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SDLRC - Scientific Articles all years by Author - Ch-Ck
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Kahoui, M., Kemainsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton.
Journal of African Earth Sciences, In press available, 46p.
Kahoui, M., Kaminsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into lithospheric mantle beneath the north eastern edge of the west African Craton.
Journal of African Earth Sciences, Vol. 63, Feb. pp. 1-11.
Journal of Petrology , Vol. 60, 6, pp. 1119--1134.
India
carbonatite
Abstract: There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr-Nd-Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan-Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (?2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.
Abstract: Similar to Earth, many large planetesimals in the Solar System experienced planetary-scale processes such as accretion, melting and differentiation. As their cores cooled and solidified, substantial chemical fractionation occurred due to solid metal-liquid metal fractionation. Iron meteorites—core remnants of these ancient planetesimals—record a history of this process. Recent iron isotope analyses of iron meteorites found their 57Fe/54Fe ratios to be heavier than chondritic by approximately 0.1 to 0.2 per mil for most meteorites, indicating that a common parent body process was responsible. However, the mechanism for this fractionation remains poorly understood. Here we experimentally show that the iron isotopic composition of iron meteorites can be explained solely by core crystallization. In our experiments of core crystallization at 1,300?°C, we find that solid metal becomes enriched in the heavier iron isotope by 0.13 per mil relative to liquid metal. Fractional crystallization modelling of the IIIAB iron meteorite parent body shows that observed iridium, gold and iron compositions can be simultaneously reproduced during core crystallization. The model implies the formation of complementary sulfur-rich components of the iron meteorite parental cores that remain unsampled by meteorite records and may be the missing reservoir of isotopically light iron. The lack of sulfide meteorites and previous trace element modelling predicting substantial unsampled volumes of iron meteorite parent cores support our findings.
Abstract: Numerous mafic dykes, sills and laccoliths crop out in the southern part of the Tafilalt basin (Eastern Anti-Atlas, Morocco). These rocks intrude the mildly folded Ordovician to Early Carboniferous formations, consisting mainly of lamprophyric dolerites and camptonites with minor gabbros and syenodiorites. Previous geochemical studies have shown that the Tafilalt magmatism of sodic-alkaline affinity has been produced by low degrees of partial melting from an enriched deep mantle source within the garnet stability field. However, the age and the geodynamic context of these rocks were presently unknown since no isotopic dating had so far been made of the Tafilalt dolerites. To resolve this issue, we present here the first 40Ar/39Ar biotite and U-Pb zircon dating from the Tafilalt alkaline magmatism. Three samples (biotite separates) yielded well-defined 40Ar/39Ar plateau ages of 264.2?±?2.7 Ma, 259.0?±?6.3 Ma and 262.6?±?4.5 Ma whereas 206Pb/238U dating of zircon from one of these samples yielded an age of 255?±?3 Ma. These ages coincide within the dating error, and indicate that this magmatism occurred in the late Permian. Considering geochronological and geochemical data, we propose that the Tafilalt magmatism reflects an early-rift magmatic activity that preceded the Triassic rifting heralded by the Central Atlantic Magmatic Province. This magmatic activity is recorded in both sides of the future Atlantic Ocean by small-volume alkaline magmatism that started in the late Permian and extends into the Triassic. The alkaline magmas are probably generated in response to an increase in the mantle potential temperature underneath the Pangea supercontinent.
Earth and Planetary Science Letters, Vol. 473, pp. 44-51.
Africa, Zimbabwe
deposit - Marange
Abstract: Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently (Luth and Stachel, 2014) that diamond formation in peridotites typically does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH4 and CO2 components in a water-rich fluid react to form diamond (CO2 + CH4 = 2C + 2H2O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH4CH4- and CO2-bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH4 or CO2) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (<1‰) enrichment of diamond in 13C as crystallization proceeds. This is in contrast to diamond precipitation by rock-buffered redox processes from a fluid containing only a single carbon species, which can result in either progressive 13C enrichment (CO2 or carbonate fluids) or View the MathML sourceC13 depletion (CH4 fluids) in the diamond. (2) Fluid speciation is the key factor controlling diamond ?13C?13C values; as XCO2 (XCO2 = CO2/[CO2 + CH4]) in the initial fluid increases from 0.1 to 0.9 (corresponding to an increase in fO2fO2 of 0.8 log units), the carbon isotope composition of the first-precipitated diamond decreases by 3.7‰. The tight mode in ?13C of ?5 ±1‰?5 ±1‰ for diamonds worldwide places strict constraints on the dominant range of XCO2 in water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with ?13C values in the range ?4 to ?6‰ from mantle-derived fluids with an average ?13C value of ?5‰ (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (XCO2 = 0.1–0.5). Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe (Smit et al., 2016), which contain CH4 fluid inclusions, allows us to perfectly match the observed co-variations in ?13C?13C, ?15N?15N and N content and at the same time explain the previously counter-intuitive observation of progressive View the MathML sourceC13 enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive View the MathML sourceC13 depletion trends within diamonds likely reflects ubiquitous precipitation from CH4- and CO2-bearing water-rich fluids, rather than diamond formation exclusively by carbonate-bearing and CH4-free oxidized fluids or melts.
Heaman, L.M., Creaser, R.A., Cookenboo, H.O., Chacko, T.
Multi stage modification of the northern Slave mantle lithosphere: evidence from zircon and diamond bearing eclogite xenoliths entrained in Jericho kimberlite.
Journal of Petrology, Vol. 47, 4, April pp. 821-858.
Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A delta 13 C-N study of eclogite hosted diamonds from the Jericho kimberlite.
Geochimica et Cosmochimica Acta, Vol. 75, pp. 6027-6047.
Smart, K.A., Chacko, T., Simonetti, A., Sharp, Z.D., Heaman, L.M.
A record of Paleoproterozoic subduction preserved in the northern Slave cratonic mantle: Sr-Pb-O isotope and trace element investigations of eclogite xenoliths from the Jericho and Muskox kimberlites.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
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.
Canadian Journal of Earth Sciences, Vol. 55, pp. 295-307.
Canada, Alberta
geophysics - electromagnetics, magnetotellurics
Abstract: The study of ancient plate boundaries can provide insights into the past and present-day tectonic processes. Here, we describe a magnetotellurics (MT) study of the Precambrian basement of the Hay River Fault (HRF) in northwest Alberta, which is the southwest segment of the Great Slave Lake shear zone. New broadband MT data were collected to give a clearer image of the crustal structure. The Western Canada Sedimentary Basin was imaged as a low-resistivity layer above the resistive crystalline basement. Four basement conductors were defined, and correlate with the terrane boundaries delineated with aeromagnetic data. These are (1) a major conductor in the Kiskatinaw domain, (2) a conductor on the boundary of the Ksituan and Chinchaga domains, (3) a conductor on the boundary of the Chinchaga and Buffalo Head domains, and (4) a conductor near the HRF. Both (1) and (2) correspond to areas of high seismic reflectivity. The low resistivity can be explained by interconnected grain boundary graphite or sulfide phases deposited by metamorphic fluid migration. The HRF was not definitively located in previous studies. The new data show that the HRF could be thin (1 km) or wide (10 km) and located at the boundary of the contrasting aeromagnetic anomalies or further to the north. Various tectonic processes are proposed to interpret the possible locations of the HRF. No electrical anisotropy structure is required to interpret the MT data in this study.
The thermochemical conditions of the Diavik lower crust: a kimberlite-hosted xenolith study.
2018 Yellowknife Geoscience Forum , p. 25-26. abstract
Canada, Northwest Territories
deposit - Diavik
Abstract: Thermochemical variables such as lower crustal heat production and Moho temperatures in cratonic regions offer critical insight in constraining the thermal and geodynamic evolution of the lithosphere. In this study, 15 lower crustal granulite xenoliths erupted via the A154N kimberlite at the Diavik mine in the NWT, Canada were studied to quantify the thermal properties of the local Moho and the effects of different heat production models on geotherm models. We quantitatively constrain the thermal properties of the local Moho and the effects of different heat production models on ancient Moho temperatures, the effects of crustal thickness on Moho temperatures, and potential lower crustal compositions. We evaluate the effect of these parameters on total lithospheric thickness estimates. In order to test the accuracy of deep crust thermal calculations, we estimated the ambient temperature of the lower crust at the time of kimberlite eruption through garnet-biotite Fe-Mg exchange geothermometry (Ferry & Spear, 1978). Rim compositions from touching garnet-biotite pairs were used in the calculations and yielded temperatures of 524 ± 77°C (n=20). These represent a maximum estimate of the ambient lower crustal temperature as the closure temperature of garnet-biotite Fe-Mg exchange between garnet and biotite may be higher than the ambient temperature. The primary objective of this study is to quantify lower crustal heat production and its effects on the thermal architecture of cratons. The concentrations of the main heat-producing elements (HPEs) U, Th, and K were quantified via LA-ICP-MS and EPMA in multiple mineral phases per xenolith. By combining these measurements with mineral modes, we derived reconstructed bulk-rock HPE concentrations that were utilized to calculate a range of lower crustal heat production values. This method is preferred over whole-rock analyses as 1) kimberlite is generally enriched in HPEs (Tappe et al. 2013) and can bias trace-element data for their xenoliths and 2) data on individual minerals allows for theoretical lower crustal compositions to be calculated on an idealized basis. A lower crust comprising exclusively mafic granulite (garnet, plagioclase, clinopyroxene ± orthopyroxene) provides a lower bound to heat production (0.07 ± 0.04 W/m3) whereas a lower crust made exclusively of high-grade metasedimentary rocks yields an upper bound (0.42 ± 0.08 W/m3). Both endmembers are present as xenoliths in the A154N kimberlite but mafic granulites predominate following the worldwide trend (Rudnick, 1992). We model the lower crust comprising 20% metasedimentary granulites and 80 % depleted mafic granulites, in accordance with the present xenolith collection. Using this preferred crustal model, we calculate an average heat production of 0.12 ± 0.05 W/m3) for the lower crust beneath Lac de Gras. Utilizing heat flow measurements (Russell et thickness estimates (Mareschal et al. 2004) in conjunction with these HPE determinations, the Moho temperature underlying A-154N can be calculated to be 502 ± 10°C. Using these values along with available mantle xenolith thermobaromtetry (Hasterok & Chapman, 2011) the geotherm is extrapolated to present a mantle potential temperature of 1365°C, at 200 km (FITPLOT, Mather et al, 2011).
Geochemical Perspective Letters, Vol. 10, pp. 43-48. doi:10.7185/ geochemlet.1904
Mantle
xenoliths
Abstract: Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.
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.
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.
Geochemical Perspectives Letters, Vol. 14, pp. 1-6.
Global
Tectonics
Abstract: The tectonic regime of the early Earth has proven enigmatic due to a scarcity of preserved continental crust, yet how early continents were generated is key to deciphering Earth’s evolution. Here we show that a compilation of data from 4.3 to 3.4 Ga igneous and detrital zircons records a secular shift to higher 176Hf/177Hf after ~3.8-3.6 Ga. This globally evident shift indicates that continental crust formation before ~3.8-3.6 Ga largely occurred by internal reworking of long-lived mafic protocrust, whereas later continental crust formation involved extensive input of relatively juvenile magmas, which were produced from rapid remelting of oceanic lithosphere. We propose that this secular shift in the global hafnium isotope record reflects a gradual yet widespread transition from stagnant-lid to mobile-lid tectonics on the early Earth.
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.
Lithos, doi.org/10.1016/ j.lithos.2020.105889 13p. Pdf
Canada, Northwest Territories
deposit - Diavik A-154
Abstract: Ambient Moho temperatures and lower crustal heat production are surprisingly poorly constrained in cratons. Here we address these problems using 15 lower crustal xenoliths from the Diavik A-154 kimberlite, Slave craton, Canada. Iron?magnesium exchange geothermometry on small biotite and amphibole inclusions in garnet indicates that the Slave craton lower crust was at a temperature of ?500 °C at the time of kimberlite eruption (~55 Ma). The ambient lower crustal temperature was likely lower than 500 °C because the thermometers record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs. New measurements of K, U and Th concentrations in the constituent minerals, together with xenolith modes, allow reconstruction of the heat-producing element (HPE) K, U, and Th budget of the Slave craton lower crust. Metasedimentary granulites have an average heat production of 0.29 ± 0.01 ?W/m3 (n = 3) whereas mafic granulites have an average heat production of 0.13 ± 0.03 ?W/m3 (n = 12). Our new data clearly show that plagioclase abundance in both lithologies has a major influence on overall lower crustal heat production, being an important reservoir of all three HPE. Combining the heat production of mafic and metasedimentary granulites in their observed 80:20 proportions results in an average heat production value for the Slave craton lower crust of 0.16 ± 0.03 ?W/m3. Using these heat production estimates, modeled Moho temperatures beneath Diavik of ~450-470 °C are broadly consistent with maximum lower crustal temperatures indicated by geothermometry. The low HPE contents predicted for cratonic lower crust must result in lower temperatures in the deep crust and mantle lithosphere, and in turn higher estimates for the thickness of mantle lithosphere. This effect becomes larger as the thickness of the low-HPE lower crustal layer increases. In the specific case of the central Slave craton, we find that model estimates of the diamond potential of the mantle lithosphere, as judged by the proportion of lithospheric mantle in the diamond stability field, are not strongly affected by small variations in lower crustal heat production and Moho temperature.
