <5% for SiO2, TiO2, total Fe2O3, MnO, MgO, CaO, K2O, P2O5, Zn, Rb, Sr, Zr, Nb, Ba, Nd, and U; <8% for Sc, V, Cr, Co, Y, Cs, La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Pb, and Th; <11% for Ni and Cu. These data clearly demonstrate that high analytical repeatability can be achieved by the fsLA-ICP-MS technique with glass beads made from 0.5 g larger samples.
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.
Abstract: The trace element compositions of inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite, Brazil, are presented. Literature data for mineral/melt partition coefficients were collated, refitted and employed to interpret inclusion compositions. As part of this process an updated empirical model for predicting the partitioning behaviour of trivalent cations for garnet-melt equilibrium calibrated using data from 73 garnet-melt pairs is presented. High levels of trace element enrichment in inclusions interpreted as former calcium silicate perovskite and majoritic garnet preclude their origin as fragments of an ambient deep mantle assemblage. Inclusions believed to represent former bridgmanite minerals also display a modest degree of enrichment relative to mantle phases. The trace element compositions of ‘NAL’ and ‘CF phase’ minerals are also reported. Negative Eu, Ce, and Y/Ho anomalies alongside depletions of Sr, Hf and Zr in many inclusions are suggestive of formation from a low-degree carbonatitic melt of subducted oceanic crust. Observed enrichments in garnet and ‘calcium perovskite’ inclusions limit depths of melting to less than ~ 600 km, prior to calcium perovskite saturation in subducting assemblages. Less enriched inclusions in sub-lithospheric diamonds from other global localities may represent deeper diamond formation. Modelled source rock compositions that are capable of producing melts in equilibrium with Juina-5 ‘calcium perovskite’ and majorite inclusions are consistent with subducted MORB. Global majorite inclusion compositions suggest a common process is responsible for the formation of many superdeep diamonds, irrespective of geographic locality. Global transition zone inclusion compositions are reproduced by fractional crystallisation from a single parent melt, suggesting that they record the crystallisation sequence and melt evolution during this interaction of slab melts with ambient mantle. All observations are consistent with the previous hypothesis that many superdeep diamonds are created as slab-derived carbonatites interact with peridotitic mantle in the transition zone.
Abstract: In this study, 660 diamonds from Igarapé Lajes Diggings (Roosevelt and Aripuanã Park indigenous areas), in Amazonian craton, Rondônia State, Brazil, were investigated. Their morphological, optical and surface characteristics were described using optical and scanning electron microscopy (SEM), cathodoluminescence (CL) and infrared spectroscopy (FTIR). The results demonstrated a predominance of resorbed crystals with many surface corrosion features, generally colorless, and led to the identification of four distinct groups: G1, G2, G3 and G4. Group G1 presents features of secondary sources while G2 and G4 show only primary features, some of which are not described in literature. Group G3 is similar to the other groups, however, is composed of less resorbed specimens with primary octahedral morphology relatively well preserved, indicating shorter time of exposure to dissolution effects. Cathodoluminescence in G2 is attributed to features of plastic deformation and to low contents of nitrogen (< 100 ppm, Type II) and high aggregation (IaB). G4 shows homogeneous blue CL, high contents of nitrogen (700 to 1000 ppm) and intermediate aggregation (IaAB). G1 presents luminescence influenced by radiation effects and populations with N contents and aggregation in the same ranges of G2 and G4, suggesting that the primary sources of the three groups can be the same. The relationship of nitrogen content versus aggregation state indicates higher temperatures of formation for G2 and lower for G4. The obtained data suggests that diamonds of G2 originated in sublithospheric mantle as has also been reported in nearby deposits (Machado River and Juína). The employed techniques were also effective in distinguishing diamonds from Roosevelt Reserve and from other localities, indicating that they could be used for improvement of certification procedures of diamonds of unknown origin.
Abstract: The South American platform is the stable part of the South American plate, unaffected by the orogenesis of the Andes and the Caribbean. Its basement is composed of Archean and Proterozoic cratonic blocks amalgamated by mobile belts, and can be separated in two large domains or continental masses: 1) The Amazonian, Northwest-west portion, including the Amazonian craton, related to the Laurentia supercontinent; and 2) the extra-Amazonian, Central-southeast or Brasiliano domain, related to West Gondwana, formed of several paleocontinental fragments, where the São Francisco and Rio de La Plata cratons and the Paranapanema block are the largest. It has been suggested that these two domains are separated by the Transbrasiliano Lineament to the south and the Araguaia Fold Belt to the north. Teleseismic P waves from 4,989 earthquakes recorded by 339 stations operated mainly in Brazil in the last 25 years have been used for relative-time tomography. The Amazonian domain is predominantly characterized by higher velocities. The SW (extra-Amazonian) domain is characterized by several blocks with high velocities, such as in and around the Sao Francisco Craton, and the Paranapanema block. Results of P-wave travel time tomography allowed to observe a strong low-velocity anomaly near 100-200 km depth following the Araguaia-Paraguay fold belt. This strong low-velocity anomaly could be considered the limit between these two domains, reaching lithospheric depths, and does not necessarily follow the Transbrasiliano lineament, especially in its southern portion.
Geophysical Journal International, Vol. 201, 1, pp. 61-69.
South America, Brazil
Geophysics - seismic
Abstract: Results of P-wave traveltime seismic tomography in central Brazil unravel the upper-mantle velocity structure and its relationship with the tectonic framework. Data were recorded between 2008 and 2012 at 16 stations distributed over the study area, and were added to the database used by Rocha et al. to improve the resolution of anomalies, and to image the surrounding regions. The main objective was to observe the upper-mantle boundary zone between the Amazonian and São Francisco cratons, represented by mobile belts, inside the Tocantins Province, and to study the lithosphere related to the collision between these two cratons during the Neoproterozoic. A set of low-velocity anomalies was observed crossing the study area in the NE–SW direction, in agreement with the main trend of the Transbrasiliano lineament. The region where the anomalies are located was interpreted as the zone separating the Amazonian and São Franciscan palaeoplates. There is a good correlation between the low-velocity anomalies and the high seismicity of this region, suggesting that it is a region of weakness, probably related to lithospheric thinning. High velocities were observed under the Amazonian and São Francisco cratons. A model is proposed for the lithospheric subsurface in central Brazil, emphasizing the boundary zone between the main palaeoplates in the study area. After merging both databases, the low-velocity anomalies in the central part of the study area suggest tectonic partitioning of the lithosphere. Synthetic tests show that the tomography results are robust.
Aravanis, T., Chen, J., Fuechsle, M., Grujic, M., Johnston, P., Kok, Y., Magaraggia, R., Mann, A., Mann, L., McIntoshm S., Rheinberger, G., Saxey, D., Smalley, M., van Kann, F., Walker, G., Winterflood, J.
Abstract: The minerals exploration industry’s demand for a highly precise airborne gravity gradiometer has driven development of the VK1TM Airborne Gravity Gradiometer, a collaborative effort by Rio Tinto and the University of Western Australia. VK1TM aims to provide gravity gradient data with lower uncertainty and higher spatial resolution than current commercial systems. In the recent years of VK1TM development, there have been significant improvements in hardware, signal processing and data processing which have combined to result in a complete AGG system that is approaching competitive survey-ready status. This paper focuses on recent improvements. Milestone-achieving data from recent lab-based and moving-platform trials will be presented and discussed, along with details of some advanced data processing techniques that are required to make the most use of the data.
Geochemistry, Geophysics, Geosystems: G3, Vol. 16, 11, Nov. pp. 3924-3945.
Mantle
Convection
Abstract: Numerical models show that small-scale convection (SSC) occurring atop a mantle plume is a plausible mechanism to rejuvenate the lithosphere. The triggering of SSC depends on the density contrast and on the rheology of the unstable layer underlying the stagnant upper part of the thermal boundary layer (TBL). Partial melting may change both properties. We analyze, using 2-D numerical simulations, how partial melting influences the dynamics of time-dependent SSC instabilities and the resulting thermo-mechanical rejuvenation of an oceanic plate moving atop of a plume. Our simulations show a complex behavior, with acceleration, no change, or delay of the SSC onset, due to competing effects of the latent heat of partial melting, which cools the plume material, and of the buoyancy increase associated with both melt retention and depletion of residue following melt extraction. The melt-induced viscosity reduction is too localized to affect significantly SSC dynamics. Faster SSC triggering is promoted for low melting degrees (low plume temperature anomalies, thick lithosphere, or fast moving plates), which limit both the temperature reduction due to latent heat of melting and the accumulation of depleted buoyant residue in the upper part of the unstable layer. In contrast, high partial melting degrees lead to a strong temperate decrease due to latent heat of melting and development of a thick depleted layer within the sublithospheric convecting layer, which delay the development of gravitational instabilities. Despite differences in SSC dynamics, the thinning of the lithosphere is not significantly enhanced relatively to simulations that neglect partial melting.
Physics of the Earth and Planetary Interiors, Vol. 269, 1, pp. 112-132.
Mantle
subduction
Abstract: The present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate. A 2D thermo-mechanical model is used to simulate a subduction channel, made of oceanic crust, free to evolve. Convergence at constant rate is imposed under a 100 km thick upper plate. Pseudo-brittle and non-Newtonian behaviours are modelled. The influence of the subduction channel strength, parameterized by the difference in activation energy between crust and mantle (?Ea) is investigated to examine in detail the variations in depth of the subduction plane down-dip extent, zcoup . First, simulations show that numerical resolution may be responsible for an artificial and significant shallowing of zcoup if the weak crustal layer is not correctly resolved. Second, if the age of the subducting plate is 100 Myr, subduction occurs for any ?Ea . The stiffer the crust is, that is, the lower ?Ea is, the shallower zcoup is (60 km depth if ?Ea = 20 kJ/mol) and the hotter the fore-arc base is. Conversely, imposing a very weak subduction channel (?Ea > 135 J/mol) leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions. Partial kinematic coupling at the fore-arc base occurs if ?Ea = 145 kJ/mol. If the incoming plate is 20 Myr old, subduction can occur under the conditions that the crust is either stiff and denser than the mantle, or weak and buoyant. In the latter condition, cold crust plumes rise from the subduction channel and ascend through the upper lithosphere, triggering (1) partial kinematic coupling under the fore-arc, (2) fore-arc lithosphere cooling, and (3) partial or complete hindrance of wet mantle melting. zcoup then ranges from 50 to more than 250 km depth and is time-dependent if crust plumes form. Finally, subduction plane dynamics is intimately linked to the regime of subduction-induced corner flow. Two different intervals of ?Ea are underlined: 80-120 kJ/mol to reproduce the range of slab surface temperature inferred from geothermometry, and 10-40 kJ/mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near-primary arc magmas and seismic tomographies. Therefore, an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations. A mantle viscosity reduction, by a factor 4-20, caused by metasomatism in the mantle wedge is proposed. From these results, I conclude that the subduction channel down-dip extent, zcoup , should depend on the subduction setting, to be consistent with the observed variability of sub-arc depths of the subducting plate surface.
Physics of the Earth and Planetary Interiors, Vol. 269, 1, pp. 112-132.
Mantle
subduction
Abstract: The present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate. A 2D thermo-mechanical model is used to simulate a subduction channel, made of oceanic crust, free to evolve. Convergence at constant rate is imposed under a 100 km thick upper plate. Pseudo-brittle and non-Newtonian behaviours are modelled. The influence of the subduction channel strength, parameterized by the difference in activation energy between crust and mantle () is investigated to examine in detail the variations in depth of the subduction plane down-dip extent, . First, simulations show that numerical resolution may be responsible for an artificial and significant shallowing of if the weak crustal layer is not correctly resolved. Second, if the age of the subducting plate is 100 Myr, subduction occurs for any . The stiffer the crust is, that is, the lower is, the shallower is (60 km depth if kJ/mol) and the hotter the fore-arc base is. Conversely, imposing a very weak subduction channel ( J/mol) leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions. Partial kinematic coupling at the fore-arc base occurs if kJ/mol. If the incoming plate is 20 Myr old, subduction can occur under the conditions that the crust is either stiff and denser than the mantle, or weak and buoyant. In the latter condition, cold crust plumes rise from the subduction channel and ascend through the upper lithosphere, triggering (1) partial kinematic coupling under the fore-arc, (2) fore-arc lithosphere cooling, and (3) partial or complete hindrance of wet mantle melting. then ranges from 50 to more than 250 km depth and is time-dependent if crust plumes form. Finally, subduction plane dynamics is intimately linked to the regime of subduction-induced corner flow. Two different intervals of are underlined: 80-120 kJ/mol to reproduce the range of slab surface temperature inferred from geothermometry, and 10-40 kJ/mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near-primary arc magmas and seismic tomographies. Therefore, an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations. A mantle viscosity reduction, by a factor 4-20, caused by metasomatism in the mantle wedge is proposed. From these results, I conclude that the subduction channel down-dip extent, , should depend on the subduction setting, to be consistent with the observed variability of sub-arc depths of the subducting plate surface.
Abstract: The presence of melt during deformation produces a drastic change in the rheological behavior of the continental crust; rock strength is decreased even for melt fractions as low as ?7%. At pressure/temperature conditions typical of the middle to lower crust, melt-bearing systems may play a critical role in the process of strain localization and in the overall strength of the continental lithosphere. In this contribution we focus on the role and dynamics of melt flow in two different mid-crustal settings formed during the Brasiliano orogeny: (i) a large-scale anatectic layer in an orthogonal collision belt, represented by the Carlos Chagas anatexite in southeastern Brazil, and (ii) a strike-slip setting, in which the Espinho Branco anatexite in the Patos shear zone (northeast Brazil) serves as an analogue. Both settings, located in eastern Brazil, are part of the Neoproterozoic tectonics that resulted in widespread partial melting, shear zone development and the exhumation of middle to lower crustal layers. These layers consist of compositionally heterogeneous anatexites, with variable former melt fractions and leucosome structures. The leucosomes usually form thick interconnected networks of magma that reflect a high melt content (>30%) during deformation. From a comparison of previous work based on detailed petrostructural and AMS studies of the anatexites exposed in these areas, we discuss the rheological implications caused by the accumulation of a large volume of melt “trapped” in mid-crustal levels, and by the efficient melt extraction along steep shear zones. Our analyses suggest that rocks undergoing partial melting along shear settings exhibit layers with contrasting competence, implying successive periods of weakening and strengthening. In contrast, regions where a large amount of magma accumulates lack clear evidence of competence contrast between layers, indicating that they experienced only one major stage of dramatic strength drop. This comparative analysis also suggests that the middle part of both belts contained large volumes of migmatites, attesting that the orogenic root was partially molten and encompassed more than 30% of granitic melt at the time of deformation.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Eocene shoshonitic mafic dykes intruding the Monashee Complex, British Columbia: a petrogenetic relationship with the Kam loops Group volcanic sequence.
Canadian Journal of Earth Sciences, Vol. 42, 1, pp. 11-24.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Abstract: Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42? in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/?Fe in spinel record a S6+Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and ?34S (+?7 to +?11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.
Melilite-type and melilite related compounds: structural variations along the join Sr2a, Bax, MgS2iO7 and high pressure behaviour of the two end members.
Physics and Chemistry of Minerals, Vol. 39, 3, pp.199-211.
Abstract: This study gives an analysis of the effect of high temperature annealing on the infrared and photoluminescence (PL) features as well as the inclusions of two hydrogen-rich diamond plates from Zimbabwe that were cut from the same rough. The samples showed strong inclusion-related zoning known as hydrogen clouds which consist of micron-sized particles of as yet undetermined structure. This allowed hydrogen-rich and hydrogen-poor areas to be compared throughout the annealing study. The diamond plates were annealed to temperatures of 300oC, 600oC, 800oC, 1000oC, 1400oC, and 1700oC. The infrared and PL, and Raman maps were collected after every temperature step to study the effects of heat on the defects, and photomicrographs were collected to study the inclusions. Several photoluminescence features were seen to decrease in size including the 637 nm peak, which is the negatively charged nitrogen-vacancy center [NV-] and the 503.2 nm peak, known as the H3 and consists of two nitrogen atoms and vacancy in the neutral charge state and normally has a high thermal stability. The H2 defect at 986.2 nm, which is the negative form of the H3, was shown to increase after annealing. The hydrogen clouds underwent dramatic changes in apparent color and particle size, going from a light translucent gray appearance to an opaque black. The particle size grew from less than one micron to an average of fourteen microns, and the hexagonal outline of the particles became noticeable. Spatial raman spectroscopy was used to show that the color change and size change were due to graphitization of the included particles.
Abstract: Blue diamonds are among the rarest and most valuable of naturally occurring gemstones. In this study, 12 rough naturally-sourced type IIb diamonds were subjected to HPHT annealing, three different irradiation energies, and then all were stepwise annealed from 200 °C to 1100 °C and the optical defects were documented by changes in phosphorescence and photoluminescence spectroscopy. Several optical features that are removed from natural type IIb diamonds by HPHT processing, such as 3H, 648.2 nm peak, 776.4 nm peak, and 660 nm band (red) phosphorescence, can be reintroduced into these diamonds with subsequent electron irradiation and annealing at low-to-moderate temperatures. The thermal stability of these centers along with their spatial distribution provided additional insights into their configuration and distinguished them from nitrogen-bearing diamonds.
Abstract: This study gives an analysis of the effect of high temperature annealing on the infrared and photoluminescence (PL) features as well as the inclusions of two hydrogen-rich diamond plates from Zimbabwe that were cut from the same rough. The samples showed strong inclusion-related zoning known as hydrogen clouds which consist of micron-sized particles of as yet undetermined structure. This allowed hydrogen-rich and hydrogen-poor areas to be compared throughout the annealing study. The diamond plates were annealed to temperatures of 300oC, 600oC, 800oC, 1000oC, 1400oC, and 1700oC. The infrared and PL, and Raman maps were collected after every temperature step to study the effects of heat on the defects, and photomicrographs were collected to study the inclusions. Several photoluminescence features were seen to decrease in size including the 637 nm peak, which is the negatively charged nitrogen-vacancy center [NV-] and the 503.2 nm peak, known as the H3 and consists of two nitrogen atoms and vacancy in the neutral charge state and normally has a high thermal stability. The H2 defect at 986.2 nm, which is the negative form of the H3, was shown to increase after annealing. The hydrogen clouds underwent dramatic changes in apparent color and particle size, going from a light translucent gray appearance to an opaque black. The particle size grew from less than one micron to an average of fourteen microns, and the hexagonal outline of the particles became noticeable. Spatial raman spectroscopy was used to show that the color change and size change were due to graphitization of the included particles.
Diamond and Related Materials, Vol. 71, pp. 20-29,
Africa, Zimbabwe
Deposit - Marange
Abstract: This study gives an analysis of the effect of low-pressure, high-temperature annealing on the infrared, Raman, and photoluminescence (PL) features, as well as the inclusion characteristics, of cubo-octahedral diamond plates from the Marange deposits in Zimbabwe. The samples showed strong inclusion-related zoning which consists of micron-sized particles identified as graphite and these grew noticeably larger with annealing at temperatures of 300 °C to 1700 °C. Within the natural diamonds, the graphite inclusions (detected by Raman spectroscopy) had a grain size of approximately 1 ?m, which increased to 3 ?m after 1200 °C and 14 ?m after 1700 °C annealing and their hexagonal morphology was discernible. From the geometry of these grains, we determined that they were oriented within the {111} family of planes. The infrared absorption and PL spatial maps were collected after every temperature step to study the effects of annealing on the defects, and photomicrographs and Raman spectra were collected to study the graphite inclusions. The graphitic inclusions grew much larger as the stressed diamond surrounding them converted to graphite. Many nitrogen-related optical centers, including NV? and H3 are no longer detected after high temperature annealing within the cuboid regions as these may have been transformed to hydrogen-bearing complexes such as NVH and N2VH. The presence of CH4 is detected in the unannealed Marange diamonds, but was no longer observed in Raman spectra after 1200 °C annealing. This CH4 disappearance along with changes in inclusion morphology could provide a method to detect heat treatment if these mixed-habit samples are sourced to create treated black gem diamond.
Diamond and Related Materials, Vol. 77, pp. 159-170.
Technology
diamond morphology
Abstract: Low-pressure, high-temperature (LPHT) annealing of yellow-to-brown type Ia natural diamonds was performed to monitor its effects on optical centers within diamond, changes in the observed color, and to assess the process's viability as a commercial gem treatment. With LPHT annealing only, the mostly brown diamonds showed a shift towards yellow coloration; Vis-NIR absorption spectra showed this change was due to a modest increase in H3 intensity. Even at long annealing times (24 h at 1800 °C) or annealing at high temperatures (2000 °C for five minutes), the diamonds did not significantly lose brown coloration. LPHT annealing showed itself as an ineffective means to break apart the vacancy clusters causing the brown color or causing nitrogen disaggregation, which resulted in only a small H3 generation. With LPHT annealing, “amber centers”—a group of several independent bands in the IR between 4200 and 4000 cm? 1 that disappear with HPHT annealing—were seen to anneal out gradually at various temperatures from 1700 to 2000 °C. In contrast, high-pressure, high-temperature (HPHT) annealing effectively removes brown color at similar time/temperature conditions. Without the high stabilizing pressure provided by HPHT annealing techniques, the LPHT annealing showed pronounced damage on inclusions and dramatic surface etching. In subsequent experiments, LPHT annealing was used as a follow-up to laboratory irradiation. The irradiation-related vacancies created greater concentrations of H3 and the vacancy-assisted disaggregation of nitrogen created donors which led to a high concentration of H2 centers. This combination of defects resulted in a pronounced and favorable shift towards saleable yellow colors due to an increase in H3 and a dramatic increase in the H2 center, which led to the suppression of the remaining brownish component. The annealing characteristics for many centers detected by Vis-NIR absorption spectroscopy, FTIR absorption spectroscopy, and photoluminescence spectroscopy were chronicled throughout the study and compared with other LPHT annealing studies and HPHT annealing experiments.
Abstract: Diamond is one of Earth’s most extraordinary materials. It represents the pinnacle for several material and physical properties. As a gem, however, it is the near-perfect examples—diamonds attaining the D-Flawless distinction—and those with imperfections resulting in a vibrant or surprising color that create the most enduring impressions. Fancy-color natural diamonds are among the most highly valued gemstones due to their attractiveness and great rarity. The 18.96 ct Winston Pink Legacy, with a color grade of Fancy Vivid pink, recently made history by selling at over $50 million, its $2.6 million per carat price an all-time high for a pink diamond (Christie’s, 2018).
Abstract: Natural Fancy white and Fancy black diamonds are not routinely submitted to GIA for grading (fewer than 2,000 since 2008). These fancy-color diamonds are distinctive since the causes of color generally are not atomic-scale defects, but nanometer- to micrometer-sized inclusions that reduce the diamond’s transparency by scattering or absorbing light (some exceptions exist among Fancy black diamonds). To clarify, Fancy white diamonds are those rare stones colored by inclusions that give a “whitish” appearance, and are distinct from “colorless” diamonds on the D-to-Z scale. These two colors, often thought of as opposites in the color world, are grouped here as outliers within the colored diamond world. Both can be colored by inclusions so numerous the stone would fall below the I3 grade on the clarity scale, demonstrating that inclusions, often perceived as a negative quality factor, can create a distinctive appearance. Among the Fancy white diamonds examined for this study, the vast majority (82%) were type IaB, making them a rare subset of a rare diamond type. Based on prior geological research, these are surmised to be mostly sublithospheric in origin (i.e., forming more than 250 km below the earth’s surface). The Fancy white diamonds generally have a different chemistry from D-to-Z type IaB diamonds, with greater quantities of several hydrogen- and nickel-related defects. Among Fancy black diamonds, the major causes of color are either micrometer-sized dark crystal inclusions, nanometer-sized inclusions clustered into clouds, or a combination of the two. For these two colors of diamond, we summarize their gemological properties along with the absorption and luminescence spectra of a representative subset of diamonds from each color, examining how they deviate from the standard grading methodology. Because of their rarity, there has been very little systematic study of either of these color categories, and never a sample set of this quantity, which includes data for ~500 Fancy white and ~1,200 Fancy black diamonds.
Abstract: The majority of natural pink diamonds have a color origin due to absorption from a broad 550?nm band that has been associated with plastic deformation. One consistent feature in the photoluminescence spectra of these pink diamonds is a wide emission band extending from ~600 to 750?nm, with a series of smaller oscillations overlaid on the larger emission band. This "pink emission band" is seen in diamonds colored by the 550?nm absorption band; the absorption band often, but not always, shows similar oscillations at ~600?nm (called the 609?nm system by previous researchers). This emission band served as a proxy for the 550?nm absorption band as we performed spatial mapping to chronicle the differences between the uniform coloration in type IIa pink diamonds and the pronounced banding in type Ia pink diamonds. We also used Raman spectroscopy to identify the internal crystal inclusions present in type IIa pink diamonds and determined that the majority have a sub-lithospheric origin.
Abstract:
Colorless to light yellow or brown diamonds with a “D-to-Z” color grade make up the overwhelming majority of the world’s gem diamond trade. Besides clarity features (such as inclusions) and fluorescence observations, however, comparatively little has been explored and published regarding the distinguishing characteristics of these diamonds. The vast majority are type Ia, with infrared spectra showing very high concentrations of nitrogen aggregates. This population of diamonds could not have been subjected to HPHT decolorizing treatment or been laboratory grown, and thus they have been spectroscopically scrutinized in much less detail than the far more rare natural diamonds of types IIa, IIb, and IaB, which need to be investigated as potentially color-treated or synthetic. This study examines a large sample set comprising the full complement of D-to-Z diamonds submitted to GIA laboratories during a significant portion of 2017. The data were evaluated on the basis of diamond type properties, as well as distribution among various grading quality factors, to provide an unprecedented glimpse into the role of these diamond types and differences in their geologic conditions of formation.
GIAcommunications @gia.edu, gia.org and knowledge sessions
Global
diamond genesis
Abstract: G&G’s most recent issue captured the past, present and future of the gem industry - with an overview of European royal jewelry sales (including the sale of Marie Antoinette’s jewelry), in-depth coverage of D-Z diamond knowledge (such as causes of color and formation) and a journey into Vietnamese pearl farming. Tune in as G&G contributors Troy Ardon and Nicole Ahline touch upon these and other highlights from the most recent publication of GIA’s prestigious scientific journal.
Abstract: Did you know that certain diamonds can temporarily change color when exposed to heat, ultraviolet light, or even when kept in the dark? Some natural greenish diamonds are known as “chameleon” diamonds due to this property. Other natural pink diamonds and some color-treated and laboratory-grown diamonds can also change color in unexpected ways. Before this phenomenon was known, there were stories of customers returning diamonds they purchased because the diamonds turned out to be the “wrong” color! What exactly causes these interesting diamonds to shift their hues? Find out as GIA senior manager of diamond research Dr. Ulrika D'Haenens-Johansson and senior research scientist Dr. Mike Breeding dive into the mystery of these ultra-cool gems.
Abstract: The phase relations in the system Na2CO3?CaCO3?MgCO3 have been studied at 3?GPa and 700-1285?°C using a Kawai-type multianvil press. At 700?°C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800?°C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850?°C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875?±?25?°C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925?±?25 °C. Mg-bearing shortite melts incongruently at 950?±?50?°C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000?°C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850?°C and 52Na2CO3?48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3?GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 () eitelite (0.1?GPa)???Na2(Ca0.97-0.98Mg0.02-0.03)4(CO3)5 (P63mc), Na2(Ca?0.91Mg?0.09)3(CO3)4 (P1n1), Na2(Ca???0.81?Mg0?0.19)(CO3)2 () nyerereite, Na2(Ca0.77-0.93Mg0.07-0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90-0.98Mg0.02-0.10)(CO3)3 (Ia3d), Na2(Mg?0.9Ca0?0.1)(CO3)2 (P21ca) eitelite (3?GPa)???Na2(Ca?0.87Mg0?0.13)4(CO3)5 (P63mc), Na2(Ca?0.89Mg?0.11)3(CO3)4 (P1n1), Na4(Ca???0.7?Mg0?0.3)(CO3)3 (Ia3d), Na2(Mg?0.92Ca0?0.08)(CO3)2 (P21ca) eitelite (6?GPa). Using the present results at 3?GPa and previous data at 6?GPa in the Na2CO3?CaCO3?MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200?km toward sodic dolomitic with the ~52?mol% Na2CO3 at 80-120?km depths.
Abstract: The phase relations in the system Na2CO3?CaCO3?MgCO3 have been studied at 3?GPa and 700-1285?°C using a Kawai-type multianvil press. At 700?°C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800?°C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850?°C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875?±?25?°C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925?±?25 °C. Mg-bearing shortite melts incongruently at 950?±?50?°C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000?°C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850?°C and 52Na2CO3?48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3?GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 () eitelite (0.1?GPa)???Na2(Ca0.97-0.98Mg0.02-0.03)4(CO3)5 (P63mc), Na2(Ca?0.91Mg?0.09)3(CO3)4 (P1n1), Na2(Ca???0.81?Mg0?0.19)(CO3)2 () nyerereite, Na2(Ca0.77-0.93Mg0.07-0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90-0.98Mg0.02-0.10)(CO3)3 (Ia3d), Na2(Mg?0.9Ca0?0.1)(CO3)2 (P21ca) eitelite (3?GPa)???Na2(Ca?0.87Mg0?0.13)4(CO3)5 (P63mc), Na2(Ca?0.89Mg?0.11)3(CO3)4 (P1n1), Na4(Ca???0.7?Mg0?0.3)(CO3)3 (Ia3d), Na2(Mg?0.92Ca0?0.08)(CO3)2 (P21ca) eitelite (6?GPa). Using the present results at 3?GPa and previous data at 6?GPa in the Na2CO3?CaCO3?MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200?km toward sodic dolomitic with the ~52?mol% Na2CO3 at 80-120?km depths.
Abstract: Subsolidus and melting phase relationships in the system Na2CO3-CaCO3-MgCO3 have been studied at 6?GPa and 900-1250?°C using a Kawai-type multianvil press. At 900 and 1000?°C, the system has four intermediate compounds: Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2Mg(CO3)2 eitelite. The Na-Ca compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percents of Ca component: Na2(Ca?0.91Mg?0.09)4(CO3)5, Na2(Ca?0.94Mg?0.06)3(CO3)4, Na4(Ca?0.67Mg?0.33)(CO3)3, and Na2(Mg?.93Ca?0.07)(CO3)2. At 1050?°C, the system is complicated by an appearance of dolomite. Na-Ca burbankite decomposes at 1075?±?25?°C to aragonite plus Na2Ca3(CO3)4. Na4Ca(CO3)3 and eitelite disappear via congruent melting between 1200 and 1250?°C. Na2Ca3(CO3)4 remains stable through the whole studied temperature range. The liquidus projection of the studied ternary system has eight primary solidification phase regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2CO3 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus at 1050?°C and 48Na2CO3•52(Ca0.75Mg0.25)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which a liquid transforms into three solid phases upon cooling: Na2(Ca0.94Mg0.06)3(CO3)4, Na4(Ca0.67Mg0.33)(CO3)3, and Na2(Mg0.93Ca0.07)(CO3)2 eitelite. Since at 6?GPa, the system has a single eutectic, there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100?°C within the lherzolite substrate will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40 and shifting the Na2CO3 content to ?40?mol%. In the case of the eclogitic wall rock, the cooling will be accompanied by dolomite crystallization keeping calcium number of the melt at 60-65 and shifting the Na2CO3 content to ?30?mol%.
Abstract: Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6?GPa and 900-1500?°C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3•60Ca0.55Mg0.24Fe0.21CO3?+?1.9?mol% SiO2?+?0.7?mol% TiO2?+?1.4?mol% Al2O3 at the solidus established near 1000?°C. Molecular CO2 liberates at 1100?°C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200?°C. This melt contains (mol/wt%): SiO2?=?57.0/52.4, TiO2?=?1.8/2.3, Al2O3?=?8.5/13.0, FeO?=?1.4/1.6, MgO?=?1.9/1.2, CaO?=?3.8/3.2, Na2O?=?3.2/3.0, K2O?=?10.5/15.2, CO2?=?12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3•76Ca0.59Mg0.21Fe0.20CO3?+?3.0?mol% SiO2?+?1.6?mol% TiO2?+?1.4?mol% Al2O3. Both melts remain stable to at least 1500?°C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4-1.5?wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali?carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180-200?km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.
Abstract: Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6?GPa and 900-1500?°C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3•60Ca0.55Mg0.24Fe0.21CO3?+?1.9?mol% SiO2?+?0.7?mol% TiO2?+?1.4?mol% Al2O3 at the solidus established near 1000?°C. Molecular CO2 liberates at 1100?°C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200?°C. This melt contains (mol/wt%): SiO2?=?57.0/52.4, TiO2?=?1.8/2.3, Al2O3?=?8.5/13.0, FeO?=?1.4/1.6, MgO?=?1.9/1.2, CaO?=?3.8/3.2, Na2O?=?3.2/3.0, K2O?=?10.5/15.2, CO2?=?12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3•76Ca0.59Mg0.21Fe0.20CO3?+?3.0?mol% SiO2?+?1.6?mol% TiO2?+?1.4?mol% Al2O3. Both melts remain stable to at least 1500?°C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4-1.5?wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali?carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180-200?km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.
Abstract: To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000-1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3-0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3•6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°?, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900-1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.
Abstract: The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed. We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K?carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2-0.3 wt%), and higher K2O (0.5-1.0 wt%), whereas the Na?carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3?60Ca0.5Mg0.4Fe0.1CO2 + 0.6-1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100?Ca/(Ca + Mg + Fe). We have also found that alkali-poor (? 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures. Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100-1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3?70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40-45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ? 30-40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM. Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30-40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40-60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.
Abstract: Compositional ranges of carbonate melts stable under P-T conditions corresponding to the base of subcontinental lithospheric mantle (SCLM) are limited by alkali-rich near-eutectic compositions. In the present work, we investigated the interaction of such melts with the natural eclogite of Group A. It was found that the interaction is accompanied by decreasing Ca# in the melt (L) and increasing Ca# in garnet (Grt) according to the reaction: 3CaCO3 (L) + Mg3Al2Si3O12 (Grt) = 3MgCO3 (Mgs and/or L) + Ca3Al2Si3O12 (Grt), where Mgs is magnesite. The interaction with the Na-Ca-Mg-Fe carbonate melt increases amount of jadeite component in clinopyroxene (Cpx) consuming Al2O3 from garnet and Na2O from the melt according to the reaction: Na2CO3 (L) + CaCO3 (L) + 2Mg3Al2Si3O12 (Grt) + 2CaMgSi2O6 (Cpx) = 2NaAlSi2O6 (Cpx) + Ca3Al2Si3O12 (Grt) + 2MgCO3 (Mgs, L) + 3Mg2SiO4 (Ol). As a result, garnet and omphacite compositions evolve from eclogite Group A to eclogite Group B. A byproduct of the reaction is olivine (Ol), which may explain the formation of inclusions of “mixed” eclogite (garnet + omphacite) and peridotite (olivine) paragenesis in lithospheric diamonds. The interaction with the K-Ca-Mg-Fe carbonate melt increases the K2O content in clinopyroxene to 0.5-1.2 wt%, while the Na2O content lowers to 0.3 wt%. The following range of carbonatite melt compositions can be in equilibrium with eclogite at the base of SCLM (1100-1200 °C and 6 GPa): 18(Na0.97K0.03)2CO3?82(Ca0.63Mg0.30Fe0.07)CO2-42(Na0.97K0.03)2CO3?58(Ca0.46Mg0.45Fe0.09)CO2. Our results also suggest that the partial melting of ‘dry’ carbonated eclogite, if any, at 1100 °C and 6 GPa yields the formation of a carbonate melt with the following composition (mol%) 25(Na0.96K0.04)2CO3?75(Ca0.64Mg0.31Fe0.05)CO2, corresponding to 18-27 wt% Na2O in the melt on a volatile-free basis.
Abstract: To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000-1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3-0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3•6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°?, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900-1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.
Contributions to Mineralogy and Petrology, Vol. 176, 34 21p. Pdf
Mantle
carbonatites
Abstract: The reactions between pyroxenes and carbonates have been studied in the CaMgSi2O6 + MgCO3 (Di + 2Mgs), CaMgSi2O6 + NaAlSi2O6 + 2MgCO3 (Di + Jd + 2Mgs), CaMgSi2O6 + Na2Mg(CO3)2 (Di + Eit), and CaMgSi2O6 + K2Mg(CO3)2 (Di + K2Mg) systems at pressures of 3.0 and 4.5 GPa in the temperature range 850-1300 °C and compared with those established previously at 6.0 GPa. The Di + 2Mgs solidus locates at 1220 °C / 3 GPa and 1400 °C / 6 GPa. Near-solidus melt is carbonatitic with SiO2 < 4 wt% and Ca# 56. The Di + Jd + 2Mgs solidus locates near 1050 °C at 3 GPa, rises to 1200 °C at 4.5 GPa, and 1350 °C at 6 GPa. The solidus is controlled by the reaction: 4NaAlSi2O6.2CaMgSi2O6 (clinopyroxene) + 12MgCO3 (magnesite) = 2MgAl2SiO6.5Mg2Si2O6 (clinopyroxene) + 2[Na2CO3.CaCO3.MgCO3] (liquid) + 6CO2. As pressure increases, the composition of solidus melt evolves from 26Na2CO3?74Ca0.58Mg0.42CO3 at 3 GPa to 10Na2CO3?90Ca0.50Mg0.50CO3 at 6 GPa. Melting in the Di + Eit and Di + K2Mg systems is controlled by the reactions: CaMgSi2O6 (clinopyroxene) + 2(Na or K)2 Mg(CO3)2 (eitelite) = Mg2Si2O6 (orthopyroxene) + 2[(Na or K)2CO3?Ca0.5Mg0.5CO3] (liquid). The Di + Eit solidus locates at 925 °C / 3 GPa and 1100 °C / 6 GPa, whereas the Di + K2Mg solidus is located at 50 °C lower. The resulting melts have alkali-rich carbonate compositions, (Na or K)2CO3?Ca0.4Mg0.6CO3. The obtained results suggest that most carbonates belong to the ultramafic suite would survive during subduction into the deep mantle and experience partial melting involving alkaline carbonates, eitelite or K2Mg(CO3)2, under geothermal conditions of the subcontinental lithospheric mantle (35-40 mW/m2). On the other hand, the jadeite component in clinopyroxene would be an important fluxing agent responsible for the partial melting of carbonated rocks under the rift margin geotherm (60 mW/m2) at a depth of about 100 km, yielding the formation of Na-carbonatite melt.
Abstract: It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0-6.5 GPa, corresponding to depths of 100-200 km. We found that at ? 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation. The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0-6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite. Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.
Abstract: The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3-6 GPa and 850-1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B. The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120-170 km, and solidify as temperature decreases below 950-1050 °C according to the following solidus reactions: pyrope + diopside + melt ? phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt ? phlogopite + magnesite + grossular, at 6 GPa. The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent-continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7-5.2 GPa and 970-990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.
Earth and Planetary Science Letters, Vol. 581, 11p.
Mantle
metasomatism
Abstract: It is generally accepted that carbonatite metasomatism in the subcontinental lithospheric mantle (SCLM) inevitably causes wehrlitization of the primary lherzolite substrate. However, the K-rich carbonatite inclusions in kimberlitic diamonds containing orthopyroxene indicate that this is not always the case. In the present study, we equilibrated natural garnet lherzolite with carbonate melts containing 33-38 wt% K2O with various Ca# = 10, 20, 30, and 40 at 6 GPa and 1200-1500 °C, where Ca# = 100?Ca/(Ca+Mg+Fe). The original ratio of peridotite to carbonate was 58 to 42 by weight. In the studied temperature range, the melt retains essentially carbonate composition with silica content increasing from 1 to 11-12 wt%. The melt with Ca# 10 alters lherzolite to harzburgite, replacing clinopyroxene by orthopyroxene and decreasing CaO content in garnet below 4 wt%. The melts with Ca# 20-30 also consume clinopyroxene; although CaO content in garnet remains in the range of lherzolitic compositions. The melt with Ca# 40 yields wehrlitization, consuming orthopyroxene, increasing clinopyroxene fraction, and increasing CaO content in garnet above 6 wt%. After the interaction, the Ca# of the melt changes as follows 10 ? 16-28, 20 ? 20-33, 30 ? 27-34, and 40 ? 30-34. The olivine + orthopyroxene + clinopyroxene + garnet assemblage was found in equilibrium with carbonatite melt with Ca# 34 at 1200 °C and Ca# 30 at 1400 °C. Thus, K-rich (26-35 wt% K2O) carbonatite melts with Ca# = 30-34 can appear in equilibrium with garnet lherzolite, while the melts with Ca# < 30 and > 34 can be in equilibrium with harzburgite and wehrlite, respectively, at 6 GPa and 1200-1400 °C. Considering that Ca-Mg-Fe carbonates do not melt at the geothermal conditions of the SCLM, while sodic, dolomitic melt causes wehrlitization, high-Mg (Ca# < 35) K-rich dolomitic melt is the only possible carbonatite fluids that are thermodynamically stable in equilibrium with garnet harzburgites and lherzolites in the SCLM at a depth of about 200 km. At higher temperatures corresponding to the underlying asthenosphere, the high alkalinity ceases to be a requirement for the stability of the carbonate melt. Nevertheless, the regularities established here for the K-rich melts remain valid for less alkaline (4-15 wt% Na2O+K2O) primary kimberlite (i.e., mantle carbonatite) melts in the sublithospheric mantle.
New gemstone treatments - a coming crisis. Overview of treatments - more specific to range of all gemstones (not diamonds in particular). Scientific explanations
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Russia, Siberia
craton
Abstract: The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.
Mertanen, S., Vuollo, J.I., Huhma, H., Arestova, N.A., Kovalenko, A.
Early Paleoproterozoic Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield - new paleomagnetic, isotope age, geochemical investigations.
Precambrian Research, Vol. 144, 3-4, Feb. 10, pp. 239-260.
Abstract: We estimate the mantle source compositions for mantle plumes, and by implication Earth’s lower mantle, by: (a) measuring trace (e.g, Sc, V, Cu) and minor (e.g., Ca, Mn, Ni) element concentrations of high forsterite olivine grains from several plume localities, (b) estimating the parent liquid compositions from which they crystallized, (c) calculating mantle potential temperatures and degrees of partial melting and (d) estimating trace element compositions of depleted and enriched mantle sources. Our sample set includes two continental flood basalt provinces (Emeishan and Deccan), a flood basalt that erupted in a continental rift setting (Baffin Island), our type example of a thermal mantle plume (Hawaii) and lavas from the Siqueiros Transform at the East Pacific Rise, which represent the mid-ocean ridge system. We also present olivine compositions for the peridotite xenoliths from Kilbourne Hole, New Mexico, USA, which are commonly used as primary and secondary analytical standards. We find that trace elements in lava-hosted olivine grains are too far removed from their mantle source to provided anything but greatly hindered views of such. Olivine compositions reflect not only evolving liquid compositions (including partial melting conditions and later fractionation), but also evolving Ol+liq partition coefficients, which mostly increase with decreasing T during crystallization. Mantle compositions, delimited by maximum forsterite contents and estimates of parental magmas (and experimentally determined partition coefficients) indicate that our selected plumes reflect some combination of (1) a depleted mantle source that is quite similar to that obtained by other methods, and (2) a variably enriched plume source that is more enriched than current estimates of pyrolite. The enriched plume mantle sources can be explained remarkably well as a mixture of subducted mid-ocean ridge basalt (MORB; Gale et al. 2013) and depleted MORB mantle (DM; Salters and Stracke 2004), with MORB:DM ratios of 1:5 to 1:4. These ratios are most sensitive to estimates of melt fraction where plume parental magmas are last equilibrated with their mantle source, but are nonetheless consistent across a wide range of chemically very different elements, and estimates of MORB and DM obtained by very different means. Baffin Island is of particular interest. Like prior studies, we verify a high mantle potential temperature (Tp) of 1630oC (compared to Tp = 1320-1420oC for MORB from Cottrell and Kelley 2011 for Ol of Fo89.3-91.4). The Baffin source is also within error the same as DM with respect to trace elements, although still isotopically distinct; Baffin appears to be sourced in something that is akin to DM that lies at the base of the mantle, where plumes acquire their excess heat. Thus while part of our analysis supports the concept of a "slab graveyard" at the bottom of the lower mantle (e.g., Wyession 1996), that cemetery is by no means ubiquitous at the CMB: subducted slabs are either unevenly interred, or efficiently excavated by later upwellings.
Skeleton, A., Whitmarsh, R., Arghe, F., Crill, P., Koyi, H.
Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes.
Skelton, A., Whitmarsh, R., Arghe, F., Crill, P., Koyi, H.
Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes.
Physics and Chemistry of Minerals, Vol. 45, 3, pp. 237-247.
Technology
xenolths
Abstract: We determined elastic constants of a single-crystal chromian spinel at temperatures from ?15 to 45 °C through the Rectangular Parallelepiped Resonance method. The sample is a natural chromian spinel, which was separated from a mantle xenolith. Elastic constants at an ambient temperature (T = 24.0 °C) are C 11 = 264.8(1.7) GPa, C 12 = 154.5(1.8) GPa and C 44 = 142.6(0.3) GPa. All the elastic constants decrease linearly with increasing temperature. The temperature derivatives are dC 11/dT = ?0.049(2) GPa/°K, dC 12/dT = ?0.019(1) GPa/°K and dC 44/dT = ?0.020(1) GPa/°K. As an implication of the elastic constants, we applied them to the correction of a fluid inclusion geobarometry, which utilizes residual pressure of fluid inclusion as a depth scale. Before entrainment by a magma, the fluid inclusions must have the identical fluid density in constituent minerals of a xenolith. It has been, however, pointed out that fluid density of fluid inclusions significantly varies with host mineral species. The present study elucidates that elastic constants and thermal expansion coefficients cannot explain the difference in fluid density among mineral species. The density difference would reflect the difference in the degree of plastic deformation in the minerals.
high pressure EXPERIMENTAL STUDIES ONA KATUNGITE and THEIR BEARING on the GENESIS of SOME POTASSIUM RICH MAGMAS of the WEST BRANCH of the AFRICAN RIFT.
Journal of PETROLOGY, Vol. 24, PT. 2, PP. 166-187.
Fluorine and Chlorine Contents of Phlogopites Crystallized from ultrapotassic Rock Compositions in High Pressure Experiments- Implications for halogen Reservoirs in Source Regions.
American MINERALOGIST., Vol. 70, No. 5-6, PP. 529-536.
Canada, Ontario, United States, State Line, Wyoming, Leucite Hills, Uganda
Fluorine and Chlorine Contents of Phlogopites in High Pressure Melting experiments on Ultrapotassic Rocks: Implications for Fluorine and Chlorine rservoirs in Mantle Source Regions.
Geological Association of Canada, Vol. 10, P. A 16, (abstract.).
Barium rich phlogopite in a mantle derived xenolith of the Upper Canada mine kimberlite, Ontario, Canada;implications for BA- reservoir in the uppermantle
Journal of Japanese Association of Mineralogists, Petrologists and Economic, Vol. 83, No. 6, pp. 217-231
Bose, S., Dunkley, D.J., Dasgupta, S., Das, K., Arima, M.
India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: evidence from new zircon U Pb and monzazite chemical age data
Geological Society of America Bulletin, Vol. 123, 9/10 pp. 2031-2049.
Physics and Chemistry of Minerals, in press available 9p.
Technology
Garnet morphology
Abstract: We present a systematic experimental study on the phase transition, lattice microstrain, and order-disorder of cations for garnets in the majorite-pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson-Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg2+ + Si4+ = 2Al3+) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain.
Toft, P.B., Scowen, A.H., Arkani-Hamed, J., Francis, D.
Demagnetization by hydration in deep crustal rocks in the Grenville Province of Quebec, Canada: implications for magnetic anomalies of continental collision zones
A Brief Account of the Discovery and Investigation and the Official Reports of Geologist and Mining Engineer on the Occurrence of Diamonds in Pike County, Arkansas.
Tectonic setting of allochthonous upper mantle ultramafic rocks in the Cache Creek terrane of The north western Canadian Cordillera
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A4. Abstract
Petrology of Alkaline Rocks in the Carbonatite Complex at Iron Hill, Powderhorn District, Gunnison County, Colorado - New Geochemical and isotopic Data.
Geological Society of America (GSA), Vol. 17, No. 3, FEBRUARY P. 149. (abstract.).
Abstract: Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unravelling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U-Pb and Hf data, and a substantial new database of ?15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages > 1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra?Nuna/Columbia sedimentary basin that we name the Itremo?Muva?Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Journal of Asian Earth Sciences, in press available, 47p.
India, Africa, Madagascar
tectonis
Abstract: It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile ?Hf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of ?Hf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on ?Hf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
Journal of the Geological Society of London, in press available 25p.
Africa, Madagascar
thermochronology
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana, and preserves a record of several Neoproterozoic events that can be linked to orogenesis of the East African Orogen. We integrate remote sensing and field data to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south to south-west directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during a ~E-W shortening event. Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. Apatite U-Pb and muscovite and biotite Rb-Sr thermochronometers indicate that much of central Madagascar was thermally reset to at least ~500oC at c. 500 Ma. Deformation in west-central Madagascar occurred between c. 750 Ma and c. 550 Ma, and we suggest this deformation formed in response to the c. 650 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, which formed in response to the final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India.
Journal of the Geological Society, Vol. 177, pp. 784-798.
Africa, Madagascar
geothermometry
Abstract: Madagascar occupied an important place in the amalgamation of Gondwana and preserves a record of several Neoproterozoic events that are linked to orogenesis of the East African Orogen. In this study, we integrate remote sensing, field data and thermochronology to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south- to SW-directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during an approximately east-west shortening event (D3). Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. We show that the Itremo and Ikalamavony domains were deformed together in the same orogenic system, which we interpret as the c. 630 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, and probably formed in response to final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India. Apatite U-Pb and novel laser ablation triple quadrupole inductively coupled plasma mass spectrometry (LA-QQQ-ICP-MS) muscovite and biotite Rb-Sr thermochronology indicates that much of central Madagascar cooled through c. 500°C at c. 500 Ma.
Abstract: Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic-Cambrian (1000-520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
Earth Science Reviews , Vol. 214, 103477, 44p. Pdf
Mantle
plate tectonics, Rodinia, Gondwana
Abstract: Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic-Cambrian (1000-520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 47-81.
Australia
Supercontinents
Abstract: The Australian continent records c. 1860-1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860-1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790-1760 Ma terminated the rapid growth of the Australian continent.
Abstract: Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1-2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g?1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid - country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
Klein, E.L., Luzardo, R., Moura, Lobato, Brito, Armstrong
Geochronology, Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of Sao Luis Craton, northern Brazil: tectonics and crustal
Journal of South American Earth Sciences, Vol. 27, 2-3, pp. 129-145.
New developments in diamond recovery and security procedures - preserving the dollars. Excellent pictoral- x-ray and optical sorting, material handling
Diamonds in Kimberley Symposium & Trade Show, Bristow and De Wit held August 23-24, Kimberley, South Africa, GSSA Diamond Workshop CD slides 1-94.
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.
Abstract: The Karowe mine, owned and operated by Lucara Diamond Corporation, located in the Republic of Botswana, achieved commercial diamond production in July 2012. The AK06 kimberlite discovered in 1969 is the ore source at Karowe. The AK06 kimberlite within the Orapa Kimberlite Field is a roughly north-south elongate kimberlite body with a near surface expression of approximately 3.3 ha and a maximum area of approximately 7 ha at approximately 120 m below surface. The body comprises three geologically distinct, coalescing pipes that taper with depth. These pipes are referred to as the North Lobe, Centre Lobe, and South Lobe. The AK6 kimberlite is an opaque-mineral-rich monticellite kimberlite, texturally classified primarily as fragmental volcaniclastic kimberlite with lesser macrocrystic hypabyssal facies kimberlite of the Group 1 variety. The nature of the kimberlite differs between each lobe with distinctions apparent in the textural characteristics. The South Lobe is considered to be distinctly different from the North and Centre Lobes that are similar to each other in terms of their geological characteristics. The North and Centre Lobes exhibit internal textural complexity whereas the bulk of the South Lobe is more massive and internally homogeneous. The South Lobe forms the majority of the resource and displays the coarsest diamond size distribution of the three lobes. In three years of production, Karowe has established a continuing production of high value diamonds including coloured diamonds. In March 2013 a 239 ct gem quality diamond was recovered which was the first in a continuing population of large high value Type IIa diamonds recovered from the Centre Lobe, and more importantly the South Lobe, of the Karowe mine. Large diamonds, 50 ct in size, are spatially distributed horizontally and vertically within the South Lobe. Since commissioning to mid-December 2015, approximately 1.5 Mct have been produced and specials (diamonds greater than 10.8 ct in weight) represent circa 4.6% by weight of all diamond production. Life of Mine average stone size for the specials is 29.6 ct/stone. In 2015, a plant optimization project was completed to modify the process plant to treat harder kimberlite at depth and improve the recovery of exceptional diamonds via bulk sorters for primary run of mine large diamond recovery. In November 2015, the Karowe mine recovered an 813 ct stone and the world’s second largest gem quality diamond in over 100 years weighing 1,111 ct
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.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0604-9 12p.
Africa, Botswana
deposit - Karowe
Abstract: Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4-16 wt%) of garnets and the Mg# (69-92) and jadeite contents (14-48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39-40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere-transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.
Exploration and geology of the Qilalugaq kimberlites, Rae Isthmus, Nunavut, Canada.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 67-78.
Abstract: Size frequency distributions are the principal tool for predicting the macro-diamond grade of new kimberlite discoveries, based on micro-diamonds (i.e., diamond ? 0.5 mm) recovered from small exploration samples. Lognormal size frequency distributions – as observed for the Artemisia kimberlite (Slave Craton, Canada) – suggest a common source for micro- and macro-diamonds recovered from single samples, an implication that has never been conclusively tested. We analyzed 209 diamonds between 0.2 and 2 mm in size from the Artemisia kimberlite for their carbon isotopic compositions and nitrogen characteristics to determine the nature of the micro-/macro-diamond relationship.-Despite overall similarity in the ?13C distributions of micro- and macro-diamonds – both are bimodal with peaks in classes ? 5.0 to ? 4.5‰ and ? 3.5 to ? 3.0‰ – rare diamonds with ?13C between ? 14.2 and ? 24.5‰ of presumed eclogitic origin are restricted to macro-diamonds, whereas positive values are only observed for micro-diamonds. In addition, a shift in main mode and median value in ?13C of about +1‰ is observed for micro- relative to macro-diamonds. Fundamental differences between micro- and macro-diamonds at Artemisia were revealed through the analysis of nitrogen concentrations: 68% of micro-diamonds are Type II (“nitrogen free”) versus 21% of macro-diamonds, and only 19% of micro-diamonds have nitrogen contents > 100 atomic ppm versus 43% of macro-diamonds. Similarly, the presence of a detectable hydrogen related peak (at 3107 cm? 1) increases from 40% for micro-diamonds to 94% for macro-diamonds.-Previous studies on diamond populations from individual deposits have documented that single batches of ascending kimberlite or lamproite magma sample multiple diamond subpopulations formed during distinct growth events in compositionally variable sources and at various depth levels. The Artemisia data clearly show that even over a fairly narrow size interval, spanning the micro- to macro-diamond transition, the specific diamond subpopulations present and their relative proportions may vary significantly with diamond size. At Artemisia, we conclude that the observed lognormal size distribution is not a reflection of an entirely common origin of micro- and macro-diamonds.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and "Recent" (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and “Recent” (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0632-5 20p.
Canada, Nunavut, Melville Peninsula
deposit - Pelly Bay, Darby, Aviat, Qilalugaq
Abstract: Detailed geochronology along with petrographic, mineralogical and geochemical studies have been conducted on recently found diamond-bearing kimberlitic and related rocks in the Rae Craton at Aviat and Qilalugaq, Melville Peninsula, north-east Canada. Magmatic rocks from the Aviat pipes have geochemical (both bulk rock and isotopic) and mineralogical signatures (e.g., core to rim Al and Ba enrichment in phlogopite) similar to Group I kimberlite. In contrast, Aviat intrusive sheets are similar to ‘micaceous’ Group II kimberlite (orangeite) in their geochemical and mineralogical characteristics (e.g., phlogopite and spinel compositions, highly enriched Sr isotopic signature). Qilalugaq rocks with the least crustal contamination have geochemical and mineralogical signatures [e.g., high SiO2, Al2O3 and H2O; low TiO2 and CO2; less fractionated REE (rare earth elements), presence of primary clinopyroxene, phlogopite and spinel compositions] that are similar to features displayed by olivine lamproites from Argyle, Ellendale and West Greenland. The Naujaat dykes, in the vicinity of Qilalugaq, are highly altered due to extensive silicification and carbonation. However, their bulk rock geochemical signature and phlogopite chemistry are similar to Group I kimberlite. U-Pb perovskite geochronology reveals that Aviat pipes and all rocks from Qilalugaq have an early Cambrian emplacement age (540-530 Ma), with the Aviat sheets being ~30 Ma younger. This volatile-rich potassic ultramafic magmatism probably formed by varying degrees of involvement of asthenospheric and lithospherically derived melts. The spectrum of ages and compositions are similar to equivalent magmatic rocks observed from the nearby north-eastern North America and Western Greenland. The ultimate trigger for this magmatism could be linked to Neoproterozoic continental rifting during the opening of the Iapetus Ocean and breakup of the Rodinia supercontinent.
Nature, doi.org/101038/ s41586-021-03395-5 5p. Pdf
Canada, Northwest Territories
craton
Abstract: Cratons are Earth’s ancient continental land masses that remain stable for billions of years. The mantle roots of cratons are renowned as being long-lived, stable features of Earth’s continents, but there is also evidence of their disruption in the recent1,2,3,4,5,6 and more distant7,8,9 past. Despite periods of lithospheric thinning during the Proterozoic and Phanerozoic eons, the lithosphere beneath many cratons seems to always ‘heal’, returning to a thickness of 150 to 200 kilometres10,11,12; similar lithospheric thicknesses are thought to have existed since Archaean times3,13,14,15. Although numerous studies have focused on the mechanism for lithospheric destruction2,5,13,16,17,18,19, the mechanisms that recratonize the lithosphere beneath cratons and thus sustain them are not well understood. Here we study kimberlite-borne mantle xenoliths and seismology across a transect of the cratonic lithosphere of Arctic Canada, which includes a region affected by the Mackenzie plume event 1.27 billion years ago20. We demonstrate the important role of plume upwelling in the destruction and recratonization of roughly 200-kilometre-thick cratonic lithospheric mantle in the northern portion of the Slave craton. Using numerical modelling, we show how new, buoyant melt residues produced by the Mackenzie plume event are captured in a region of thinned lithosphere between two thick cratonic blocks. Our results identify a process by which cratons heal and return to their original lithospheric thickness after substantial disruption of their roots. This process may be widespread in the history of cratons and may contribute to how cratonic mantle becomes a patchwork of mantle peridotites of different age and origin.
Abstract: Still today, diamond growth in the mantle is difficult to understand. It may implicate different processes but there is an agreement to involve fluids as diamonds parents. The composition of these fluids is supposed to be variable depending of the the settings and depths. Natural diamonds also exhibit dissolution features, possibly mantle-derived and not only due to kimberlite-induced resorption during magma ascent [1]. We present experimental results devoted to understand diamond growth versus dissolution mechanisms in the lithosphere. Experiments are performed using multianvil presses at 7 GPa, 1300-1675°C for a few hours (4 to 27 hrs). As starting materials we use mixtures of water, carbonates, natural lherzolite or MORB, graphite and diamonds seeds resulting in hydrous-carbonate-silicate fluids at high pressure and temperature. For similar pressure and temperature conditions, results show that diamonds are formed or dissolved in these fluids, depending on the redox conditions. Focussed ion beam preparations of the diamonds evidence that when they grow, they trap multi-phased inclusions similar to those observed in fibrous, coated and monocrystalline natural diamonds, in agreement with previous studies [2-4].
Abstract: Zinc-rich chromite [(Fe,Zn)Cr2O4] is an important repository for chromium (Cr) that has been observed sporadically in kimberlite-bearing deposits worldwide. As another source reservoir for Cr, the green uvarovite garnet [ideally Ca3Cr2(SiO4)3] is the rarest variety among anhydrous garnets. Despite being reported from a wide range of localities, the occurrences of uvarovite are predominately restricted to hydrothermal and metamorphic settings rarely associated with kimberlite. Here, we present a detailed petrographic, mineralogical, and geochemical characterization of 71 uvarovite garnets with zinc-bearing chromite cores recovered from the Pikoo Property (central eastern Saskatchewan), which also hosts recently discovered kimberlites proven to be diamondiferous. In this work, euhedral to anhedral unzoned chromite occurs as kernels or cores and, in some cases, as irregular inclusions enclosed by uvarovite mantles. They contain moderate to high Cr [41.63-66.70 wt.% Cr2O3; Cr/(Cr+Al) = 0.64-0.99], Fe2+ (16.71-28.67 wt.% FeO) and Zn (1.64-15.52 wt.% ZnO) contents (Fig. 1), accompanied by an appreciable amount of Mn (0.63-2.32 wt.% MnO). The core with the highest Zn content gave structural formula (Zn0.409Fe2+0.555Mg0.018Mn0.019)1.00(Cr1.174Al0.674Fe3+0.152)2.00O4, which corresponds to Zn-rich chromite with a minor proportion of other end-members (e.g., hercynite, FeAl2O4). The garnets are compositionally zoned and occasionally devoid of inclusions. Formula calculations indicate that they are mainly members of the uvarovite-grossular series (up to 93% mol.% Uv) enriched in Ca (22.99-35.57 wt.% CaO) and Cr (up to 28.10 wt.% Cr2O3), but consistently depleted in Mg (mean = 0.10 wt.% MgO) and Ti (mean = 0.26 wt.% TiO2). Most garnets exhibit a core-rim zoning pattern, whereas the remainder are irregularly zoned and show evidence of resorption. The core to rim trend is characterized by an increase in grossular proportion at the expense of the uvarovite component. Morphological characteristics, textural interrelations, and compositional trends suggest that uvarovite garnet formed through interaction of Zn-rich chromite with late metasomatic (Ca,Al)-enriched hydrothermal fluids capable of precipitating secondary grossular.
Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil: subducted protoliths, carbonated melts and primary kimberlite ..
Contributions to Mineralogy and Petrology, Vol. 160, 4, pp. 489-50.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 141-151.
Mantle
carbon
Abstract: One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of fO2 from ?IW?0.4 to ?IW?3.5 at 1.2 to 3 GPa, and 1400?°C or 1600?°C, where ?IW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning ( ) depends chiefly on fO2, decreasing from to with decreasing fO2. also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients () show no evidence of a pressure dependence but diminish with temperature. At 1400?°C, partition coefficients increase linearly with decreasing fO2 from to At 1600?°C, however, they increase from ?IW?0.7 to ?IW?2 ( to ) and decrease from ?IW?2 to ?IW?3.3 . Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH4). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (?IW?0.4 to ?2.2), N is more compatible in core-forming metal than in molten silicate ( ), while at more reduced conditions (?IW?2.2 to ?IW?3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated (). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.
Jelsma, H.,Krishnan, S.U., Perritt, S.,Kumar, M., Preston, R., Winter, F., Lemotlo, L., Costa, J., Van der Linde, G., Facatino, M., Posser, A., Wallace, C., Henning, A., Joy, S., Chinn, I., Armstrong, R., Phillips, D.
Kimberlites from central Angola: a case stidy of exploration findings.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Jelsma, H., Krishnan, U., Perritt, S., Preston, R., Winter, F., Lemotlo, L., van der Linde, G., Armstrong, R., Phillips, D., Joy, S., Costa, J., Facatino, M., Posser, A., Kumar, M., Wallace, C., Chinn, I., Henning, A.
Kimberlites from central Angola: a case study of exploration findings.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, pp. 173-190.
Mineralogy and Petrology, Vol. 110, 2, pp. 399-420.
Indonesia
Megacrysts
Abstract: Zircon megacrysts (± gem corundum) appear in basalt fields of Indo-Pacific origin over a 12,000 km zone (ZIP) along West Pacific continental margins. Age-dating, trace element, oxygen and hafnium isotope studies on representative zircons (East Australia-Asia) indicate diverse magmatic sources. The U-Pb (249 to 1 Ma) and zircon fission track (ZFT) ages (65 to 1 Ma) suggest thermal annealing during later basalt transport, with?
Grantham, G.H., Manhica, A.D.S.T., Armstrong, R.A., Kruger, F.J., Loubser, M.
New SHRIMP, Rb/Sr and Sm/Nd isotope and whole rock chemical dat a from central Mozambique and western Dronning Maud Land: implications for eastern Kalahari
Journal of African Earth Sciences, Vol. 59, 1, pp.74-100.
Neoproterozoic inheritance during Cainozoic rifting in the western and southwestern branches of the East African Rift system: evidence from carbonatite alkaline
Tanzanian Journal of Earth Science, Vol. 1, Dec. pp. 29-37.
New U-Pb SHRIMP ages from the Lubango region, sw Angola: insights into the Paleoproterozoic evolution of the Angolan shield, southern Congo craton, Africa.
Journal of the Geological Society, Vol. 170, pp. 353-363.
Abstract: Gem-quality diamonds have been found in several alluvial deposits across central and southern Borneo. Borneo has been a known source of diamonds for centuries, but the location of their primary igneous source remains enigmatic. Many geological models have been proposed to explain their distribution, including: the diamonds were derived from a local diatreme; they were brought to the surface through ophiolite obduction or exhumation of UHP metamorphic rocks; they were transported long distances southward via major Asian river systems; or, they were transported from the Australian continent before Borneo was rifted from its northwestern margin in the Late Jurassic. To assess these models, we conducted a study of the provenance of heavy minerals from Kalimantan's Cempaka alluvial diamond deposit. This involved collecting U Pb isotopic data, fission track and trace element geochemistry of zircon as well as major element geochemical data of spinels and morphological descriptions of zircon and diamond. The results indicate that the Cempaka diamonds were likely derived from at least two sources, one which was relatively local and/or involved little reworking, and the other more distal which records several periods of reworking. The distal diamond source is interpreted to be diamond-bearing pipes that intruded the basement of a block that: (1) rifted from northwest Australia (East Java or SW Borneo) and the diamonds were recycled into its sedimentary cover, or: (2) were emplaced elsewhere (e.g. NW Australia) and transported to a block (e.g. East Java or SW Borneo). Both of these scenarios require the diamonds to be transported with the block when it rifted from NW Australia in the Late Jurassic. The local source could be diamondiferous diatremes associated with eroded Miocene high-K alkaline intrusions north of the Barito Basin, which would indicate that the lithosphere beneath SW Borneo is thick (~ 150 km or greater). The ‘local’ diamonds could also be associated with ophiolitic rocks that are exposed in the nearby Meratus Mountains.
Earth and Planetary Science Letters, Vol. 401, pp. 132-147.