Abstract: The formation and preservation of cratons-the oldest parts of the continents, comprising over 60 per cent of the continental landmass-remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses-the cratons.
Abstract: The term craton has a complex and confused etymology. Despite originally specifying only strength and stability - of the crust - the term craton, within the context of diamond exploration, has widely come to refer to a region characterised by crustal basement older than 2.5 Ga, despite the fact that some such “cratons” no longer possess their deep lithospheric root. This definition often precluded regions with deep lithospheric roots but basement younger than 2-2.5 Ga. Viscous, buoyant lithospheric mantle roots are key to the survival and stability of continental crust. Here we use a revised craton definition (Pearson et al., 2021, in press), that includes the requirement of a deep (~150 km or greater) and intact lithospheric root, to re-examine the link between cratons and diamonds. The revised definition has a nominal requirement for tectonic stability since ~ 1 Ga and recognises that some regions are “modified cratons” - having lost their deep roots, i.e., they may have behaved like cratons for an extended period but subsequently lost much of their stabilising mantle roots during major tectono-thermal events. In other words, despite being long-lived features, cratons are not all permanent. The 150 km lithospheric thickness cut-off provides an optimal match to crustal terranes with 1 Ga timescale stability. In terms of regional diamond exploration, for a given area, the crucial criterion is when a deep mantle root was extant, i.e., over what period was the lithospheric geotherm suitable for diamond formation, stability and sampling? A thick lithospheric root is key to the formation of deep-seated magmas such as olivine lamproites and to the evolution of sub-lithospheric sourced proto-kimberlites, all capable of carrying and preserving diamonds to Earth's surface. This criterion appears essential even for sub-lithospheric diamonds, that still require a diamond transport mechanism capable of preserving the high-pressure carbon polymorph via facilitating rapid transport of volatile-charged magma to the surface, without dilution from additional melting that takes place beneath thinner (<120 km) lithospheric "lids". Seismology can help to define the lateral extent of today's cratons, but a detailed understanding of the regional geological history, kimberlite eruption ages and geothermal conditions is required to evaluate periods of past diamond potential, no-longer evident today. This revised craton concept broadens the target terranes for diamond exploration away from only the Archean cores of cratons and an associated mentality that "the exception proves the rule". The revised definition is compatible with numerous occurrences of diamond in Proterozoic terranes or Archean terranes underpinned by Proterozoic mantle.
Youbi, N., Kouyate, D., Soderlund, U., Ernst, R.E., Soulaimani, A., Hafid, A., Ikenne, M., El Bahat, A., Betrand, H., Chaham, K.R., Ben Abbou, M., Mortaji, A., El Ghorfi, M., Zouhair, M., El Janati, M.
The 1750 Ma magmatic event of the West African Craton ( Anti-Atlas) Morocco.
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.
Diamond & Related Materials, Vol. 113, 108284, 8p. Pdf
Global
carbon
Abstract: A new orthorhombic carbon crystal denoted oI20?carbon possessing the Immm space group was designed. Its structure is formed by stacking of a cage structure, which consists of 32 carbon atoms. Its stability and structural, mechanical and electronic properties were investigated by first-principles simulations. Density functional theory calculations show that this new carbon allotrope is thermodynamically stable (even more stable than synthesized T?carbon and supercubane). Ab initio molecular dynamics (AIMD) simulations show that it can maintain the structure above a temperature of 1000 K, indicating its excellent thermal stability. oI20?carbon can also maintain dynamic stability under a high pressure of 100 GPa. It is an anisotropic superhard material with a Vickers hardness of 46.62 GPa. Notably, the cage structure gives it a low density, which has a really small value among superhard carbon allotropes. In addition, it is worth noting that oI20?carbon has an indirect ultrawide band structure with a bandgap of 4.55 eV (HSE06), which is higher than that of most previously reported superhard carbon allotropes. All these outstanding properties show that it is a potential material for high-temperature, high-frequency electronic devices and the aerospace industry.
Russian Geology and Geophysics, Vol. 59, 11, pp. 1450-1460.
Russia, Aldan
lamproite
Abstract: We consider a hypothesis for the origin of PGE-bearing ultramafic rocks of the Inagli massif (Central Aldan) through fractional crystallization from ultrabasic high-potassium magma. We studied dunites and wehrlites of the Inagli massif and olivine lamproites of the Ryabinovy massif, which is also included into the Central Aldan high-potassium magmatic area. The research is focused on the chemistry of Cr-spinels and the phase composition of Cr-spinel-hosted crystallized melt inclusions and their daughter phases. Mainly two methods were used: SEM-EDS (Tescan Mira-3), to establish different phases and their relationships, and EPMA, to obtain precise chemical data on small (2-100 ?m) phases. The obtained results show similarity in chromite composition and its evolutionary trends for the Inagli massif ultramafites and Ryabinovy massif lamproites. The same has been established for phlogopite and diopside from crystallized melt inclusions from the rocks of both objects. Based on the results of the study, the conclusion is drawn that the ultramafic core of the Inagli massif resulted from fractional crystallization of high-potassium melt with corresponding in composition to low-titanium lamproite. This conclusion is consistent with the previous hypotheses suggesting an ultrabasic high-potassium composition of primary melt for the Inagli ultramafites.
Abstract: The development of sophisticated sample environments to control temperature, pressure, and magnetic field has grown in parallel with neutron source and instrumentation development. High-pressure apparatus, with high- and low-temperature capability, novel designs for diamond cells, and large volume presses are matched with next-generation neutron sources and moderator designs to provide unprecedented neutron beam brightness. Recent developments in sample environments are expanding the pressure-temperature space accessible to neutron scattering experiments. Researchers are using new capabilities and an increased understanding of the fundamentals of structural and magnetic transitions to explore new territories, including hydrogenous minerals (e.g., ices and hydrates) and magnetic structural phase diagrams.
Abstract: Manitoba, with its 400 000 km2 of exposed Precambrian basement, remains the most conspicuous "white spot" on the map of Canadian kimberlites. The apparent absence of these rocks from the regional geological record seems all the more paradoxical, given the existence of large Phanerozoic kimberlite fields just across the provincial border in eastern Saskatchewan, and abundant evidence of mantle-derived carbonate-rich magmatism (carbonatites and ultramafic lamprophyres) across central Manitoba. Interestingly, rocks of this type were first identified in the Province in 1983 at Wekusko Lake, where they crosscut supracrustal assemblages of the Paleoproterozoic Flin Flon belt, and were tentatively logged as kimberlites. This interpretation, based to a large extent on their high Cr + Ni contents and the presence of indicator minerals in their modal composition, was challenged in subsequent research. Similar rocks have been recognized recently in similar settings south of Wekusko Lake. These discoveries expanded not only the area of known post-Paleoproterozoic mantle magmatism, but also the petrographic and geochemical spectrum of its products. The primary carbonate phase in these rocks is dolomite that shows a variable degree of subsolidus isotopic re-equilibration under CO2-rich conditions. Fluid-rock interaction was also responsible for the replacement of olivine, phlogopite and groundmass perovskite by secondary minerals and deposition of hydrothermal carbonates in fractures, although the relative timing of these processes with respect to dike emplacement is poorly understood at present. Notably, indicator minerals indistinguishable from those in bona fide kimberlites are common in all of the examined dikes. These recent discoveries may hold key to understanding the genetic relations between kimberlites and primitive carbonatites, and have practical implications for heavy-mineral-based diamond exploration.
Geochemistry and mineralogy of HFSE in intracratonic carbonatites: implications for their economic potential (on the example of Kola alkaline province).
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Chakhmouradian, A.R., Demeny, A., Reguir, E.P., Hegner, E., Halden, N.M., Yang, P.
'Kimberlite' from Wekusko Lake, Manitoba: re-assessment and implications for further exploration. Beforsite ( primary dolomite carbonatite)... 'notion' could be
Abstract: The composition of calcite and dolomite from several carbonatite complexes (including a large set of petrographically diverse samples from the Aley complex in Canada) was studied by electron-microprobe analysis and laser-ablation inductively-coupled-plasma mass-spectrometry to identify the extent of substitution of rare-earth and other trace elements in these minerals and the effects of different igneous and postmagmatic processes on their composition. Analysis of the newly acquired and published data shows that the contents of rare-earth elements (REE) and certain REE ratios in magmatic calcite and dolomite are controlled by crystal fractionation of fluorapatite, monazite and, possibly, other minerals. Enrichment in REE observed in some samples (up to ~2000 ppm in calcite) cannot be accounted for by coupled substitutions involving Na, P or As. At Aley, the REE abundances and chondrite-normalized (La/Yb)cn ratios in carbonates decrease with progressive fractionation. Sequestration of heavy REE from carbonatitic magma by calcic garnet may be responsible for a steeply sloping "exponential" pattern and lowered Ce/Ce* ratios of calcite from Magnet Cove (USA) and other localities. Alternatively, the low levels of Ce and Mn in these samples could result from preferential removal of these elements by Ce4+- and Mn3+-bearing minerals (such as cerianite and spinels) at increasing f(O2) in the magma. The distribution of large-ion lithophile elements (LILE = Sr, Ba and Pb) in rock-forming carbonates also shows trends indicative of crystal fractionation effects (e.g., concomitant depletion in Ba + Pb at Aley, or Sr + Ba at Kerimasi), although the phases responsible for these variations cannot be identified unambiguously at present. Overall, element ratios sensitive to the redox state of the magma and its complexing characteristics (Eu/Eu*, Ce/Ce* and Y/Ho) are least variable and in both primary calcite and dolomite, approach the average chondritic values. In consanguineous rocks, calcite invariably has higher REE and LILE levels than dolomite. Hydrothermal reworking of carbonatites does not produce a unique geochemical fingerprint, leading instead to a variety of evolutionary trends that range from light-REE and LILE enrichment (Turiy Mys, Russia) to heavy-REE enrichment and LILE depletion (Bear Lodge, USA). These differences clearly attest to variations in the chemistry of carbonatitic fluids and, consequently, their ability to mobilize specific trace elements from earlier-crystallized minerals. An important telltale indicator of hydrothermal reworking is deviation from the primary, chondrite-like REE ratios (in particular, Y/Ho and Eu/Eu*), accompanied by a variety of other compositional changes depending on the redox state of the fluid (e.g., depletion of carbonates in Mn owing to its oxidation and sequestration by secondary oxides). The effect of supergene processes was studied on a single sample from Bear Lodge, which shows extreme depletion in Mn and Ce (both due to oxidation), coupled with enrichment in Pb and U, possibly reflecting an increased availability of Pb2+ and (UO2)2+ species in the system. On the basis of these findings, several avenues for future research can be outlined: (1) structural mechanisms of REE uptake by carbonates; (2) partitioning of REE and LILE between cogenetic calcite and dolomite; (3) the effects of fluorapatite, phlogopite and pyrochlore fractionation on the LILE budget of magmatic carbonates; (4) the cause(s) of coupled Mn-Ce depletion in some primary calcite; and (5) relations between fluid chemistry and compositional changes in hydrothermal carbonates.
Abstract: Carbonatites are nominally igneous rocks, whose evolution commonly involves also a variety of postmagmatic processes, including exsolution, subsolidus re-equilibration of igneous mineral assemblages with fluids of different provenance, hydrothermal crystallization, recrystallization and tectonic mobilization. Petrogenetic interpretation of carbonatites and assessment of their mineral potential are impossible without understanding the textural and compositional effects of both magmatic and postmagmatic processes on the principal constituents of these rocks. In the present work, we describe the major (micro)textural characteristics of carbonatitic calcite and dolomite in the context of magma evolution, fluid-rock interaction, or deformation, and provide information on the compositional variation of these minerals and its relation to specific evolutionary processes.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 5-21.