Africa, South Africa
metasomatism
Abstract: The Karoo igneous rocks (174-185 Ma) of southern Africa represent one of the largest continental flood basalt provinces on Earth. Available evidence indicates that Karoo magmas either originated in the asthenosphere and were extensively modified by interaction with the lithospheric mantle prior to emplacement in the upper crust; or were produced by partial melting of enriched mantle lithosphere. However, no direct evidence of interaction by Karoo melts (or their precursors) with lithospheric mantle rocks has yet been identified in the suites of mantle xenoliths sampled by post-Karoo kimberlites in southern Africa. Here we report U-Pb ages for lindsleyite-mathiasite (LIMA) titanate minerals (crichtonite series) from three metasomatised, phlogopite and clinopyroxene-rich peridotite xenoliths from the ?84 Ma Bultfontein kimberlite (Kimberley, South Africa), located in the southern part of the Karoo magmatic province. The LIMA minerals appear to have formed during metasomatism of the lithospheric mantle by fluids enriched in HFSE (Ti, Zr, Hf, Nb), LILE (K, Ba, Ca, Sr) and LREE. LIMA U-Pb elemental and isotopic compositions were measured in situ by LA-ICP-MS methods, and potential matrix effects were evaluated by solution-mode analysis of mineral separates. LIMA minerals from the three samples yielded apparent U-Pb ages of , and (). A single zircon grain extracted from the ?190 Ma LIMA-bearing sample produced a similar U-Pb age of , within uncertainty of the LIMA ages. These data provide the first robust evidence of fluid enrichment in the lithospheric mantle beneath the Kimberley region at ?180-190 Ma, and suggest causation of mantle metasomatism by Karoo melts or their precursor(s). The results further indicate that U-Pb dating of LIMA minerals provides a new, accurate tool for dating metasomatic events in the lithospheric mantle.
Proterozoic mantle under Quesnellia: variably reset rubidium-strontium (Rb-Sr) mineral isochrons in ultramafic nodules carried up in Cenozoic volcanic vents of the s. OminecaBelt
Canadian Journal of Earth Sciences, Vol. 28, No. 8, August pp. 1239-1253
Armytage, R.M.G., Brandon, A.D., Peslier, A.H., Lapen, T.J.
Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concommitant production of juvenile crust in Dish Hill, CA peridotite xenoliths.
Geochimica et Cosmochimica Acta, Vol. 137, pp. 113-133.
Abstract: We explore the shear-wave lithospheric velocity structure of the Eastern Caribbean and Northern Venezuela using ambient noise tomography with stations deployed around the study area. We construct cross-correlation functions from continuous seismic records, and measure phase velocities of fundamental-mode Rayleigh waves. These velocities are further projected onto 0.6°x0.6° phase velocity grids for each period between 5 s and 50 s. The pseudo-dispersion curve at each grid point is inverted for 1D shear velocity profiles by using a Markov Chain Monte Carlo scheme. The interpolated 3D velocity model shows that the mean shear velocity of the Eastern Caribbean lithospheric mantle is lower than the global average, which is in agreement with values reported in other large igneous provinces. We interpret that low velocities in the lithospheric keel are associated with an anomalous composition and/or an elevated thermal state; this gives the Caribbean plate a high buoyancy that determines the subduction polarities in the region. The results also indicate that: (a) the mantle beneath Northern Venezuela retains compositional anomalies related to extension processes of different ages; (b) the overriding of the Caribbean plate by the Great Antilles arc seems to be much slower than previously suggested; and (c) the localized volcanism in the center of the Lesser Antilles arc is related to asthenospheric flow through the tear induced on the subducted slab by major strike-slip faults.
Abstract: The study of sediment cores allows for the reconstruction of past climate and environment through physical-chemical analysis. Nevertheless, this interpretation suffers from many drawbacks that can be overcome with the newest technologies. Hyperspectral imaging is one of these and allows a fast, high resolution, and non-destructive analysis of sediment cores. In this study, we use visible and near-infrared hyperspectral imaging to predict particle size fractions and distribution (PSD) at a resolution of 200??m on a previously well-studied sediment core taken from Lake Bourget (Western Alps, France). These predictions agree with previous studies on this core. Then, the PSD was used to estimate sedimentary deposit sources using the PSD unmixing algorithm AnalySize. It permitted estimation of the contribution of five sources (micrite, small and large bio-induced calcite crystals, diatom frustules, detrital particles), which had previously been characterized. The spatial dimension allowed for laminae to be discretized and counted, in agreement with the age-depth model previously established. We then evaluated the particle size and spectral signatures of each of these annual laminae, hence characterizing their physico-chemical composition. These high-resolution data also allowed for estimation of the accumulation rate (cm/year) of each of the main sources in the laminated unit and inferring the trophic status and the presence of instantaneous events of the lake.
Abstract: The Lower Cretaceous Jacupiranga complex, in the central-southeastern portion of the South American Platform, includes carbonatites in close association with silicate rocks (i.e. strongly and mildly silica-undersaturated series). Here we document the first hafnium isotope data on the Jacupiranga complex, together with new trace element and Pb isotope compositions. Even though liquid immiscibility from a carbonated silicate melt has been proposed for the genesis of several Brazilian carbonatites, isotopic and geochemical (e.g., Ba/La ratios, lack of pronounced Zr-Hf and Nb-Ta decoupling) information argues against a petrogenetic relationship between Jacupiranga carbonatites and their associated silicate rocks. Thus, an origin by direct partial melting of the mantle is considered. The isotopic compositions of the investigated silicate samples are coherent with a heterogeneously enriched subcontinental lithospheric mantle (SCLM) source of rather complex evolution. At least two metasomatic processes are constrained: (1) a first enrichment event, presumably derived from slab-related fluids introduced into the SCLM during Neoproterozoic times, as indicated by consistently old TDM ages and lamprophyre trace signatures, and (2) a Mesozoic carbonatite metasomatism episode of sub-lithospheric origin, as suggested by ?Nd-?Hf values inside the width of the terrestrial array. The Jacupiranga parental magmas might thus derive by partial melting of distinct generations of metasomatic vein assemblages that were hybridized with garnet peridotite wall-rocks.
Abstract: The Lower Cretaceous Jacupiranga complex, in the central-southeastern portion of the South American Platform, includes carbonatites in close association with silicate rocks (i.e. strongly and mildly silica-undersaturated series). Here we document the first hafnium isotope data on the Jacupiranga complex, together with new trace element and Pb isotope compositions. Even though liquid immiscibility from a carbonated silicate melt has been proposed for the genesis of several Brazilian carbonatites, isotopic and geochemical (e.g., Ba/La ratios, lack of pronounced Zr-Hf and Nb-Ta decoupling) information argues against a petrogenetic relationship between Jacupiranga carbonatites and their associated silicate rocks. Thus, an origin by direct partial melting of the mantle is considered. The isotopic compositions of the investigated silicate samples are coherent with a heterogeneously enriched subcontinental lithospheric mantle (SCLM) source of rather complex evolution. At least two metasomatic processes are constrained: (1) a first enrichment event, presumably derived from slab-related fluids introduced into the SCLM during Neoproterozoic times, as indicated by consistently old TDM ages and lamprophyre trace signatures, and (2) a Mesozoic carbonatite metasomatism episode of sub-lithospheric origin, as suggested by ?Nd-?Hf values inside the width of the terrestrial array. The Jacupiranga parental magmas might thus derive by partial melting of distinct generations of metasomatic vein assemblages that were hybridized with garnet peridotite wall-rocks.
Abstract: Zircon age peaks are commonly interpreted either as crustal production peaks or as selective preservation peaks of subduction-produced crust selectively preserved during continent-continent collision. We contribute to this ongoing debate, using the Nd isotopic compositions of felsic igneous rocks and their distribution during the accretionary and collisional phases of orogens. The proportion of juvenile input into the continental crust is estimated with a mixing model using arc-like mantle and reworked continental crust end members. Orogen length and duration proxies for juvenile crustal volume show that the amount of juvenile crust produced and preserved at zircon age peaks during the accretionary phase of orogens is ?3 times that preserved during the collisional phase of orogens. The fact that most juvenile crust is both produced and preserved during the accretionary phase of orogens does not require craton collisions for its preservation, thus favoring the interpretation of zircon age peaks as crustal production peaks. Most juvenile continental crust older than 600 Ma is produced and preserved before final supercontinent assembly and does not require supercontinent assembly for its preservation. Episodic destabilization of a compositionally heterogeneous layer at the base of the mantle may produce mantle plume events leading to enhanced subduction and crustal production. Our Nd isotope model for cumulative continental growth based on juvenile crust proxies for the past 2.5 b.y. suggests a step-like growth curve with rapid growth in accretionary orogens at the times of zircon age peaks.
Formation of dunite xenoliths in kimberlites and allikites, petrographic and mineral compositions from a deformed xenolith in the Majuagaa kimberlite dike, Greenland.
Abstract: Recent studies conclude that plate tectonics started 3 b.y. ago in the mid Archean. A transition from a "presubduction" regime to modern plate tectonics is said to be marked by changes in trace-element or isotopic ratios, the appearance of eclogitic inclusions in diamonds, or an apparent change in upper crust composition. Behind these arguments is the notion that subduction was intermittent or impossible early in Earth history when the mantle was hotter. If so, a mechanism other than subduction must have created granitoids of Archean continental crust. In the sagduction model, the base of thick oceanic crust converts to eclogite, founders, and melts to generate granitic magma. Here we evaluate two crucial constraints on the sagduction process: to generate granitic magma requires that water and basalt is taken deep into the mantle; thick oceanic crust is internally differentiated into uppermost layers of hydrated basalt and lower mafic-ultramafic cumulates. Our numerical modelling shows that any deformation within thick, differentiated crust is restricted to the lower cumulates that lack ingredients essential to generate granitic magma. Emplacement of hot intrusions heats the lower crust which was hot and anhydrous. We conclude that the sagduction model is flawed. Recent re-evaluation gives temperatures in ambient Archean upper mantle only moderately higher than in modern mantle, which deflates arguments that subduction was impossible in the Archean. We conclude that Archean continental crust was generated in subduction zones and that plate tectonics started in the early Archean.
Abstract: H2O strongly influences physical properties of the mantle and its ability to melt or convect and can trace recycling of surface reservoirs down to the deep mantle1,2. This makes knowledge of water content in the Earth's interior and its evolution through time crucial to understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from high-degree mantle melting at high pressures3 and thus are excellent probes of H2O contents in the deep mantle. A significant excess of H2O over elements of similar geochemical behavior during mantle melting (e.g. Ce) was recently found in melt inclusions in the most Mg-rich olivine in 2.7 Ga old komatiites from Canada4 and Zimbabwe5. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchean time. In this paper we confirm the mantle source of this H2O by measurement of deuterium to hydrogen ratios in these melt inclusions and present similar data for 3.3 Ga old komatiites from the Barberton Greenstone Belt. Using hydrogen isotopes, we show that the mantle sources of these melts contained excess H2O which implies that a deep mantle hydrated reservoir has been present in the Earth's interior at least since the Paleoarchean. The reconstructed initial hydrogen isotope composition of komatiites is significantly more depleted in deuterium than all surface reservoirs and typical mantle but resembles that in dehydrated subducted slabs. Together with a significant excess of chlorine and a temporal trend of Pb/Ce in the mantle sources of komatiites, these results argue that lithosphere recycling into the deep mantle, arguably via subduction, started before 3.3 Ga. (a un-reviewed version of the manuscript accepted for publication in Nature magazine).
Geochimica et Cosmochimica Acta, in press available. 13p.
Mantle
plate tectonics
Abstract: Many recent studies conclude that plate tectonics started about 3 billion years ago in the mid Archean. The transition from a pre-subduction regime to modern plate tectonics is reported to be marked by changes in trace element ratios or isotopic compositions that monitor the rate of growth of the continental crust, the appearance of eclogitic inclusions in diamonds, or an apparent change in the composition of the upper crust. Behind most of these arguments is the hypothesis that, early in Earth history when the mantle was hotter, subduction was intermittent or impossible. If so, a mechanism other than subduction must have created the granitoids that dominate Archean continental crust. One alternative, commonly referred to as sagduction, proposes that the base of thick oceanic crust founders and partially melts to generate granitic magma. Here we evaluate the sagduction process, starting by discussing two crucial concepts: (1) thick oceanic crust is internally differentiated, with hydrated basalt being restricted to the uppermost layers, (2) the generation of granitic magma requires that water and basalt is present in the lower part of the crust or is taken deep into the mantle. We present the results of numerical modelling that demonstrates that when intrusion is taken into account, the lower portion of the crust is well above dehydration temperatures and therefore essentially dry. We show that any deformation within thick, differentiated crust is restricted to the lowermost layers of dry, infertile mafic-ultramafic cumulates that lack the ingredients essential for the generation of granitic magma. Given the implausibility of the sagduction process, we suggest that subduction was the main mechanism that generated granitoid magmas, in the Archean as today.
Abstract: Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (~Fo88) but highly variable minor element contents (e.g. 500-2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid-peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
Abstract: Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (~Fo88) but highly variable minor element contents (e.g. 500-2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid-peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
Abstract: Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source1, 2. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes3, 4, 5, 6 while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes7, 8, 9. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5?mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone10. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.
Abstract: Moore proposes in his Comment (Moore, 2017) that marginal zones in olivine grains in kimberlites (Fig. 1a) are produced by crystallization from kimberlite melt. He suggests that the chemical zones observed in these marginal zones (inner transition zones and outer margins, illustrated in his fig. 1) result from abrupt changes in distribution coefficients during crystallization. He proposes that the transition zones, characterized by variable Fo at constant and high Ni contents, are produced by crystallization with high KdFe-Mg (= 0•45) and low DNi (= 4) whereas the margins, characterized by a sharp drop in Ni content at nearly constant Fo (Fig. 1b), are produced by crystallization with higher DNi owing to a sudden change in physical conditions of crystallization (P,…
geochemicalperspectives.org, Vol. 6, 1, April, 187p. Pdf 28 MB
Global
geochemistry
Abstract: Some scientists and journalists, and many members of the general public, have been led to believe that the world is rapidly running out of the metals on which our modern society is based. Advocates of the peak metal concept have predicted for many decades that increasing consumption will soon lead to exhaustion of mineral resources. Yet, despite ever-increasing production and consumption, supplies of minerals have continued to meet the needs of industry and society, and lifetimes of reserves remain similar to what they were 30-40 years ago. In this volume, we discuss the reasons for this apparent paradox using our broad experience and expertise on both academic and industrial sides of the minerals sector. Many misconceptions arise from flawed estimates of the size of global mineral resources which stem from a lack of understanding of the critical difference between reserves and resources. Some authors use quoted reserves – the amount of metal proven to exist and to be economic for mining at present – when predicting imminent shortages. Resources – the amount that may be accessible in the upper few kilometres of the crust – are far larger.Over the last 150 years, improved technologies, economies of scale and increased efficiency have combined to reduce costs hence allowing lower-grade ore to be mined economically. The net result is that the long-term inflation-adjusted price of most metals has decreased more or less in parallel with increasing production, a second apparent paradox that frequently is not well understood. Using copper as the principal example and other metals as appropriate, we summarise the latest research on ore deposits and the activities of the minerals industry. Following a description of the numerous geological processes that form ore deposits, we outline the scientific methods used by the minerals industry to explore for new deposits. We also discuss how resources are mined and how minerals are processed, as well as recent efforts to reduce related environmental impacts. Economic and societal factors influence supply, and these are as important as the actual presence of a resource. Finally, we discuss the critical roles that geoscientists will play in assuring continued supplies of minerals. These include the development of new concepts and techniques that will assist the discovery, mining, processing, remediation, and management of mineral resources. It is essential that researchers help to educate the general public about the need for continued exploration to find new resources to meet growth in world living standards. We demonstrate that global resources of copper, and probably of most other metals, are much larger than most currently available estimates, especially if increasing efficiencies and higher prices allow lower-grade ores to be mined. These observations indicate that supplies of important mineral commodities will remain adequate for the foreseeable future.
Earth Planetary Science Letters, Vol. 491, pp. 148-159.
Africa, Australia, Canada, Europe
geothermometry
Abstract: The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ?0.2 wt% water plus potassium to form phlogopite, or ?5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200?°C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Mantle
eclogites
Abstract: Some high-Mg eclogite xenoliths, entrained by kimberlites from the mantle lithospheres of ancient continental cores, and rare orogenic eclogites and ophiolites, exhumed or obducted during the closure of palaeo-ocean basins, have elemental and isotopic compositions indicative of protoliths that formed as little-differentiated melts erupted in ancient ocean floors. Despite metamorphism and, in part, partial melt loss, these samples of ancient mid-ocean ridge basalt and picrite retain a memory of the chemical and physical state of their protoliths' ambient convecting mantle sources. Published data show that, when filtered to exclude specimens with cumulate protoliths or showing evidence for later enrichment (metasomatism), the samples lack Y or Al 2 O 3 depletion relative to TiO 2 and MgO. This indicates melt segregation of the protolith predominantly from a garnet-free peridotite source and implies intersection of the solidus at low pressures (?3 GPa). Given the dependence of melt composition and volume on source composition (assumed to be similar to modern depleted mantle) and mantle potential temperature (T P), we calculate moderate average melt fractions F (?0.22 ± 0.01) from the Ti contents of the least differentiated samples in three sample suites with 2.6 to 2.9 Ga ages. This converts to T P of ?1410 ± 10 • C assuming a final pressure of melting of 0.5 GPa, melt productivity of 10%/GPa and mantle adiabat of 0.4 • C/km, and using a mantle solidus parameterisation. Though model-dependent, the results are in agreement with recent work advocating moderate Archaean mantle T P. Estimates drop to F = 0.19 and T P = 1380 • C at 1.9 Ga and F = 0.12 and T P = 1310 at 0.6 Ga, corresponding to a decrease in T P of only ?100 • C over the last 3 Ga. A less depleted mantle source yields higher F and T P , but the above estimates are in better agreement with qualitative evidence from Al 2 O 3 and Y, and with Nd-Hf and Sr isotope compositions of orogenic eclogite and granulite suites and mantle eclogites, respectively, which indicate that portions of the Meso-to Neoarchaean mantle were depleted. Moderate T P supports early plate strengthening and a possible transition to plate tectonics in the Mesoarchaean if not earlier. Moreover, moderate temperatures in Archaean subduction zones may have facilitated deep recycling of volatiles that would otherwise have been lost from subducting slabs at shallow depths.
Geochemistry, Geophysics, Geosystems, Vol. 19, 9, pp. 3140-3163.
United States, California
subduction
Abstract: Mantle convection shapes Earth's surface by generating dynamic topography. Observational constraints and regional convection models suggest that surface topography could be sensitive to mantle flow for wavelengths as short as 1,000 and 250 km, respectively. At these spatial scales, surface processes including sedimentation and relative sea?level change occur on million?year timescales. However, time?dependent global mantle flow models do not predict small?scale dynamic topography yet. Here we present 2?D spherical annulus numerical models of mantle convection with large radial and lateral viscosity contrasts. We first identify the range of Rayleigh number, internal heat production rate and yield stress for which models generate plate?like behavior, surface heat flow, surface velocities, and topography distribution comparable to Earth's. These models produce both whole?mantle convection and small?scale convection in the upper mantle, which results in small?scale (<500 km) to large?scale (>104 km) dynamic topography, with a spectral power for intermediate scales (500 to 104 km) comparable to estimates of present?day residual topography. Timescales of convection and the associated dynamic topography vary from five to several hundreds of millions of years. For a Rayleigh number of 107, we investigate how lithosphere yield stress variations (1050 MPa) and the presence of deep thermochemical heterogeneities favor small?scale (200500 km) and intermediate?scale (500104 km) dynamic topography by controlling the formation of small?scale convection and the number and distribution of subduction zones, respectively. The interplay between mantle convection and lithosphere dynamics generates a complex spatial and temporal pattern of dynamic topography consistent with constraints for Earth.
Earth and Planetary Science Letters, Vol. 547, 15p. Pdf
Mantle
geodynamics
Abstract: Mantle plumes provide valuable information about whole-mantle convection: they originate at the core-mantle boundary, cross Earth's mantle and interact with the lithosphere. For instance, it has been proposed that the mobility/stability of plumes depends on plume intrinsic properties, on how slabs interact with the basal boundary layer, on mantle flow, or on their proximity to mid-ocean ridges. Here, we use 3D-spherical models of mantle convection generating self-consistent plate-like behaviour to investigate the mechanisms linking tectonics and mantle convection to plume dynamics. Our models produce fully-dynamic mantle plumes that rise vertically with deflection and present excess temperatures, rising speeds, buoyancy and heat fluxes comparable to observations. In the absence of plate tectonics, plumes are stable and their lifetime exceeds hundreds of million years. With plate tectonics, plumes are more mobile, and we identify four physical mechanisms controlling their stability. 1/ Fixed plumes are located at saddle points of basal mantle flow. 2/ Plumes moving at speeds between 0.5-1 cm yr?1 are slowly entrained by passive mantle flow. 3/ Fast plume motions between 2-5 cm yr?1 lasting several tens of million years are caused by slab push. 4/ Plumes occasionally drift at speeds >5 cm yr?1 over <10 Myr through plume merging. We do not observe systematic anchoring of plumes to mid-oceanic ridges. Independent of the presence of a dense basal layer, plate-like regimes decrease the lifetime of plumes compared to stagnant-lid models. Plume age, temperature excess or buoyancy flux are not diagnostic of plume lateral speed. The fraction of plumes moving by less than 0.5 cm yr?1 is >25%, which suggests that fixed hotspot reference frames can be defined from carefully selected hotspot tracks.
Galimov, E., Kudin, A., Skorobogatskii, V., Plotnichenko, V., Bondarev, O., Zarubin, B., Strazdovskii, V., Aronin, A., Fisenko, A., Bykov, I., Barinov, A.
Experimental corrobation of the synthesis of diamond in the cavitation process.
Doklady Physical Chemistry, Vol. 49, 3, pp. 150-153.
Age correlation of endogenic processes of the Slave (Canada) and Middle Peri Dneiper (Ukraine) cratons in connection with diamond bearing ability problems.
Gems & Gemology, abstracts Mineralogical Journal (Ukraine) Vol. 26, 1, pp. 18-23. *** in English, Vol. 41, 2, Summer p. 194. abstract only
Abstract: Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean-Proterozoic Siberian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is < 0.02 t/m3 or < 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. ‘Pristine’ cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8-3.0% (and SPT density of 3.29-3.33 t/m3 as compared to 3.39 t/m3 of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0-1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38-3.40 t/m3 at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimberlites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.
Abstract: Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean-ocean, ocean-continent, and continent-continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M > 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent-continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent-ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean-ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels.
Abstract: The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500-700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness. We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37-3.41 g/cm3 in the Cape Fold and Namaqua-Natal fold belts, ca. 3.34-3.35 g/cm3 in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34-3.37 g/cm3 in the Limpopo Belt, and ca. 3.32-3.33 g/cm3 in the Kaapvaal and southern Zimbabwe cratons.The results indicate that 0.5-1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. + 20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100-200 °C in a ca. 100-150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5-1.0 km to the regional topographic uplift.
Abstract: We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30? × 30? grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5-1.0 km, and have uncertainty ranging from ca. 0.01 g/cm3 for the Archean terrains to ca. 0.03 g/cm3 for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31-3.33 g/cm3) and the western (3.32-3.34 g/cm3) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34-3.35 g/cm3) with values typical of Proterozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34-3.37 g/cm3) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua-Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm3) with local (100-300 km across) low-density (down to 3.34 g/cm3) and high-density (up to 3.41 g/cm3) anomalies. High (3.40-3.42 g/cm3) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle, such as associated with subducted oceanic slabs. We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm3) are not sampled by kimberlites and may represent the "pristine" Archean mantle. (2) Young (< 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm3) than the older Group II kimberlites (3.33 ± 0.01 g/cm3), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32-3.35 g/cm3), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35-40 km) and low-density (3.32-3.33 g/cm3) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40-50 km), and denser mantle (3.34-3.36 g/cm3). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100-150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.
Russian Geology and Geophysics, Vol. 59, 11, pp. 1389-1409.
Russia
craton
Abstract: Precambrian cratons cover about 70% of the total continental area. According to a large volume of geomorphological, geological, paleontological, and other data for the Pliocene and Pleistocene, these cratons have experienced a crustal uplift from 100-200 m to 1000-1500 m, commonly called the recent or Neotectonic uplift. Shortening of the Precambrian crust terminated half a billion years ago or earlier, and its uplift could not have been produced by this mechanism. According to the main models of dynamic topography in the mantle, the distribution of displacements at the surface is quite different from that of the Neotectonic movements. According to seismic data, there is no magmatic underplating beneath most of the Precambrian cratons. In most of cratonic areas, the mantle lithosphere is very thick, which makes its recent delamination unlikely. Asthenospheric replacement of the lower part of the mantle lithosphere beneath the Precambrian cratons might have produced only a minor part of their Neotectonic uplifts. Since the above mechanisms cannot explain this phenomenon, the rock expansion in the crustal layer is supposed to be the main cause of the recent uplift of Precambrian cratons. This is supported by the strong lateral nonuniformity of the uplift, which indicates that expansion of rocks took place at a shallow depth. Expansion might have occurred in crustal rocks that emerged from the lower crust into the middle crust with lower pressure and temperature after the denudation of a thick layer of surface rocks. In the dry state, these rocks can remain metastable for a long time. However, rapid metamorphism accompanied by expansion of rocks can be caused by infiltration of hydrous fluids from the mantle. Analysis of phase diagrams for common crustal rocks demonstrates that this mechanism can explain the recent crustal uplift of Precambrian cratons.
Europe, Fennoscandia, Kola Peninsula, Karelia, Canada
craton
Abstract: Rapid glacio-isostatic rebound in Fennoscandia and Canada that is nonuniform in time and space indicates that there is a layer with strongly decreased viscosity at shallow crustal depths. The upper boundary of the layer is near the depth of 15 km, which corresponds to the maximum depth of earthquake hypocenters in the Precambrian cratons of the Kola Peninsula and Karelia. The position of the lower boundary is less distinct; however, most likely it is located near the base of the crust. The formation of such a layer in the Pliocene-Quaternary occurred due to infiltration of a large volume of mantle fluids into the crust. In many regions, this has led to retrograde metamorphism with rock expansion and a strong decrease in rocks viscosity.
Abstract: New petrographic, geochemical, and isotopic (Sr, Nd, and ?18?) data on olivine and pyroxene phenocrysts provide constraints on the composition and crustal evolution of primary melts of Paleoproterozoic (2.40 Ga) picrodoleritic sills in the northwest Kola province, Fennoscandian Shield. The picrodolerites form differentiated sills with S-shaped compositional profiles. Their chilled margins comprise porphyritic picrodolerite (upper margin) and olivine gabbronorite (bottom) with olivine and clinopyroxene phenocrysts. Analysis of the available data allows us to recognize three main stages in the crystallization of mineral assemblages. The central parts of large (up to 2 mm) olivine phenocrysts (Ol-1-C) crystallized at the early stage. This olivine (Mg# 85-92) is enriched in Ni (from 2845 to 3419 ppm), has stable Ni/Mg ratio, low Ti, Mn and Co concentrations, and contains tiny (up to 10 ?m) diopside-spinel dendritic lamella that probably originated due to the exsolution from high Ca- and Cr- primary magmatic olivine. All these features of Ol-1-C are typical of olivine from primitive picritic and komatiitic magmas (De Hoog et al., 2010; Asafov et al., 2018). Ol-1-C contains large (up to 0.25 mm) crystalline inclusions of high-Al enstatite (Mg# 80-88) and clinopyroxene (Mg# 82-90), occasionally in association with Ti-pargasite and chromian spinel (60.4 wt.% Al2O3). These inclusions are regarded as microxenoliths of wall rock that were captured by primary melt at depths more than 30 km and preserved due to the conservation in magmatic olivine. The second stage was responsible for the crystallization of Ol-1 rim (Ol-1-R), small (up to 0.3 mm) olivine (Ol-2, Mg# 76-85) grains, and central parts of large (up to 1.5 mm) clinopyroxene (Cpx-C) phenocrysts in the mid-crustal transitional magma chamber (at a depth of 15-20 km) at 1160-1350°C. At the third stage, Cpx-C phenocrysts were overgrown by low-Mg rims (Mg# 70-72) similar in composition to the groundmass clinopyroxene from chilled picrodolerite and gabbro-dolerite in the central parts of the sills. This stage likely completed the evolution of picrodoleritic magma and occurred in the upper crust at a depth of about 5 km. All stages of picrodoleritic magma crystallization were accompanied by contamination. Primary melts were contaminated by upper mantle and/or lower crust as recognized from xenocrystic inclusions in Ol-1-C. The second contamination stage is supported by the negative values of ?Nd(2.40) = -1.1 in clinopyroxene phenocrysts. At the third stage, contamination likely occurred in the upper crust when ascending melts filled gentle fractures. This caused vertical whole-rock Nd heterogeneity in the sills (Erofeeva et al., 2019), and difference in Nd isotopic composition of clinopyroxene phenocrysts and doleritic groundmass. It was also recognized that residual evolved melts are enriched in radiogenic strontium but have neodymium isotopic composition similar to other samples. It could be explained by the interaction of the melts with fluid formed via decomposition of biotite from surrounding gneisses under the effect of high-temperature melts.
Stratigraphy and Geological Correlation, Vol. 28, 1, pp. 1-34.
Europe, Fennoscandia
magmatism
Abstract: The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak-Pechenga-Imandra-Varzuga belt and dikes and sills of the Murmansk and Kola-Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06-1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra-Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra-Varzuga zones intruded at 2010-1970 and 1970-1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra-Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 124-147.
Baltic Shield, Fennoscandia
Carbonatite, alkaline rocks
Abstract: The paper presents the results of a study of the large Paleozoic ore-magmatic system in the northeastern Fennoscandian Shield comprising the Khibiny and Lovozero plutons, the Kurga intrusion, volcanic rocks, and numerous alkaline dike swarms. As follows from the results of deep drilling and 3D geophysical simulation, large bodies of rocks pertaining to the ultramafic alkaline complex occur at the lower level of the ore-magmatic system. Peridotite, pyroxenite, melilitolite, melteigite, and ijolite occupy more than 50 vol % of the volcanic-plutonic complex within the upper 15 km accessible to gravity exploration. The proposed model represents the ore-magmatic system as a conjugate network of mantle magmatic sources localized at different depth levels and periodically supplying the melts belonging to the two autonomous groups: (1) ultramafic alkaline rocks with carbonatites and (2) alkali syenites-peralkaline syenites, which were formed synchronously having a common system of outlet conduits. With allowance for the available isotopic datings and new geochronological evidence, the duration of complex formation beginning from supply of the first batches of melt into calderas and up to postmagmatic events, expressed in formation of late pegmatoids, was no less than 25 Ma.
Arzamastsev, A.A., Bea, F., Arzamastseva, L.V., Montero, P.
Devonian plume magmatism in the NE Baltic Shield: rare earth elements in rocks and minerals of ultrabasic alkaline series as indicators of magma evolution.
Deep Seated Magmatism, magmatism sources and the problem of plumes., pp. 42-68.
Abstract: Results from thermochronological studies have multiple applications to various problems in tectonics and landform evolution However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectonothermal evolution of the region Here, we use both new and previously published multimineral 40Ar/39Ar data (amphibole, mica, and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900–360 Ma Using the apatite fission track method as well as a numerical model of the heating?cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360–0 Ma According to our model, since Lapland?Kola orogenesis (1930–1905 Ma) northeastern Fennoscandia experienced a quasi?monotonous cooling with the average rate of ~0 15 °C/Myr, which is equal to an exhumation rate of ~1–2 m/Myr New apatite fission track data and time?temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360 and 300 Ma This we attribute to an elevated geothermal gradient due to Baltica's drift over the African large low shear?wave velocity province in the lowest mantle and/or thermal blanketing by insulating Devonian?Carboniferous sedimentary/volcanic cover Our model is further supported by evidence of Late Devonian?Carboniferous rifting in the East and South?Western Barents Basin, as well as various 360–300 Ma magmatic events within SW Fennoscandia and the Baltic countries
Abstract: Results from thermochronological studies have multiple applications to various problems in tectonics and landform evolution. However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectonothermal evolution of the region. Here, we use both new and previously published multimineral 40Ar/39Ar data (amphibole, mica, and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900-360 Ma. Using the apatite fission track method as well as a numerical model of the heating?cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360-0 Ma. According to our model, since Lapland?Kola orogenesis (1930-1905 Ma) northeastern Fennoscandia experienced a quasi?monotonous cooling with the average rate of ~0.15 °C/Myr, which is equal to an exhumation rate of ~1-2 m/Myr. New apatite fission track data and time?temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360 and 300 Ma. This we attribute to an elevated geothermal gradient due to Baltica's drift over the African large low shear?wave velocity province in the lowest mantle and/or thermal blanketing by insulating Devonian?Carboniferous sedimentary/volcanic cover. Our model is further supported by evidence of Late Devonian?Carboniferous rifting in the East and South?Western Barents Basin, as well as various 360-300 Ma magmatic events within SW Fennoscandia and the Baltic countries.
Abstract: New data on the age of dolerite dikes in the NE part of the Kola province of the Fennoscandinavian shield and the picrodolerite sills that cut the dikes are presented. The results of U-Pb ID-TIMS baddeleyite dating indicate that dolerites were formed between 2508 ± 6 and 2513 ± 16 Ma ago, simultaneously with the intrusions of the Monchegorsk group. A comparison of the composition of the dolerites studied with dykes of the same age found in other Archean cratons shows their significant similarity and suggests their formation at the same large magmatic province. The age of baddeleyite from the picrodolerites sills at 2403 ± 12 Ma ago indicates an event of basic magmatism that was not previously established in this part of the Fennoscandinavian shield. It is possible that, along with dolerite dykes with an age of 2405 Ma and komatiites of the Vetreny belt of the Karelian craton, sills of the Kola province are a component of a unified large magmatic event.
Arzamastsev, A.A., Bea, F., Arzamastseva, L.V., Montero, P.
Devonian plume magmatism in the NE Baltic Shield: rare earth elements in rocks and minerals of ultrabasic alkaline series as indicators of magma evolution.
Deep Seated Magmatism, magmatism sources and the problem of plumes., pp. 42-68.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 124-147.
Baltic Shield, Fennoscandia
Carbonatite, alkaline rocks
Abstract: The paper presents the results of a study of the large Paleozoic ore-magmatic system in the northeastern Fennoscandian Shield comprising the Khibiny and Lovozero plutons, the Kurga intrusion, volcanic rocks, and numerous alkaline dike swarms. As follows from the results of deep drilling and 3D geophysical simulation, large bodies of rocks pertaining to the ultramafic alkaline complex occur at the lower level of the ore-magmatic system. Peridotite, pyroxenite, melilitolite, melteigite, and ijolite occupy more than 50 vol % of the volcanic-plutonic complex within the upper 15 km accessible to gravity exploration. The proposed model represents the ore-magmatic system as a conjugate network of mantle magmatic sources localized at different depth levels and periodically supplying the melts belonging to the two autonomous groups: (1) ultramafic alkaline rocks with carbonatites and (2) alkali syenites-peralkaline syenites, which were formed synchronously having a common system of outlet conduits. With allowance for the available isotopic datings and new geochronological evidence, the duration of complex formation beginning from supply of the first batches of melt into calderas and up to postmagmatic events, expressed in formation of late pegmatoids, was no less than 25 Ma.
Abstract: H2O strongly influences physical properties of the mantle and its ability to melt or convect and can trace recycling of surface reservoirs down to the deep mantle1,2. This makes knowledge of water content in the Earth's interior and its evolution through time crucial to understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from high-degree mantle melting at high pressures3 and thus are excellent probes of H2O contents in the deep mantle. A significant excess of H2O over elements of similar geochemical behavior during mantle melting (e.g. Ce) was recently found in melt inclusions in the most Mg-rich olivine in 2.7 Ga old komatiites from Canada4 and Zimbabwe5. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchean time. In this paper we confirm the mantle source of this H2O by measurement of deuterium to hydrogen ratios in these melt inclusions and present similar data for 3.3 Ga old komatiites from the Barberton Greenstone Belt. Using hydrogen isotopes, we show that the mantle sources of these melts contained excess H2O which implies that a deep mantle hydrated reservoir has been present in the Earth's interior at least since the Paleoarchean. The reconstructed initial hydrogen isotope composition of komatiites is significantly more depleted in deuterium than all surface reservoirs and typical mantle but resembles that in dehydrated subducted slabs. Together with a significant excess of chlorine and a temporal trend of Pb/Ce in the mantle sources of komatiites, these results argue that lithosphere recycling into the deep mantle, arguably via subduction, started before 3.3 Ga. (a un-reviewed version of the manuscript accepted for publication in Nature magazine).
Abstract: Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source1, 2. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes3, 4, 5, 6 while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes7, 8, 9. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5?mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone10. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.
Partitioning of FeO between magnesiwustite and liquid iron at high pressures and temperatures: implications for the composition of the Earth's outer core.
Earth and Planetary Science Letters, Vol. 257, 3-4, May 30, pp. 435-449.
Geochemistry of the rare lithophile elements Zirconium, Hafnium, Niobium, Tantalum, Thorium, and Uranium and variations in their ratios during fraction.
Alkaline Magmatism -problems mantle source, pp. 216-22.
Abstract: The determination of the chemical composition of meimechites which are unique and rarely occurring ultra-high MgO igneous rocks can be complicated due to their porphyric structure, the presence of acid-insoluble minerals, and wide variation of major and trace element contents. In the present study the optimal analytical strategy based on a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) methods was suggested for the determination of the elemental composition of meimechites. The preparation of glass beads using a lithium tetraborate and metaborate mixture proved to be suitable for the XRF determination of major oxides. A comparative study of the sample decomposition procedures for the determination of trace elements by ICP-MS clearly showed that fusion with lithium metaborate was the most appropriate sample preparation technique for complete digestion of meimechites. The open beaker HF-HNO3-HClO4 acid digestion was insufficient because the results for Nb, Ta, V, Zr, Cr and Hf were underestimated by 20-80% compared to those determined using the fusion method due to the presence in the rock samples of acid-resistant accessory minerals. It is shown that using analytical data from acid digestion may lead to erroneous interpretation of geochemical data.
Geochemistry: Exploration, Environment, Analysis, Vol. 19, pp. 233-243. pdf
Mantle
meimechites
Abstract: The determination of the chemical composition of meimechites which are unique and rarely occurring ultra-high MgO igneous rocks can be complicated due to their porphyric structure, the presence of acid-insoluble minerals, and wide variation of major and trace element contents. In the present study the optimal analytical strategy based on a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) methods was suggested for the determination of the elemental composition of meimechites. The preparation of glass beads using a lithium tetraborate and metaborate mixture proved to be suitable for the XRF determination of major oxides. A comparative study of the sample decomposition procedures for the determination of trace elements by ICP-MS clearly showed that fusion with lithium metaborate was the most appropriate sample preparation technique for complete digestion of meimechites. The open beaker HF-HNO3-HClO4 acid digestion was insufficient because the results for Nb, Ta, V, Zr, Cr and Hf were underestimated by 20-80% compared to those determined using the fusion method due to the presence in the rock samples of acid-resistant accessory minerals. It is shown that using analytical data from acid digestion may lead to erroneous interpretation of geochemical data.
Abstract: The determination of the chemical composition of meimechites which are unique and rarely occurring ultra-high MgO igneous rocks can be complicated due to their porphyric structure, the presence of acid-insoluble minerals, and wide variation of major and trace element contents. In the present study the optimal analytical strategy based on a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) methods was suggested for the determination of the elemental composition of meimechites. The preparation of glass beads using a lithium tetraborate and metaborate mixture proved to be suitable for the XRF determination of major oxides. A comparative study of the sample decomposition procedures for the determination of trace elements by ICP-MS clearly showed that fusion with lithium metaborate was the most appropriate sample preparation technique for complete digestion of meimechites. The open beaker HF-HNO3-HClO4 acid digestion was insufficient because the results for Nb, Ta, V, Zr, Cr and Hf were underestimated by 20-80% compared to those determined using the fusion method due to the presence in the rock samples of acid-resistant accessory minerals. It is shown that using analytical data from acid digestion may lead to erroneous interpretation of geochemical data.
Geochemistry International, Vol. 57, 11, pp. 1125-1146.
Russia, Siberia
carbonatite
Abstract: We report the first combined investigation (neutron activation, X-ray fluorescence, and electron microprobe analysis) of mineral forms of Au and Ag and noble metal distribution in the sulfide-bearing phoscorites and carbonatites of the Guli alkaline ultrabasic massif (Polar Siberia) and magnetite and sulfide separates from these rocks. The highest noble metal contents were observed in the sulfide separates from the carbonatites: up to 2.93 Pt, 61.6 Au, and 3.61 ppm Ag. Pyrrhotite, djerfisherite, chalcopyrite, and pyrite are the most abundant sulfides and the main hosts for Au and Ag. The latest assemblage of chalcopyrite, Ag-rich djerfisherite, lenaite, sternbergite, and native silver shows significant Ag concentrations. The wide occurrence of K sulfides and presence of multiphase inclusions in pyrrhotite consisting of rasvumite, K?Na–Ca carbonate, carbocernaite, strontianite, galena, chalcopyrite, sternbergite, lenaite, and native silver suggest that the sulfides were formed at high activities of K, Na, Sr, LREE, F, Cl, and S. Chlorine shows high complex-forming capacity to Ag and could be an agent of noble metal transport in the carbonatites. Crystallization of the early djerfisherite–pyrrhotite assemblages of the phoscorites and carbonatites began at a temperature not lower than 500°C and continued up to the formation of late Ag-bearing sulfides at temperatures not higher than 150°C. The carbonatite-series rocks could be enriched in Au and Ag during late low-temperature stages and serve as a source for Au placers.
Tectonic setting of allochthonous upper mantle ultramafic rocks in the Cache Creek terrane of The north western Canadian Cordillera
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A4. Abstract
Geochimica et Cosmochimica Acta, Vol. 222, Feb. 1, pp. 447-466.
Mantle
Thermodynamics
Abstract: Oxygen fugacity of the mantle is a crucial thermodynamic parameter that controls such fundamental processes as planetary differentiation, mantle melting, and possible core-mantle exchange. Constraining the evolution of the redox state of the mantle is of paramount importance for understanding the chemical evolution of major terrestrial reservoirs, including the core, mantle, and atmosphere. In order to evaluate the secular evolution of the redox state of the mantle, oxygen fugacities of six komatiite systems, ranging in age from 3.48 to 2.41?Ga, were determined using high-precision partitioning data of the redox-sensitive element vanadium between liquidus olivine, chromite and komatiitic melt. The calculated oxygen fugacities range from ?0.11?±?0.30 ?FMQ log units in the 3.48?Ga Komati system to +0.43?±?0.26 ?FMQ log units in the 2.41?Ga Vetreny system. Although there is a slight hint in the data for an increase in the oxygen fugacity of the mantle between 3.48 and 2.41?Ga, these values generally overlap within their respective uncertainties; they are also largely within the range of oxygen fugacity estimates for modern MORB lavas of +0.60?±?0.30 ?FMQ log units that we obtained using the same technique. Our results are consistent with the previous findings that argued for little change in the mantle oxygen fugacity since the early Archean and indicate that the mantle had reached its nearly-present day redox state by at least 3.48?Ga.
Geochimica et Cosmochimica Acta, Vol. 222, 1, pp. 447-466.
Canada, Ontario
komatiites
Abstract: Oxygen fugacity of the mantle is a crucial thermodynamic parameter that controls such fundamental processes as planetary differentiation, mantle melting, and possible core-mantle exchange. Constraining the evolution of the redox state of the mantle is of paramount importance for understanding the chemical evolution of major terrestrial reservoirs, including the core, mantle, and atmosphere. In order to evaluate the secular evolution of the redox state of the mantle, oxygen fugacities of six komatiite systems, ranging in age from 3.48 to 2.41 Ga, were determined using high-precision partitioning data of the redox-sensitive element vanadium between liquidus olivine, chromite and komatiitic melt. The calculated oxygen fugacities range from -0.11 ± 0.30 ?FMQ log units in the 3.48 Ga Komati system to +0.43 ± 0.26 ?FMQ log units in the 2.41 Ga Vetreny system. Although there is a slight hint in the data for an increase in the oxygen fugacity of the mantle between 3.48 and 2.41 Ga, these values generally overlap within their respective uncertainties; they are also largely within the range of oxygen fugacity estimates for modern MORB lavas of +0.60 ± 0.30 ?FMQ log units that we obtained using the same technique. Our results are consistent with the previous findings that argued for little change in the mantle oxygen fugacity since the early Archean and indicate that the mantle had reached its nearly-present day redox state by at least 3.48 Ga.
Geochimica et Cosmochimica Acta, Vol. 250, 1, pp. 49-75.
Mantle
picrites
Abstract: The oxygen fugacities of nine mantle-derived komatiitic and picritic systems ranging in age from 3.55?Ga to modern day were determined using the redox-sensitive partitioning of V between liquidus olivine and komatiitic/picritic melt. The combined set of the oxygen fugacity data for seven systems from this study and the six komatiite systems studied by Nicklas et al. (2018), all of which likely represent large regions of the mantle, defines a well-constrained trend indicating an increase in oxygen fugacity of the lavas of ?1.3 ?FMQ log units from 3.48 to 1.87?Ga, and a nearly constant oxygen fugacity from 1.87?Ga to the present. The oxygen fugacity data for the 3.55?Ga Schapenburg komatiite system, the mantle source region of which was previously argued to have been isolated from mantle convection within the first 30?Ma of the Solar System history, plot well above the trend and were not included in the regression. These komatiite’s anomalously high oxygen fugacity data likely reflect preservation of early-formed magma ocean redox heterogeneities until at least the Paleoarchean. The observed increase in the oxygen fugacity of the studied komatiite and picrite systems of ?1.3 ?FMQ log units is shown to be a feature of their mantle source regions and is interpreted to indicate secular oxidation of the mantle between 3.48 and 1.87?Ga. Three mechanisms are considered to account for the observed change in the redox state of the mantle: (1) recycling of altered oceanic crust, (2) venting of oxygen from the core due to inner core crystallization, and (3) convection-driven homogenization of an initially redox-heterogeneous primordial mantle. It is demonstrated that none of the three mechanisms alone can fully explain the observed trend, although mechanism (3) is best supported by the available geochemical data. These new data provide further evidence for mantle involvement in the dramatic increase in the oxygen concentration of the atmosphere leading up to the Great Oxidation Event at ?2.4?Ga.
Abstract: New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (?530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar–Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3–4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5–6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.
Compositional heterogeneity of the continental lithospheric mantle beneath the Early Precambrian and Phanerozoic structures: evidence from mantle xenoliths.
Geochemistry International, Vol. 45, 11, pp. 1077-1102.
The provenance of fertile off craton lithospheric mantle: Sr Nd isotope chemical composition of garnet and spinel peridotite xenoliths from Vitim, Siberia.
Chemical Geology, Vol. 217, 1-2, April 15, pp. 41-75.
Economic Geology Research Institute 2015, Vol. 17,, # 2121, 1p. Abstract
Technology
Barometer
Abstract: Original monomineral thermobarometers for mantle peridotites for clinopyroxene, garnet, chromite and ilmenites for the mantle peridotites were statistically calibrated on the PT estimates for mantle peridotites [Ashchepkov et al., 2010] were tested using the mineral phases obtained in high pressure experiments with the natural peridotites (380 runs) [Brey et al.,1990; 2008 etc] and eclogites (240 runs)[Dasgupta et al., 2006 etc]. In the original program of that written on FORTRAN are assembled the most reliable methods of mineral thermometers (45) and barometers (36) and oxybarometers (5), including original monomineral and methods [Ashchepkov, 2003 Ashchepkov et al., 2008; 2009; 2010; 2011] for the mantle peridotites bases on the compositions of on clinopyroxene, garnet, chromite and ilmenite. Program reads the text files, which converted from Excel. Original data include standard silicate compositions for 12 components in standard order. The text file includes 15 columns of 8 symbols. The first is file name which is the same for all the minerals in the association. The second is indicator symbol for phases. E- enstatite, D - diopside, O-olivine, S-spinel, G- garnet, I –ilmenite, A- amphibole, F – phlogopite, P-plagioclase, L- liquid, R- bulk rock. Then follow oxides: SiO2, TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, CaO, Na2O, K2O, NiO, V2O3 written with 2–3 decimals. The last column may contain description of the mineral or association up to 64 symbols. Monomineral methods use calculated values for Fe#Ol or Fe#Cpx. The input from console includes file name (8 symbols) (A8), then amount of PT pairs of numbers thermometers and barometers (2I2) and one for FO2 method. Program allows input of the iteration numbers (to 25 by default). It allow to choose whether to use the calculated Fe3+ for the minerals and also. It is possible also to put fixed values of T and P (default 1000oC and 40 kbar ).
Ashchepkov, I., Ntaflos, T., Logvinova, A., Vladykin, N., Ivanov, A., Spetsius, Z., Stegnitsky, Y., Kostrovitsky, S., Salikhov, R., Makovchuk, I., Shmarov, G., Karpenko, M., Downes, H., Madvedev, N.
Abstract: The PTX diagrams for the separate phases in Sytykanskaya (Ashchepkov et al., 2016) Dalnyaya (Ashchepkov et al., 2017), pipes shows that the PK show the relatively simple P-X trends and geotherms and shows more contrast and simple layering. The PK contain most abundant material from the root of the magma generation they are dunitic veins as the magma feeders represented by the megacrysts. New results for the Aykhal, Zarya and Komsomolskaya pipes in Alake field and Zarnitsa and Udachnaya pipes in Daldyn field show that evolution is accompanied by the developing of metasomatites and branching and veining of the wall rock peridotites . In Aykhal pipe in PK the Gar- dunites prevail, the xenoliths from the dark ABK "Rebus" contain Cr-Ti - rich garnets and ilmenites, more abundant compared with the grey carbonited breccia Nearly the same features were found for Yubileinaya pipe. The example of Komsomolskya pipes show that the ABK contain more eclogitic xenolith than PK. The developing of the magma channel shown in satellite Chukukskaya and Structurnaya pipe was followed by the separation of some parts of the magmatic feeders and crystallization of abundant Gar megacrysts near o the walls blocking the peridotites from the magma feeder. This drastically decrease diamond grade of pipes. Such blocking seems to be the common features for the latest breccias. In Zarnitsa pipe, the dark PK and ABK also contain fresh xenoliths but not only dunites but also sheared and metasomatic varieties and eclogites. Most of dark ABK in Yakutia contain the intergrowth of ilmenites with brown Ti- Cpx showing joint evolution trends. The late breccia contains completely altered peridotite xenoliths mainly of dunite- harzburgite type. The comparison of the trace elements of the coexisting minerals in megacryst show that they were derived from the protokimberlites but are not in complete equilibrium as well as other megacrystalline phases. Ilmenites show inflections of the trace element patterns of most Ilmenites but more regular for the Cpx and Garnets revealing the sub parallel patterns elevating LREE with the rising TRE. But commonly these are not continuous sequances because they developed in the pulsing moving systems like beneath Zarnitsa. The minerals from the feeders like dunites also show the inflected or S-type REE patterns. From the earlier to later phases the TRE compositions became more evolved reflecting the evolution of protokimberlites. The wall rocks also often show the interaction with the more evolved melts and sometimes "cut" spectrums due to the dissolution some phases and repeated melting events So we could suggest the joint evolution of the mantle column protokimberlites and megacrysts composition and type of kimberlites with the diamond grade. The mantle lithospheric base captured by the PK. The developing and rising protokimbelrites was followed by the crystallization of the diamonds in the gradient in FO2 zone in wall rocks due to reductions of C -bearing fluids and carbonatites (> 1 QMF) on peridotites ((< -2 -5 QMF). The most intensive reactions are near the graphite - diamond boundary where protokimberlites are breaking and where most framesites are forming.
Abstract: Variations of the structure and composition of mantle terranes in the terminology of the Siberian craton were studied using database (>60000) EPMA of kimberlite xenocrysts from the pipes of Yakutian kimberlite province (YKP) by a team of investigators from IGM, IGH, IEC and IGBM SB RAS and ALROSA company. The monomineral thermobarometry (Ashchepkov et al., 2010, 2014, 2017) Geochemistry of minerals obtained LA ICP MS was used to determine the protolith, melting degree, Type of the metasomatism . The mantle stratification commonly was formed by 6-7 paleosubduction slabs, separated by pyroxenite, eclogite, and metasomatic horizons and dunite lenses beneath kemberltes . We built mantle sections across the kimberlite field and transects of craton. Within the established tectonic terrains strengthening to thousands km (Gladkochub et al, 2006), the collage of microplates was determined at the mantle level. Under the shields of Anabar and Aldan lower SCLM consist of 3 -4 dunites dunites with Gar-Px-Ilm- Phl nests. Terranes framing protocratons like suture Khapchanskyare are saturated in eclogites and pyroxenites, sometimes dominated probably represent the ascending bodies of igneous eclogites intruding mantle lithosphere (ML). The ubiquitous pyroxenite layer at the level of 3.5-4.5 GPa originated in the early Archaean when melted eclogites stoped stoped subdction. Beneath the Early Archaean granite-greenstone terranes - Tunguskaya, Markhinskaya, Birektinskaya, Shary-Zhalgaiskaya (age to~3.8-3.0 GA) (Gladkochub et al., 2018) the SCLM is less depleted and often metasomatized having flat structures in some subterrains. Daldyn and Magan granulite-orthogneisic terranes have a layered and folded ML seen in N-S sections from Udachnaya to Krasnopresnenskaya less pronounced in latitudinal direction. From Daldyn to Alakit field increases the degree of Phl metasomatism and Cpx alkalinity. The most productive Aykhal and Yubleynaya pipes confined to the dunite core. Within the Magan terrane, the thin-layered SCLM have depleted base horizon. Granite-greenstone Markha terrane contains pelitic eclogites. Central and Northern craton parts show slight inclination of paleoslabs to West. The formation of SCLM in Hadean accompanied by submelting (Perchuk et al., 2018, Gerya, 2014.) had no deep roots. Ultrafine craton nuclei like Anabar shield was framed by steeper slab. During 3.8-3.0 GA craton keel growth in superplume periods (Condie, 2004) when melted eclogites and peridotites acquiring buoyancy of the sinking plate melted. For peridotites, the melting lines calculated from the experimental data (Herzberg, 2004) mainly lie near 5-6 GPA (Ionov et al., 2010; 2015). In classical works all geotherms are conductive (Boyd, 1973), but this is quite rare. The garnet pyroxene geotherms for (Ashchepkov et al., 2017) calculated with most reliable methods (Nimis, Taylor, 2000; McGregor , 1974; Brey Kohler, Nickel Green, 1985; Ashchepkov et al., 2010; 2017) give are sub-adiabatic and are formed during the melt percolation superplume vent often in presence of volatiles (Wyllie, Ryabchikov, 2000) and therefore, after superplumes trends P-Fe# of garnet are smoothed and change the tilts.
Abstract: Synchrotron Mössbauer Source (SMS) spectroscopy (ESRF, Grenoble, France) has high spatial resolution (~20 microns) and has been successfully applied to measuring Fe3+ concentrations in diamond inclusions [1,2]. Over the last few decades a number of studies have been conducted on individual minerals from mantle xenoliths in order to determine the oxidation state of the upper mantle [3,4]. These studies were conducted using ?50 mg of handpicked grains as opposed to individual crystals. In this study, we applied SMS to measure ferric iron contents of individual spinels, orthopyroxenes, clinopyroxenes and garnets from 5 spinel peridotite xenoliths and 1 pyroxenite and 2 eclogite xenoliths. Spinel xenoliths derive from Kilbourne hole, Mont Briançon and Ichinomegata. Spinels from these xenoliths were previously analysed by Mössbauer spectroscopy on bulk separates [4]. Eclogite xenoliths (UAS 1055, UAS 1525) and pyroxenite xenolith (UAS 510) were obtained from Udachnaya kimberlite pipe in Siberia. In spinel peridotites measured ratios range between 0.04- 0.14 Fe3+/Fetot for Opx, 0.14-0.19 Fe3+/Fetot for Cpx and between 0.15-0.23 for Spl. These values are broadly in agreement with previous measurements [3]. In eclogites and pyroxenite, the ratios range between 0.05-0.16 for garnet and 0.07-0.17 for Cpx, showing DGrt/Cpx for Fe3+ of 0.8-1.9. Oxygen fugacities derived from the spinel-olivineorthopyroxene oxybarometer are consistent with previous results for the continental lithosphere fO2 of between -1 and +1 log units relative to the FMQ buffer [5]. Nevertheless we observe small differences between our results on individual grains and previous data on bulk separates.
Minerals, Vol. 10, 9, 740 10.3390/ min10090755 29p. Pdf
Russia, Yakutia
deposit - Zapplyarnaya
Abstract: Minerals from mantle xenoliths in the Zapolyarnaya pipe in the Upper Muna field, Russia and from mineral separates from other large diamondiferous kimberlite pipes in this field (Deimos, Novinka and Komsomolskaya-Magnitnaya) were studied with EPMA and LA-ICP-MS. All pipes contain very high proportions of sub-calcic garnets. Zapolyarnaya contains mainly dunitic xenoliths with veinlets of garnets, phlogopites and Fe-rich pyroxenes similar in composition to those from sheared peridotites. PT estimates for the clinopyroxenes trace the convective inflection of the geotherm (40-45 mW•m?2) to 8 GPa, inflected at 6 GPa and overlapping with PT estimates for ilmenites derived from protokimberlites. The Upper Muna mantle lithosphere includes dunite channels from 8 to 2 GPa, which were favorable for melt movement. The primary layering deduced from the fluctuations of CaO in garnets was smoothed by the refertilization events, which formed additional pyroxenes. Clinopyroxenes from the Novinka and Komsomolskaya-Magnitnaya pipes show a more linear geotherm and three branches in the P-Fe# plot from the lithosphere base to the Moho, suggesting several episodes of pervasive melt percolation. Clinopyroxenes from Zapolyarnaya are divided into four groups according to thermobarometry and trace element patterns, which show a stepwise increase of REE and incompatible elements. Lower pressure groups including dunitic garnets have elevated REE with peaks in Rb, Th, Nb, Sr, Zr, and U, suggesting mixing of the parental protokimberlitic melts with partially melted metasomatic veins of ancient subduction origin. At least two stages of melt percolation formed the inclined PT paths: (1) an ancient garnet semi-advective geotherm (35-45 mW•m?2) formed by volatile-rich melts during the major late Archean event of lithosphere growth; and (2) a hotter megacrystic PT path (Cpx-Ilm) formed by feeding systems for kimberlite eruptions (40-45 mW•m?2). Ilmenite PT estimates trace three separate PT trajectories, suggesting a multistage process associated with metasomatism and formation of the Cpx-Phl veinlets in dunites. Heating associated with intrusions of protokimberlite caused reactivation of the mantle metasomatites rich in H2O and alkali metals and possibly favored the growth of large megacrystalline diamonds.
Journal of Asian Earth Sciences, Vol. 213, 104756, 22p.pdf
Russia, Siberia
deposit - Zarnitsa
Abstract: Zarnitsa kimberlite pipe in Central Yakutia contains pyrope garnets with Cr2O3 ranging from 9 to 19.3 wt% derived from the asthenospheric mantle. They show mostly S-shaped, inflected rare earth element (REE) patterns for dunitic and harzburgitic, lherzolitic and harzburgitic varieties and all are rich in high field strength elements (HFSE) due to reaction with protokimberlite melts. Lithospheric garnets (<9 wt% Cr2O3) show a similar division into four groups but have more symmetric trace element patterns. Cr-diopsides suggest reactions with hydrous alkaline, protokimberlitic and primary (hydrous) partial melts. Cr-diopsides of metasomatic origin have inclined REE patterns and high LILE, U, Th and Zr concentrations. Four groups in REE of Ti-rich Cr-diopsides, and augites have asymmetric bell-like REE patterns and are HFSE-rich. Mg-ilmenites low in REE were formed within dunite conduits. Ilmenite derived from differentiated melts have inclined REE patterns with LREE ~ 100 × chondrite levels. Thermobarometry for dunites shows a 34 mWm?2 geotherm with a HT branch (>50 mWm?2) at 6-9 GPa, and a stepped HT geotherm with heated pyroxenite lenses at four levels from 6.5 to 3.5 GPa. Parental melts calculated with KDs suggest that augites and high-Cr garnets in the lithosphere base reacted with essentially carbonatitic melts while garnets from lower pressure show subduction peaks in U, Ba and Pb. The roots of the Zarnitsa pipe served to transfer large portions of deep (>9 GPa) protokimberlite melts to the lithosphere. Smaller diamonds were dissolved due to the elevated oxidation state but in peripheral zones large diamonds could grow.
Geological Society of London Special Publications, 513, 34p. Pdf
Russia
deposit - Upper Muna
Abstract: Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe-Mg-rich rocks in the Yakutian kimberlite province (average FeOtotal = 8.4 wt%, MgO = 32.36 wt%, TiO2 = 1.6 wt.%). Striking mineralogical features of Upper Muna kimberlites are: 1) abundance of monticellite and perovskite in the groundmass; 2) rare occurrence of Mg-ilmenite; 3) abundance of phlogopite megacrysts (up to 8 cm across); 4) coexistence of low-Cr (0.1-4wt. % Cr2O3, with 0.8-1.2 wt.% TiO2), and high-Cr (3-8 wt.% Cr2O3, with 0.1-0.6 wt.% TiO2) garnet megacrysts with contrasting REE patterns. The compositional features of groundmass minerals, the relatively low CaO and CO2 contents in kimberlites, and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite-harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide P-T range (40-65 kbar and 900-1350 °C). Mineral geochemistry, trace element distribution and Sr-Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.
Abstract: A database of microprobe EPMA and ICP MS analysis of the kimberlite indicator minerals > 20,000 of the Lunda and Kukumbi-Kwango kimberlite regions were used for construction of series PTCFO2 diagrams for mantle section beneath major kimberlite pipes the and profile through the subcontinental lithospheric mantle (SLM) beneath the NE Angolan Kasai craton within the Lucapa tectonic "corridor", which controls the kimber-lite volcanism in the North of Angola. The general construction of the mantle sections are similar for most pipes but details of the structure refer t the mineralogy and degree of the hydrous metasomatism. The vertical and lat-eral heterogeneity of the mantle in this region and the PTCFO2 parameters in mantle beneath diamond-bearing kimberlite pipes were calculated. There is the inclination of the general mantle structure toward the SW marked by the depleted layer of mantle peridotites. The local dome-like structures are found near the lithosphere base beneath Catoca, Luaxe field and other kimberlite regions. Mapping of the upper mantle beneath the Angolan Archaean cratons of is a relatively pioneer direction in the regional diamond forecasting, developed by the authors.
Ionov, D.A., Ashchepkov, I.V., Stosch, H.G., et al.
Garnet peridotite xenoliths from the Vitim volcanic field, Baikal region:the nature of the garnet-spinel peridotite transition zone in the continentalmantle.
Journal of Petrology, Vol. 34, No. 6, pp. 1141-1175.
Petrology and mineralogy of disintegrated mantle inclusions of kimberlite like diatremes from the Aldan Shield ( Chompolo field): mantle reconstructions.
Alkaline Magmatism and the problems of mantle sources, pp. 161-176.
Variations of the oxygen conditions in mantle column beneath Siberian kimberlite pipes and it's application to lithospheric structure of feeding systems.
Vladykin: VI International Workshop, held Mirny, Deep seated magmatism, its sources and plumes, pp. 125-144.
Wyoming craton mantle lithosphere: reconstructions based on xenocrysts from Sloan and Kelsey Lake kimberlites.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 13-27.
Abstract: Mantle xenoliths (>150) and concentrates from late autolithic breccia and porphyritic kimberlite from the Sytykanskaya pipe of the Alakit field (Yakutia) were analyzed by EPMA and LAM ICP methods. In P-T-X-f(O2) diagrams minerals from xenoliths show widest variations, the trends P-Fe#-CaO, f(O2) for minerals from porphyric kimberlites are more stepped than for xenocrysts from breccia. Ilmenite PTX points mark moving for protokimberlites from the lithosphere base (7.5 GPa) to pyroxenite lens (5-3.5 GPa) accompanied by Cr increase by AFC and creation of two trends P-Fe#Ol ?10-12% and 13-15%. The Opx-Gar-based mantle geotherm in Alakit field is close to 35 mW/m2 at 65 GPa and 600 °C near Moho was determined. The oxidation state for the megacrystalline ilmenites is lower for the metasomatic associations due to reduction of protokimberlites on peridotites than for uncontaminated varieties at the lithosphere base. Highly inclined linear REE patterns with deep HFSE troughs for the parental melts of clinopyroxene and garnet xenocrysts from breccia were influenced by differentiated protokimberlite. Melts for metasomatic xenoliths reveal less inclined slopes without deep troughs in spider diagrams. Garnets reveal S-shaped REE patterns. The clinopyroxenes from graphite bearing Cr-websterites show inclined and inflected in Gd spectrums with LREE variations due to AFC differentiation. Melts for garnets display less inclined patterns and Ba-Sr troughs but enrichment in Nb-Ta-U. The 40Ar/39Ar ages for micas from the Alakit mantle xenoliths for disseminated phlogopites reveal Proterozoic (1154 Ma) age of metasomatism in early Rodinia mantle. Veined glimmerites with richterite - like amphiboles mark ?1015 Ma plume event in Rodinia mantle. The ?600-550 Ma stage manifests final Rodinia break-up. The last 385 Ma metasomatism is protokimberlite-related.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 203-232.
Russia, Yakutia
Deposit - Sytykanskaya
Abstract: The concentrate from two phases of the kimberlite (breccia and porphyritic kimberlite) and about 130 xenoliths from the Sytykanskaya pipe of the Alakit field (Yakutia) were studied by EPMA and LAM ICP methods. Reconstructions of the PTXfO2 mantle sections were made separately for the two phases. The porphyritic kimberlites and breccia show differences in the minerals although the layering and pressure interval remains the same. For the porphyritic kimberlite the trends P- Fe# - CaO in garnet, fO2 are sub-vertical while the xenocrysts from the breccia show stepped and curved trends possibly due to interaction with fluids. Minerals within xenoliths show the widest variation in all pressure intervals. PT points for the ilmenites which trace the magmatic system show splitting of the magmatic source into two levels at the pyroxenite lens (4GPa) accompanied by peridotite contamination and an increase in Cr in ilmenites. Two groups of metasomatites with Fe#Ol ~ 10-12% and 13-15% were created by the melts derived from protokimberlites and trace the mantle columns from the lithosphere base (Ilm - Gar - Cr diopside) to Moho becoming essentially pyroxenitic (Cr-diopside with Phl). The first Opx-Gar-based mantle geotherm from the Alakit field has been constructed from15 associations and is close to 35 mw/m2 in the lower part of mantle section but deviates to high temperatures in the upper part of the mantle section. The oxidation state for the protokimberlite melts determined from ilmenites is higher than for the other pipes in the Yakutian kimberlite province which probably accounts for the decrease in the diamond grade of this pipe. The geochemistry of the minerals (garnets and clinopyroxenes) from breccias, metasomatic peridotite xenoliths and pyroxenites systematically differ. Xenocrysts from the breccia were produced by the most differentiated melts and enriched protokimberlite or carbonatite; they show highly inclined nearly linear REE patterns and deep troughs of HFSE. Minerals of the metasomatic xenoliths are less inclined with lower La/Cen ratios and without troughs in spider diagrams. The garnets often show S-shaped patterns. Garnets from the Cr websterites show round REE patterns and deep troughs in Ba-Sr but enrichment in Nb-Ta-U. The clinopyroxenes reveal the inclined and inflected on Gd spectrums with variations in LREE due to AFC differentiation. The 40Ar-39Ar ages for micas from the Alakit field reveal three intervals for the metasomatism. The first (1154 Ma) relates to dispersed phlogopites found throughout the mantle column, and probably corresponds to the continental arc stage in the early stage of Rodinia. Veined highly alkaline and Ti-rich veins with richterite ~1015 Ma corresponds to the plume event within the Rodinia mantle. The ~600-550 Ma stage marks the final Rodinia break-up. The last one near 385 Ma is protokimberlite related.
Abstract: Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature (P-T) ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE) and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE), sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P-Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6-7 linear arrays of P-Fe# in the lower part of the mantle section at 7.5-3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P-Fe# trend indicating widespread metasomatism is associated with decreased diamond grades. Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel. Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins. Incorporation of island arc peridotites from an eroded arc is a possible scenario.