Russia, Siberia
Deposit - Murun
Abstract: Vladykinite, ideally Na3Sr4(Fe2+Fe3+)Si8O24, is a new complex sheet silicate occurring as abundant prismatic crystals in a dike of coarse-grained peralkaline feldspathoid syenite in the north-central part of the Murun complex in eastern Siberia, Russia (Lat. 58° 22? 48? N; Long. 119° 03? 44? E). The new mineral is an early magmatic phase associated with aegirine, potassium feldspar, eudialyte, lamprophyllite, and nepheline; strontianite (as pseudomorphs after vladykinite) and K-rich vishnevite are found in the same assemblage, but represent products of late hydrothermal reworking. Vladykinite is brittle, has a Mohs hardness of 5, and distinct cleavage on {100}. In thin section, it is colorless, biaxial negative [a = 1.624(2), b = 1.652(2), g = 1.657(2), 2Vmeas = 44(1)°, 2Vcalc = 45(1)°] and shows an optic orientation consistent with its structural characteristics (X^a = 5.1° in b obtuse, Z^c = 4.7° in b acute, Y = b). The Raman spectrum of vladykinite consists of the following vibration modes (listed in order of decreasing intensity): 401, 203, 465, 991, 968, 915, 348, 167, 129, 264, 1039, and 681 cm–1; O-H signals were not detected. The Mössbauer spectrum indicates that both Fe2+ and Fe3+ are present in the mineral (Fe3+/FeS = 0.47), and that both cations occur in a tetrahedral coordination. The mean chemical composition of vladykinite (acquired by wavelength-dispersive X?ray spectrometry and laser-ablation inductively-coupled-plasma mass-spectrometry), with FeS recast into Fe2+ and Fe3+ in accord with the Mössbauer data, gives the following empirical formula calculated to 24 O atoms: (Na2.45Ca0.56)S3.01(Sr3.81 K0.04Ba0.02La0.02Ce0.01)S3.90(Fe2+0.75Fe3+0.66Mn0.26Zn0.16Al0.12Mg0.05Ti0.01)S2.01(Si7.81Al0.19)S8.00O24. The mineral is monoclinic, space group P21/c, a = 5.21381(13), b = 7.9143(2), c = 26.0888(7) Å, b = 90.3556(7)°, V = 1076.50(5) Å3, Z = 2. The ten strongest lines in the powder X?ray diffraction pattern are [dobs in Å (I) (hkl)]: 2.957 (100) (123, 123); 2.826 (100) (117, 117); 3.612 (58) (114, 114); 3.146 (37) (120); 2.470 (32) (210, 01.10); 4.290 (30) (111, 111); 3.339 (30) (106, 115, 106); 2.604 (28) (200); 2.437 (25) (034); 1.785 (25) (21.10, 234). The structure of vladykinite, refined by single-crystal techniques on the basis of 3032 reflections with Fo > 4sFo to R1 = 1.6%, consists of tetrahedral sheets parallel to (100) and consisting of (Si8O24)16– units incorporating four-membered silicate rings and joined into five- and eight-membered rings by sharing vertices with larger tetrahedra hosting Fe2+, Fe3+, Mn, Zn, Al, Mg, and Ti. Larger cations (predominantly Na, Sr, and Ca) are accommodated in octahedral and square-antiprismatic interlayer sites sandwiched between the tetrahedral sheets. Structural relations between vladykinite and other sheet silicates incorporating four-, five-, and eight-membered rings are discussed. The name vladykinite is in honor of Nikolay V. Vladykin (Vinogradov Institute of Geochemistry, Russia), in recognition of his contribution to the study of alkaline rocks. Holotype and co-type specimens of the mineral were deposited in the Robert B. Ferguson Museum of Mineralogy in Winnipeg, Canada.
Mineralogical Magazine, Vol. 79, 5, pp. 1231-1244.
Russia
Carbonatite
Abstract: Anzaite-(Ce), ideally Formula Fe2+Ti6O18(OH)2, is a new, structurally complex mineral occurring as scarce minute crystals in hydrothermally altered silicocarbonatites in the Afrikanda alkali-ultramafic complex of the Kola Peninsula, Russia. The mineral is a late hydrothermal phase associated with titanite, hibschite, clinochlore and calcite replacing the primary magmatic paragenesis. The rare-earth elements (REE) (dominated by Ce), Ti and Fe incorporated in anzaite-(Ce) were derived from primary Ti oxides abundant in the host rock. Anzaite-(Ce) is brittle and lacks cleavage; the density calculated on the basis of structural data is 5.054(6) g cm?3. The mineral is opaque and grey with a bluish hue in reflected light; its reflectance values range from 15-16% at 440 nm to 13-14% at 700 nm. Its infrared spectrum shows a prominent absorption band at 3475 cm?1 indicative of OH? groups. The average chemical composition of anzaite-(Ce) gives the following empirical formula calculated on the basis of 18 oxygen atoms and two OH? groups: (Ce2.18Nd0.85La0.41Pr0.26Sm0.08Ca0.36Th0.01)?4.15Fe0.97(Ti5.68Nb0.22Si0.04)?5.94O18(OH)2. The mineral is monoclinic, space group C2/m, a = 5.290(2), b = 14.575(6), c = 5.234(2) Å, ? = 97.233(7)°, V = 400.4(5) Å3, Z = 1. The ten strongest lines in the X-ray micro-diffraction pattern are [dobs in Å (I) hkl]: 2.596 (100) 002; 1.935 (18) 170; 1.506 (14) 133; 1.286 (13) 1.11.0; 2.046 (12) 2?41; 1.730 (12) 003; 1.272 (12) 0.10.2; 3.814 (11) 1?11; 2.206 (9) 061; 1.518 (9) 172. The structure of anzaite-(Ce), refined by single-crystal techniques to R1 = 2.1%, consists of alternating layers of type 1, populated by REE (+ minor Ca) in a square antiprismatic coordination and octahedrally coordinated Fe2+, and type 2, built of five-coordinate and octahedral Ti, stacked parallel to (001). This atomic arrangement is complicated by significant disorder affecting the Fe2+, five-coordinate Ti and two of the four anion sites. The order-disorder pattern is such that only one half of these positions in total occupy any given (010) plane, and the disordered (010) planes are separated by ordered domains comprising REE, octahedral Ti and two anion sites occupied by O2?. Structural and stoichiometric relations between anzaite-(Ce) and other REE-Ti (±Nb, Ta) oxides are discussed. The name anzaite-(Ce) is in honour of Anatoly N. Zaitsev of St Petersburg State University (Russia) and The Natural History Museum (UK), in recognition of his contribution to the study of carbonatites and REE minerals. The modifier reflects the prevalence of Ce over other REE in the composition of the new mineral.
Abstract: The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to ? 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12-15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6-0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3-2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.
Abstract: On the basis of extensive studies of synthetic perovskite-structured compounds it is possible to derive a hierarchy of hettotype structures which are derivatives of the arisotypic cubic perovskite structure (ABX3), exemplified by SrTiO3 (tausonite) or KMgF3 (parascandolaite) by: (1) tilting and distortion of the BX6 octahedra; (2) ordering of A- and B-site cations; (3) formation of A-, B- or X-site vacancies. This hierarchical scheme can be applied to some naturally-occurring oxides, fluorides, hydroxides, chlorides, arsenides, intermetallic compounds and silicates which adopt such derivative crystal structures. Application of this hierarchical scheme to naturally-occurring minerals results in the recognition of a perovskite supergroup which is divided into stoichiometric and non-stoichiometric perovskite groups, with both groups further divided into single ABX3 or double A2BB?X6 perovskites. Subgroups, and potential subgroups, of stoichiometric perovskites include: (1) silicate single perovskites of the bridgmanite subgroup; (2) oxide single perovskites of the perovskite subgroup (tausonite, perovskite, loparite, lueshite, isolueshite, lakargiite, megawite); (3) oxide single perovskites of the macedonite subgroup which exhibit second order Jahn-Teller distortions (macedonite, barioperovskite); (4) fluoride single perovskites of the neighborite subgroup (neighborite, parascandolaite); (5) chloride single perovskites of the chlorocalcite subgroup; (6) B-site cation ordered double fluoride perovskites of the cryolite subgroup (cryolite, elpasolite, simmonsite); (7) B-site cation ordered oxide double perovskites of the vapnikite subgroup [vapnikite, (?) latrappite]. Non-stoichiometric perovskites include: (1) A-site vacant double hydroxides, or hydroxide perovskites, belonging to the söhngeite, schoenfliesite and stottite subgroups; (2) Anion-deficient perovskites of the brownmillerite subgroup (srebrodolskite, shulamitite); (3) A-site vacant quadruple perovskites (skutterudite subgroup); (4) B-site vacant single perovskites of the oskarssonite subgroup [oskarssonite]; (5) B-site vacant inverse single perovskites of the cohenite and auricupride subgroups; (6) B-site vacant double perovskites of the diaboleite subgroup; (7) anion-deficient partly-inverse B-site quadruple perovskites of the hematophanite subgroup.
Abstract: Many carbonatites are associated both spatially and temporally with large igneous provinces (LIPs), and considered to originate from a mantle plume source lacking any contribution from recycled crustal materials. Here, we report an occurrence of carbonatite enriched in rare-earth elements (REE) and associated with the Tarim LIP in northwestern China. The Tarim LIP comprises intrusive and volcanic products of mantle plume activity spanning from ~ 300 to 280 Ma. The carbonatites at Wajilitage in the northwestern part of Tarim are dominated by calcite and dolomite varieties, and contain abundant REE minerals (principally, monazite and REE-fluorcarbonates). Th-Pb age determination of monazite yielded an emplacement age of 266 ± 5.3 Ma, i.e. appreciably younger than the eruption age of flood basalts at ~ 290 Ma. The carbonatites show low initial 87Sr/86Sr (0.7037-0.7041) and high ?Nd(t) (1.2-4) values, which depart from the isotopic characteristics of plume-derived basalts and high-Mg picrites from the same area. This indicates that the Wajilitage carbonatites derived from a mantle source isotopically distinct from the one responsible for the voluminous (ultra)mafic volcanism at Tarim. The carbonatites show ?26MgDSM3 values (? 0.99 to ? 0.65‰) that are significantly lower than those in typical mantle-derived rocks and rift carbonatites, but close to marine sediments and orogenic carbonatites. We propose that the carbonatites in the Tarim LIP formed by decompressional melting of recycled sediments mixed with the ambient mantle peridotite. The enriched components in the Tarim plume could be accounted for by the presence of recycled sedimentary components in the subcontinental mantle.
Abstract: Zoned crystals of carbocernaite occur in hydrothermally reworked burbankite-fluorapatite-bearing calcite carbonatite at Bear Lodge, Wyoming. The mineral is paragenetically associated with pyrite, strontianite, barite, ancylite-(Ce), and late-stage calcite, and is interpreted to have precipitated from sulfate-bearing fluids derived from an external source and enriched in Na, Ca, Sr, Ba, and rare-earth elements (REE) through dissolution of the primary calcite and burbankite. The crystals of carbocernaite show a complex juxtaposition of core-rim, sectoral, and oscillatory zoning patterns arising from significant variations in the content of all major cations, which can be expressed by the empirical formula (Ca0.43–0.91Sr0.40–0.69REE0.18–0.59Na0.18–0.53Ba0–0.08)?1.96–2.00(CO3)2. Interelement correlations indicate that the examined crystals can be viewed as a solid solution between two hypothetical end-members, CaSr(CO3)2 and NaREE(CO3)2, with the most Na-REE-rich areas in pyramidal (morphologically speaking) growth sectors representing a probable new mineral species. Although the Bear Lodge carbocernaite is consistently enriched in light REE relative to heavy REE and Y (chondrite-normalized La/Er = 500–4200), the pyramidal sectors exhibit a greater degree of fractionation between these two groups of elements relative to their associated prismatic sectors. A sample approaching the solid-solution midline [(Ca0.57Na0.42)?0.99(Sr0.50REE0.47Ba0.01)?0.98(CO3)2] was studied by single-crystal X-ray diffraction and shown to have a monoclinic symmetry [space group P11m, a = 6.434(4), b = 7.266(5), c = 5.220(3) Å, ? = 89.979(17)°, Z = 2] as opposed to the orthorhombic symmetry (space group Pb21m) proposed in earlier studies. The symmetry reduction is due to partial cation order in sevenfold-coordinated sites occupied predominantly by Ca and Na, and in tenfold-coordinated sites hosting Sr, REE, and Ba. The ordering also causes splitting of carbonate vibrational modes at 690–740 and 1080–1100 cm?1 in Raman spectra. Using Raman micro-spectroscopy, carbocernaite can be readily distinguished from burbankite- and ancylite-group carbonates characterized by similar energy-dispersive spectra.
Abstract: Many carbonatites are associated both spatially and temporally with large igneous provinces (LIPs), and considered to originate from a mantle plume source lacking any contribution from recycled crustal materials. Here, we report an occurrence of carbonatite enriched in rare-earth elements (REE) and associated with the Tarim LIP in northwestern China. The Tarim LIP comprises intrusive and volcanic products of mantle plume activity spanning from ~ 300 to 280 Ma. The carbonatites at Wajilitage in the northwestern part of Tarim are dominated by calcite and dolomite varieties, and contain abundant REE minerals (principally, monazite and REE-fluorcarbonates). Th–Pb age determination of monazite yielded an emplacement age of 266 ± 5.3 Ma, i.e. appreciably younger than the eruption age of flood basalts at ~ 290 Ma. The carbonatites show low initial 87Sr/86Sr (0.7037–0.7041) and high ?Nd(t) (1.2–4) values, which depart from the isotopic characteristics of plume-derived basalts and high-Mg picrites from the same area. This indicates that the Wajilitage carbonatites derived from a mantle source isotopically distinct from the one responsible for the voluminous (ultra)mafic volcanism at Tarim. The carbonatites show ?26MgDSM3 values (? 0.99 to ? 0.65‰) that are significantly lower than those in typical mantle-derived rocks and rift carbonatites, but close to marine sediments and orogenic carbonatites. We propose that the carbonatites in the Tarim LIP formed by decompressional melting of recycled sediments mixed with the ambient mantle peridotite. The enriched components in the Tarim plume could be accounted for by the presence of recycled sedimentary components in the subcontinental mantle.
Abstract: On the basis of extensive studies of synthetic perovskite-structured compounds it is possible to derive a hierarchy of hettotype structures which are derivatives of the arisotypic cubic perovskite structure (ABX3), exemplified by SrTiO3 (tausonite) or KMgF3 (parascandolaite) by: (1) tilting and distortion of the BX6 octahedra; (2) ordering of A- and B-site cations; (3) formation of A-, B- or X-site vacancies. This hierarchical scheme can be applied to some naturally-occurring oxides, fluorides, hydroxides, chlorides, arsenides, intermetallic compounds and silicates which adopt such derivative crystal structures. Application of this hierarchical scheme to naturally-occurring minerals results in the recognition of a perovskite supergroup which is divided into stoichiometric and non-stoichiometric perovskite groups, with both groups further divided into single ABX3 or double A2BB?X6 perovskites. Subgroups, and potential subgroups, of stoichiometric perovskites include: (1) silicate single perovskites of the bridgmanite subgroup; (2) oxide single perovskites of the perovskite subgroup (tausonite, perovskite, loparite, lueshite, isolueshite, lakargiite, megawite); (3) oxide single perovskites of the macedonite subgroup which exhibit second order Jahn-Teller distortions (macedonite, barioperovskite); (4) fluoride single perovskites of the neighborite subgroup (neighborite, parascandolaite); (5) chloride single perovskites of the chlorocalcite subgroup; (6) B-site cation ordered double fluoride perovskites of the cryolite subgroup (cryolite, elpasolite, simmonsite); (7) B-site cation ordered oxide double perovskites of the vapnikite subgroup [vapnikite, (?) latrappite]. Non-stoichiometric perovskites include: (1) A-site vacant double hydroxides, or hydroxide perovskites, belonging to the söhngeite, schoenfliesite and stottite subgroups; (2) Anion-deficient perovskites of the brownmillerite subgroup (srebrodolskite, shulamitite); (3) A-site vacant quadruple perovskites (skutterudite subgroup); (4) B-site vacant single perovskites of the oskarssonite subgroup [oskarssonite]; (5) B-site vacant inverse single perovskites of the cohenite and auricupride subgroups; (6) B-site vacant double perovskites of the diaboleite subgroup; (7) anion-deficient partly-inverse B-site quadruple perovskites of the hematophanite subgroup.
Abstract: Calcic garnets are an important – although somewhat neglected – member of the garnet group. Typically, these mineral are members of complex solid solutions involving largely substitutions in the Fe3+/Al and Si sites and at least eight different end-members. The absolute majority of garnets in this family are Ti-Mg-Fe2+(± Al ± Zr)-bearing andradite transitional to morimotoite and schorlomite. Importantly, these garnets occur as common accessory minerals in a wide range of igneous and rocks, including nepheline syenites, alkali feldspar syenites, melteigite-urtites, nephelinites, melilitolites, melilitites, calcite carbonatites, ultramafic lamprophyres, orangeites, contaminated kimberlites, skarns and rodingites. Calcic garnets have a great capacity for atomic substitutions involving high-field-strength elements and, even more importantly, rare earths (up to 4000 ppm, including Y), Th and U (both up to 100 ppm) at low levels of common Pb. Their (La/Yb)cn ratio varies over two orders of magnitude (from < 0.01 to ~1), making these minerals a sensitive indicator of crystal fractionation, degassing and other magma-evolution processes. Given these unique compositional characteristics and surprising lack of interest in these minerals in the previous literature, we explored the possibility of using calcic garnets as a U-Pb geochronometer. For this purpose, we selected samples of well-crystallized igneous garnet from four very different rock types of different age, including: carbonatite (Afrikanda) from the Devonian Kola Alkaline Province, carbonatite from the Neoproterozoic Belaya Zima complex (Central-Asian mobile belt), ijolite from the Chick Ordovician igneous complex (Central-Asian mobile belt), granitic pegmatite from the Eden Lake complex in the Paleoproterozoic Trans-Hudson orogen, and feldspathoid syenite from the Cinder Lake alkaline complex in the Archean Knee Lake greenstone belt. U-Pb TIMS ages of the studied garnets are mostly concordant and reveal perfect correspondence with reported U-Pb zircon or perovskite ages as well as Sm-Nd isochrone age for these complexes. Therefore we can advertise calcic garnets as a promising tool for U-Pb geochronological studies.
Reguir, E.P., Chakhmouradian, A.R., Zaitsev, A.N., Yang, P.
Trace element variations and zoning in phlogopite from carbonatites and phoscorites.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 8-9.
carbonatites
Abstract: Phlogopite from carbonatites and phoscorites worldwide shows three major types of core-to-rim trends of compositional variation: Ba+Al-, Fe and Fe+Al enrichment. These major-element trends are accompanied by largely consistent changes in traceelement abundances. Uptake of Rb, Sr, Ba, Sc, V, Mn and HFSE by phlogopite is susceptible to changes in the availability of these elements due to precipitation of other early silicate and oxide phases (especially, magnetite, apatite and niobates). In rare cases, more complex oscillatory and sector patterns are juxtaposed over the principal evolutionary trend, indicating kinetic and crystal-chemical controls over element uptake. Phlogopite is a common accessory to major constituent of carbonatites and genetically related rocks (including phoscorites). Major-element variations of phlogopite from these rocks have been addressed in much detail in the literature (for references, see Reguir et al. 2009), whereas its trace-element characteristics and zoning patterns have so far received little attention. In this work, we examined the compositional variation of phlogopite from 23 carbonatite and phoscorite localities worldwide. The major-element compositions were determined using wavelength-dispersive X-ray spectrometry (WDS) and trace-element abundances by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS). Previously, two major core-to-rim zoning trends have been identified in micas from calcite carbonatites (Reguir et al. 2009, 2010). Phlogopite from Oka (Canada) and Iron Hill (USA), for example, involves an increase in kinoshitalite component rim-ward, accompanied by enrichment in high-field-strength elements (HFSE = Zr, Nb, Ta), Sr and Sc. At most other carbonatite localities (e.g., Kovdor in Russia, or Prairie Lake in Canada), phlogopite crystals exhibit rim-ward enrichment in Fe. In the present work, we confirmed these two common types of zoning, and identified new patterns that have not been reported in the previous literature. In addition to the common Fe-enrichment trend, which occurs in both carbonatites (e.g., Guli in Russia and Sokli in Finland) and phoscorites (e.g., Aley in Canada), we identified a Fe-Al-enrichment subtype of this zoning pattern observed, for example, in samples from the Shiaxiondong calcite carbonatite (China). Overall, the Fe-enrichment pattern is accompanied by rim-ward depletion in Ba, Rb and HFSE, coupled with enrichment in Mn. Other trace elements exhibit no consistent variation among the studied samples. The Shiaxiondong material is characterized by the highest recorded Rb values, ranging from 1120 to 660 ppm. Phlogopite from the Kovdor calcite-forsterite-magnetite phoscorite contains the highest recorded levels of Nb and Ta, ranging from 320 ppm and 40 ppm, respectively, in the core to 85 ppm and 4 ppm in the rim. The maximum levels of Zr (up to 50 ppm) were observed in the core of Prairie Lake phlogopite, whereas its rim contains the highest measured Mn content (up to 4100 ppm). The levels of Sc are typically below 100 ppm in samples from calcite and dolomite carbonatites, but may reach 280 ppm in phoscorites. Interestingly, phlogopite from phoscorites shows rim-ward enrichment in Sc, whereas the opposite trend is observed in carbonatitic micas. Phlogopite from calcite carbonatites at Zibo (China) and Valentine Township (Canada), and from phoscorites at Aley (Canada) shows an unusual zoning pattern involving depletion in Fe, which is accompanied by a decrease in Al, Ba, Sr, Zr, Hf, Y, Sc and V abundances. The concentrations of other trace elements, including Nb and Ta show inconsistent variations. In the Aley phoscorite, phlogopite is enriched in Ba (up to 15000 ppm in the core and < 7500 ppm in the rim), but poor in Sr (80 and 35 ppm in the core and rim, respectively) relative to those from the Zibo and Valentine carbonatites. Zirconium levels reach 200 and 170 ppm in the core, and drop to < 40 and 60 ppm in the rim of the Valentine and Zibo samples, respectively. In the Aley sample, the content of Zr does not exceed 55 ppm. The Zibo sample is also enriched in V (up to 230 and 160 ppm in the core and rim, respectively) relative to the two other samples (< 100 ppm V). The Sc and Hf levels are consistently low (less than 30 and 4 ppm, respectively). In addition to simple core-rim patterns, phlogopite from carbonatites and phoscorites may exhibit oscillatory zoning, which involves periodic variations in Fe/Mg ratio. Iron-rich zones are relatively depleted in Mn, but enriched in Nb. One sample of phoscoritic phlogopite (Aley) exhibits striking sector zoning juxtaposed over the overall Feenrichment trend and Fe-Mg oscillations. In terms of major elements, basal sectors perpendicular to [001] are enriched in Fe and Al, but depleted in Mg and K relative to the flank sectors. This enrichment is accompanied by higher Ba, Sr and HFSE levels in the basal sector. Our data confirm that there is no universal pattern of zoning in carbonatitic or phoscoritic phlogopite, and variations in the content of most trace elements are strongly coupled to major-element patterns. Three major core-to-rim variation trends, as well as juxtaposed oscillatory and sector patterns, can be recognized. The observed compositional variations indicate that, in the majority of cases, the trace-element composition of phlogopite is controlled by partitioning of Rb, Sr, Ba, Sc, V, Mn and HFSE between this mineral, its parental magma, and co-precipitating early phases. Among the latter, magnetite, apatite and niobates appear to exert the greatest influence on element distributions. More complex oscillatory and sector patterns imply the presence of kinetic and crystal-chemical controls over element uptake in certain carbonatitic systems
Abstract: The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the world’s largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361-913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent ?Nd(1.3Ga) values (0.3 ± 0.6) close to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower 87Sr/86Sr ratios (0.7024-0.7030) than the recrystallized dolomite (0.7038-0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks.