Abstract: New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (?530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar-Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3-4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.
Abstract: The Dalnyaya kimberlite pipe (Yakutia, Russia) contains mantle peridotite xenoliths (mostly lherzolites and harzburgites) that show both sheared porphyroclastic (deformed) and coarse granular textures, together with ilmenite and clinopyroxene megacrysts. Deformed peridotites contain high-temperature Fe-rich clinopyroxenes, sometimes associated with picroilmenites, which are products of interaction of the lithospheric mantle with protokimberlite related melts. The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe. Coarse granular xenoliths fall on a geotherm of 35 mWm?2 whereas deformed varieties yield a 45 mWm?2 geotherm in the 2-7.5 GPa pressure interval. The chemistry of the constituent minerals including garnet, olivine and clinopyroxene shows trends of increasing Fe# (=Fe/(Fe + Mg)) with decreasing pressure. This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels. Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals, determined by LA-ICP-MS. Orthopyroxenes, some clinopyroxenes and rare garnets are depleted in Ba, Sr, HFSE and MREE and represent relic lithospheric mantle. Re-fertilized garnet and clinopyroxene are more enriched. The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene. Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets. Olivines show decreases in Ni and increases in Al, Ca and Ti from Mg-rich varieties to the more Fe-rich, deformed and refertilized ones. Minerals showing higher Fe# (0.11-0.15) are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels. In P-f(O2) diagrams, garnets and Cr-rich clinopyroxenes indicate reduced conditions at the base of the lithosphere at ?5 log units below a FMQ buffer. However, Cr-poor clinopyroxenes, together with ilmenite and some Fe-Ca-rich garnets, demonstrate a more oxidized trend in the lower part of lithosphere at ?2 to 0 log units relative to FMQ. Clinopyroxenes from xenoliths in most cases show conditions transitional between those determined for garnets and megacrystalline Cr-poor suite. The relatively low diamond grade of Dalnyaya kimberlites is explained by a high degree of interaction with the oxidized protokimberlite melts, which is greater at the base of the lithosphere.
Abstract: Mantle xenolith from Komsomolskya and Zarnitsa pie were used for the reconstryctions of mantle columns beneath theses kimberlite pipes. Relatively fresh mantle xenolith from Zarnitsa and Komsomolskaya pipes we used for PTX reconstructions of mantle sections. In Zarnitsa dunites - harburgites with richterite, Phl-Ilm veins, sheared lherzolites, pyroxenites (with amphibole) and eclogites and deformed peridotites. Mg -rich Gar and Opx formed stepped P-Fe# trend, Fe- enriched Cpx with Ilm were created mostly by protkimberlites. Sub Ca garnets rarely show U spikes while Ti rich show Th, U, Ta, Nb, Zr and peaks Many minerals demonstrate Th enrichment due to carbonitites. In mantle of Komsomolskaya pipe Phl is wide spreadin periditites from lherzolites ti dunites and in eclogites. There are 6 intervals with sharp division at 5 GPa.Mg eclogites prevae in lower part while fe- enriched in middle part. The Fe# rise is detevcted in lower and upper parts of mantle section. The TRE spiderdiagrams of grnets shows U -pb subduction peaks But Cpx mainly show n Th- peak. The ages of eclogites ogive 500-600 Ma (one 1525 MA) which is much less than in Zarnitsa or Udachnaya having Proterozoic - Archean ages.
International Geology Review, in press available 24p. Pdf
Europe, Ukraine
deposit - Priazovie
Abstract:
Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: Ist layer at 2.5-1 GPa is enriched mantle (Fe#Ol ~ 0.11 ? 0.14) with Gar- pyroxenites and Sp peridotites; IInd at 2.5-3.2 GPa - Gar-Sp (Fe#Ol 0.08 ? 0.10) peridotite. IIId at 4.3-3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 ? 0.095. IVth at 3.2-5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 ? 0.125); Vth at 5.8 ? 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VIth layer at 5.8-6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VIIth layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets. Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Ybn, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.
Vestnik Otdelenia nauk o Zemle RAN *** ENG, Vol. 3, doi:10.2205/2011NZ000138, 5p. Pdf * note date
Mantle
geobarometry, peridotites
Abstract: Original monomineral thermobarometers for mantle peridotites for clinopyroxene, garnet, chromite and ilmenites for the mantle peridotites were statistically calibrated on the PT estimates for mantle peridotites [Ashchepkov et al., 2010] were tested using the mineral phases obtained in high pressure experiments with the natural peridotites (380 runs) [Brey et al.,1990; 2008 etc] and eclogites (240 runs)[Dasgupta et al., 2006 etc]. In the original program of that written on FORTRAN are assembled the most reliable methods of mineral thermometers (45) and barometers (36) and oxybarometers (5), including original monomineral and methods [Ashchepkov, 2003 Ashchepkov et al., 2008; 2009; 2010; 2011] for the mantle peridotites bases on the compositions of on clinopyroxene, garnet, chromite and ilmenite. Program reads the text files, which converted from Excel. Original data include standard silicate compositions for 12 components in standard order. The text file includes 15 columns of 8 symbols. The first is file name which is the same for all the minerals in the association. The second is indicator symbol for phases. E- enstatite, D - diopside, O-olivine, S-spinel, G- garnet, I -ilmenite, A- amphibole, F - phlogopite, P-plagioclase, L- liquid, R- bulk rock. Then follow oxides: SiO2, TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, CaO, Na2O, K2O, NiO, V2O3 written with 2-3 decimals. The last column may contain description of the mineral or association up to 64 symbols. Monomineral methods use calculated values for Fe#Ol or Fe#Cpx. The input from console includes file name (8 symbols) (A8), then amount of PT pairs of numbers thermometers and barometers (2I2) and one for FO2 method. Program allows input of the iteration numbers (to 25 by default). It allow to choose whether to use the calculated Fe3+ for the minerals and also. It is possible also to put fixed values of T and P (default 1000o C and 40 kbar ).
Solid Earth Discussions, Vol. 5, pp. 1-75. pdf * note date
Russia, Yakutia
picroilmenites
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages.
High-pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10-100)/PM with La / Ybn ~ 10-25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet-spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn-Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Revista Brasileira de Geociencas*** ENG, Vol. 31, 4, pp. 653-660. pdf
South America, Brazil
kimberlites
Abstract: Garne ts from couc eru ratc from the vargcm l kimberl ite pipe show a long compos itional range and reveallong lincar tre nds within the lherzolite field in a Cr~Ol - CaO% dia gram (Sobolcv et til. 1974) (lip (0 11% MgO). fon ned by grains of different dimensions with fcw deviations to harzburg itcs . Larger grains (fraction +3) arc higher in CaO with less Cr~01 (to 5.5%). TIle Cr20 1 freq uen cy reduc es in hyperbo lic function for each fraction . IImenites reve;1142-56% Ti0 2l..'Olllpositionai range with linear FeO - MgO correhuions but 3(4) separate groups for A I ~01 suggest different proport ion of co-prccipimted gimlet , probably due to polybn ric Irncnonanon. lncreasing Cr~O l nnd r"t..-Q% conte nt (fractionation uegn:e ) with red ucing TiO~ is in accord with Ar c mod el.. Ganict xenolith fnnnldnin II pipe with large Ga r- Cpxgrains and fine Mica-Curb bearing mat rix refer to 60 kbcr and 35 mv/m2 gcothcrm . 11displays enr iched trace c lement pat ter ns but not completely equilibrated compositions for Ga r anti Cpx. sugges ting low degree me lting of rela tively fertile mantle. St udied uuuc rinlmay s uggcsrmcrasomu tized, relat ively fertile and irre gularly heated mantle bene ath Sombcrn Bra zil as found by (Carvalho & Lccnnrdos 1997).
Lithos, Vol. 406-407. doi: 10.1016/j.lithos.2021.106499 77p. Pdf
Russia
kimberlite genesis
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Geological Society of London Special Publication 513, pp. 71-102.
Russia, Siberia
deposit - Muna
Abstract: Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe-Mg-rich rocks in the Yakutian kimberlite province (average FeOtotal = 8.4 wt%, MgO = 32.36 wt%, TiO2 = 1.6 wt%). Striking mineralogical features of Upper Muna kimberlites are: (1) abundance of monticellite and perovskite in the groundmass; (2) rare occurrence of Mg-ilmenite; (3) abundance of phlogopite megacrysts (up to 8 cm across); and (4) coexistence of low-Cr (0.1-4 wt% Cr2O3, with 0.8-1.2 wt% TiO2) and high-Cr (3-8 wt% Cr2O3, with 0.1-0.6 wt% TiO2) garnet megacrysts with contrasting rare earth element patterns. The compositional features of groundmass minerals, the relatively low CaO and CO2 contents in kimberlites and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite-harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide pressure and temperature range (40-65 kbar and 900-1350°?). The mineral geochemistry, trace element distribution and Sr-Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.
Geological Society of London Special Publication 513, pp. 71-102.
Russia, Siberia
deposit - Muna
Abstract: Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe-Mg-rich rocks in the Yakutian kimberlite province (average FeOtotal = 8.4 wt%, MgO = 32.36 wt%, TiO2 = 1.6 wt%). Striking mineralogical features of Upper Muna kimberlites are: (1) abundance of monticellite and perovskite in the groundmass; (2) rare occurrence of Mg-ilmenite; (3) abundance of phlogopite megacrysts (up to 8 cm across); and (4) coexistence of low-Cr (0.1-4 wt% Cr2O3, with 0.8-1.2 wt% TiO2) and high-Cr (3-8 wt% Cr2O3, with 0.1-0.6 wt% TiO2) garnet megacrysts with contrasting rare earth element patterns. The compositional features of groundmass minerals, the relatively low CaO and CO2 contents in kimberlites and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite-harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide pressure and temperature range (40-65 kbar and 900-1350°?). The mineral geochemistry, trace element distribution and Sr-Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.
Jounral of Earth System Science, Vol. 131 81 doi.org/10/1007/s12040-022-01814-3 19p. Pdf
Russia
deposit - Kayla
Abstract: Origin of abundant alkaline and related lamproite massifs and dykes in Aldan shield have no explanation and the geochemistry of rocks and their xenocrysts is used for the explanation. Bulk-rock geochemistry, mineral chemistry data of the Kayla lamproites of Russia and mineral chemical data (trace and rare elements) of the mantle xenocrysts found in these lamproites was studied using ICP MS and electron microprobe analyses (EPMA). The trace element spectrum of Kayla tuffs and breccias show the similarity with the olivine lamproites and belong to the orogenic type according to Th-U-Nb systematics. Primitive mantle normalized trace element (TRE) spider diagrams show right-leaning patterns with the peaks in large ion lithophile elements Sr, Pb, U, and troughs in Ta, Nb suggesting melting of original peridotites mixed with the ancient EMI (according to Nd, Sr isotopes) source probably belonging to eclogites or lower crust. The age of the emplacement is 132-134 Ma, similar to the Chompolo lamprophyres and many other alkaline Aldan complexes. Thermo-barometric estimation from Cr-diopsides and chromites xenocrysts suggest the origin from the spinel-garnet transition in the lithospheric mantle region. The P-T estimates derived from low-Cr-clinopyroxene xenocrysts, and related to lamproites show a high heat flow of 90 mW/m2 due to interaction with the plume-related magma. The Cr-diopsides and chromites give 45 mW/m2 geotherm similar to regional heat flow. The chondrite normalized rare earth element (REE) pattern for chrome-diopsides is steeper, compared to the low-chrome varieties. Primitive mantle normalized spidergram of Cr-diopsides displays peaks for Sr, U, and Th, and deep troughs of Nd, Nb, Ta. REE. The trace element spider diagrams of both types of xenocrysts show that they were equilibrated with the lamproitic melts and reconstructed parental melts of low-Cr-clinopyroxene coincides with the lamproite spectrums.
International Geology Review, Vol. 63, 10, pp. 1288-1309.
Europe, Ukraine
deposit - Priazovie
Abstract: Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: Ist layer at 2.5-1 GPa is enriched mantle (Fe#Ol ~ 0.11 ? 0.14) with Gar- pyroxenites and Sp peridotites; IInd at 2.5-3.2 GPa - Gar-Sp (Fe#Ol 0.08 ? 0.10) peridotite. IIId at 4.3-3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 ? 0.095. IVth at 3.2-5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 ? 0.125); Vth at 5.8 ? 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VIth layer at 5.8-6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VIIth layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets. Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Ybn, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.
Chemical Reviews, Vol. 120, 4, 10.1021/ acs.chemrev.9b00578 50p. Pdf
Global
HPHT, CVD, synthetics
Abstract: Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methods: high pressure high temperature (HPHT) growth and chemical vapor deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g., by implantation or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamond alone and in association with vacancies, hydrogen, and transition metal atoms. Among these, the negatively charged nitrogen-vacancy (NV-) defect in diamond is attracting particular current interest in account of the many new and exciting opportunities it offers for, for example, quantum technologies, nanoscale magnetometry, and biosensing.
Andreoli, M.A.G., Hart, R.J., Ashwal, L.D., Coetzee, H.
Correlations between U, Th content and metamorphic grade in the Western Namaqualand Belt, South Africa: with implications for radioactive heating of the crust.
Abstract: Plate tectonics plays a vital role in the evolution of our planet. Geochemical analysis of Earth’s oldest continental crust suggests that subduction may have begun episodically about 3.8 to 3.2 billion years ago, during the early Archaean or perhaps more than 3.8 billion years ago, during the Hadean. Yet, mantle rocks record evidence for modern-style plate tectonics beginning only in the late Archaean, about 3 billion years ago. Here we analyse the nitrogen abundance, as well as the nitrogen and carbon isotopic signatures of Archaean placer diamonds from the Kaapvaal craton, South Africa, which formed in the upper mantle 3.1 to 3.5 billion years ago. We find that the diamonds have enriched nitrogen contents and isotopic compositions compared with typical mantle values. This nitrogen geochemical fingerprint could have been caused by contamination of the mantle by nitrogen-rich Archaean sediments. Furthermore, the carbon isotopic signature suggests that the diamonds formed by reduction of an oxidized fluid or melt. Assuming that the Archaean mantle was more reduced than the modern mantle, we argue that the oxidized components were introduced to the mantle by crustal recycling at subduction zones. We conclude, on the basis of evidence from mantle-derived diamonds, that modern-style plate tectonics operated as early as 3.5 billion years ago.
Nature Communications, Jan. 31, doi 10:1038/ncomms1048
Africa, Mauritius
Hot spots
Abstract: A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7?Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0?Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ?700?km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.
South African Journal of Geology, Vol. 122, 4, pp. 397-420.
India
Gondwana
Abstract: On the last page of his 1937 book "Our Wandering Continents" Alex Du Toit advised the geological community to develop the field of "comparative geology", which he defined as "the study of continental fragments". This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called "ARCs") and their Deformed equivalents (called "DARCs"), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian - Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called “Mauritia”, and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge “jumps”. This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.
South African Journal of Geology, Vol. 122, 4, pp. 397-420.
India
alkaline, carbonatites
Abstract:
On the last page of his 1937 book “Our Wandering Continents” Alex Du Toit advised the geological community to develop the field of “comparative geology”, which he defined as “the study of continental fragments”. This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called “ARCs”) and their Deformed equivalents (called “DARCs”), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian - Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called "Mauritia", and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge "jumps". This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.
South African Journal of Geology, Vol. 124, pp. 421-442.
Africa, South Africa
magmatism
Abstract: At least four spatially overlapping Large Igneous Provinces, each of which generated ~1 x 106 km3 or more of basaltic magmas over short time intervals (<5 m.y.), were emplaced onto and into the Kaapvaal Craton between 2.7 and 0.18 Ga: Ventersdorp (2 720 Ma, ~0.7 x 106 km3), Bushveld (2 056 Ma, ~1.5 x 106 km3), Umkondo (1 105 Ma, ~2 x 106 km3) and Karoo (182 Ma, ~3 x 106 km3). Each of these has been suggested to have been derived from melting of sub-continental lithospheric mantle (SCLM) sources, but this is precluded because: (1) each widespread heating event sufficient to generate 1 to 2 x 106 km3 of basalt from the Kaapvaal SCLM (volume = 122 to 152 x 106 km3) would increase residual Mg# by 0.5 to 2 units, depending on degree of melting, and source and melt composition, causing significant depletion in already-depleted mantle, (2) repeated refertilization of the Kaapvaal SCLM would necessarily increase its bulk density, compromising its long-term buoyancy and stability, and (3) raising SCLM temperatures to the peridotite solidus would also have repeatedly destroyed lithospheric diamonds by heating and oxidation, which clearly did not happen. It is far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of continental crustal components. Combined Sr and Nd isotopic data (n = 641) for the vast volumetric majority of Karoo low-Ti tholeiitic magmatic products can be successfully modelled as an AFC mixing array between a plume-derived parental basalt, with <10% of a granitic component derived from 1.1 Ga Namaqua-Natal crust. Archaean crustal materials are far too evolved (?Nd ~ -35) to represent viable contaminants. However, a very minor volume of geographically-restricted (and over-analysed) Karoo magmas, including picrites, nephelinites, meimechites and other unusual rocks may represent low-degree melting products of small, ancient, enriched domains in the Kaapvaal SCLM, generated locally during the ascent of large-volume, plume-derived melts. The SCLM-derived rocks comprise the well-known high-Ti (>2 to 3 wt.% TiO2) magma group, have ?Nd, 182 values between +10.5 and -20.9, and are characteristically enriched in Sr (up to 1 500 ppm), suggesting a possible connection to kimberlite, lamproite and carbonatite magmatism. These arguments may apply to continental LIPs in general, although at present, there are insufficient combined Sr + Nd isotopic data with which to robustly assess the genesis of other southern African LIPs, including Ventersdorp (n = 0), Bushveld (n = 55) and Umkondo (n = 18).
Two cratons and an orogen. Excursion guidebook and review articles for afield workshop through selected Archean terranes of Swaziland, SouthAfrica, Zimbabwe
International Geological Correlation Programme (IGCP) Project 280- the oldest rocks on earth, available from Witwatersrand, approx. 400p. $ 40.00 United States incl. postage
Dampare, S.B., Asiedu, D.K., Osea, S., Nyarko, B.J.B., Banoeng-Yakubo, B.
Determination of rare earth elements by neutron activation analysis in altered ultramafic rocks from the Akwatia district of Birim Diamondiferous field.
Journal of Radioanalytical and Nuclear Chemistry , Vol. 265, 1, pp. 101-106.
Journal of Volcanology and Geothermal Research, Vol. 398, 106872 17p. Pdf
United States, Hawaii
melting
Abstract: The syneruptive decompression rate of basaltic magma in volcanic conduits is thought to be a critical control on eruptive vigor. Recent efforts have constrained decompression rates using models of diffusive water loss from melt embayments (Lloyd et al. 2014; Ferguson et al. 2016), olivine-hosted melt inclusions (Chen et al. 2013; Le Voyer et al. 2014), and clinopyroxene phenocrysts (Lloyd et al. 2016). However, these techniques are difficult to apply because of the rarity of melt embayments and clinopyroxene phenocrysts suitable for analysis and the complexities associated with modeling water loss from melt inclusions. We are developing a new magma ascent chronometer based on syneruptive diffusive water loss from olivine phenocrysts. We have found water zonation in every olivine phenocryst we have measured, from explosive eruptions of Pavlof, Seguam, Fuego, Cerro Negro and Kilauea volcanoes. Phenocrysts were polished to expose a central plane normal to the crystallographic `b' axis and volatile concentration profiles were measured along `a' and `c' axes by SIMS or nanoSIMS. Profiles are compared to 1D and 3D finite-element models of diffusive water loss from olivine, with or without melt inclusions, whose boundaries are in equilibrium with a melt undergoing closed-system degassing. In every case, we observe faster water diffusion along the `a' axis, consistent with the diffusion anisotropy observed by Kohlstedt and Mackwell (1998) for the so-called `proton-polaron' mechanism of H-transport. Water concentration gradients along `a' match the 1D diffusion model with a diffusivity of 10-10 m2/s (see Plank et al., this meeting), olivine-melt partition coefficient of 0.0007-0.002 (based on melt inclusion-olivine pairs), and decompression rates equal to the best-fit values from melt embayment studies (Lloyd et al. 2014; Ferguson et al. 2016). Agreement between the melt embayment and water-in-olivine ascent chronometers at Fuego, Seguam, and Kilauea Iki demonstrates the potential of this new technique, which can be applied to any olivine-bearing mafic-intermediate eruption using common analytical tools (SIMS and FTIR). In theory, each crystal is a clock, with the potential to record variable ascent in the conduit, over the course of an eruption, and between eruptions.
Abstract:
Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ? Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase ? spinel ? garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.
Mosenfelder, J.L., Asimow, P.D., Frost, D.J., Rubie, D.C., Ahrens, T.J.
The MgSiO3 system at high pressure: thermodynamic properties of perovskite, postperovskite and melt from global inversion of shock and static compression data.
Journal of Geophysical Research, Vol. 114, B1 B01203.
Geochimica et Cosmochimica Acta, Vol. 266, pp. 307-311.
United States, California, Africa, Morocco
metasomatism
Abstract: We investigate the potential to use concentrations and zoning patterns of phosphorus (P) in clinopyroxene as indicators of the rates of igneous and metasomatic processes, comparable to recent applications of P in olivine but applicable to more evolved rocks and lower temperatures of crystallization. Few high-P pyroxenes have been previously reported, and none have been analyzed in detail for the mechanism of P enrichment or the implications for mineral growth kinetics. Here, we report the discovery and characteristics of exotic phosphorus-rich secondary clinopyroxene in glassy pockets and veins in composite mantle xenoliths from the Cima Volcanic Field (California, USA) and the Middle Atlas Mountains (Morocco, West Africa). These glass-bearing xenoliths preserve evidence of melt infiltration events and the contrasting behavior of P in their pyroxene crystals constrains the different rates of reaction and extents of equilibration that characterized infiltration in each setting. We report optical petrography and chemical analysis of glasses and minerals for major elements by electron microprobe microanalyzer and trace elements by laser-ablation Inductively Coupled Plasma Mass Spectrometry. The Cima Volcanic Field specimen shows one end-member behavior, with unzoned P-rich clinopyroxene in a melt pocket. We attribute this occurrence to a slow crystallization process that occurred after the melt temperature reached near-equilibrium with the host rock and during which the P concentration in the melt was buffered by apatite saturation. In the Morocco xenolith, by contrast, clinopyroxene exhibits zonation with P increasing all the way to the rim, in contact with the glass. We ascribe this feature to a rapid growth process in which excess P was incorporated into the growing clinopyroxene from a diffusive boundary layer. We demonstrate quantitative agreement between the enrichment of P and other trace elements and their expected diffusion and partitioning behavior during rapid growth. We suggest that P has not been widely reported in clinopyroxene in large part because it has rarely been looked for and that its analysis offers considerable promise as a kinetic indicator both in xenoliths and volcanic rocks.
Abstract: The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide?angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30?km?thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo?reconstructions of the Gondwana. This 30?km?thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle?derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
American Mineralogist, doi:10.2138/am-2022-8186 in press
Mantle
perovskite
Abstract: Perovskite, CaTiO3, originally described as a cubic mineral, is known to have a distorted (orthorhombic) crystal structure. We herein report on the discovery of natural cubic perovskite. This was identified in gehlenite rocks occurring in a pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), in the vicinity of the Dead Sea, Negev Desert, Israel. The mineral is associated with native ?-(Fe,Ni) metal, schreibersite (Fe3P) and Si-rich fluorapatite. The crystals of this perovskite reach 50 ?m in size and contain many micron sized inclusions of melilite glass. The mineral contains significant amounts of Si substituting for Ti (up to 9.6 wt.% SiO2) corresponding to 21 mol.% of the davemaoite component (cubic perovskite-type CaSiO3), in addition to up to 6.6 wt.% Cr2O3. Incorporation of trivalent elements results in the occurrence of oxygen vacancies in the crystal structure; this being the first example of natural oxygen-vacant ABO3 perovskite with the chemical formula Ca(Ti,Si,Cr)O3-? (? ~ 0.1). Stabilization of cubic symmetry (space group Pm?3m) is achieved via the mechanism not reported so far for CaTiO3, namely displacement of an oxygen atom from its ideal structural position (site splitting). The mineral is stable at atmospheric pressure to 1250±50 °C; above this temperature its crystals fuse with the embedded melilite glass, yielding a mixture of titanite and anorthite upon melt solidification. The mineral is stable upon compression to at least 50 GPa. The a lattice parameter exhibits continuous contraction from 3.808(1) Å at atmospheric pressure to 3.551(6) Å at 50 GPa. The second-order truncation of the Birch-Murnaghan equation of state gives the initial volume V0 equal to 55.5(2) Å3 and room temperature isothermal bulk modulus K0 of 153(11) GPa. The discovery of oxygen-deficient single perovskite suggests previously unaccounted ways for incorporation of almost any element into the perovskite framework up to pressures corresponding to those of the Earth’s mantle.
American Mineralogist, doi:10.2138/am-2022-8186 in press
Mantle
perovskite
Abstract: Perovskite, CaTiO3, originally described as a cubic mineral, is known to have a distorted (orthorhombic) crystal structure. We herein report on the discovery of natural cubic perovskite. This was identified in gehlenite rocks occurring in a pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), in the vicinity of the Dead Sea, Negev Desert, Israel. The mineral is associated with native ?-(Fe,Ni) metal, schreibersite (Fe3P) and Si-rich fluorapatite. The crystals of this perovskite reach 50 ?m in size and contain many micron sized inclusions of melilite glass. The mineral contains significant amounts of Si substituting for Ti (up to 9.6 wt.% SiO2) corresponding to 21 mol.% of the davemaoite component (cubic perovskite-type CaSiO3), in addition to up to 6.6 wt.% Cr2O3. Incorporation of trivalent elements results in the occurrence of oxygen vacancies in the crystal structure; this being the first example of natural oxygen-vacant ABO3 perovskite with the chemical formula Ca(Ti,Si,Cr)O3-? (? ~ 0.1). Stabilization of cubic symmetry (space group Pm?3m) is achieved via the mechanism not reported so far for CaTiO3, namely displacement of an oxygen atom from its ideal structural position (site splitting). The mineral is stable at atmospheric pressure to 1250±50 °C; above this temperature its crystals fuse with the embedded melilite glass, yielding a mixture of titanite and anorthite upon melt solidification. The mineral is stable upon compression to at least 50 GPa. The a lattice parameter exhibits continuous contraction from 3.808(1) Å at atmospheric pressure to 3.551(6) Å at 50 GPa. The second-order truncation of the Birch-Murnaghan equation of state gives the initial volume V0 equal to 55.5(2) Å3 and room temperature isothermal bulk modulus K0 of 153(11) GPa. The discovery of oxygen-deficient single perovskite suggests previously unaccounted ways for incorporation of almost any element into the perovskite framework up to pressures corresponding to those of the Earth’s mantle.
Watremez, L., Leroy, S., d'Acremont, E., Roche, V., Evain, M., Lepretre, A., Verrier, F., Aslanian, D., Dias, N., Afilhado, A., Schnurle, P., Castilla, R., Despinois, F., Moulin, M.
Abstract: A variety of structures results from the interplay of evolving far-field forces, plate kinematics, and magmatic activity during continental break-up. The east Limpopo transform margin, offshore northern Mozambique, formed as Africa and Antarctica separated during the mid-Jurassic period break-up of the Gondwana supercontinent. The nature of the crust onshore has been discussed for decades in an effort to resolve issues with plate kinematic models. Two seismic refraction profiles with coincident multichannel seismic reflection profiles allow us to interpret the seismic velocity structures across the margin, both onshore and offshore. These seismic profiles allow us to (a) delineate the major regional crustal domains; (b) identify widespread indications of magmatic activity; and (c) map crustal structure and geometry of this magma-rich transform margin. Careful examination of the profiles allows us to make the following observations and interpretations: (a) on land, continental crust is overlain by a >10-km thick volcano-sedimentary wedge related to an early rifting stage, (b) offshore, thick oceanic crust formed due to intense magmatic activity, and between the two (c) a 50-60-km wide transform zone where the crustal structures are affected by intense magmatic activity and faulting. The prominent presence of intrusive and extrusive igneous units may be attributed to the combination of a deep-seated melting anomaly and a trans-tensional fault zone running through thinned lithosphere that allowed melt to reach the surface. A comparison of the crustal thinning along other transform margins shows a probable dependence with the thermal and/or tectonic history of the lithosphere.
Earth and Planetary Science Letters, Vol. 506, pp. 1-7.
Mantle
geodynamics
Abstract: We present a theoretical investigation, based on ab initio calculations and the quasi-harmonic approximation, on the stability properties of magnesium (MgCO3) and calcium (CaCO3) carbonates at high temperatures and pressures. The results indicate that those carbonates should be stable in the Earth's lower mantle, instead of dissociating into other minerals, in chemical environments with excess of SiO2, MgO, or MgSiO3. Therefore, considering the lower mantle chemical composition, consisting mostly of the MgSiO3 and MgO minerals, calcium and magnesium carbonates are the primary candidates as carbon hosts in that region. For the thermodynamic conditions of the mantle, the results also indicate that carbon should be primarily hosted on MgCO3, contrasting with what was found by other theoretical studies, which neglected temperature effects. Finally, the results indicate that carbon, in the form of free CO2, is unlikely in the lower mantle.
Abstract: Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ? 6.1 wt%), alkalies (Na2O + K2O ? 10 wt%) and sulfur (S ? 0.50 wt%), referred to as a “salty” kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average ?34S of sulfides is ? 1.4 ± 2.2‰ in the salty kimberlite, 2.1 ± 2.7‰ in the non-salty kimberlites and 14.2 ± 5.8‰ in the breccia. The average ?34S of sulfates in the salty kimberlites is 11.1 ± 1.8‰ and 27.3 ± 1.6‰ in the breccia. In contrast, the ?34S of potential contaminants range from 20 to 42‰ for hydrothermal sulfides, from 16 to 34‰ for hydrothermal sulfates, 34‰ for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides – this explains the removal of salts, alkali-carbonates and sulfates, as well as the minor olivine serpentinization; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average ?34S of 7.9 ± 3.4‰ and chalcopyrites with average ?34S of 38.0 ± 0.4‰) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with ?34S values within the mantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate is more abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate-rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earth's mantle.
Abstract: Formed billions of years ago deep below the surface of the earth, natural diamonds have always had an ethereal mysticism about them. Once reserved only for royals, red carpets, and life’s most celebratory moments, diamonds were seen as symbols of wealth and prosperity that only few could access. But with a new century comes a new expression of luxury, as a new crop of young designers and heritage jewelry houses alike including Sabyasachi, Messika, Anita Ko, Boucheron, and Fernando Jorge are celebrating natural diamonds as an everyday indulgence. Today’s tastemakers are incorporating diamonds into their designs in unexpected ways, unafraid to play around with interesting cuts and colorful hues. In telling the story of today’s expression of natural diamonds and their continous impact on the world, this new volume dives into tales of the world’s most captivating stones, from the Hope Diamond to the legend of the Beau Sancy Diamond. Featuring stunning images; tall tales; and interviews with top designers, tastemakers, and enthusiasts alike; Diamonds is the definitive book on the world’s most sought-after jewel.
Abstract: The South American platform is the stable part of the South American plate, unaffected by the orogenesis of the Andes and the Caribbean. Its basement is composed of Archean and Proterozoic cratonic blocks amalgamated by mobile belts, and can be separated in two large domains or continental masses: 1) The Amazonian, Northwest-west portion, including the Amazonian craton, related to the Laurentia supercontinent; and 2) the extra-Amazonian, Central-southeast or Brasiliano domain, related to West Gondwana, formed of several paleocontinental fragments, where the São Francisco and Rio de La Plata cratons and the Paranapanema block are the largest. It has been suggested that these two domains are separated by the Transbrasiliano Lineament to the south and the Araguaia Fold Belt to the north. Teleseismic P waves from 4,989 earthquakes recorded by 339 stations operated mainly in Brazil in the last 25 years have been used for relative-time tomography. The Amazonian domain is predominantly characterized by higher velocities. The SW (extra-Amazonian) domain is characterized by several blocks with high velocities, such as in and around the Sao Francisco Craton, and the Paranapanema block. Results of P-wave travel time tomography allowed to observe a strong low-velocity anomaly near 100-200 km depth following the Araguaia-Paraguay fold belt. This strong low-velocity anomaly could be considered the limit between these two domains, reaching lithospheric depths, and does not necessarily follow the Transbrasiliano lineament, especially in its southern portion.
International Geology Review, Vol. 57, 11-12, pp. 1341-1348.
Mantle
Supercontinents
Abstract: There is a correlation of global large igneous province (LIP) events with zircon age peaks at 2700, 2500, 2100, 1900, 1750, 1100, and 600 and also probably at 3450, 3000, 2000, and 300?Ma. Power spectral analyses of LIP event distributions suggest important periodicities at 250, 150, 100, 50, and 25?million years with weaker periodicities at 70-80, 45, and 18-20?Ma. The 25?million year periodicity is important only in the last 300?million years. Some LIP events are associated with granite-forming (zircon-producing) events and others are not, and LIP events at 1900 and 600?Ma correlate with peaks in craton collision frequency. LIP age peaks are associated with supercontinent rifting or breakup, but not dispersal, at 2450-2400, 2200, 1380, 1280, 800-750, and ?200?Ma, and with supercontinent assembly at 1750 and 600?Ma. LIP peaks at 2700 and 2500?Ma and the valley between these peaks span the time of Neoarchaean supercraton assemblies. These observations are consistent with plume generation in the deep mantle operating independently of the supercontinent cycle and being controlled by lower-mantle and core-mantle boundary thermochemical dynamics. Two processes whereby plumes can impact continental assembly and breakup are (1) plumes may rise beneath supercontinents and initiate supercontinent breakup, and (2) plume ascent may increase the frequency of craton collisions and the rate of crustal growth by accelerating subduction.
Journal of Asian Earth Sciences, in press available, 17p.