Abstract: Rare-earth deposits associated with intrusive carbonatite complexes are the world’s most important source of these elements (REE). One of the largest deposits of this type is Maoniuping in the Mianning-Dechang metallogenic belt of eastern Tibet (Sichuan, China). In the currently mined central part of the deposit (Dagudao section), REE mineralization is hosted by a structurally and mineralogically complex Late Oligocene (26.4 ±?1.2 Ma, 40Ar/39Ar age of fluorphlogopite associated with bastnäsite) hydrothermal vein system developed in a coeval syenite intrusion. Low-grade stockworks of multiple veinlets and breccias in the lower part of the orebody grade upwards into progressively thicker veins (up to 12 m in width) that are typically zoned and comprise ferromagnesian micas (biotite to fluorphlogopite), sodium clinopyroxenes (aegirine to aegirine-augite), sodium amphiboles (magnesio-arfvedsonite to fluororichterite), K-feldspar, fluorite, barite, calcite, and bastnäsite. The latter four minerals are most common in the uppermost 80 m of the Dagudao section and represent the climax of hydrothermal activity. Systematic variations in the fluid inclusion data indicate a continuous hydrothermal evolution from about 230-400 °C (fluid inclusions in feldspar, clinopyroxene, and amphibole) to 140-240 °C (fluid inclusions in bastnäsite, fluorite, calcite). Hydrothermal REE transport was probably controlled by F?, (SO4)2?, Cl?, and (CO3)2? as complexing ligands. We propose that at Dagudao, silicate magmas produced orthomagmatic fluids that explored and expanded a fissure system generated by strike-slip faulting. Initially, the fluids had appreciable capacity to transport REE and, consequently, no major mineralization developed. The earliest minerals to precipitate were alkali- and Fe-rich silicates containing low levels of F, which caused progressive enrichment of the fluid in Ca, Mg, F, Cl, REE, (SO4)2?, and (CO3)2?, leading to the crystallization of aegirine-augite, fluororichterite, fluorphlogopite, fluorite, barite, calcite, and bastnäsite gradually. Barite, fluorite, calcite, and bastnäsite are the most common minerals in typical ores, and bastnäsite generally postdates these gangue minerals. Thus, it is very probable that fluid cooling and formation of large amount of fluorite, barite, and calcite triggered bastnäsite precipitation in the waning stage of hydrothermal activity.
Contributions to Mineralogy and Petrology, Vol. 173, 12, pp. 106-
Russia, Kola Peninsula
deposit - Afrikanda
Abstract: Perovskite is a common accessory mineral in a variety of mafic and ultramafic rocks, but perovskite deposits are rare and studies of perovskite ore deposits are correspondingly scarce. Perovskite is a key rock-forming mineral and reaches exceptionally high concentrations in olivinites, diverse clinopyroxenites and silicocarbonatites in the Afrikanda alkaline-ultramafic complex (Kola Peninsula, NW Russia). Across these lithologies, we classify perovskite into three types (T1-T3) based on crystal morphology, inclusion abundance, composition, and zonation. Perovskite in olivinites and some clinopyroxenites is represented by fine-grained, equigranular, monomineralic clusters and networks (T1). In contrast, perovskite in other clinopyroxenites and some silicocarbonatites has fine- to coarse-grained interlocked (T2) and massive (T3) textures. Electron backscatter diffraction reveals that some T1 and T2 perovskite grains in the olivinites and clinopyroxenites are composed of multiple subgrains and may represent stages of crystal rotation, coalescence and amalgamation. We propose that in the olivinites and clinopyroxenites, these processes result in the transformation of clusters and networks of fine-grained perovskite crystals (T1) to mosaics of more coarse-grained (T2) and massive perovskite (T3). This interpretation suggests that sub-solidus processes can lead to the development of coarse-grained and massive perovskite. A combination of characteristic features identified in the Afrikanda perovskite (equigranular crystal mosaics, interlocked irregular-shaped grains, and massive zones) is observed in other oxide ore deposits, particularly in layered intrusions of chromitites and intrusion-hosted magnetite deposits and suggests that the same amalgamation processes may be responsible for some of the coarse-grained and massive textures observed in oxide deposits worldwide.
Abstract: Subduction zones are an important way for crustal materials to enter deep parts of the Earth. Therefore, carbonatites in orogenic belt are of great significance in revealing deep carbon cycling pathways. To date, mantle-derived carbonatites have been identified in many orogenic belts, and their origin is considered to be related to subducted sediments. However, almost all orogenic carbonatites are composed of calcite, and their C isotopic compositions show typical mantle values, lacking any evidence of sedimentary origin. Here, we report decoupling of C and Sr isotopes between intimately associated dolomite and forsterite-calcite carbonatites from Caotan in the Qinling orogen, central China. The dolomite carbonatite is mainly composed of dolomite (plus minor apatite and magnetite), which has elevated ?13CPDB values (-3.1 to -3.6 ‰) and low 87Sr/86Sr ratios (0.7026-0.7042). The forsterite-calcite carbonatite consists of calcite (60-65 vol. %), forsterite and its replacement products (30-35 vol. %), and magnetite. The calcite shows mantle-like ?13CPDB (-6.2 to -7.2 ‰) but high initial 87Sr/86Sr values (0.7053-0.7076). Neodymium and Pb isotopic compositions are comparable in the two carbonatite types. The forsterite-calcite carbonatite is interpreted to have formed by metasomatic interaction of primary dolomitic melts with eclogite in thickened lower crust during collision of the North and South China cratons. The reaction resulted in decarbonation and depletion of the carbonatitic magma in 13C. Because of its initially low REE and Pb contents, the Nd-Pb isotopic signature of the primary dolomitic melt was preserved in the forsterite-calcite carbonatite. We propose that some orogenic calcite carbonatites may not be primary mantle-derived rocks and their mantle-like ?13CPDB values may be misleading.
Contributions to Mineralogy and Petrology, Vol. 175, 13p. Pdf
Russia
perovskite
Abstract: The exceptional accumulation of perovskite in the alkaline-ultramafic Afrikanda complex (Kola Peninsula, Russia) led to the study of polymineralic inclusions hosted in perovskite and magnetite to understand the development of the perovskite-rich zones in the olivinites, clinopyroxenites and silicocarbonatites. The abundance of inclusions varies across the three perovskite textures, with numerous inclusions hosted in the fine-grained equigranular perovskite, fewer inclusions in the coarse-grained interlocked perovskite and rare inclusions in the massive perovskite. A variety of silicate, carbonate, sulphide, phosphate and oxide phases are assembled randomly and in variable proportions in the inclusions. Our observations reveal that the inclusions are not bona fide melt inclusions. We propose that the inclusions represent material trapped during subsolidus sintering of magmatic perovskite. The continuation of the sintering process resulted in the coarsening of inclusion-rich subhedral perovskite into inclusion-poor anhedral and massive perovskite. These findings advocate the importance of inclusion studies for interpreting the origin of oxide minerals and their associated economic deposits and suggest that the formation of large scale accumulations of minerals in other oxide deposits may be a result of annealing of individual disseminated grains.
Mineralogy and Petrology, doi.org/10.1007/ s00710-021-00736-0 11p. Pdf
Europe, Norway
carbonatite
Abstract: Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirmed by Raman microspectroscopy) as inclusions in calcite, dolomite and, less commonly, fluorapatite and pyrochlore. Typically the inclusions occur in the core of calcite or dolomite grains relatively unaffected by subsolidus processes, and are associated with Fe-poor dolomite or Sr-rich calcite, respectively. Burbankite does not exceed 30?×?50 ?m in size and is characteristically absent from the peripheral areas of carbonate grains affected by recrystallization or interaction with fluids. Compositionally, the mineral falls within the following range: (Na1.51-2.16Ca0.58-1.21)(Sr1.50-2.42Ca0.28-0.57LREE0.05-0.64Ba0.06-0.41)(CO3)5 and contains low Th, but no detectable Mg, Fe or F (LREE?=?light rare-earth elements: Ce?>?La?>?Nd?>?Pr?>?Sm). Burbankite inclusions at Fen are interpreted as primary and indicative of Na enrichment in their parental carbonatitic magma. Dissociation of burbankite during subsolidus re-equilibration of its host phases with fluids undoubtedly served as one of the sources of LREE for the development of late-stage mineralization in the Fen complex.
Flourine , yttrium and lanthaide rich cerianite (Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosive craters Gregory Rift Tanzania.
Mineralogical Magazine, Vol. 75, 6, pp. 2813-2822.
Abstract: Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Abstract: Mantle xenoliths from the Middle-Late Jurassic Obnazhennaya kimberlite are often compared with those from the Udachnaya kimberlite (ca. 367 Ma) to inform the evolution of the Siberia craton. However, there are no direct constraints on the timing of the Obnazhennaya kimberlite eruption. Such uncertainty of the kimberlite age precludes a better understanding of the mantle xenoliths from the Obnazhennaya pipe, and thus also of the evolution of the Siberia craton. This paper reports U-Pb ages for both perovskite from the Obnazhennaya kimberlite and rutile in an Obnazhennaya eclogite xenolith. The fresh perovskite formed during the early stage of magmatic crystallization and yields a U-Pb age of 151.8 ± 2.5 Ma (2?). Rutile in the eclogite xenolith yields an overlapping U-Pb age of 154.2 ± 1.9 Ma (2?). Because rutile has a Pb closure temperature lower than the inferred residence temperature of the eclogite prior to eruption, the U-Pb isotope system in rutile was not closed until the host eclogite was entrained and delivered to the surface by the kimberlite and therefore records the timing of kimberlite eruption. These data provide the first direct constraints on the emplacement age of the Obnazhennaya kimberlite and add to the global ‘kimberlite bloom’ from ca. 250-50 Ma as well as to the largest pulse of kimberlite volcanism in Siberia from ca. 171-144 Ma. The timing of this Jurassic-Cretaceous pulse coincides with the closure of the Mongol-Okhotsk Ocean, but the depleted Sr-Nd isotopic characteristics of 171-144 Ma kimberlites are inconsistent with a subduction-driven model for their petrogenesis. Thus, the closure of the Mongol-Okhotsk Ocean may act as a trigger for the initiation of 171-144 Ma kimberlite emplacement of Siberia, but was not the source.
Abstract: Mineralogy and geochemistry of the Udirpikonda lamprophyre, located within the Mesoproterozoic diamondiferous Wajrakarur kimberlite field (WKF), towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin are presented. The lamprophyre is characterised by a panidiomorphic-porphyritic texture imparted by clinopyroxene, olivine and biotite set in a groundmass of feldspar and spinel. Olivine occurs as the microphenocrysts with a composition range of Fo87-78. Clinopyroxenes display reverse as well as oscillatory optical zoning and are diopsidic in nature with a variation in the composition from core (Wo47 En28 Fs20Ac5) to rim (Wo46En41Fs11Ac3). Biotite (Mg# < 0.6) is the only mica present and spinels are titano-magnetites showing ulvospinel- magnetite solid solution. Plagioclase is the dominant feldspar with a variable compositional range of An41-8Ab82-56Or33-3. Based on the mineralogy, the lamprophyre can be classified to be of calc-alkaline variety but its geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. K2O/Na2O ranges from 1.49 to 2.79, making it distinctly potassic and highlights its shoshonitic character. Moderate Mg# (60-65), Ni (110-200 ppm) and Cr (110-260 ppm) contents in the bulk-rock indicate substantial fractional crystallization of olivine and clinopyroxene. Fractionated chondrite normalized REE patterns (average (La/Yb)N = 37.56) indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative Ta-Nb-Ti and Hf anomalies displayed on the primitive mantle normalized multi-element spider gram highlight involvement of a subducted component in the mantle source. Given the spatial disposition of the studied lamprophyre, the age of the emplacement is considered to be coeval with WKF kimberlites (~ 1.1 Ga) and the initial 143Nd/144Nd (0.510065-0.510192) and 87Sr/86Sr (0.705333-0.706223) are strikingly similar to those observed for the Smoky Butte lamproites, Montana, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N < 1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.
Abstract: Major, trace element concentrations and Nd, Sr and Ca stable isotopic compositions (?44/40Ca and ?44/42Ca w.r.t. NIST SRM915a) of carbonatites and associated igneous silicate rocks from the ~65?Ma old Ambadongar carbonatite complex and the surrounding Phenai Mata igneous complex of western India are reported. Samples of fluorspar from Ambadongar and the Bagh Limestone and Sandstone, which are part of the country rocks at Ambadongar, have also been analysed. The Ambadongar carbonatites are primarily calcio- and ferro-carbonatites while the silicate rocks from these two complexes are alkaline and tholeiitic in composition. The ?44/40Ca values of the carbonatites (0.58-1.1‰, n?=?7) and the associated igneous silicate rocks (0.50-0.92‰, n?=?14) show a broad range. The low K/Ca values of the carbonatites (<0.2) and silicate rocks (<2) along with their young eruption age (~65 Ma) rule out any effect of radiogenic 40Ca ingrowth due to decay of 40K on the ?44/40Ca values. The lack of correlations between ?44/40Ca and Mg# as well as La/Yb(N) values suggest that the variability in ?44/40Ca is not controlled by the degree of partial melting. The ?44/40Ca values of the carbonatites (0.58-1.1‰) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the ?44/40Ca value of the Bagh Limestone (0.66‰) suggesting that assimilation of these crustal limestones by the magma is unlikely to have caused the variability in ?44/40Ca of the carbonatites. In plots of ?44/40Ca versus ?Nd(t) and 87Sr/86Sr(t), the igneous silicate rocks from the Ambadongar and Phenai Mata complexes plot on a mixing trend between a primitive (plume) mantle source and the continental crustal basement suggesting the role of continental crustal contamination during eruption of the Reunion plume. While simple binary mixing calculations yield unrealistically high amounts of crustal contamination (40%), assimilation and fractional crystallization (AFC) models suggest up to 20% contribution from a heterogeneous basement for these igneous silicate rocks. The role of continental crustal contamination in the genesis of the igneous silicate rocks is further supported by their unradiogenic ?Nd(t), radiogenic 87Sr/86Sr(t) and low Ce/Pb values. In contrast, carbonatites plot away from the mixing trend between a primitive mantle (plume) source and continental crust in Ca-Sr-Nd isotopic diagrams suggesting that the Ca isotopic variability of carbonatites is not caused by continental crustal contamination. In contrast, the isotopic composition of the carbonatites can be explained by mixing of the plume end-member with up to 20% of ~160?Ma-old recycled carbonates suggesting their derivation from a highly heterogeneous, recycled carbonate-bearing plume mantle source. The composition of one carbonatite sample showing unusually high ?44/40Ca and highly radiogenic 87Sr/86Sr(t) is explained by hydrothermal alteration which is also invoked for the formation of massive fluorspar deposits with high ?44/40Ca (1.44‰) at Ambadongar. In a plot of ?44/40Ca versus K/Rb, the carbonatites plot towards the phlogopite end-member (?44/40Ca?=?1‰, K/Rb?=?40-450) while the igneous silicate rocks plot towards the amphibole end-member (?44/40Ca?=?0.44‰, K/Rb >1000). Phlogopite, especially if F-rich, is stable at greater depths in the mantle compared to amphibole. Hence, the correlated ?44/40Ca and K/Rb values of the carbonatites and associated igneous silicate rocks suggest the derivation of these carbonatites from a relatively deeper mantle source compared to the silicate rocks, both within the Reunion mantle plume. The origin of the carbonatites from the F-rich phlogopite-bearing mantle is also consistent with the occurrence of large fluorspar deposits within the Ambadongar carbonatite complex.
A geochemical and Nd, Sr and stable Ca isotopic study of carbonatites and associated silicate rocks from the ~65 Ma old Ambadongar carbonatite complex and the Phenai Mata igneous complex, Gujarat, India: implications for crustal contamination, carbonate r
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Geochemical and Nd-Sr-Ca isotopic compositions of carbonatites and alkaline igneous rocks from the Deccan Igneous Province: role of recycled carbonates, crustal assimilation and plume heterogeneity.
Neues Jahbuch fur Mineralogy, Vol. 196, p2, pp. 149-177.
India
camptonite
Abstract: We report petrology and geochemistry (including Sr and Nd isotopes) of a fresh lamprophyre at Ankiraopalli area at the north-western margin of Paleo-Mesoproterozoic Cuddapah basin, eastern Dharwar craton, southern India. Ankiraopalli samples possess a typical lamprophyre porphyritic-panidiomorphic texture with phenocrysts of kaersutite and diopside set in a plagioclase dominant groundmass. Combined mineralogy and geochemistry classify it as alkaline lampro- phyre in general and camptonite in particular. Contrary to the calc-alkaline and/or shoshonitic orogenic nature portrayed by lamprophyres occurring towards the western margin of the Cuddapah basin, the Ankiraopalli samples display trace element composition revealing striking similarity with those of ocean island basalts, Italian alkaline lamprophyres and highlights an anorogenic character. However, the87 Sr/86 Srinitial (0.710316 to 0.720016) and ?Ndinitial (- 9.54 to - 9.61) of the Ankiraopalli lamprophyre show derivation from an 'enriched' mantle source showing long term enrichment of incompatible trace elements and contrast from those of (i) OIB, and (ii) nearby Mahbubnagar alkaline mafic dykes of OIB affinity. Combining results of this study and recent advances made, multiple mantle domains are identified in the Eastern Dharwar craton which generated distinct Mesoproterozoic lamprophyre varieties. These include (i) Domain I, involving sub-continental lithospheric mantle source essentially metasomatized by subduction-derived melts/fluids (represented by orogenic calcalkaline and/or shoshonitic lamprophyres at the Mudigubba, the Udiripikonda and the Kadiri); (ii) Domain II, comprising a mixed sub-continental lithospheric and asthenospheric source (represented by orogenic-anorogenic, alkaline to calc-alkaline transitional lamprophyres at the Korakkodu), and (iii) Domain III, representing a sub-continental lithospheric source with a dominant overprint of an asthenospheric (plume) component (represented by essentially alkaline lamprophyres at the Ankiraopalli). Our study highlights the varied mantle source heterogeneities and complexity of geodynamic processes involved in the Neoarchean-Paleo/Mesoproterozoic evolution of the Eastern Dharwar craton.
Abstract: We report a 40Ar/39Ar Mesoproterozoic radiometric age for a calc-alkaline lamprophyre dyke from the Mudigubba area towards the western margin of the Cuddapah Basin, Eastern Dharwar Craton (EDC), Southern India. Amphibole phenocryst separates from this lamprophyre yielded a plateau age of 1169 ± 8 Ma (2? ), which is almost 50 million years older than the majority of radiometric dates available for the Wajrakarur field kimberlites which are proximal to this dyke. Bulk-rock Sr-Nd isotopic analyses of the Mudigubba lamprophyre dykes (?Nd(t) between -13.3 and -12.4) reveal their derivation from an old, enriched, continental lithospheric mantle unlike the kimberlites (bulk-rock and perovskite in situ ?Nd(t) between -0.77 and +7.93), which originated either from a chondritic or depleted mantle source. This study provides further evidence for emplacement of compositionally distinct, mantle-derived Mesoproterozoic alkaline magmas in the EDC and highlights the extremely heterogeneous character of the lithospheric mantle beneath this craton.
Journaof Mineralogical and Petrological Sciences, Vol. 116, pp. 121-128 pdf
India
deposit - Newania
Abstract: This study presents first report of the sulfur isotopic compositions of carbonatites from the Mesoproterozoic Newania complex of India along with their stable C and O isotope ratios. The ?34SV-CDT (?1.4 to 2‰) and ?33S (?0.001 to ?0.13‰) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth’s mantle. Additionally, the ?13CV-PDB and ?18OV-SMOW values of these carbonatites also show overlapping compositions to that of Earth’s mantle. Based on these mantle-like stable isotopic compositions of carbonatites along with their higher crystallization temperature (~ 600 °C) compared to a hydrothermal fluid (<250 °C), we suggest that the sulfide minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the ?34S, ?13C- ?18O, and 87Sr/86Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sulfide phase (pyrrhotite) with magnesite in these carbonatites suggests that the sulfide phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the ?34S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and reflect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of ?34S of carbonatites from Newania and other complexes worldwide indicates limited variability in the isotopic composition for carbonatites older than 400 Ma, which broadly overlaps with Earth’s asthenospheric mantle composition. This contrasts with the larger variability in ?34S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.
Enigmatic association of the carbonatite and alkali pyroxenite along the Northern Shear Zone, Purulia, West Bengal: a saga of primary magmatic carbonatite.
Journal of Geological Society of India, Vol. 76, 5, pp.399-402.
Abstract: The formation of transitional agpaitic rocks is not a well understood process as there are few studies of miaskitic to agpaitic transitions. The Mesoproterozoic Sushina Hill complex (India) provides a suitable site to investigate these "transitions" as this complex hosts diverse miaskitic and agpaitic nepheline syenites, together with syenites containing exotic mineral assemblages. In this study, we have used mineralogical and geochemical data to describe the evolution of the transitional agpaitic rocks occurring at Sushina Hill. In common with other occurrences, high field strength elements (HFSE) in miaskitic nepheline syenites are mainly sequestered by primary zircon and magnetite. In contrast, the major HFSE carriers in agpaitic nepheline syenites (agpaitic unit-I) are late-magmatic eudialyte and rinkite-(Ce) - nacareniobsite-(Ce), formed at T between 825° - 784ºC and aSiO2 in the range of 0.41 - 0.44. With decreasing temperature (? 575ºC) and aSiO2(0.30), coupled with an increase in aH2O, this assemblage has undergone extensive subsolidus alteration leading to the decomposition of late-magmatic eudialyte to wöhlerite - marianoite, alkali-zirconosilicates (catapleiite/gaidonnyaite, hilairite), and pectolite - serandite. Decomposition of late-magmatic eudialyte resulted in a more alkaline fluid by increasing the a(Na+)/a(Cl-) ratio, facilitating crystallization of hydrothermal eudialyte replacing late-magmatic eudialyte. Crystallization of hydrothermal eudialyte leads to evolving fluids which are less alkaline, resulting in the crystallization of a transitional agpaitic assemblage of pyrochlore + zircon + niobokupletskite + wadeite in agpaitic unit-II in the temperature range 547º - 455ºC with aSiO2 in the range 0.27 - 0.25. Regional scale deformation contemporaneous with the subsolidus alteration stage leads to separation of the evolving fluid from the system, resulting in extensive albitization, with superposition of a new miaskitic-like assemblage in syenite I in the form of late-stage zircon - magnetite - xenotime - monazite-(Ce) upon the early assemblage of primary zircon and magnetite. During deformation, syenite unit-II composed of eudialyte - albite - aegirine was also formed and considered as a later stage pegmatitic offshoot of agpaitic unit I. The mineralogical changes are also complemented by variations in the bulk-rock composition in which the total REE, Nb, U and Th concentrations increase in order from: miaskitic unit ? agpaitic unit I ? syenite unit II, -I ? agpaitic unit II at constant Zr concentration. This suggests that the REE-Nb are mainly mobilized in agpaitic unit-II during the agpaitic - to - transitional agpaitic assemblage transformation in a relatively less alkaline environment.
Abstract: Pyrochlore-group minerals are common in the Neoproterozoic Sevattur carbonatite complex. This complex is composed of dolomite-, calcite-, banded- and blue carbonatite together with pyroxenite, albitite and diverse syenites. This work reports the paragenetic-textural types and compositional variation of pyrochlore hosted by dolomite carbonatite, banded carbonatite and albitite together with that of alteration assemblages containing belkovite and baotite. On the basis of composition, five different types of pyrochlore are recognised and termed Pcl-I through to Pcl-V. The Pb-rich Pcl-I are present exclusively as inclusions in U-rich Pcl-IIa in dolomite carbonatite. The alteration assemblages of Pb-poor Pcl-IIb + Ba-rich or Ba-Si- rich Pcl-IV + belkovite (dolomite carbonatite) and Si-rich Pcl-V + baotite (banded carbonatite) formed after Pcl-IIa differ in these carbonatites. The albitite hosts extremely U-Ti-rich Pcl-III, mantled by Ba-rich potassium feldspar. In common with the banded carbonatite, Pcl-V is formed by alteration of Pcl-III where this mantle is partially, or completely broken. The Ba-Si-enrichment of Pcl-IV and Pcl-V together with the ubiquitous presence of baryte in all Sevattur lithologies suggests late-stage interaction with a Ba-Si-rich acidic hydrothermal fluid. This fluid was responsible for leaching silica from the associated silicates and produced Pcl-V in the silicate-rich lithologies of the banded carbonatite and albitite. The absence of Pcl-V in dolomite carbonatite is a consequence of the low modal abundance of silicates. The complex compositional diversity and lithology specific pyrochlore alteration assemblages suggest that all pyrochlore (Pcl-I to Pcl-IV) were formed initially in an unknown source and transported subsequently in their respective hosts as altered antecrysts.
Abstract: The Neoproterozoic Sevattur complex is composed essentially of calcite and dolomite carbonatites together with pyroxenites and diverse syenites. This work reports the compositions and paragenesis of different pyrochlore generations hosted by albitite veins in this complex. The pyrochlore are distinctive, being exceptionally rich in uranium (26 to 36 wt.% UO2). Five types of pyrochlore (Pcl-I to Pcl-V) are recognised on the basis of composition and texture. With the exception of Pcl-V, the majority of the pyrochlore (Pcl-II to Pcl-IV) are surrounded by a thick orbicular mantle of Ba-rich potassium feldspar. This mantle around Pcl-V is partially-broken. Pcl-I is restricted to the cores of crystals, and associated with Pcl-II and -III and is relatively rich in Nb (0.53-0.62 apfu) together with more A-site vacancies (0.37-0.71 apfu) compared to Pcl-II to Pcl-IV. Other pyrochlore (Pcl-II to Pcl-IV) are characterised by elevated Ca and Ti compared to Pcl-I, which are related to the (3Nb5+ + Na+ ? 3Ti4+ + U4+) and (2Nb5+ ? 2Ti4+ + Ca2+) substitutions, respectively. These substitutions represent replacement of Pcl-II to Pcl-IV. Alteration and Ba-enrichment in all the pyrochlore are marked by interaction with an externally-derived Ba-rich hydrothermal fluid following the (2Nb5+ ? 2Ti4+ + Ba2+) substitution. This substitution, coupled with extensive metamictisation leads to the formation of Ba-rich (15.9-16.3 wt.% BaO) patchy-zoned Pcl-V. The orbicular mantles around Pcl-I to Pcl-IV have prevented extensive metamictisation and extensive secondary alteration compared to Pcl-V, where mantling is partially disrupted. The compositional and textural variation suggests that Pcl-II to Pcl-IV form by nucleation on Pcl-I, and are transported subsequently as antecrysts in the host albitite.