India
geodynamics
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Journal of Asian Earth Sciences, Vol. 157, pp. 218-234.
India
Craton
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Abstract: The first small manufactured industrial diamonds were produced in 1953 by the Swedish company ASEA but their accomplishment went unannounced. In 1970, General Electric (GE) produced synthetic diamond crystals using the HPHT method with a belt type of press and created a 0.78ct polished RBC colorless diamond. In the 1980’s and 1990’s Russians used their own technology (“BARS” and “TOROID” high pressure apparatus (HPA) with high pressure presses of up to 25 MN load) to grow industrial and crystals up to 2.00ct in polished size, mostly orange to yellow in colour. In the last 15 years, companies including Lucent, Chatham, AOTC, Gemesis (now IIa Technologies) and many other producers in China, Germany, India, Russia, Ukraine, USA and Taiwan have improved the technology yet again and used their expertise to successfully grow diamond crystals that cut to 1.00ct up to 2.00ct in size. This “next generation” of diamonds exhibited high clarities (VS and VVS) and colours (D-H), as well as new blue and pink colours (after irradiation). Other companies (ref. 2-3) including Scio Diamonds, Washington Diamonds, Taidiam, PDC diamonds and Pure Grown Diamonds (selling arm of IIa technologies) are also using a very different technology/process of Chemical Vapour Deposition (CVD) to produce laboratory-grown diamonds up to 3.00 ct in size (table 1).
Abstract: A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume?fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
Abstract: We present original results that contribute to the understanding of lithospheric structures modification of regions that have witnessed superimposition of multiple tectonic events throughout their geological history. We analyze satellite gravity data through two-dimensional radially-averaged power spectral analysis as well as passive seismic data through thermal modeling to image the depth to the Moho and the lithosphere - asthenosphere boundary (LAB beneath the Central Africa Orogenic Belt (CAOB). The CAOB is an ENE-trending deformation belt extending from Cameroon in the west to Sudan in the east. In Cameroon, it is found on the northern edge of the Congo craton represented by the Oubanguides orogenic belt (the Western Cameroon, the Adamawa - Yade, and the Yaoundé domains). It coincides with the Adamawa plateau and the Benue Trough, and it is spotted by the Cenozoic Cameroon Volcanic Line (CVL). The CAOB was formed during the Precambrian Greater Gondwana assembly but was reactivated during the Mesozoic as a result of Gondwana breakup. We find deeper Moho and LAB) beneath Congo craton and the Yaoundé domain reaching ~50?km and ~200?km, respectively. We map shallower Moho and LAB beneath the CAOB (together with the Adamawa plateau and the Benue trough) reaching ~25?km and ~70?km, respectively. We interpret the shallower LAB beneath the CAOB as due to zonal sub-continental lithospheric mantle (SCLM) delamination along the northern edge of the Congo craton that occurred in association with collisional assembly of Greater Gondwana. This allowed for channelization of mantle flow during the Cenozoic resulting in the formation of the CVL and the uplift of the Adamawa plateau. Our approach can be used to understand the modification of lithospheric structures beneath other terrains that have long tectonic history.
Abstract: We present original results that contribute to the understanding of lithospheric structures modification of regions that have witnessed superimposition of multiple tectonic events throughout their geological history. We analyze satellite gravity data through two-dimensional radially-averaged power spectral analysis as well as passive seismic data through thermal modeling to image the depth to the Moho and the lithosphere - asthenosphere boundary (LAB beneath the Central Africa Orogenic Belt (CAOB). The CAOB is an ENE-trending deformation belt extending from Cameroon in the west to Sudan in the east. In Cameroon, it is found on the northern edge of the Congo craton represented by the Oubanguides orogenic belt (the Western Cameroon, the Adamawa - Yade, and the Yaoundé domains). It coincides with the Adamawa plateau and the Benue Trough, and it is spotted by the Cenozoic Cameroon Volcanic Line (CVL). The CAOB was formed during the Precambrian Greater Gondwana assembly but was reactivated during the Mesozoic as a result of Gondwana breakup. We find deeper Moho and LAB) beneath Congo craton and the Yaoundé domain reaching ~50?km and ~200?km, respectively. We map shallower Moho and LAB beneath the CAOB (together with the Adamawa plateau and the Benue trough) reaching ~25?km and ~70?km, respectively. We interpret the shallower LAB beneath the CAOB as due to zonal sub-continental lithospheric mantle (SCLM) delamination along the northern edge of the Congo craton that occurred in association with collisional assembly of Greater Gondwana. This allowed for channelization of mantle flow during the Cenozoic resulting in the formation of the CVL and the uplift of the Adamawa plateau. Our approach can be used to understand the modification of lithospheric structures beneath other terrains that have long tectonic history.
Ramp-flat geometry within the central Kapuskasing structural zone? evidence from potential field modelling
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC)/SEG Annual Meeting May 27-29. Toronto, Ontario, Abstract, Vol. 16, p. A5. Abstract
Geophysical Research Letters, Vol. 42, 20, pp. 8398-8405.
Africa, Botswana
Geophysics - gravity
Abstract: Rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins. In order to decipher the characteristics of rifting at its earliest stage, we conduct the first teleseismic crustal study of one of the world's youngest continental rifts, the Okavango Rift Zone (ORZ), where the magma has not yet breached the surface. Results from receiver function stacking and gravity modeling indicate that the crust/mantle boundary beneath the ORZ is uplifted by 4-5 km, and the initiation of the ORZ is closely related to lithospheric stretching. Possible decompression melting of the subcrustal lithosphere occurs beneath the ORZ, as evidenced by a relatively low upper mantle density based on the gravity modeling.
Journal of African Earth Sciences, Vol. 129, pp. 94-107.
Africa, Cameroon
Craton, Congo
Abstract: We used the World Gravity Map (WGM 2012) data to investigate the Archean Congo craton and the Oubanguides orogenic belt in Cameroon. The Oubanguides orogenic belt constitutes, from northwest to southeast, the Neoproterozoic West Cameroon domain, the Paleoproterozoic-Neoproterozoic Adamawa-Yade domain, and the dominantly Neoproterozoic Yaoundé domain (the crustal expression of the suture zone between the Congo craton and the orogenic terranes). We analyzed the WGM 2012 data to identify different gravity anomalies. We also applied the two-dimensional (2D) radially-averaged power spectral analysis to the WGM 2012 data to estimate the Moho depth. Additionally, we developed a 2D forward gravity model along a Nsbnd S profile to image the lithospheric structure of the Precambrian entities. We found that: (1) the Congo craton, the Yaoundé domain, the southeastern part of the West Cameroon domain, and the northern part of the Adamawa-Yade domain are characterized by low gravity anomaly. (2) the southern part of the Adamawa-Yade domain is marked by a pronounced E-W trending high gravity anomaly. (3) the crust is thicker beneath the Congo craton, the Yaoundé domain and the southern part of the Adamawa-Yade domain. (4) the presence of a denser lower crust material beneath the southern part of the Adamawa-Yade domain. We propose that this denser crustal material is an under-thrusted portion of the Congo craton that has been densified through metacratonization processes that accompanied collision between the craton and the orogenic terranes. This is in good agreement with geological and geochemical observations indicating that the northern edge of the Congo craton and the Adamawa-Yade domain had undergone metacratonization during the Neoproterozoic. Our suggestion is also in good agreement with observations which show that the margins of many cratons worldwide have been decratonized due to subduction processes. Our work highlights the importance of potential field geophysical data in mapping the metacratonized margins of cratons.
Abstract: Continental rifts most often nucleate within orogenic belts. However, some studies in the East African Rift System (EARS) have shown that continental rifts can also develop withincratons. This work investigated the ~1.5 Ma Eyasibasin,which propagates in a WSW direction into the Tanzanian craton. The basin is located where the Eastern Branch of the EARS transitions from a narrow rift (~70 km wide) thewider(~300 km wide) North Tanzanian Divergence. Unlike the rest of the Eastern Branch segments, the Eyasibasindoes not follow the Mozambique orogenic belt located on the eastern margin of the Tanzanian craton. This work generatedlithospheric?scale sections across the basinusing: (1) Digital Elevation Model to map surface rift?related brittle structures; (2) Aeromagnetic data to determine the depth to the Precambrian basement;and (3) World Gravity Model 2012 to estimatecrustal and lithospheric thickness by applying the two?dimensional(2D) radially?averaged power spectral analysis and 2D forward gravity modeling. These cross?sectionsshow that the Eyasibasinnucleates within a previously unidentified suture zone within the Tanzanian cratonand that this suture zone is characterized by thinner lithospherethat can be as thin as ~95 km. This zone ofthinner lithosphere is offset southeastwardfrom the surface expression of the Eyasibasinand might have facilitated the formation of other basins further south. Furthermore, the lithospheric thickness map indicates that the Tanzanian craton is heterogeneous and possibly composed of multiplesmaller cratonic fragments.
Abstract: A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume?fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
Abstract: Botswana experienced a Mw 6.5 earthquake on 3rd April 2017, the second largest earthquake event in Botswana's recorded history. This earthquake occurred within the Limpopo-Shashe Belt, ~350 km southeast of the seismically active Okavango Rift Zone. The region has no historical record of large magnitude earthquakes or active faults. The occurrence of this earthquake was unexpected and underscores our limited understanding of the crustal configuration of Botswana and highlight that neotectonic activity is not only confined to the Okavango Rift Zone. To address this knowledge gap, we applied a regularized inversion algorithm to the Bouguer gravity data to construct a high-resolution crustal thickness map of Botswana. The produced crustal thickness map shows a thinner crust (35-40 km) underlying the Okavango Rift Zone and sedimentary basins, whereas thicker crust (41-46 km) underlies the cratonic regions and orogenic belts. Our results also show localized zone of relatively thinner crust (~40 km), one of which is located along the edge of the Kaapvaal Craton within the MW 6.5 Botswana earthquake region. Based on our result, we propose a mechanism of the Botswana Earthquake that integrates crustal thickness information with elevated heat flow as the result of the thermal fluid from East African Rift System, and extensional forces predicted by the local stress regime. The epicentral region is therefore suggested to be a possible area of tectonic reactivation, which is caused by multiple factors that could lead to future intraplate earthquakes in this region.
Abstract: Burmese sapphires are among the most coveted colored gemstones in the world. The historical importance of this source and the fine quality of its high-grade material contribute to the legendary status of these gems. Since Mogok is such a long-known source, there are many classic studies available, but modern analytical data are often missing or not up to current standards. This article summarizes the characteristics of Burmese sapphires, including standard gemological properties, inclusion observations, and spectroscopic and trace element analyses. This information was collected from hundreds of blue sapphires that GIA's field gemologists sampled while visiting different mining regions in Mogok over the past decade. Our observations indicate that these sapphires show a wide range of blue color intensities but very consistent inclusion scenes. Trace element chemistry did not show any significant differences between various regions apart from a wider range of Fe concentrations in sapphires from north of Mogok. Rare observations such as orange fluorescence and unusual FTIR spectra can be attributed to the chemical compositions of the sapphires.
Abstract: We report mineral chemistry and whole-rock major and trace-element geochemistry for a recent find of Mesoproterozoic (~1.4 Ga) lamproites from the Garledinne (Banganapalle) cluster, south-western part of the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The Garledinne lamproites occur as WNW-ESE-trending dykes that have undergone varying degree of pervasive silicification and carbonate alteration. Nevertheless, their overall texture and relict mineralogy remain intact and provide important insights into the nature of their magmas. The lamproite dykes have porphyritic to weakly porphyritic textures comprising pseudomorphed olivine macrocrysts and microphenocrysts, titanian phlogopite microphenocrysts, spinel having a compositional range from chromite to rarely magnesiochromite, Sr-rich apatite and niobian rutile. The Garledinne and other Cuddapah Basin lamproites (Chelima and Zangamarajupalle) collectively lack sanidine, clinopyroxene, potassic richterite, and titanite and are thus mineralogically distinct from the nearby Mesoproterozoic lamproites (Krishna and Ramadugu) in the Eastern Dharwar Craton, southern India. The strong correlation between various major and trace elements coupled with high abundances of incompatible and compatible trace elements imply that alteration and crustal contamination have had a limited effect on the whole-rock geochemistry (apart from K2O and CaO) of the Garledinne lamproites and that olivine fractionation played an important role in their evolution. The Garledinne lamproites represent small-degree partial melts derived from a refractory (previously melt extracted) peridotitic mantle source that was subsequently metasomatised (enriched) by carbonate-rich fluids/melts within the garnet stability field. The involvement of multiple reservoirs (sub-continental lithospheric mantle and asthenosphere) has been inferred in their genesis. The emplacement of the Garledinne lamproites is linked to extensional events, across the various Indian cratons, related to the break-up of the Proterozoic supercontinent of Columbia.
Mineralogy and Petrology, in press available, 25p.
India
Lamproites - Nuapada field
Abstract: We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045?±?9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an ‘enriched’ source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical characteristics of lamproite and kamafugite (also displayed by two other lamproites of the Nuapada field at Amlidadar and Parkom) are linked to the emplacement in a unique geological setting at the craton-mobile belt contact and hence of geodynamic significance.
Mineralogy and Petrology, in press available, 25p.
India
Deposit - Sakri Nuapada
Abstract: We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045?±?9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an ‘enriched’ source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical characteristics of lamproite and kamafugite (also displayed by two other lamproites of the Nuapada field at Amlidadar and Parkom) are linked to the emplacement in a unique geological setting at the craton-mobile belt contact and hence of geodynamic significance.
Kimberlites and diamonds in the Northwest Territories -explorationhighlights.
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 68.
Northwest Territories kimberlites and diamonds: exploration highlights andimplications.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 89-94.
The Discovery and Evaluation of the Ellendale and Argyle Lamproite Diamond Deposits, Kimberley, Western Australia.
Society for Mining, Metallurgy and Exploration (SME)-American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME)., SYMPOSIUM OUTLINE FALL MEETING DENVER OCTOBER 26TH. No. 84-3
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 49-64.
Lithostratigraphy and composition of 2.1 Ga greenstone belts of the West African craton and their bearing on crustal evolution and the Archean-Proterozoic Boundary
Abstract: Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt-associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000-550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2?, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, ?FMQ, at 1 GPa) of Archean sample suites (?FMQ -1.19 ± 0.33, 2?) is significantly lower than that of post-Archean sample suites, including MORB (?FMQ -0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.
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: Geomagnetic jerks are abrupt changes in the second time derivative "the secular acceleration" of Earth’s magnetic field that punctuate ground observatory records. As their dynamical origin has not yet been established, they represent a major obstacle to the prediction of geomagnetic field behaviour for years to decades ahead. Recent jerks have been linked to short-lived, temporally alternating and equatorially localized pulses of secular acceleration observed in satellite data, associated with rapidly alternating flows at Earth’s core surface. Here we show that these signatures can be reproduced in numerical simulations of the geodynamo that realistically account for the interaction between slow core convection and rapid hydromagnetic waves. In these simulations, jerks are caused by the arrival of localized Alfvén wave packets radiated from sudden buoyancy releases inside the core. As they reach the core surface, the waves focus their energy towards the equatorial plane and along lines of strong magnetic flux, creating sharp interannual changes in core flow and producing geomagnetic jerks through the induced variations in magnetic field acceleration. The ability to numerically reproduce jerks offers a new way to probe the physical properties of Earth’s deep interior.
Abstract: We explore the shear-wave lithospheric velocity structure of the Eastern Caribbean and Northern Venezuela using ambient noise tomography with stations deployed around the study area. We construct cross-correlation functions from continuous seismic records, and measure phase velocities of fundamental-mode Rayleigh waves. These velocities are further projected onto 0.6°x0.6° phase velocity grids for each period between 5 s and 50 s. The pseudo-dispersion curve at each grid point is inverted for 1D shear velocity profiles by using a Markov Chain Monte Carlo scheme. The interpolated 3D velocity model shows that the mean shear velocity of the Eastern Caribbean lithospheric mantle is lower than the global average, which is in agreement with values reported in other large igneous provinces. We interpret that low velocities in the lithospheric keel are associated with an anomalous composition and/or an elevated thermal state; this gives the Caribbean plate a high buoyancy that determines the subduction polarities in the region. The results also indicate that: (a) the mantle beneath Northern Venezuela retains compositional anomalies related to extension processes of different ages; (b) the overriding of the Caribbean plate by the Great Antilles arc seems to be much slower than previously suggested; and (c) the localized volcanism in the center of the Lesser Antilles arc is related to asthenospheric flow through the tear induced on the subducted slab by major strike-slip faults.
Upper mantle structure underlying the diamondiferous Slave craton from teleseismic body-wave tomography.
2018 Yellowknife Geoscience Forum , p.104-105. abstract
Canada, Northwest Territories
tomography
Abstract: Cratons are, by definition, the most tectonically stable and oldest parts of the continental lithosphere on Earth. The Archean Slave craton is located in the northwestern part of the Canadian Shield. The propensity of diamondiferous kimberlite pipes in the central Slave craton raises many questions regarding their structural environment and source. Here, we provide the most robust teleseismic P and S body wave tomography models over the Slave craton region based on 20,547 P-wave delay times, 6,140 direct S-wave delay times and 3,381 SKS delay times. The P-wave model reveals an alternating pattern of relative positive and negative anomalies over a fine broad scale region within the central Slave craton. Furthermore, the P-wave model revealed two fine structures located in the lithosphere beneath the Lac de Gras kimberlite cluster, with relatively slow anomalies (B - C) that extend from 75 km to 350 km depths with an apparent dip to the north. These relatively slow P- and S-wave anomalies are associated with metasomatised regions within the lithosphere. The S-wave model displays a slow S-wave anomaly lying from 300 km depth to the transition zone beneath the central Slave craton. This anomaly is located beneath the Lac de Gras kimberlite cluster. We suggest that this anomaly is not the cause of the actual kimberlites at the surface since last eruption occurred 75-45 Ma ago but may be related to a potential kimberlite magma ascent in the asthenosphere.
Abstract: Cratons are, by definition, the most tectonically stable and oldest parts of the continental lithosphere on Earth. The Archean Slave craton is located in the northwestern part of the Canadian Shield. The propensity of diamondiferous kimberlite pipes in the central Slave craton raises many questions regarding their structural environment and source. Here, we provide the most robust teleseismic P and S body wave tomography models over the Slave craton region based on 20,547 P-wave delay times, 6,140 direct S-wave delay times and 3,381 SKS delay times. The P-wave model reveals an alternating pattern of relative positive and negative anomalies over a fine broad scale region within the central Slave craton. Furthermore, the P-wave model revealed two fine structures located in the lithosphere beneath the Lac de Gras kimberlite cluster, with relatively slow anomalies (B - C) that extend from 75 km to 350 km depths with an apparent dip to the north. These relatively slow P- and S-wave anomalies are associated with metasomatised regions within the lithosphere. The S-wave model displays a slow S-wave anomaly lying from 300 km depth to the transition zone beneath the central Slave craton. This anomaly is located beneath the Lac de Gras kimberlite cluster. We suggest that this anomaly is not the cause of the actual kimberlites at the surface since last eruption occurred 75-45 Ma ago but may be related to a potential kimberlite magma ascent in the asthenosphere.
Abstract: Hf-isotope data of greater than 1100 detrital zircon grains from the Palaeozoic, south-central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8-4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust-addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville-age igneous belt can be postulated along the palaeo-margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircons were mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.
Abstract: Hf-isotope data of >1100 detrital zircon grains from the Palaeozoic, south-central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8-4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust-addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville-age igneous belt can be postulated along the palaeo-margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.
Aulbach, S., Stachel, T., Craeser, R.A., Heaman, L.M., Shirey, S.B., MUehlenbachs, K., Eichenberg, D., Harris
Sulphide survival and diamond genesis during formation and evolution of Archean subcontinental lithosphere: a comparison between the Slave and Kaapvaal cratons.
Earth and Planetary Science Letters, Vol. 431, pp. 274-286.
Africa, South Africa
Deposit - Lace
Abstract: Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ?2.5 Ga age (in situ Pb-Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ?REE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ?0.20-0.25?0.20-0.25 that left a spinel peridotite residue at pressures ?2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ?1420 to 1470?°C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.24.7±1.2; n=11n=11) indicating lower fO2fO2 (?1.9 relative to the fayalite-magnetite-quartz buffer=?FMQbuffer=?FMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (?1.5 GPa) during formation of their crustal protoliths. Extrapolation to 1 GPa, similar to the average pressure of present-day MORB generation, yields ?FMQ of ?1.7±1.1?1.7±1.1, corresponding to Fe3+# of 0.07±0.040.07±0.04. If these results are correct, they suggest an Archaean ambient mantle more reducing than at present, with implications for the speciation of volatiles, which affects the mantle solidus and the composition of volcanic gases. This has implications for the Archaean atmospheric redox evolution and the recycling of carbon and other volatiles.
Reviews in Mineralogy and Geochemistry, Vol. 81, pp. 239-304.
Mantle
Mineralogy
Abstract: Cratonic lithospheric mantle is composed of predominantly refractory materials that formed at higher mantle potential temperatures (TP) than recorded in non-cratonic peridotites. It also shows stronger depletion and fractionation of Pd and Pt from Ru, Os and Ir than oceanic, supra-subduction zone or off-cratonic lithospheric mantle, as well as some of the lowest Se and Te contents. The varied response of the highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au), and their embedded radioactive decay systems, to changes in oxygen fugacity (fO2), sulfur fugacity (fS2) and pressure (P)-in particular through the impact of these parameters on the stability of the main HSE-bearing sulfide and alloy phases makes them potentially powerful tracers of their melting environment. Therefore, investigation of the HSE systematics of cratonic mantle peridotites, in combination with information from Re–Os isotopes on time-integrated enrichment or depletion, can help us to understand processes leading to mantle differentiation and continental lithosphere formation in the Archean, which are controversial subjects despite decades of research. The longevity of the cratonic lithosphere implies that there was ample opportunity for secondary overprint, obscuring our view of earlier processes. For example, destabilization of platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) alloy leading to depletions in the compatible PGE, and perhaps Pt, in some cratonic mantle samples may occur in an oxidizing mantle wedge or through interaction with oxidizing small-volume, volatile-rich melts that typically invade cratonic roots. Such melts may eventually deposit S, Pd, Pt and Re and also capture remaining PGE alloys, consistent with the anomalous S-rich character of many kimberlite-borne xenoliths. Their basalt-borne counterparts show additional late effects of subaerial degassing that can deplete volatile elements (S, Re, Os). Basaltic melts can also scavenge PGE alloys at depth, while still sulfide-undersaturated. Such melts, may, on ascent, add sulfides when they become sulfur-saturated and, during the process, refertilize the mantle and modify major-element and modal compositions. The investigation of minor lithologies in the cratonic lithosphere, such as eclogites and pyroxenites, which are expressions of tectonothermal events ranging from subduction to melt infiltration, can enhance our understanding of the effects of these processes on HSE redistribution. Thus, three major topics will be discussed, using HSE systematics in cratonic mantle samples: (1) How did the HSE behave during the (in part) extreme degrees of partial melt extraction experienced by cratonic lithospheric mantle; (2) What were the effects of the secular metasomatic overprint of the cratonic mantle; (3) What was the composition of the Archean convecting mantle, for which cratonic mantle samples may afford better insight than modern samples, provided, of course, that we have an accurate grasp of how HSE are redistributed during partial melting and metasomatism. Models based on experiments done under controlled pressure (P), temperature (T), fO2 and fS2 conditions can help place the data in context and to distinguish between melt- and metasomatism-related processes. Disentangling the various primary and secondary effects is only possible when HSE are studied in combination with lithophile elements, with due attention to petrography and mineralogy. This adds many layers of complexity, but ultimately allows a more complete understanding of the variegated processes that have shaped the cratonic lithosphere through time. In this review, we commence by discussing the peculiarities and complexities of continental lithospheric mantle origin, evolution and current state. We then introduce the database used in this contribution, followed by a brief review of the mineral hosts of HSE in peridotite and of the diverse approaches to isolate the HSE for measurement. We examine the behavior of the HSE during the formation of cratonic lithospheric mantle under non-uniformitarian conditions, where the application of the Re–Os isotope system has afforded particularly useful information on the timing of initial melt depletion and the stabilization of cratonic roots. We then turn to the effects of mantle metasomatism, both during intra-plate and craton-margin processes (see also Gannoun et al. 2016, this volume), on HSE systematics in cratonic mantle. We also discuss the data in the context of melt extraction modelling that shed light on the primary versus secondary HSE signatures in cratonic mantle rocks. Finally, we evaluate the possibility that the HSE in cratonic mantle retain a memory of core formation and subsequent accretionary processes.
Geochimica et Cosmochimica Acta, Vol. 179, pp. 156-176.
Africa, South Africa
Deposit - Lace
Abstract: Diamond- and kyanite-bearing eclogites from the Lace kimberlite on the Kaapvaal craton have common picritic to gabbroic oceanic protoliths with bimineralic eclogites, lying on arrays of Eu? and ?REE that are consistent with accumulation and fractionation of plagioclase and olivine. However, they also show significant compositional differences, such as more grossular-rich garnet and aluminous clinopyroxene (cpx), which require the operation of additional processes. Their nature is elucidated using mineral major- and trace-element compositions, as well as Sr isotope ratios determined by in situ techniques.Highly variable major-element compositions across the co-genetic eclogite suites exert a strong effect on the trace-element distribution between garnet and cpx, whereby Sc, Ge, Sr, Y, Cd, REE, Th and U partition more strongly into garnet with increasing grossular-content. Thus, significant differences between the trace-element compositions of garnet can ensue from crystal-chemical effects alone, making their use as petrogenetic indicators potentially ambiguous. After correcting for these compositional effects, garnet in kyanite-/diamond eclogites, and in eclogites devoid of accessory minerals but with similar signatures, shows depletion (or dilution) in Sc, Ge, Y, In, Zr, Hf and the HREE, and enrichment in the LREE and Th compared to garnet in bimineralic eclogites. This is interpreted as the signature of a pelite-derived melt, which was transferred by addition of aluminous cpx that later exsolved kyanite and garnet, as observed in other aluminous eclogite suites. Continental input can explain initial (at 2.9 Ga) 87Sr/86Sr ? 0.714 measured in cpx in eleven samples with low 87Rb/86Sr (<0.01). The association of diamond with kyanite suggests that diamond formation is also linked to this event, possibly due to diamond formation by oxidation of reduced carbon, such as methane, and attendant reduction of Fe3+ in garnet. This model of sediment melt-oceanic crust interaction reconciles evidence for both low- and high-pressure igneous processes in some aluminous eclogites. We suggest that a subduction mélange is a favourable setting for the transfer of a sediment-derived signature into oceanic crust, leading to formation of diamondiferous kyanite-eclogites from bimineralic eclogites. Diapirism, fluxed by the presence of partial melt, may have facilitated dispersal of the eclogites in the lithosphere column, consistent with their widely varying equilibration pressures ranging from ?5 to 8 GPa.
Abstract: There is a growing body of evidence for the origin of cratonic mantle eclogite xenoliths by low-pressure formation in now-recycled ocean floors. Because they have protoliths ultimately derived from the convecting mantle, their study can potentially yield unprecedented insights into as yet little-understood palaeo-geodynamic regimes, once primary (fractional crystallisation, accumulation, mixing) and secondary processes (kimberlite infiltration, metasomatism) affecting their compositions are understood. This is achieved using diagnostic concentrations or ratios of the analytically and geologically most robust elements (major and minor elements, transition metals, REE), and aided by comparison to natural and modelled analogues. Here, mineral compositions taken from the literature were used to reconstruct bulk rocks and assign the samples to eclogites (further divided into high-Mg, low-Mg and high-Ca types), pyroxenites and their gabbroic (Eu* > 1.05) counterparts. Various protolith types - formed predominantly by < 1 GPa crystallisation from broadly picritic magmas leaving garnet-poor mantle sources - are identified: (1) Many high-Mg eclogites lie on modelled crystallisation trends between 0.5 and < 1 GPa. Some have elevated FeO contents with lower SiO2 and CaO possibly requiring Fe-rich pyroxenite heterogeneities in their mantle source. (2) Many high-Ca eclogites may be the differentiated (higher Na2O, TiO2 and FeO at lower MgO) equivalents of high-Mg eclogites, following modelled crystallisation trends at somewhat lower pressure (0.05 to 0.5 GPa). Other high-Ca eclogites with low FeO were produced during interaction with fluids and melts in mélange-type settings. (3) Low-Mg eclogites, with intermediate MgO content, are too FeO-rich to be intermediary crystallisation products of the same parental melt and are ascribed to melting out of Fe-rich lithologies possibly related to recycling of eclogite and/or contamination with ferromanganese sediments. (4) The positive Eu anomalies in gabbroic eclogites require accumulation of substantial amounts of plagioclase, consistent with their low FeO and TiO2 contents, but their simultaneously low MgO contents suggest that they interacted with residual melts. (5) The elevated CaO and low Al2O3 in pyroxenite may indicate clinopyroxene-rich high- or low-pressure cumulate protoliths, but high Cr2O3 and MgO, combined with low HREE and high LREE in many of these samples, suggests formation by hybridisation of eclogite-derived melt with peridotite.
Abstract: Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt-associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000-550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2?, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, ?FMQ, at 1 GPa) of Archean sample suites (?FMQ -1.19 ± 0.33, 2?) is significantly lower than that of post-Archean sample suites, including MORB (?FMQ -0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.
Abstract: Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities. LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to ~ 200-250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton). Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past magmatic-metasomatic activity, or fabric inherited from cratonisation. They may accumulate as layers at, or as subvertical veins above, the depth at which melt flow transitions from pervasive to focussed flow at the mechanical boundary layer, causing azimuthal and radial anisotropy. Thermodynamic calculations investigating the depth range in which small-volume melts can be produced relative to the field of phlogopite stability and the presence of MLDs show that phlogopite precipitates at various pressures as a function of age-dependent thermal state of the cratonic mantle, thus explaining variable MLD depths. Even if not directly observed, such metasomes have been shown to be important ingredients in small-volume volatile-rich melts typically penetrating cratonic lithospheres. The apparent sparseness of evidence for phlogopite-rich assemblages in the mantle xenolith record at geophysically imaged MLD depths, if not due to preferential disaggregation in the kimberlite or alteration, may relate to vagaries of both kimberlite and human sampling.
Re-Os isotope systematics of sulphide inclusions in diamonds from Victor ( Superior craton) document mobilization of volatiles and Os during Rodinia break up.
Abstract: Mantle potential temperature (TP) and composition are crucial parameters that regulate terrestrial dynamics and geochemical cycles, ranging from controls on the peridotite solidus and consequent geochemical differentiation, to plate stiffness conducive to the operation of plate tectonics, and the recycling efficiency of volatiles in subduction zones. Earth’s mantle has been cooling, but there is little agreement on the rate at which this proceeded. It is also unclear whether the Archaean ambient mantle was similar to, or more or less depleted than that giving rise to modern MORB. Since the ambient convecting mantle is most reliably sampled at spreading ridges, ancient kimberlite-borne eclogite xenoliths with low-pressure oceanic crustal protoliths, together with orogenic eclogites and (meta)basalts from allochtonous greenstone belts, may be used to constrain some characteristics of the convecting mantle sources from which their protoliths were ultimately derived. Carefully screened eclogite suites up to 3 Ga in age have TiO2-REE relationships consistent with fractionation of olivine±plagioclase during formation of picritic protoliths from a melt that separated from a garnet-free peridotite source, implying intersection of the solidus at ?3.0 GPa. Low melt fractions (F<0.25), calculated from samples with the least fractionated protoliths using the batch melting equation, further argue against deep intersection of the mantle solidus. This is contingent on correctly identifying the mantle source (C0) as depleted, which is supported by depleted initial 176Hf/177Hf in 2.9 to 2.6 Ga orogenic eclogite suites. Inversion of melt fractions for temperature suggests moderately elevated TP of ~1420-1470º C, significantly lower than some estimates for the ambient convecting mantle at that time. If these results are accurate, the unusual degree of melt depletion experienced by cratonic lithospheric mantle (F = 0.3-0.5) underpinning Earth’s oldest continental cores requires formation at excess TP and/or from fertile mantle which sustains longer melting columns and higher melt productivity, either during plume-ridge interactions or plume subcretion. This matches increasing evidence that ancient continental crust formed by melting at the base of oceanic plateau-like enriched oceanic crust. A moderate Mesoarchaean TP also argues for early plate strengthening that would support plate tectonics and topography.
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 ?18O 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 ?18O 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 ?18O 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.
Earth and Planetary Science Letters, Vol. 474, pp. 283-295.
Africa, South Africa
deposit - Lace
Abstract: Reconstructing the redox state of the mantle is critical in discussing the evolution of atmospheric composition through time. Kimberlite-borne mantle eclogite xenoliths, commonly interpreted as representing former oceanic crust, may record the chemical and physical state of Archaean and Proterozoic convecting mantle sources that generated their magmatic protoliths. However, their message is generally obscured by a range of primary (igneous differentiation) and secondary processes (seawater alteration, metamorphism, metasomatism). Here, we report the Fe3+/?Fe ratio and ?18 O in garnet from in a suite of well-characterised mantle eclogite and pyroxenite xenoliths hosted in the Lace kimberlite (Kaapvaal craton), which originated as ca. 3 Ga-old ocean floor. Fe3+/?Fe in garnet (0.01 to 0.063, median 0.02; n = 16) shows a negative correlation with jadeite content in clinopyroxene, suggesting increased partitioning of Fe3+ into clinopyroxene in the presence of monovalent cations with which it can form coupled substitutions. Jadeite-corrected Fe3+/?Fe in garnet shows a broad negative trend with Eu*, consistent with incompatible behaviour of Fe3+ during olivine-plagioclase accumulation in the protoliths. This trend is partially obscured by increasing Fe3+ partitioning into garnet along a conductive cratonic geotherm. In contrast, NMORB-normalised Nd/Yb - a proxy of partial melt loss from subducting oceanic crust (<1) and metasomatism by LREE-enriched liquids (>1) - shows no obvious correlation with Fe3+/?Fe, nor does garnet ?18OVSMOW (5.14 to 6.21‰) point to significant seawater alteration. Median bulk-rock Fe3+/?Fe is roughly estimated at 0.025. This observation agrees with V/Sc systematics, which collectively point to a reduced Archaean convecting mantle source to the igneous protoliths of these eclogites compared to the modern MORB source. Oxygen fugacites (fO2) relative to the fayalite-magnetite-quartz buffer (FMQ) range from ?log ? fO2 = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility of carbon in the fluids released by dehydration is higher than in fluids closer to FMQ. The implication is that Archean processes of C transport and deposition would have differed from those known in modern-style subduction zones, and diamond would have formed from methane-rich fluids. In addition, such reducing material could drive redox melting or freezing upon deep recycling and migration of CH4-bearing fluids into the ambient mantle.
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 ?18O 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 ?18O 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 ?18O 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: A comprehensive petrographic and in situ major and trace element study of rutile, ilmenite and Ti-magnetite was undertaken in six lower crustal xenoliths of metabasaltic (?underplate) and metasedimentary (subduction) origin from the Diavik kimberlites (central Slave Craton, Canada). The aims of the study were to improve our understanding of trace element incorporation into these Ti-minerals, and to use these systematics to obtain insights into lower continental crust formation and evolution. Abundant (oxy)exsolution of titanomagnetite lamellae, blocky rutile, as well as minor pleonaste and zircon in ilmenite from metabasaltic granulites are proposed to reflect cooling from magmatic or metamorphic temperatures and subsequent secular mantle cooling. This explains the large spread in Zr-in-rutile temperatures (>200°C) and may partly be responsible for the substantial heterogeneity of other trace element concentrations in rutile and ilmenite. Even after accounting for trace element heterogeneity and modal uncertainties, mass-balance calculations indicate that both Ti and Nb in lower crustal granulites are largely controlled by rutile and ilmenite. Rutile U-Pb data define discordia arrays that yield upper intercept ages broadly coincident with the 1•27 Ga giant Mackenzie dike swarm event, suggesting reheating of the lower crust above the rutile U-Pb closure temperature, whereas lower intercept ages roughly correspond to the age of Cretaceous to Eocene kimberlite magmatism. Subsequent cooling led to partial resetting and data spread along the concordia. Closer inspection reveals that inter-grain concentrations of elements that are compatible in rutile (Nb, Ta, W, U), but highly incompatible in the abundant silicate minerals (in equilibrium with melt), are heterogeneous and contrast with the more homogeneous concentrations of the transition metals (NiO, V). This may indicate that local reaction partners for diffusive homogenization of these element concentrations were absent. Nb/Ta is also highly variable at the sample scale. This may be explained by prograde growth from high-Nb/Ta mineral precursors (e.g. biotite) in the metasedimentary granulites and crystallization of the protoliths to the metabasaltic granulites from a mafic magma that had experienced fractionation of ilmenite with low Nb/Ta in a crustal magma chamber. Thus, (Fe)-Ti minerals represent high field strength element ‘islands’ in the granulite silicate matrix. The lack of homogenization and persistence of high-energy grain boundaries, such as exsolution lamellae, further indicate that the lower continental crust remained essentially dry and did not recrystallize, possibly since Neoarchaean metamorphism.
Earth Planetary Science Letters, Vol. 490, pp. 77-87.
Canada, Ontario
deposit - Victor
Abstract: The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re-Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n = 14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (?Os -3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012-0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic. The ?720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.
Abstract: The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ?100-170?km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150?Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ?90-110?km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550?Ma extended to >210?km depth, and suggests loss of ?40?km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (?90-110?km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.
Mineralogy and Petrology, in press available, 12p.
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Jurassic Victor kimberlite (Attawapiskat Field) was emplaced into an area of the central Superior Craton that was affected by a lithosphere-scale thermal event at ~1.1 Ga. Victor diamonds formed ca. 400 million years after this event, in a lithospheric mantle characterized by an unusually cool model geotherm (37-38 mW/m2; Hasterok and Chapman 2011). The bulk of Victor diamonds derives from a thin (<10 km thick) layer that is located at about 180 km depth and represents lherzolitic substrates (for 85% of diamonds). Geothermobarometric calculations (average pressure and temperature at the 1 sigma level are 57?±?2 kbar and 1129?±?16 °C) coupled with typical fluid metasomatism-associated trace element patterns for garnet inclusions indicate diamond precipitation under sub-solidus (lherzolite + H2O) conditions. This conclusion links the presence of a diamond-rich lherzolitic layer in the lithospheric mantle, just above the depth where ascending melts would freeze, to the unusually low paleogeotherm beneath Attawapiskat, because along an average cratonic geotherm (40 mW/m2) lherzolite in the presence of hydrous fluid would melt at depths >140 km.
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 ?18O 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 ?18O 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 ?18O 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: Cratons represent modern Earth’s thickest, coldest and most depleted lithospheres. Reintroduction of volatiles led to gradual re-oxidation and refertilisation of initially highly refractory and reducing cratonic mantle, enabling the eventual deposition of carbonates and hydrous minerals, which can cause seismic velocity reductions appearing as mid-lithospheric discontinuities (MLDs). Ubiquitous small-volume potassic magmas erupted since at least the Palaeoproterozoic testify to the presence of such metasomes. Attendant rheological weakening and densification led to reworking up to complete loss of deep cratonic roots, i.e. meta-cratonisation. The depths of meta-cratonic lithosphere-asthenosphere boundaries (LABs),concentrated at ~80 to 150 km, strikingly overlap those of MLDs, which may have provided lithospheric weak zones along which the deep root could be removed during continent collision, flat subduction or rifting. Since most cratonic lithospheres are too cold for melt to be presently stable, some MLDs associated with reduced resistivities may point to a role for mantle brines, which bears further investigation. If small volumes of melt are required to generate the rarely observed seismic discontinuities at the depths of intact cratonic LABs (180-260 km), their presence places lower bounds on the amount of solidus-depressing volatiles in the underlying mantle volume, whereas their absence places upper bounds if temperature can be independently constrained.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (36?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.5713.52) and low 238U/204Pb (1.01.5) give single-stage model Pb dates of 2.902.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp.319-342.
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Mantle
eclogites
Abstract: Some high-Mg eclogite xenoliths, entrained by kimberlites from the mantle lithospheres of ancient continental cores, and rare orogenic eclogites and ophiolites, exhumed or obducted during the closure of palaeo-ocean basins, have elemental and isotopic compositions indicative of protoliths that formed as little-differentiated melts erupted in ancient ocean floors. Despite metamorphism and, in part, partial melt loss, these samples of ancient mid-ocean ridge basalt and picrite retain a memory of the chemical and physical state of their protoliths' ambient convecting mantle sources. Published data show that, when filtered to exclude specimens with cumulate protoliths or showing evidence for later enrichment (metasomatism), the samples lack Y or Al 2 O 3 depletion relative to TiO 2 and MgO. This indicates melt segregation of the protolith predominantly from a garnet-free peridotite source and implies intersection of the solidus at low pressures (?3 GPa). Given the dependence of melt composition and volume on source composition (assumed to be similar to modern depleted mantle) and mantle potential temperature (T P), we calculate moderate average melt fractions F (?0.22 ± 0.01) from the Ti contents of the least differentiated samples in three sample suites with 2.6 to 2.9 Ga ages. This converts to T P of ?1410 ± 10 • C assuming a final pressure of melting of 0.5 GPa, melt productivity of 10%/GPa and mantle adiabat of 0.4 • C/km, and using a mantle solidus parameterisation. Though model-dependent, the results are in agreement with recent work advocating moderate Archaean mantle T P. Estimates drop to F = 0.19 and T P = 1380 • C at 1.9 Ga and F = 0.12 and T P = 1310 at 0.6 Ga, corresponding to a decrease in T P of only ?100 • C over the last 3 Ga. A less depleted mantle source yields higher F and T P , but the above estimates are in better agreement with qualitative evidence from Al 2 O 3 and Y, and with Nd-Hf and Sr isotope compositions of orogenic eclogite and granulite suites and mantle eclogites, respectively, which indicate that portions of the Meso-to Neoarchaean mantle were depleted. Moderate T P supports early plate strengthening and a possible transition to plate tectonics in the Mesoarchaean if not earlier. Moreover, moderate temperatures in Archaean subduction zones may have facilitated deep recycling of volatiles that would otherwise have been lost from subducting slabs at shallow depths.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3-6?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.57-13.52) and low 238U/204Pb (1.0-1.5) give single-stage model Pb dates of 2.90-2.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
Contributions to Mineralogy and Petrology, Vol. 174, 23p.
Europe, Greenland
metasomatism
Abstract: We report highly siderophile element (HSE) abundances and Re-Os isotope compositions, obtained by isotope dilution induc-tively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014-0.62, and have both depleted and enriched 187 Os/ 188 Os (0.1139-0.2724) relative to chondrite that are not correlated with 187 Re/ 188 Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1-1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<< 1 ppm) during more recent wehrlitisation and phlogopite-introduction, and control of the Os isotopic signature by older Os-rich BMS that precipitated from the basaltic melt. Depletions in compatible HSE (< 0.5 × PUM for Ru, Ir, Os) in all, but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic BMS inclusions) and/or loss due to sulphide dissolution into oxidising small-volume melts that invaded the lithosphere during recurrent rifting, the latter supported by similar depletions in published bulk peridotite data. Combined, these multiple metasomatic events destroyed all vestiges of Mesoarchaean or older inheritance in the olivine separates investigated here, and highlight that caution is needed when interpreting Proterozoic Os model ages in terms of Proterozoic lithosphere stabilisation.
Contributions to Mineralogy and Petrology, Vol. 174, 23p.
Europe, Greenland
kimberlites
Abstract: We report highly siderophile element (HSE) abundances and Re-Os isotope compositions, obtained by isotope dilution inductively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014-0.62, and have both depleted and enriched 187Os/188Os (0.1139-0.2724) relative to chondrite that are not correlated with 187Re/188Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1-1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<
Contributions to Mineralogy and Petrology, Vol. 174, 5, 27p.
Africa, Ethiopia
xenoliths
Abstract: The behavior of sub-continental lithospheric mantle (SCLM) in extensional settings, up to successful rifting, plays an important role in geodynamics and in the global carbon cycle, yet the underlying processes and rates of lithosphere destruction remain poorly constrained. We determined platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) abundances and Re-Os-isotope systematics for well-characterized mantle xenoliths hosted in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia to provide new insights on the nature and timing of processes leading to the formation and transformation of the off-cratonic lithospheric mantle beneath the East Africa rift system (EARS). The whole-rock PGE concentrations are highly variable, with total PGE abundances ranging from 6.6 to 12.6 ppb for Gundeweyn, 11.5 to 23.3 ppb for Dillo, and 9.9 to 19.4 ppb for Megado mantle xenoliths. The 187Os/188Os ratios of the whole-rock mantle xenoliths vary from 0.1180 to 0.1287 for Gundeweyn, 0.1238 to 0.1410 for Dillo and 0.1165 to 0.1277 for Megado, compared to 0.130 for the Afar plume and???0.14 for the Kenya plume, with Re depletion ages up to 1.45 Ga for Gundeweyn, 0.64 Ga for Dillo, and 1.65 Ga for Megado mantle xenoliths. The regional differences between refertilizing agents recorded in mantle xenoliths from the plateau area and the rift systems reflect distinct tectonomagmatic settings: (1) low PGE abundances, with some retention of low 187Os/188Os in Gundeweyn peridotites, are ascribed to scavenging by early small-volume oxidizing melts, generated in the convecting mantle ahead of the arrival of the Afar plume. (2) Percolation of late-stage silicate/basaltic melts, associated with the arrival of hot mantle plume and lithosphere thinning in the rift setting, locally led to refertilization and sulfide precipitation and partial replenishment of the PGE (Dillo), with convecting mantle-like 187Os/188Os. Local enclaves of older, cryptically metasomatised mantle with unradiogenic Os (Megado) attest to the heterogeneous nature of melt-peridotite interaction at this stage (pervasive vs. focused melt flow). Highly depleted abundances of the compatible PGE are characteristic of SCLM affected by incipient rifting and percolation of oxidizing melts, here associated with the Afar and Kenya plume beneath the East Africa rift, and may be precursors to advanced degrees of lithosphere destruction/transformation.
Abstract: Bimineralic eclogites and pyroxenites (n?=?75?±?accessory rutile, ilmenite, sulphide, apatite) from the Koidu kimberlite (West African Craton) were investigated for mineral major and trace elements and mineral Sr-Nd isotope compositions to constrain (1) the nature and age of their basaltic to picritic protoliths, and (2) the effect, timing and source of mantle metasomatism. Consistent with published work, samples are grouped into low-Mg eclogites with Mg# from 0•49 to 0•73 (median 0•59; n?=?40) and high-Mg eclogites with Mg# from 0•60 to 0•88 (median 0•75; n?=?14), plus pyroxenites [clinopyroxene Na/(Na + Ca) <0•2; n?=?8] and gabbroic eclogites and pyroxenite (Eu/Eu* of reconstructed bulk-rocks >1•05; n?=?8), with five unclassifiable samples. Reconstructed low-Mg and gabbroic eclogites have major and trace element systematics (Eu/Eu*-heavy rare earth elements-Y) indicating crustal protolith crystallisation, confirming an origin as subducted oceanic crust. Their high FeO contents at MgO >?10?wt % require an Fe-rich source, the high melt productivity of which led to the formation of thicker crust, perhaps in a plateau-like setting. This is consistent with SiO2-MgO relationships indicating differentiation at ?0•5?GPa. Unradiogenic Sr in some clinopyroxene (87Sr/86Sr of 0•7010-0•7015), combined with light rare earth element (LREE) depletion relative to normal mid-ocean ridge basalt (N-MORB) for the majority of samples (average N-MORB-normalised Nd/Yb of unmetasomatised samples = 0•51), suggests eclogitisation and partial melt loss in the Neoarchaean, possibly coeval with and parental to 2•7?Ga overlying continental crust. Most reconstructed high-Mg eclogites and some pyroxenites formed by metasomatic overprinting of low-Mg eclogites and gabbroic eclogites, as indicated by the preservation of positive Eu anomalies in some samples, and by the Mg-poorer composition of included versus matrix minerals. Coupled enrichment in MgO, SiO2 and Cr2O3 and in incompatible elements (Sr, LREE, Pb, Th and U) is ascribed to metasomatism by a kimberlite-like, small-volume, carbonated ultramafic melt, mediated by addition of clinopyroxene from the melt (i.e. stealth metasomatism). Strontium-Nd isotope systematics suggest a Neoproterozoic age for this metasomatic event, possibly linked to Rodinia break-up, which facilitated intrusion of asthenospheric carbonated melts with an ocean island basalt-like 87Sr/86Sri of ?0•7035. Cretaceous kimberlite magmatism (including Koidu), with more radiogenic 87Sr/86Sr (?0•7065, intermediate between Kaapvaal kimberlites and orangeites), may have been partially sourced from associated Neoproterozoic metasomes. The presence of diamonds in low-Mg eclogites, but absence in high-Mg eclogites, indicates the diamond-destructive nature of this event. Nevertheless, the moderate proportion of affected eclogites (?35%) suggests preservation of a sizeable diamond-friendly mantle eclogite reservoir beneath Koidu.
Geochimica et Cosmochimica Acta, in press available, 42p.
Canada, Northwest Territories
deposit -Diavik A154 N & S
Abstract: Kimberlite-borne granulite xenoliths provide rare insights into the age, chemical composition and tectonothermal evolution of the otherwise largely inaccessible deep cratonic crust. The formation and transformation of the lower continental crust (LCC) beneath the central Slave craton (Canada) is here illuminated using whole-rock trace-element and Sr-Nd isotope compositions of nine metabasaltic (MBG), one gabbroic (MGG) and two metasedimentary/hybrid (MSG) granulite xenoliths. On the one hand, published sulphide Re-Os and a few zircon U-Pb data indicate that at least a portion of the LCC beneath the central Slave craton has a Palaeoarchaean origin (?3.3?Ga), which apparently coincides with a period of juvenile crust and deep lithospheric mantle formation during plume impingement beneath the pre-existing cratonic nucleus. On the other hand, enrichment in Li, Sr, LREE, Pb and Th, but relative depletion in Ti, Hf and HREE, suggest formation of (picro)basaltic protoliths by partial melting of a subduction-modified garnet-bearing source, Crystallisation in the crust after fractionation of plagioclase is inidicated by their Sr and Eu negative anomalies, which are complementary to the positive anomalies in the MGG. Samarium-Nd isotopes in MBG and MGG show large scatter, but fall on Neo- or Mesoarchaean age arrays. These elemental systematics are suggested to fingerprint deserpentinisation fluids plus small amounts of sedimentary melt as the main contaminants of the mantle source, supporting the operation of at least regional and transient subduction at 3.3?Ga. Evidence for quasi-coeval plume impingement and subduction beneath the central Slave craton in the Mesoarchaean is reconcilable in a dynamic regime where vertical tectonics, though waning, was still active and plate interactions became increasingly important. Unradiogenic 87Sr/86Sr (down to 0.7017) is consistent with significant loss of Rb and probably other heat-producing elements (K, Th, U) plus H2O during Neoarchaean metamorphism, which helped to enhance LCC viscosity and stabilise the cratonic lithosphere.
Abstract: he science of studying diamond inclusions for understanding Earth history has developed significantly over the past decades, with new instrumentation and techniques applied to diamond sample archives revealing the stories contained within diamond inclusions. This chapter reviews what diamonds can tell us about the deep carbon cycle over the course of Earth’s history. It reviews how the geochemistry of diamonds and their inclusions inform us about the deep carbon cycle, the origin of the diamonds in Earth’s mantle, and the evolution of diamonds through time.
IN: Deep carbon: past to present, Orcutt, Daniel, Dasgupta eds., pp. 89-128.
Mantle
geodynamics
Abstract: The science of studying diamond inclusions for understanding Earth history has developed significantly over the past decades, with new instrumentation and techniques applied to diamond sample archives revealing the stories contained within diamond inclusions. This chapter reviews what diamonds can tell us about the deep carbon cycle over the course of Earth’s history. It reviews how the geochemistry of diamonds and their inclusions inform us about the deep carbon cycle, the origin of the diamonds in Earth’s mantle, and the evolution of diamonds through time.
IN: Deep Carbon: past to present. Editors Orcutt, Danielle, Dasgupta, pp. 89-128.
Mantle
geodynamics
Abstract: The science of studying diamond inclusions for understanding Earth history has developed significantly over the past decades, with new instrumentation and techniques applied to diamond sample archives revealing the stories contained within diamond inclusions. This chapter reviews what diamonds can tell us about the deep carbon cycle over the course of Earth’s history. It reviews how the geochemistry of diamonds and their inclusions inform us about the deep carbon cycle, the origin of the diamonds in Earth’s mantle, and the evolution of diamonds through time.
Nature Research Scientific Reports, https://doi.org/10.1038/ s41598-019-55743-1 11p. Pdf
Mantle
melting, redox
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Oxygen fugacity (fO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present fO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/?Fe and ƒf2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/?Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/?Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/?Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated fO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-fO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 99-122.
Mantle
carbon
Abstract: Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ?3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ?3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total ?¹³C data range from ?17.1 to ?1.9 ‰ (P = ?8.4 to ?1.9 ‰; E = ?17.1 to ?2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in ?¹³C of peridotitic diamonds has been at ?5 (±2) ‰ since the earliest diamond growth ?3.5 Ga, and this mode is also observed in the eclogitic diamond record since ?3 Ga. The skewness of eclogitic diamonds’ ?¹³C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal ?¹³C mode of ?5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Geochimica et Cosmochimica Acta, Vol. 278, pp. 78-83.
Canada, Northwest Territories
kimberlites
Abstract: Kimberlite-borne granulite xenoliths provide rare insights into the age, chemical composition and tectonothermal evolution of the otherwise largely inaccessible deep cratonic crust. The formation and transformation of the lower continental crust (LCC) beneath the central Slave craton (Canada) is here illuminated using whole-rock trace-element and Sr-Nd isotope compositions of nine metabasaltic (MBG), one gabbroic (MGG) and two metasedimentary/hybrid (MSG) granulite xenoliths. On the one hand, published sulphide Re-Os and a few zircon U-Pb data indicate that at least a portion of the LCC beneath the central Slave craton has a Palaeoarchaean origin (?3.3?Ga), which apparently coincides with a period of juvenile crust and deep lithospheric mantle formation during plume impingement beneath the pre-existing cratonic nucleus. On the other hand, enrichment in Li, Sr, LREE, Pb and Th, but relative depletion in Ti, Hf and HREE, suggest formation of (picro)basaltic protoliths by partial melting of a subduction-modified garnet-bearing source, Crystallisation in the crust after fractionation of plagioclase is inidicated by their Sr and Eu negative anomalies, which are complementary to the positive anomalies in the MGG. Samarium-Nd isotopes in MBG and MGG show large scatter, but fall on Neo- or Mesoarchaean age arrays. These elemental systematics are suggested to fingerprint deserpentinisation fluids plus small amounts of sedimentary melt as the main contaminants of the mantle source, supporting the operation of at least regional and transient subduction at 3.3?Ga. Evidence for quasi-coeval plume impingement and subduction beneath the central Slave craton in the Mesoarchaean is reconcilable in a dynamic regime where vertical tectonics, though waning, was still active and plate interactions became increasingly important. Unradiogenic 87Sr/86Sr (down to 0.7017) is consistent with significant loss of Rb and probably other heat-producing elements (K, Th, U) plus H2O during Neoarchaean metamorphism, which helped to enhance LCC viscosity and stabilise the cratonic lithosphere.
Abstract: Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO?=?7.3 to 14.5?wt%; n?=?43) with accessory diamond ± corundum and kyanite; garnet grossular content (median Ca#?=?0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29?ppm) and have low median Cr2O3 (0.06?wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO?=?14.0 to 20.0?wt%; n?=?29), which are barren and enriched in LREE (median La 1.39?ppm), Cr2O3 (0.25?wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr?>?0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3?Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton. Based on the proportions of MgO-poor and -rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135?±?15?km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ?20?km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenite, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.
Abstract: A coesite-bearing diamondiferous eclogite from the Udachnaya kimberlite (Daldyn field, Siberian craton) has been studied to trace its complex evolution recorded in rock-forming and minor mineral constituents. The eclogite sample is composed of rock-forming omphacite (60 vol%), garnet (35 vol%) and quartz/coesite (5 vol%) and contains intergranular euhedral zoned olivine crystals, up to 200 µm long, coexisting with phlogopite, orthopyroxene, clinopyroxene (secondary), K-feldspar, plagioclase, spinel, sodalite and djerfisherite. Garnet grains are zoned, with a relatively homogeneous core and a more magnesian overgrowth rim. The rim zones further differ from the core in having higher Zr/Y (6 times that in the cores), ascribed to interaction with, or precipitation from, a kimberlite-related melt. Judging by pressure-temperature estimates (~1200 °C; 6.2 GPa), the xenolith originated at depths of ~180-200 km at the base of the continental lithosphere. The spatial coexistence of olivine, orthopyroxene and coesite/quartz with K-Na-Cl minerals in the xenolith indicates that eclogite reacted with a deep-seated kimberlite melt. However, Fe-rich olivine, orthopyroxene and low-pressure minerals (sodalite and djerfisherite) likely result from metasomatic reaction at shallower depths during transport of the eclogite by the erupting kimberlite melt. Our results demonstrate that a mixed eclogitic-peridotitic paragenesis, reported previously from inclusions in diamond, can form by interaction of eclogite and a kimberlite-related melt.
Journal of Petrology, , https://doi.org/10. 1093/petrology/egaa065
Mantle
oxygen fugacity
Abstract: Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle-crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, while apparent clinopyroxene-garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ?250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti-Nb-Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised.
Nature Research Scientific Reports, Vol. 9:20190 doir.org/10.38 /s41598-019-55743-1, 11p. Pdf
Mantle
eclogite
Abstract: Oxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/?Fe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/?Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/?Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/?Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.
Journal of Petrology, in press available, 89p. Pdf
Mantle
eclogite
Abstract: Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle-crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, while apparent clinopyroxene-garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ?250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti-Nb-Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised.
Lithos, doi.org/10.1016/ jlithos.2020.105880, 26p. Pdf
Africa, South Africa
deposit - Bultfontein
Abstract: The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/? PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80-120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Physics of the Earth and Planetary Interiors, doi.org/10.1016/ j.pepi.2020.106638 51p. Pdf
Mantle
geophysics - magnetics
Abstract: Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200-300?km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1?wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.
Geochimica et Cosmochimica Acta, Vol. 306, pp. 143-170. pdf
Europe, Italy
subduction
Abstract: The volatile transfer in subduction zones and the role of sulfate as a vector for the mobilization of oxidized components from down-going slabs remain hotly debated issues. Orogenic spinel and garnet peridotite lenses from the Ulten Zone (Eastern Alps, Italy), exhumed as part of felsic metamorphic terranes in continental collision zones, bear witness to mass transfer processes in these pivotal environments. In this study, we carried out a multi-method investigation of mantle sulfides coexisting with four generations of carbonates, indicating coupled sulfur and carbon mobility throughout the peridotites’ metamorphic evolution as part of the Variscan subduction architecture. Detailed petrography, bulk rock measurements, in situ chemical and geochemical analyses of sulfides as well as Sr isotope analyses of associated clinopyroxene and amphibole are combined with the aim to constrain the origin, nature and effect of multiple C-O-H-S-bearing fluids and melts the peridotites interacted with. The first, pre-peak, metasomatic pulse (Stage 1) is represented by an H2S-CO2-bearing melt from the subduction-modified hot mantle wedge, which formed a pyroxenite layer hosting matrix pentlandite with ?34S of +2.77‰. Matrix carbonates occasionally occur in the coarse-grained peridotite under eclogite-facies conditions (Stage 2), with heavier ?34S (up to +3.43‰), radiogenic Sr (87Sr/86Srclinopyroxene > 0.7052) and elevated Pb abundances. These are ascribed to interaction with isotopically heavy melts carrying recycled crustal component, permissive of, but not requiring, involvement of oxidized S species. Conversely, isotopically lighter matrix pentlandite (?34S = ?1.62 to +0.67‰), and radiogenic Sr in amphibole (87Sr/86Sr = 0.7056) and associated dolomite (published data) from fine-grained garnet-amphibole peridotites may point to involvement of H2S-CO2-bearing crustal fluids, which variably equilibrated with the mantle before interacting with the peridotites. The post-peak Stage 3 marks the entrapment of peridotites into a tectonic mélange. Here, kelyphitization of garnet is catalyzed by further ingress of a S-bearing fluid (?34S = ?0.38‰), while carbonate veining with occasional sulfides bear witness to channelized fluid flow. Sulfide and amphibole grains in retrogressed spinel peridotites reveal the highest contents of fluid-mobile elements (As, Sb) and 87Sr/86Sramphibole up to 0.7074, suggesting late interactions with isotopically heavy crustal fluids at high fluid-rock ratios. Textural observations indicate that, during Stage 4, serpentinization of peridotites at low ƒS2 played an active role not only in CO2 release by conversion of dolomite to calcite + brucite intergrowths, but also in local removal of 32S during the final exhumation stage. Late channelized sulfur remobilization is evidenced by the serpentine + magnetite (±millerite ± calcite) vein carrying > 300 ppm S. Overall, the relatively narrow range of sulfur isotope composition (?34S = ?1.62 to +3.76‰) is indicative of limited interaction with isotopically heavy crustal liquids, and points to a subordinate role of subduction-derived sulfate throughout the extended fluid(melt)/rock evolution of the Ulten Zone peridotites, first in the mantle wedge and then as part of a tectonic mélange.
Abstract: Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. To shed new light on this issue, seven graphite-diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite 87Sr/86Sr, acquired in situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P-T conditions for basaltic group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4•8-6•5?GPa and temperatures of 1060-1130?°C, whereas gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4•8-5•8?GPa), but a larger range in temperature (990-1260?°C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine ± clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their light rare earth element depletion was the result of <10?% partial melting during subsequent subduction and emplacement into the cratonic lithosphere. Extremely unradiogenic 87Sr/86Sr (0•70091-0•70186 for six of seven samples) not only provides new evidence for the Archean age (2•5-2•9?Gyr) of Yakutian graphite-diamond-bearing eclogites and for formation of their protoliths in a depleted mantle source, but also suggests that they were not significantly metasomatically overprinted after their formation, despite their extended residence in the cratonic mantle lithosphere. The mineralogical and petrographic features indicate that the primary mineral association includes garnet, omphacite, ± coesite, ± kyanite, ± rutile, graphite, and diamond. Graphite occurs in the samples in the form of idiomorphic crystals (the longest dimensions being 0•4-1?mm) in garnet and kyanite and extends beyond their grain boundaries. Diamonds occur as octahedral cubic transparent, slightly colored or bright yellow crystals as large as 0•1-2?mm. Furthermore, idiomorphic and highly ordered graphite occurs as inclusions in diamond in four samples. The carbon isotope composition for diamond and graphite has a narrow range (?4 to ?6•6?‰) for both groups (gabbroic and basaltic), indicating a mantle source and limiting the role of subducted isotopically light biogenic carbon or reduction of isotopically heavy carbonate in diamond crystallization. Importantly, the presence of graphite and diamond inclusions in garnet, omphacite, and kyanite in three samples indicates a co-formation close in time to eclogitization. Combined, the petrographic and geochemical evidence suggests that both polymorphic carbon modifications can form in the diamond stability field, as also suggested by experiments and some natural examples, although the exact mechanism remains unresolved. Furthermore, this study provides natural evidence that graphite can be preserved (metastably) deep within the diamond stability field, without recrystallizing into diamond, for a long time, ?2•5?Gyr.
Abstract: The mineralogical and geochemical features of titanite and associated minerals in a rare sample of kyanite-coesite-rutile-bearing eclogite from the Udachnaya-East (Vostochnaya) kimberlite pipe have been studied in detail. Subidiomorphic titanite grains (100-300 ?m) were identified in the intergranular space. The composition of individual grains of titanite is characterized by a constant presence of Al2O3, F, P2O5, Zr, and Sr impurities but varies within the xenolith. Based on the absence of titanite inclusions in the rock-forming minerals and their presence in the intergranular space, titanite was formed in the studied sample at a late stage of its formation, most likely in the process of metasomatic action of the fluid/melt. Crystallization of rock-forming minerals (garnet + omphacite + kyanite) and accessory rutile occurred jointly at 3.5 ± 0.32 GPa and 920 ± 65°?. The value of Eu/Eu* = 1.06 in the reconstructed bulk composition of the rock, the high modal content of kyanite (~17 vol %), and the value of Ca# = Ca/(Ca + Mg + Fe + Mn) > 0.5 in garnet indicate a subduction nature of the studied eclogite. Most likely, the formation of titanite in the studied sample occurred as a result of the metasomatic action of a fluid/melt enriched in calcium, strontium, large lithophilic elements, and lead, by a mechanism similar to the formation of eclogites in the units of the Western Tien Shan.
Phanerozoic burial and unroofing of the western Slave Craton and Wopmay Orogen from apatite ( U Th/He thermochronometry, assessing links between surface/deep
American Geological Union, Fall meeting Dec. 15-19, Eos Trans. Vol. 89, no. 53, meeting supplement, 1p. abstract
Earth and Planetary Science Letters, Vol. 410, pp. 97-104.
Global, Canada
Thermochronology - Slave craton
Abstract: A variety of models are used to explain an apparent episodicity in kimberlite emplacement. Implicit in these models is the assumption that the preserved kimberlite record is largely complete. However, some cratons now mostly devoid of Phanerozoic cover underwent substantial Phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions. Here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the Slave craton, Superior craton, and cratonic western Australia. A similar pattern exists in the Kaapvaal craton, although its magmatic, deposition, and erosion history differs in key ways from the other localities. One explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism. Another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts. Even if kimberlites occurred during cratonic burial phases, the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced. In this model, kimberlite magmatism was more continuous than the preserved record suggests, implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects. Either way, the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns.
Abstract: The Geological and Geodiversity Mapping binational program along the Brazil?Guyana border zone allowed reviewing and in? tegrating the stratigraphy and nomenclature of the Roraima Supergroup along the Pakaraima Sedimentary Block present in northeastern Brazil and western Guyana. The area mapped corresponds to a buffer zone of approximately 25 km in width on both sides of the border, of a region extending along the Maú?Ireng River between Mount Roraima (the tri? ple?border region) and Mutum Village in Brazil and Monkey Mountain in Guyana. The south border of the Roraima basin is overlain exclusively by effusive and volcaniclastic rocks of the Surumu Group of Brazil and its correlated equivalent the Burro?Burro Group of Guyana.
Brazilian Journal of Geology, Vol. 47, 1, pp. 43-57.
South America, Brazil, Guyana
craton
Abstract: The Geological and Geodiversity Mapping binational program along the Brazil?Guyana border zone allowed reviewing and in? tegrating the stratigraphy and nomenclature of the Roraima Supergroup along the Pakaraima Sedimentary Block present in northeastern Brazil and western Guyana. The area mapped corresponds to a buffer zone of approximately 25 km in width on both sides of the border, of a region extending along the Maú?Ireng River between Mount Roraima (the tri? ple?border region) and Mutum Village in Brazil and Monkey Mountain in Guyana. The south border of the Roraima basin is overlain exclusively by effusive and volcaniclastic rocks of the Surumu Group of Brazil and its correlated equivalent the Burro?Burro Group of Guyana.
Brazilian Journal of Geology, Vol. 41, 1, pp. 43-57.
South America, Brazil, Guyana
Guiana shield
Abstract: The Geological and Geodiversity Mapping binational program along the Brazil-Guyana border zone allowed reviewing and integrating the stratigraphy and nomenclature of the Roraima Supergroup along the Pakaraima Sedimentary Block present in northeastern Brazil and western Guyana. The area mapped corresponds to a buffer zone of approximately 25 km in width on both sides of the border, of a region extending along the Maú-Ireng River between Mount Roraima (the triple-border region) and Mutum Village in Brazil and Monkey Mountain in Guyana. The south border of the Roraima basin is overlain exclusively by effusive and volcaniclastic rocks of the Surumu Group of Brazil and its correlated equivalent the Burro-Burro Group of Guyana.