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Journal of Earth Science System, doi:10.1007/s12040- 019-1228-0
India
geochemistry
Abstract: The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE-SSW to NE-SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5-25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of ~2.75-2.8 Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India.
Abstract: The rock record and geochemical evidence indicate that continental recycling has been occurring since the early history of the Earth. The stabilization of felsic continents in place of Earth’s early mafic crust about 3.0 to 2.0 billion years ago, perhaps due to the initiation of plate tectonics, implies widespread destruction of mafic crust during this time interval. However, the physical mechanisms of such intense recycling on a hotter, (late) Archaean and presumably plate-tectonic Earth remain largely unknown. Here we use thermomechanical modelling to show that extensive recycling via lower crustal peeling-off (delamination but not eclogitic dripping) during continent-continent convergence was near ubiquitous during the late Archaean to early Proterozoic. We propose that such destruction of the early mafic crust, together with felsic magmatism, may have caused both the emergence of silicic continents and their subsequent isostatic rise, possibly above the sea level. Such changes in the continental character have been proposed to influence the Great Oxidation Event and, therefore, peeling-off plate tectonics could be the geodynamic trigger for this event. A transition to the slab break-off controlled syn-orogenic recycling occurred as the Earth aged and cooled, leading to reduced recycling and enhanced preservation of the continental crust of present-day composition.
Journal of Metamorphic Geology, Vol. 37, 1, pp. 113-151.
India
geology
Abstract: The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c. 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c. 1.65 and c. 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c. 1.56 and c. 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c. 1.45 and c. 0.97 Ga. Based on thermobarometric studies and zircon-monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three?stage model of crustal accretion across the Singhbhum craton - GSB/North Singhbhum Mobile Belt - GC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c. 1.65 Ga in an arc setting. The earliest accretion event at c. 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton-Upper Bonai Group composite at c. 1.45 Ga. Finally, continent-continent collision at c. 0.96 Ga led to the accretion of the CGC with the Singhbhum craton-Upper Bonai Group-Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North China Orogen and the Capricorn Orogen of Western Australia, indicating that these suture zones are not correlatable.
Major, trace element compositions and Nd, Sr and stable Ca isotopic compositions of carbonatites and alkaline silicate rocks of the Amba Dongar carbonatite complex, India: role of mantle mineralogy and subducted carbonates.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 18.
India
deposit - Amba Dongar
Abstract: Carbonatites, with limited spatial but wide temporal occurrences, are unique igneous rocks with more than 50% modal carbonate. Geochemically carbonatites are characterized by high La/Yb(N) and enrichment in the Ba, Sr concentrations and superchondritic whole-rock Nb/Ta (~35) and Zr/Hf ratios (~60). Most of the global carbonatites are associated with the alkaline silicate rocks and their origin is highly debated. To understand the petrogenesis of carbonatites, we present geochemical and isotopic data, including the first ever measurements of Ca stable isotopes in the ~65 Ma old carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India We have performed a detail geochemical and isotopic investigation of the carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India. The analysed carbonatites are primarily calcitic and ankeritic in nature. The associated silicate rocks can be classified into two groups, the first being highly Krich and alkaline while the second group is tholeiitic in nature. The high La/Yb(N) ratio of carbonatites are suggestive of low degree partial melting while the enrichment in large ion lithophile elements (LILE) of the silicate rocks suggest that these rocks are derived from a LILE enriched upper mantle source. The carbonatites and the associated alkaline silicate rocks from the Amba Dongar carbonatite complex show overlapping Nd-Sr isotopic compositions with the tholeiitic rocks from the Phenai Mata complex, located approximately 16 km NW of Amba Dongar. Variability in ? 44/40Ca in Hawaiian shield stage tholeiites have been interpreted as evidence of subducted ancient marine carbonates, with very low -44/40Ca, into the Hawaiian plume (Huang et al., 2010). Boron isotopic composition of global carbonatites suggests that subducted crustal components contributed to the mantle source of relatively young carbonatites (<300 Ma old) (Hulett et al., 2016), a signature which should potentially be traceable using Ca isotopes. We report -44/40Ca of carbonatites and associated alkaline silicate rocks from the Amba Dongar complex. The samples were analyzed using a 43Ca-48Ca double spike on a Thermo Fischer Triton Plus Thermal Ionization Mass Spectrometer (TIMS) at IISc. ?44/40Ca in these rocks show a significant variation (~0.6 ‰- (w.r.t. SRM 915a), which is much larger than the variation observed in limited analyses of global carbonatites (Amini et al., 2009). Our external reproducibility, estimated from multiple analyses of NIST standards SRM 915a, SRM 915b and seawater (NASS6), is better than 0.1 - (2SD). ?44/40Ca of the ~65 Ma old Amba Dongar carbonatites shows correlations with Nb/Yb, K/Rb as well as with Sr/Nb, Sr/Zr. These variations suggest the role of phlogopite versus amphibole in the mantle source as well sas subducted carbonates in controlling the ?44/40Ca of these carbonatites.
Journal of Volcanology and Geothermal Research, in press available 15p.
Africa, Algeria
Lamproite
Abstract: The late Miocene (11-9 Ma) volcanic rocks of Kef Hahouner, ~ 40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O > 3 wt.%, K2O/Na2O > 2.5, MgO > 3-4 wt.%, SiO2 < 55-57 wt.% and SiO2/K2O < 15. All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb = 37-59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874-0.70961, 143Nd/144Nd = 0.51222-0.51223, 206Pb/204Pb = 18.54-18.60, 207Pb/204Pb = 15.62-15.70 and 208Pb/204Pb = 38.88-39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6-21.4 wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as "normal" potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into "orogenic" and "anorogenic" types appears questionable.
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.
Geological Society of London Special Publications, doi:10.1144/SP513-2021-60
geodynamics
Abstract: Our pilot study reveals potential fingerprints of Li isotopes recorded in the Mesoproterozoic (?1.4-1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine-Himalayan (SW Tibet) orogenic belts formed in response to Gondwana-Pangea amalgamation and break-up. As a result of supercontinents development, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (?7Li up to 9.7‰) similar to an ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3-6.3‰) and lamprophyres (3.3-6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e., rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards isotopically lighter component (?7Li as low as -9.5‰) comparable with the involvement of an evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.
A petrological and geochemical appraisal of the Mesoproterozoic Diamondiferous Majhgawan pipe of central India: evidence for transitional kimberlite - orangeite ( group II kimberlite) - lamproite rock type.
Chalapathi Rao, N.V., Burgess, R., Anand, M., Mainkar, D.
Evidence for a Phanerozoic (478 Ma) Diamondiferous kimberlite emplacement epoch in the Indian Shield from 40 Ar/ 39Ar dating of the Kodomali kimberlite: implications ....
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 103-106.
India, Bastar Craton, Rodinia
Tectonics - Kodomali, Pan African , Geothermometry
Dongre, A., Kamde, G., Chalapathi Rao, N.V., Kale, H.S.
Is megacrystic/xenocrystic ilmenite entrainment in the source magma responsible for the non-Diamondiferous nature of the Maddur-Kotakonda-Narayanpet kimberlites
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 72.
The Jungel Valley re-visited: evidence from the lamprophyres for the presence of a Paleoproterozoic carbonate rich metasomatised mantle in Mahakoshal belt
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 123.
India, Madhya Pradesh, Aravalli Bundelkhand Craton
Petrology, geochemistry and tectonic significance of Paleoproterozoic alkaline lamprophyres from the Jungel Valley, Mahakostal supracrustal belt, Central India.
Mineralogy and Petrology, Vol. 89, 3-4, pp. 189-215.
Petrology and geochemistry of Diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field, eastern Dharwar craton, southern India: genesis and constraints on mantle
Contributions to Mineralogy and Petrology, Vol. 157, 2, pp. 245-265.
Srivastava, R.K., Chalapathi Rao, N.V., Sinha, A.K.
Cretaceous potassic intrusives with affinities to aillikites from Jharia area: magmatic expression of metasomatically veined and thinned lithospheric mantle
Srivastava, R.K., Chalapathi Rao, N.V., Sinha, A.K.
Cretaceous potassic intrusives with affinities to aillikites from Jharia area: magmatic expression of metasomatically veined and thinned lithospheric mantle
Chalapathi Rao, N.V., Anand, M., Dongre, A., Osborne, I.
Carbonate xenoliths hosted by the Mesoproterozoic Siddanpalli kimberlite cluster ( Eastern Dharwar craton): implications for the geodynamic evolution of
International Journal of Earth Sciences, Vol. 99, pp. 1791-1804.
Mineralogy and geochemistry of kimberlites NK-2 and KK-6 Narayanpet kimberlite field, eastern Dharwar Craton, southern India: evidence for transitional ...
The Canadian Mineralogist, Vol. 47, 5, pp. 1117-1135,
Chalapathi Rao, N.V., Lehmann, B., Mainkar, D., Belyatsky, B.
Petrogenesis of the end Cretaceous Diamondiferous Behradih kimberlite, central India: implication for the plume lithosphere interactions in the Bastar craton?
International Dyke Conference Held Feb. 6, India, 1p. Abstract
Chalapathi Rao, N.V., Lehmann, B., Mainkar, D., Belyatsky, B.
Petrogenesis of the end Cretaceous Diamondiferous Behradih orangeite pipe: implications for mantle plume - lithosphere interaction in the Bastar craton, India.
Contributions to Mineralogy and Petrology, Vol. 161, pp. 721-742.
Chalapathi Rao, N.V., Lehmann, B., Belousova, E., Frei, D., Mainkar, D.
Petrology, bulk rock geochemistry, indicator mineral composition and zircon U-Pb geochronology of the end Cretaceous Diamondiferous Mainpur orangeites, Bastar Craton, Central India.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, eastern Dharwar Craton southern India: insights from groundmass mineralogy, bulk chemistry and perovskite oxybarometry.
New paleomagnetic and rock magnetic results on Mesoproterozoic kimberlites from the Eastern Dharwar craton, southern India: towards constraining India's position in Rodinia.
Petrology, bulk-rock geochemistry, indicator mineral composition and zircon U-Pb geochronology of the end-Cretaceous Diamondiferous Mainpur orangeites, Bastar craton, central India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 93-121.
Platinum group elements (PGE) geochemistry of Deccan orangeites, Bastar craton, central India: implication for a non-terrestrial origin for irridium enrichment at the K-Pg boundary.
Journal of Asian Earth Sciences, Vol. 84, pp. 24-33.
Mesoproterozoic U-Pb ages, trace element and Sr-Nd isotopic composition of perovskite from kimberlites of the Eastern Dharwar craton, southern India: distinct mantle sources and a Wide spread 1.1 Ga Tectonomagmatic event.
New paleomagnetic and rock magnetic results on Mesoproterozoic kimberlites from the Eastern Dharwar craton, southern India: towards constraining India's position in Rodinia.
Chalapathi Rao, N.V., Kumar, A., Sahoo, S., Dongre, A.N., Talukdar, D.
Petrology and petrogenesis of Mesoproterozoic lamproites from the Ramadugu field NW margin of the Cuddapah basin, eastern Dharwar craton, southern India.
Platinum-group elements ( PGE) geochemistry of Deccan orangeites, Bastar craton, central India: implication for a non-terrestrial origin for iridium enrichment at the K-Pg boundary.
Journal of Asian Earth Sciences, Vol. 84, Apr. 15, pp. 24-33.
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, 13p.
India
Deposit - Wajrakur
Abstract: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9?90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).
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.
Mineralogy and Petrology, Vol. 110, 2, pp. 295-307.
India
Deposit - Wajrakarur
Abstract: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9?90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).
Proceedings National Academy of Sciences India , Vol. 82, 3, July special issue pp. 515-536.