Abstract: In the present work, we report the chemical composition of representative emerald crystals from some of the most important worldwide deposits. Major and trace elements were investigated using Electron Microprobe Analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS) techniques. Binary, ternary and spider diagrams along with statistical analysis, i.e., Principal Component Analysis (PCA), were used to discriminate each deposit with high reliability. PCA of SiO2, Al2O3, V, Sc, B, Li content identified distinct groups. The use of binary and ternary diagrams contributed to discriminate among emerald crystals from various deposits, which are included in the same clusters of the PCA analysis. In addition, the geochemical features of each group were linked to the geological environment and genetic processes which leaded to emerald formation. In particular, the emeralds related to granitic-pegmatitic intrusions (Type-1) or those occurring in environments controlled by tectonic events (Type-2) were distinguished using the concentrations of major and trace elements. The results of this study can contribute to improve the existing genetic models and classification schemes as well as to identify useful geochemical fingerprints for provenance purposes.-
Abstract: arth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core-mantle boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core-mantle boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with heterogeneous core-mantle boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core-mantle boundary—an apparent thermal stratification despite the average heat flux across the core-mantle boundary being strongly superadiabatic.
Geochemical behaviour of rare earth elements in alterites of phosphate and titanium ore deposits in Tapira (Minas Gerais, Brasil):importance ofphosphates
Chemical Geology ( Geochem. of the Earth's surface and of min. formation, 2nd., Vol. 84, No. 1-4, July 5, pp. 377. Abstract
Brazil
Alkaline rocks, rare earth elements (REE) -phosphates
Earth and Planetary Science Letters, Vol. 467, pp. 99-107.
Mantle
chlorine
Abstract: We report concentrations of Chlorine (Cl) in synthetic wadsleyite (Wd) and ringwoodite (Rw) in the system NaCl-(Mg,?Fe)2SiO4 under hydrous and anhydrous conditions. Multi-anvil press experiments were performed under pressures (14-22 GPa) and temperatures (1100-1400?°C) relevant to the transition zone (TZ: 410-670 km depth). Cl and H contents were measured using Particle Induced X-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA) respectively. Results show that Cl content in Rw and Wd is significantly higher than in other nominally anhydrous minerals from the upper mantle (olivine, pyroxene, garnet), with up to 490 ppm Cl in anhydrous Rw, and from 174 to 200 ppm Cl in hydrous Wd and up to 113 ppm Cl in hydrous Rw. These results put constrains on the Cl budget of the deep Earth. Based on these results, we propose that the TZ may be a major repository for major halogen elements in the mantle, where Cl may be concentrated together with H2OH2O and F (see Roberge et al., 2015). Assuming a continuous supply by subduction and a water-rich TZ, we use the concentrations measured in Wd (174 ppm Cl) and in Rw (106 ppm Cl) and we obtain a maximum value for the Cl budget for the bulk silicate Earth (BSE) of 15.1 × 1022 g Cl, equivalent to 37 ppm Cl. This value is larger than the 17 ppm Cl proposed previously by McDonough and Sun (1995) and evidences that the Cl content of the mantle may be higher than previously thought. Comparison of the present results with the budget calculated for F (Roberge et al., 2015) shows that while both elements abundances are probably underestimated for the bulk silicate Earth, their relative abundances are preserved. The BSE is too rich in F with respect to heavy halogen elements to be compatible with a primordial origin from chondrites CI-like (carbonaceous chondrites CC) material only. We thus propose a combination of two processes to explain these relative abundances: a primordial contribution of different chondritic-like materials, including EC-like (enstatite chondrites), possibly followed by a distinct fractionation of F during the Earth differentiation due to its lithophile behavior compared to Cl, Br and I.
Geological Society, London Special Publication, doi.org/10.1144/SP513-2021-36 49p. Pdf
Europe, Italy, France, Spain, Serbia, Macedonia, Turkey
lamproites
Abstract: High-MgO lamproite and lamproite-like (i.e., lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine-Himalaya collisional margin, which followed the closure of the Tethys ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g., plugs, dykes, and laccoliths), and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (Northern Italy), through the Late Miocene in Corsica (Southern France) and in Murcia-Almeria (South-Eastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (Central Italy), in the Balkan peninsula (Serbia and Macedonia), and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO, and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent and it is rarely found only in the groudmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207Pb over 206Pb and high time integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatised by at least two different agents: i) a high Th/La and Sm/La (i.e., SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; ii) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterised by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterise areas that were affected by multiple Wilson cycles, as observed in the the Alpine-Himalayan realm.
Geological Society London Special Publication, doi.org/10.1144/SP513-2021-36. pdf
Mantle
lamproite
Abstract: High-MgO lamproite and lamproite-like (i.e., lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine-Himalaya collisional margin, which followed the closure of the Tethys ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g., plugs, dykes, and laccoliths), and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (Northern Italy), through the Late Miocene in Corsica (Southern France) and in Murcia-Almeria (South-Eastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (Central Italy), in the Balkan peninsula (Serbia and Macedonia), and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO, and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent and it is rarely found only in the groudmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207Pb over 206Pb and high time integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatised by at least two different agents: i) a high Th/La and Sm/La (i.e., SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; ii) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterised by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterise areas that were affected by multiple Wilson cycles, as observed in the the Alpine-Himalayan realm.
Abstract: An unusual variety of impact-related diamond from the Popigai impact structure - yakutites - is characterized by complementary methods including optical microscopy, X-ray diffraction, radiography and tomography, infra-red, Raman and luminescence spectroscopy providing structural information at widely different scales. It is shown that relatively large graphite aggregates may be transformed to diamond with preservation of many morphological features. Spectroscopic and X-ray diffraction data indicate that the yakutite matrix represents bulk nanocrystalline diamond. For the first time, features of two-phonon infra-red absorption spectra of bulk nanocrystalline diamond are interpreted in the framework of phonon dispersion curves. Luminescence spectra of yakutite are dominated by dislocation-related defects. Optical microscopy supported by X-ray diffraction reveals the presence of single crystal diamonds with sizes of up to several tens of microns embedded into nanodiamond matrix. The presence of single crystal grains in impact diamond may be explained by CVD-like growth in a transient cavity and/or a seconds-long compression stage of the impact process due to slow pressure release in a volatile-rich target. For the first time, protogenetic mineral inclusions in yakutites represented by mixed monoclinic and tetragonal ZrO2 are observed. This implies the presence of baddeleyite in target rocks responsible for yakutite formation.
The History of the State of Georgia from 1850 to 1881 Embracing the Three Important Epochs: the Decade Before the War 1861-1865; the War; the Period of Reconstruction.
Geochimica et Cosmochimica Acta, Vol. 232, pp. 82-100.
Mantle
geochemistry
Abstract: We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3?Ga) to Paleozoic (404?Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30-40‰ u?1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3?Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9?±?1.2 (1?) ‰ u?1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u?1 at 3.5?Ga to 12.9‰ u?1 at 3.3?Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7?Ga. However, further studies will be needed to confirm this observation. After 2.7?Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1?Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3?Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth's surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3?Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time.
Abstract: Nitrogen is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth’s accretion versus that subducted from the Earth’s surface is unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle ?15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative ?15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember ?15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased ?15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has ?15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume ?15N values may both be dominantly primordial features.
Abstract: The continental basement in the Eastern Mediterranean represents the northern edge of Gondwana, which has been the site of repeated crustal accretion and has subsequently been modified by consecutive rifting events. We investigated the geologic and thermal history of the North Gondwana lower crust by examining the U-Pb-Hf isotope systematics in zircons within 6 mafic granulite xenoliths from Pliocene lava cone in North Israel. The lava cone protrudes through the platform cover that seals the late Neoproterozoic junction between the Arabian-Nubian basement to the South and the Cadomian basement exposed in the Taurides to the North. The mafic granulite xenoliths are composed of plagioclase + orthopyroxene + clinopyroxene ± garnet ± spinel ± secondary amphibole. U-Pb zircon ages from the granulites vary among the different samples with distinct zircon age populations at 400-1200 Ma, 170-350 Ma, and 3.6-4.2 Ma, attesting the lower crust preserves a prolonged thermal and igneous history. While 400-550 Ma U-Pb ages are interpreted to be the result of Pb loss, the wide scatter of zircon grains aged between 550 and 1200 Ma, alongside their diverse ?Hf(t) values (?25-+10), is an extraordinary evidence for the accretion of Neoproterozoic sediments into the North Gondwana arc root lower crust. The U-Pb-Hf signature of these zircons resembles Cadomian sediments of the Tauride block to the north, indicating southward (present coordinates) subduction under North Gondwana and possible accretion of fore-arc sediments to the lower crust through relamination in the latest Neoproterozoic. One xenolith contained metamorphic-shaped zircons aged 170-350 Ma with positive ?Hf values and Hf-TDM of 0.85 Ga interpreted to reflect Paleozoic recycling of the Neoproterozoic juvenile Arabian basement, which we consider to form a major component of the lower crust in the region. An overwhelming cluster of Carboniferous zircons concentrating at 305 Ma with exclusively negative ?Hf values around ?6, was retrieved from three xenoliths. Some of these zircons portrayed igneous textures and shape. While Carboniferous igneous activity is the hallmark of Western Europe's Variscan orogeny, the latter did not affect the southern rifted edge of Neo-Tethys where our xenoliths were retrieved. The Paleozoic age-Hf composition in our xenoliths is therefore interpreted to result from syn-Variscan recycling of Neoproterozoic sedimentary remains in the lower crust, and some degree of melting in a non-orogenic environment. Rather than with horizontal plate motions and orogeny, the Carboniferous zircon ages in the xenoliths appear to coalesce with significant vertical movements that created continental scale unconformities and a broad basin and swell architecture known to develop over the entire North Gondwana margin at that time. The Carboniferous aged zircons in northern Israel lower crustal xenoliths are therefore a unique gauge of the thermal perturbation that accompanied the large-scale mantle dynamics below the then passive North African margin of Gondwana, while Variscan orogenic accretion occurred on the Eurasian margin. These lower crustal granulites xenoliths therefore contain important information with respect to the nature of the lower crust under Israel, with implications on the geodynamic setting during the Cadomian and Variscan cycles.
Abstract: Field, petrographic and geochemical data combined with in situ zircon U-Pb LA-ICP-MS ages are documented for the São Tiago Batholith (southernmost portion of the São 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 São 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 São Tiago Archean crust, representing an irregular paleo-coastline or a micro-terrane amalgamation with the São 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 São Francisco Craton.
Journal of South American Earth Sciences, Vol. 94, 102203 14p. Pdf
South America, Brazil
geophysics - magnetics
Abstract: Accretionary orogens are considered as the result of the major crust production process, and terranes are accreted material representing distinctive assemblages regarding age and evolution. Scientific advances in the last years show that the southernmost São Francisco Craton includes Archean, Paleoproterozoic and Neoproterozoic areas. We used aerogeophysical data and field geology to understand the extension of individual pieces of this tectonic puzzle. We described five magnetometric lineaments. A-, B- and C-lineaments are related to dyke swarms of different ages. D- and E-lineaments represent regional-scale tectonic structures. In a tectonic perspective, we have identified the following terranes: (i) the Archean São Tiago crust (2.67?Ga), which is part of the São Francisco proto-craton; two Siderian juvenile arcs, represented by the (ii) Cassiterita (2.47?Ga) and the (iii) Resende Costa/Lagoa Dourada (2.36-2.35?Ga) orthogneisses; and two Rhyacian arcs, the (iv) juvenile Serrinha (2.22-2.20?Ga) and the continental Ritápolis (2.19-2.10?Ga) arcs. Tectonic terranes and five magnetic subdomains were identified on the basis of (i) association of shear zones/faults with quartz veins in the field; (ii) low- and high-intensity magnetic anomalies; (iii) sharp contrast in Euler solution intensities; and (iv) high-contrast in radioelement contents in the gammaspectrometric maps. Processing of aerogeophysical data permitted us to propose a new scenario on the evolution of the southern São Francisco Craton, and in particular of the Mineiro belt. The integration between aerogeophysical, new and compiled geologic information, provides a robust model for the understanding of individual tectonic pieces of the studied area.
Journal of South American Earth Sciences, Vol. 107, 103088, 17p. Pdf
South America, Venezuela
Geophysics - seismics
Abstract: As part of the lithosphere, the crust represents Earth's rigid outer layer. Some of the tools to study the crust and its thickness are wide-angle seismic studies. To date, a series of seismic studies have been carried out in Venezuela to determine in detail the crustal thickness in the southern Caribbean, in the region of the Caribbean Mountain System in northern Venezuela, as well as along the Mérida Andes and surrounding regions. In this study, a review of the wide-angle seismic data is given, incorporating new data from the GIAME project for western Venezuela, resulting in a map of Moho depth north of the Orinoco River, which serves as the basis for future integrated models. Differences in Moho depths from seismic data and receiver function analysis are discussed. From the Caribbean plate, Moho depth increases from 20 to 25 km in the Venezuela Basin to about 35 km along the coast (except for the Falcón area where a thinning to less than 30 km is observed) and 40-45 km in Barinas - Apure and Guárico Basins, and Guayana Shield, respectively. Values of more than 50 km are observed in the Maturín Basin and in the southern part of the Mérida Andes.
Abstract: A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not "accidental" xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly "metamorphic" trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be "magmatic" rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate "metamorphic" from "magmatic" gem corundum ought to be used with caution.
Abstract: The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, ?-implantation, ?-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71?Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.
Abstract: Deciphering erosion rates over geologic time is fundamental for understanding the interplay between climate, tectonic, and erosional processes. Existing techniques integrate erosion over different time scales, and direct comparison of such rates is routinely done in earth science. On the basis of a global compilation, we show that erosion rate estimates in glaciated landscapes may be affected by a systematic averaging bias that produces higher estimated erosion rates toward the present, which do not reflect straightforward changes in erosion rates through time. This trend can result from a heavy-tailed distribution of erosional hiatuses (that is, time periods where no or relatively slow erosion occurs). We argue that such a distribution can result from the intermittency of erosional processes in glaciated landscapes that are tightly coupled to climate variability from decadal to millennial time scales. In contrast, we find no evidence for a time scale bias in spatially averaged erosion rates of landscapes dominated by river incision. We discuss the implications of our findings in the context of the proposed coupling between climate and tectonics, and interpreting erosion rate estimates with different averaging time scales through geologic time.
Earth and Planetary Science Letters, Vol. 461, pp. 105-118.
Mantle
Metasomatism
Abstract: Peridotite constitutes most of the Earth's upper mantle, and it is therefore unsurprising that most mantle-derived magmas exhibit evidence of past equilibrium with an olivine-dominated source. Although there is mounting evidence for the role of pyroxenite in magma generation within upwelling mantle plumes, a less documented non-peridotite source of melts are metasomatic veins (metasomes) within the lithospheric mantle. Here we present major and trace element analyses of 66 lavas erupted from a small Miocene shield volcano located within the Ethiopian flood basalt province. Erupted lavas are intercalated with lahars and pyroclastic horizons that are overlain by a later stage of activity manifested in small cinder cones and flows. The lavas form two distinctive petrographic and geochemical groups: (A) an olivine-phyric, low Ti group (1.7-2.7 wt.% TiO2; 4.0-13.6 wt.% MgO), which geochemically resembles most of the basalts in the region. These low Ti lavas are the only geochemical units identified in the later cinder cones and associated lava flows; (B) a clinopyroxene-phyric high Ti group (3.1-6.5 wt.% TiO2; 2.8-9.2 wt.% MgO), which resembles the Oligocene HT-2 flood basalts. This unit is found intercalated with low Ti lavas within the Miocene shield. In comparison to the low Ti group, the high Ti lavas exhibit a profound depletion in Ni, Cr, Al, and Si, and significant enrichment in Ca, Fe, V, and the most incompatible trace elements. A characteristic negative K anomaly in primitive-mantle normalized diagrams, and Na2O > K2O, suggests a source rich in amphibole, devoid of olivine, and perhaps containing some carbonate and magnetite. While melt generation during rift development in Ethiopia is strongly correlated with the thermo-chemical anomalies associated with the African Superplume, thermobaric destabilization and melting of mantle metasomes may also contribute to lithospheric thinning. In regions impacted by mantle plumes, such melts may be critical to weakening of the continental lithosphere and the development of rifts.
Active compressional tectonics in central Africa and implications for plate tectonic models: evidence from fault mechanism studies of 1998 earthquakes in the Congo B
Journal of African Earth Sciences, Vol. 35, 1, July, pp. 45-50.
Project 03-002. Synthesis of Archean geology and diamond bearing rocks in the Michipicoten Greenstone Belt: results from microdiamond extraction and geochronology.
Ontario Geological Survey Summary of Fieldwork 2005, O.F. 6172, pp. 8-1-13.
Abstract: Structure within the Earth is best studied in three dimensions and using several coincident overlays of diverse information with which one can best see where unusual properties match up. Here we use regional surfaces causing discontinuities in seismic waves a few hundred kilometers deep in the Earth, intersected and thus calibrated by rebuilt rock columns using rare rock samples erupted to the surface in two locations. Electrically conductive regions can be mapped using natural (magnetotelluric) currents. East- and west-dipping seismic discontinuity surfaces match surface structures that developed about 1.8 billion years ago marginal to the Superior crustal block. Surfaces dipping to the southeast and northwest match some boundaries between crustal blocks that are over 2.5 billion years old, but many such crustal boundaries trend more east-west. Conductive rocks appear more commonly above these discontinuity surfaces where gas-rich fluids apparently flowed and that the discontinuities somehow filtered these fluids. The mismatch in orientation and dip between the most ancient deep and exposed structures suggests that plate tectonic processes operating today differed earlier than 2.5 billion years ago.
The stability of Zirconium and Phosphorus bearing minerals in peridotite coexisting with alkaline melts. Implications for the storage of Uranium and Thorium in the mantle
Geological Association of Canada (GAC) Annual Meeting, Vol. 11, p. 101. (abstract.)
Cryptic metasomatism and creation of melts with depleted contents of the high field strength elements:coupled effects due to infiltration of melt intoharz
New Mexico Bureau of Mines Bulletin., Continental Magmatism Abstract Volume, Held, Bulletin. No. 131, p. 185. Abstract
A model of mantle metasom. by carb. alkaline melts:trace element and isotopic compositions of mantle source regions of carbonatite and cont. igneousrocks
Carbonatites -Genesis and Evolution, Ed. K. Bell Unwin Hyman Publ, pp. 464-499
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.
Journal of Geophysical Research, Vol. 120, 10, pp. 6698-6721.
India
Geophysics - craton
Abstract: Magnetotelluric-derived two-dimensional lithospheric resistivity structure of the western Dharwar craton (WDC) and adjoining Coorg block indicates isolated low-resistivity zones in the crust and three striking upper mantle conductive features within the highly resistive Archean lithosphere. The crustal conductors in the WDC show good spatial correlation with the exposed supracrustal rocks conformable with the relic schist belt channels having conductive mineral grains. Conductive zones within the Coorg crust might be related to the relatively young (933?Ma) metamorphic processes in the area and/or possible fluids derived from the Cretaceous passage of Reunion plume in the proximity of Coorg area. A near-vertical conductive structure extending from the lower crust into the upper mantle coincides with the transition zone between Coorg and WDC. This is interpreted as the suture zone between the two tectonic blocks and provides evidence for the individuality of the two Archean terrains. An anomalous upper mantle conductive zone found beneath the craton nucleus may indicate a modified cratonic lithosphere. This could have been derived due to the collision between Coorg and WDC and possibly survived by the subsequent multiple episodes of melt and fluid infiltration processes experienced in the region. Thick (~190?km) and preserved lithosphere is mapped at the eastern segment of WDC. Resistive lithosphere of ~125?km thickness is imaged for the Coorg block.
Late Ediacaran ( 605-580 Ma) post-collisional alkaline magmatism in the Arabian-Nubian shield: a case study of Serbal ring-shaped intrusion, southern Sinai, Egypt.
Journal of Asian Earth Sciences, Vol. 77, pp. 203-223.
Abstract: New and compiled data of zircon U-Pb ages and geochemical-isotopic constraints provide new insights into the orogenic evolution of the Rio Apa Terrane (RAT) and its close affinity with the Amazonia throughout the Proterozoic. Two terranes with distinct evolutionary histories built the RAT. The Porto Murtinho (2070-1940 Ma) and Amoguijá (1870-1820 Ma) magmatic arcs generated the Western Terrane which is mainly composed of short-lived crustal components. Granitoid rocks (1870 Ma) in the distal Corumbá Window indicate that the RAT is much larger in extent. The Caracol accretionary arc (1800-1740 Ma) and the associated Alto Tererê back-arc basin formed away from the Amoguijá belt, being roughly coeval with the adjoining Baía das Garças suite (1776 Ma) and Paso Bravo granitoid rocks (1774-1752 Ma). These tectonic units constitute the Eastern Terrane, whilst the NdHf isotopic constraints indicate derivation from a predominantly juvenile magma source with the minor input of crustal-derived contaminants. The youngest detrital zircon grains from the Alto Tererê samples gave 1740-1790 Ma ages and unimodal age spectra were mainly present. The basin infill was, therefore, most likely concomitant with the exhumation of the Caracol belt. Alto Tererê provenance study also included detritus from passive to active margin settings. The RAT underwent regional cooling between 1.35 and 1.27 Ga, documented mainly by 40Ar39Ar and KAr ages. This age pattern matches a collisional episode that formed the accretionary margin of Amazonia, suggesting that the RAT was a close neighbor at Ectasian times. The geodynamic interplay between them lasted until 1.1 Ga ago, highlighted by some shared-components of a LIP event.
Gasquet, D., Levresse, G., Cheilletz, A., Azizi Samir, M.R., Mouttaqi, A.
Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogeny...
Abstract: Geological observations indicate that there are only a few rocks of Archean Earth and no Hadean rocks on the surface of the present-day Earth. From these facts, many scientists believe that the primordial continents never existed during Hadean Earth, and the continental volume has kept increasing. On the other hand, recent studies reported the importance of the primordial continents on the origin of life, implying their existence. In this paper, we discussed the possible process that could explain the loss of the primordial continents with the assumption that they existed in the Hadean. Although depending on the timing of the initiation of plate tectonics and its convection style, subduction erosion, which is observed on the present-day Earth, might have carried the primordial continents into the deep mantle.
Abstract: The primordial crust on the Earth formed from the crystallization of the surface magma ocean during the Hadean. However, geological surveys have found no evidence of rocks dating back to more than 4 Ga on the Earth's surface, suggesting the Hadean crust was lost due to some processes. We investigated the subduction of one of the possible candidates for the primordial crust, anorthosite and KREEP crust similar to the Moon, which is also considered to have formed from the crystallization of the magma ocean. Similar to the present Earth, the subduction of primordial crust by subduction erosion is expected to be an effective way of eliminating primordial crust from the surface. In this study, the subduction rate of the primordial crust via subduction channels is evaluated by numerical simulations. The subduction channels are located between the subducting slab and the mantle wedge and are comprised of primordial crust materials supplied mainly by subduction erosion. We have found that primordial anorthosite and KREEP crust of up to ?50 km thick at the Earth's surface was able to be conveyed to the deep mantle within 0.1-2 Gy by that mechanism.
Azzone, R.G., Enrich, G.E.R., De Barros Gomes, C., Ruberti, E.
Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: the Juquia mafic-ultramafic alkaline-carbonatite massif, SE Brazil.
Journal of South American Earth Sciences, in press available, 17p.
Azzone, R.G., Enrich, G.E.R., De Barros Goes, C., Ruberti, E.
Trace element composition of parental magmas from mafic-ultramafic cumulates by in situ mineral analyses: the Juquia mafic-ultramafic alkaline carbonatite massif, SE Brazil.
Journal of South American Earth Sciences, Vol. 41, pp. 5-21.
Journal of South American Earth Sciences, Vol. 71, pp. 248-261.
South America, Paraguay
Carbonatite
Abstract: This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.
Journal of South American Earth Sciences, Vol. 77, pp. 286-309.
South America, Brazil
alkaline - Jacupiringa
Abstract: The Jacupiranga Complex is one of several Meso-Cenozoic alkaline intrusive complexes along the margins of the intracratonic Paraná Basin in southern Brazil. The complex encompasses a wide range of rock-types, including dunites, wehrlites, clinopyroxenites, melteigites-ijolites, feldspar-bearing rocks (diorites, syenites, and monzonites), lamprophyres and apatite-rich carbonatites. While carbonatites have been extensively investigated over the last decades, little attention has been paid to the silicate rocks. This study presents new geochonological and geochemical data on the Jacupiranga Complex, with particular emphasis on the silicate lithotypes. 40Ar/39Ar ages for different lithotypes range from 133.7 ± 0.5 Ma to 131.4 ± 0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9 ± 1.3 Ma. These ages indicate a narrow time frame for the Jacupiranga Complex emplacement, contemporaneous with the Paraná Magmatic Province. Most of the Jacupiranga rocks are SiO2-undersaturated, except for a quartz-normative monzonite. Based on geochemical compositions, the Jacupiranga silicate lithotypes may be separated into two magma-evolution trends: (1) a strongly silica-undersaturated series, comprising part of the clinopyroxenites and the ijolitic rocks, probably related to nephelinite melts and (2) a mildly silica-undersaturated series, related to basanite parental magmas and comprising the feldspar-bearing rocks, phonolites, lamprophyres, and part of the clinopyroxenites. Dunites and wehrlites are characterized by olivine compositionally restricted to the Fo83-84 interval and concentrations of CaO (0.13–0.54 wt%) and NiO (0.19–0.33 wt%) consistent with derivation by fractional crystallization, although it is not clear whether these rocks belong to the nephelinite or basanite series. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Some feldspar-bearing rocks (i.e. meladiorite and monzonite) show petrographic features and geochemical and isotope compositions indicative of crustal assimilation, although this may be relegated to a local process. Relatively high CaO/Al2O3 and La/Zr and low Ti/Eu ratios from the lamprophyres and calculated melts in equilibrium with cumulus clinopyroxene point to a lithospheric mantle metasomatized by CO2-rich fluids, suggesting vein-plus-wall-rock melting mechanisms. The chemical differences among those liquids are thought to reflect both variable contributions of melting resulting from veins and variable clinopyroxene/garnet proportions of the source.
Abstract: The Lower Cretaceous Jacupiranga complex, in the central-southeastern portion of the South American Platform, includes carbonatites in close association with silicate rocks (i.e. strongly and mildly silica-undersaturated series). Here we document the first hafnium isotope data on the Jacupiranga complex, together with new trace element and Pb isotope compositions. Even though liquid immiscibility from a carbonated silicate melt has been proposed for the genesis of several Brazilian carbonatites, isotopic and geochemical (e.g., Ba/La ratios, lack of pronounced Zr-Hf and Nb-Ta decoupling) information argues against a petrogenetic relationship between Jacupiranga carbonatites and their associated silicate rocks. Thus, an origin by direct partial melting of the mantle is considered. The isotopic compositions of the investigated silicate samples are coherent with a heterogeneously enriched subcontinental lithospheric mantle (SCLM) source of rather complex evolution. At least two metasomatic processes are constrained: (1) a first enrichment event, presumably derived from slab-related fluids introduced into the SCLM during Neoproterozoic times, as indicated by consistently old TDM ages and lamprophyre trace signatures, and (2) a Mesozoic carbonatite metasomatism episode of sub-lithospheric origin, as suggested by ?Nd-?Hf values inside the width of the terrestrial array. The Jacupiranga parental magmas might thus derive by partial melting of distinct generations of metasomatic vein assemblages that were hybridized with garnet peridotite wall-rocks.
Abstract: The Lower Cretaceous Jacupiranga complex, in the central-southeastern portion of the South American Platform, includes carbonatites in close association with silicate rocks (i.e. strongly and mildly silica-undersaturated series). Here we document the first hafnium isotope data on the Jacupiranga complex, together with new trace element and Pb isotope compositions. Even though liquid immiscibility from a carbonated silicate melt has been proposed for the genesis of several Brazilian carbonatites, isotopic and geochemical (e.g., Ba/La ratios, lack of pronounced Zr-Hf and Nb-Ta decoupling) information argues against a petrogenetic relationship between Jacupiranga carbonatites and their associated silicate rocks. Thus, an origin by direct partial melting of the mantle is considered. The isotopic compositions of the investigated silicate samples are coherent with a heterogeneously enriched subcontinental lithospheric mantle (SCLM) source of rather complex evolution. At least two metasomatic processes are constrained: (1) a first enrichment event, presumably derived from slab-related fluids introduced into the SCLM during Neoproterozoic times, as indicated by consistently old TDM ages and lamprophyre trace signatures, and (2) a Mesozoic carbonatite metasomatism episode of sub-lithospheric origin, as suggested by ?Nd-?Hf values inside the width of the terrestrial array. The Jacupiranga parental magmas might thus derive by partial melting of distinct generations of metasomatic vein assemblages that were hybridized with garnet peridotite wall-rocks.
Abstract: Alkaline complexes consist of variable mantle-derived silicate rocks, ranging from primitive alkali basalts, melilitites, nephelinites and basanites towards tephrites and more evolved phonolites, respectively their plutonic equivalents. This lithological variance is also expressed by a wide range of redox conditions that vary by several log units around the synthetic fayalite-magnetite-quartz (FMQ) buffer. However, only some of these complexes are characterized by the occurrence of carbonatites which must be related to specific formation conditions. Based on textural, mineralogical and geochemical observations, we calculated the redox conditions of carbonatites and associated silicate rocks for seven alkaline complexes (Kaiserstuhl, Sokli, Kovdor, Palabora, Magnet Cove, Oka, Jacupiranga) which are considered to represent typical carbonatite-alkaline silicate rock associations. In combination with a comprehensive literature review, we demonstrate that carbonatite-bearing alkaline complexes formed under highly oxidized conditions and hence, belong to the most oxidized alkaline rocks at all. This is consistent with the prerequisite of a carbonated mantle as the source region for carbonatite complexes, which requires redox conditions distinctively above that for mean lithospheric or asthenospheric mantle. Carbonatitemetasomatized peridotites also show high redox conditions and might not only reflect an interaction between peridotite and carbonatitic melts/fluids, but at least partly represent the carbonated mantle source for crustally emplaced carbonatite complexes. We therefore suggest that the oxidation state of carbonatites and associated silicate rocks provides direct information about an extraordinary oxidized parental mantle source.
Earth and Planetary Science Letters, Vol. 533, 11p. Pdf
Mantle
carbonatite
Abstract: A detailed investigation on seven carbonatites and associated alkaline rock complexes (Kaiserstuhl, Sokli, Kovdor, Palabora, Oka, Magnet Cove, Jacupiranga), together with a world-wide comparison between carbonatites, alkaline silicate rocks and mantle xenoliths, implies peculiar redox conditions for carbonatite-bearing alkaline complexes: Carbonatites and associated alkaline rocks in continental settings crystallize from relatively oxidized magmas, on average 1.4 log units () and 1.3 log units () above the synthetic fayalite-magnetite-quartz (FMQ) buffer. In contrast, alkaline rocks in continental settings that lack associated carbonatites reveal rather reduced conditions (mean ; ). The calculated redox conditions for carbonatites and associated silicate rocks demonstrate that these crystallize from relatively oxidized mantle-derived melts compared to the general range found for alkaline rocks in continental settings.
Abstract: Oxygen fugacity (ƒO2) conditions were established for Três Ranchos IV (TR-IV, diamond-bearing) and Limeira I (LM-I, barren) kimberlite intrusions, in Alto Paranaíba Alkaline Province, to constrain a possible correlation between fO2 and diamond instability. Temperature and pressure estimates obtained from the xenocryst assemblage composition are compatible up to garnet lherzolite levels. It suggests that both intrusions could cross the diamond-stability field. The ƒO2 of the TR-IV constrained by perovskite oxygen barometry presents an average value of -2.4 for ?NNO, with standard deviation of 1.30 (n = 120), whereas those calculations for LM-I have an average value of -1.31 for ?NNO, with standard deviation of 1.38 (n = 81). Considering these uncertainties, there is an important superposition of fO2 values for both intrusions, in which there is higher tendency of more reduced conditions for TR-IV. For the LM-I, an oxybarometer based on the composition of monticellite yielded a similar ?NNO range: -4.2 and +2.5. Some crystals and samples present trends towards more reduced conditions, while others display more oxidized conditions for each intrusion. Due to the superposition of ranges and absence of a preferential trend, the influence of fO2 for the possible instability of diamonds in the study area remains uncertain.
Abstract: The Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12-2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.
Abstract: General information is presented on ten agpaitic occurrences located in southern Brazil and at the border between Brazil and Paraguay. All the Brazilian agpaitic rocks are Late Cretaceous in age, whereas the Paraguayan ones are older than Early Triassic. The most significant occurrence is Poços de Caldas, the largest alkaline massif in South America. In general, these agpaitic rocks contain mineral assemblages that indicate presence of typical halogen-bearing Na-Ca-HFSE phases, eudialyte-, rinkite- and wöhlerite-group minerals being the most frequent ones. However, these associations are indeed more complex in terms of composition, with accessory phases in some cases consisting of various minerals, including U-Th oxides/silicates, Nb oxides, REE-Sr-Ba bearing carbonates-fluorocarbonates-phosphates-silicates and Zr-Na rich silicates. They usually form late magmatic stage to hydrothermal/deuteric assemblages linked with coarse and fine-grained, mainly silica-undersaturated evolved rocks. Data also indicate significant differences in type, amount and composition of agpaitic minerals in all investigated occurrences.
Chmyz, L., Azzone, R.G., Ruberti, E., Marks, M.A.W.
Olivines as probes into assimilation of silicate rocks by carbonate magmas: unraveling the genesis of reaction rocks from the Jacupiranga alkaline-carbonatite complex, southern Brazil.
Abstract: Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790-1165?°C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction - promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing - resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.