India
Kimberlites, lamproites
Abstract: Major highlights of researches carried out on kimberlites, lamproites, lamprophyres, carbonatites, other alkaline rocks and mafic dykes from the Indian shield during 2012-2016 are presented. New findings involving field mapping, petrology, geochemistry (including high quality mineral based in situ isotopic studies) and geophysics have provided remarkable insights on the mode of their occurrence, timing of emplacement, mineralogy and bulk-rock composition, redox conditions, relative contribution of the lithosphere and asthenosphere, as well as their economic potential. Several large-scale geodynamic aspects such as plume-lithosphere interactions, ancient subduction events, layered structure of the sub-continental lithospheric mantle, spatial extent of the Precambrian large igneous provinces and supercontinent configurations could be unraveled from these studies on deep-mantle derived small-volume magmatic rocks.
Abstract: We report groundmass perovskite U -Pb (SIMS) ages, perovskite Nd isotopic (LA-ICPMS) composition and bulk-rock geochemical data of the Timmasamudram diamondiferous kimberlite cluster, Wajrakarur kimberlite field, in the Eastern Dharwar craton of southern India. The kimberlite pipes gave similar Mesoproterozoic ages of 1086 ± 19 Ma (TK-1, microcrystic variant) and 1119 ± 12 Ma (TK-3). However, a perovskite population sampled from the macrocrystic variant of TK-1 gave a much younger Late Cretaceous age of ca. 90 Ma. This macrocrystic kimberlite phase intrudes the Mesoproterozoic microcrystic phase and has a distinct bulk-rock geochemistry. The Nd-isotope composition of the ~ 1100 Ma perovskites in the cluster show depleted ?Nd(T) values of 2.1 ± 0.6 to 6.7 ± 0.3 whereas the ~ 90 Ma perovskites have enriched ?Nd(T) values of ? 6.3 ± 1.3. The depleted-mantle (DM) model age of the Cretaceous perovskites is 1.2 Ga, whereas the DM model age of the Proterozoic perovskites is 1.2 to 1.5 Ga. Bulk-rock incompatible trace element ratios (La/Sm, Gd/Lu, La/Nb and Th/Nb) of all Timmasamudram kimberlites show strong affinity with those from the Cretaceous Group II kimberlites from the Bastar craton (India) and Kaapvaal craton (southern Africa). As the Late Cretaceous age of the younger perovskites from the TK-1 kimberlite is indistinguishable from that of the Marion hotspot-linked extrusive and intrusive igneous rocks from Madagascar and India, we infer that all may be part of a single Madagascar Large Igneous Province. Our finding constitutes the first report of Cretaceous kimberlite activity from southern India and has significant implications for its sub-continental lithospheric mantle evolution and diamond exploration programs.
Abstract: New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic (ca. 1100 Ma) and late Cretaceous (ca. 90 Ma) kimberlites in the Timmasamudram cluster (TKC) of the Wajrakarur kimberlite field (WKF), Eastern Dharwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry (phlogopite, spinel, perovskite and clinopyroxene), bulk-rock chemistry (SiO2, K2O, low TiO2, Ba/Nb and La/Sm), and perovskite Nd isotopic compositions, the TK-1 (macrocrystic variety) and TK-4 (microcrystic variety) kimberlites in this cluster are here classified as orangeites (i.e. Group II kimberlites), with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites (e.g., TK-2, TK-3 and the TK-1 microcrystic variant), are more similar to other 1100 Ma, Group I-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on published carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite.
Abstract: The article gives new experimental data on spectral characteristics of photoluminescence of natural diamonds extracted from deep horizons of Mir and Internatsionalnaya Pipes, Republic of Sakha (Yakutia) depending on composition of basic and additional optically active structural defects in crystals and on temperature during spectrum recording, considering kinetics of luminescence. It is hypothesized on applicability of low-temperature effects to enhance efficiency of photoluminescence separation of diamond crystals.
Abstract: New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic (ca. 1100 Ma) and late Cretaceous (ca. 90 Ma) kimberlites in the Timmasamudram cluster (TKC) of the Wajrakarur kimberlite field (WKF), Eastern Dharwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry (phlogopite, spinel, perovskite and clinopyroxene), bulk-rock chemistry (SiO2, K2O, low TiO2, Ba/Nb and La/Sm), and perovskite Nd isotopic compositions, the TK-1 (macrocrystic variety) and TK-4 (Macrocrystic variety) kimberlites in this cluster are here classified as orangeites (i.e. Group II kimberlites), with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites (e.g., TK-2, TK-3 and the TK-1 microcrystic variant), are more similar to other 1100 Ma, Group I-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on published carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite. Depleted mantle (TDM) Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasomatic enrichment of their source regions are broadly similar to that of the Mesoproterozoic kimberlites of the EDC. The younger, late Cretaceous (ca. 90 Ma) TK-1 (macrocrystic variant) and TK-4 kimberlites, as well as the orangeites from the Bastar Craton, share similar Nd model ages of 1100 Ma, consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent. The presence of late Cretaceous diamondiferous orangeite activity, presumably related to the location of the Marion hotspot in southern India at the time, suggests that thick lithosphere was preserved, at least locally, up to the late Cretaceous, and was not entirely destroyed during the breakup of Gondwana, as inferred by some recent geophysical models.
Abstract: Mineralogy and geochemistry of the Udirpikonda lamprophyre, located within the Mesoproterozoic diamondiferous Wajrakarur kimberlite field (WKF), towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin are presented. The lamprophyre is characterised by a panidiomorphic-porphyritic texture imparted by clinopyroxene, olivine and biotite set in a groundmass of feldspar and spinel. Olivine occurs as the microphenocrysts with a composition range of Fo87-78. Clinopyroxenes display reverse as well as oscillatory optical zoning and are diopsidic in nature with a variation in the composition from core (Wo47 En28 Fs20Ac5) to rim (Wo46En41Fs11Ac3). Biotite (Mg# < 0.6) is the only mica present and spinels are titano-magnetites showing ulvospinel- magnetite solid solution. Plagioclase is the dominant feldspar with a variable compositional range of An41-8Ab82-56Or33-3. Based on the mineralogy, the lamprophyre can be classified to be of calc-alkaline variety but its geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. K2O/Na2O ranges from 1.49 to 2.79, making it distinctly potassic and highlights its shoshonitic character. Moderate Mg# (60-65), Ni (110-200 ppm) and Cr (110-260 ppm) contents in the bulk-rock indicate substantial fractional crystallization of olivine and clinopyroxene. Fractionated chondrite normalized REE patterns (average (La/Yb)N = 37.56) indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative Ta-Nb-Ti and Hf anomalies displayed on the primitive mantle normalized multi-element spider gram highlight involvement of a subducted component in the mantle source. Given the spatial disposition of the studied lamprophyre, the age of the emplacement is considered to be coeval with WKF kimberlites (~ 1.1 Ga) and the initial 143Nd/144Nd (0.510065-0.510192) and 87Sr/86Sr (0.705333-0.706223) are strikingly similar to those observed for the Smoky Butte lamproites, Montana, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N < 1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.
Journal of the Geological Society of India, Vol. 91, pp. 395-399.
India
Alkaline - Mundwara
Abstract: The occurrence of a rare mantle-derived chrome-diopside megacryst (~8 mm), containing inclusions of olivine, in a lamprophyre dyke from the late Cretaceous polychronous (~100 - 68 Ma) Mundwara alkaline complex of NW India is reported. The olivine inclusions are forsteritic (Fo: 85.23) in composition, and their NiO (0.09 wt%) and CaO (0.13 wt%) contents imply derivation from a peridotitic mantle source. The composition of the chrome diopside (Cr2O3: 0.93 wt ) (Wo45.27 En48.47 Fs5.07 and Ac1.18) megacryst is comparable to that occurring in the garnet peridotite xenoliths found in diamondiferous kimberlites from Archaean cratons. Single pyroxene thermobarometry revealed that this chrome diopside megacryst was derived from a depth range of ~100 km, which is relatively much deeper than that of the chrome-diopside megacrysts (~40-50 km) reported in spinellherzolite xenoliths from the alkali basalts of Deccan age (ca. 66- 67 Ma) from the Kutch, NW India. This study highlights that pre- Deccan lithosphere, below the Mundwara alkaline complex, was at least ~100 km thick and, likely, similar in composition to that of the cratonic lithosphere.
Journal of the Indian Institute of Science, 15p. available in press
India
magmatism
Abstract: We report rare occurrence of lamprophyre xenoliths within a host lamprophyre from the Dongargaon area, Deccan Large Igneous Province, NW India. The lamprophyre xenoliths are distinct in texture (grain size) as well in mineralogy from those of their host rock. The clinopyroxene (diopside) in the xenoliths is depleted in Ca and Mg but substantially enriched in Fe compared to those in the host lamprophyre. Mica in the xenoliths is a phlogopite whereas that present in the host rock is compositionally a biotite; spinels in the host lamprophyre are relatively enriched in TiO2. As the host lamprophyre dyke has been dated to be of Eocene (ca. 55 Ma) age, the entrained lamprophyre xenoliths are inferred to represent an earlier pulse of lamprophyre emplacement. The recurrent lamprophyre emplacement in this domain is consistent with the recently brought out polychronous nature of Late Cretaceous alkaline magmatism at the Mundwara and Sarnu Dandali complexes in the NW India and is related to the extensional events linked with the reactivation of the Narmada rift zone.
Contributions to Mineralogy and Petrology, Vol. 173, doi.org/10.1007/ s00410-018-1493-y 27p.
India
lamproites
Abstract: Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW-ESE to NW-SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K2O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial 87Sr/86Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ?Nd range from ??10.6 to ??9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic-anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history.
Journal of the Geological Society of India, Vol. 93, 2, pp. 157-162.
India
craton
Abstract: The eastern Dharwar craton (EDC) of the southern Indian Shield hosts five geochronologically distinct Paleoproterozoic mafic dyke swarms emplaced at 2.37, 2.21, 2.18, 2.08 and 1.89 Ga. Trace element geochemical data available for these dykes display the ‘arc signals’ viz., negative Nb-Ta anomalies and elevated Zr/Nb, Th/Yb and Th/Ta values, which are conventionally interpreted to represent involvement of subduction in their genesis. It is shown that these ‘arc signals’ resulted from coupled assimilation and fractional crystallization (AFC) processes that modified these mantle-derived melts. Since, mafic dykes under study are highly evolved, an attempt has been made to estimate (using PRIMELTS2.xls software) the composition of the primary magma from the most primitive sample available from the 2.21 and 2.37 Ga swarms. The mantle potential temperature derived from the estimated primary magma compositions revealed anomalously hot mantle source regions compared to the known ambient upper mantle temperatures during Paleoproterozoic, thus implying the possible involvement of thermal plumes in their genesis.
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Abstract: We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034-0.710582) and sub-chondritic initial ?Nd (?8.6 to 2.1) are akin to those of the (i) ca. 65?Ma Ambadongar carbonatite, NW India, and (ii) ca. 65?Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564-961?Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100?km at ca. 65?Ma.
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Precambrian Research, in press available, 17p. Pdf
India
craton
Abstract: The geodynamic evolution of the eastern Dharwar craton, southern India, is widely debated with a number of contrasting models ranging from uniformitarian plate convergence to the mantle plume and their combination. We report here the petrology and geochemistry of two undeformed and unmetamorphosed lamprophyre dykes from the Mudigubba area located immediately towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin from this craton. The Mudigubba lamprophyres are free from crustal xenoliths, and have a typical porphyritic-panidiomorphic texture predominated by phenocrysts of amphibole. Clinopyroxene occurs as microphenocrysts with feldspar essentially confined to the groundmass. F-rich apatite and sphene are the other accessories. Mineral chemistry reveals that the amphiboles are of calcic variety (dominantly magnesio-hornblende), the clinopyroxene to be a diopside (Wo45.01-50.40 En36.74-44.58 Fs6.79-12.73 Ac0.42-2.24) and the albitic (Or1.12 Ab91.17 An7.70) nature of the feldspar. The lower abundance of TiO2 in both the amphibole and clinopyroxene, suggest a calc-alkaline nature of the magma. High Mg# (76.8-79.3), Ni (140-240 ppm) and Cr (380-830 ppm) contents along with (i) depletion in U, and Th, (ii) variable Ba/La and (iii) low Nb/La as well as Th/La strikes out possibility of crustal contamination and supports the primary nature of the lamprophyre magma. The presence of significant Nb-Ta, Zr-Hf and Ti negative anomalies in the primitive mantle normalized multi-element plots and their striking similarity with the global calc-alkaline lamprophyres imply the involvement of subduction-related mantle source modification. Various geochemical ratios (e.g., Hf/Sm, Ta/La, Th/Yb, Nb/Yb, La/Nb, Ba/Nb) demonstrate the source enrichment was caused by a fluid-related, rather than silicate-melt related, subduction metasomatism. Binary-mixing calculations assuming average upper crust and N-MORB as the two end members reveals ?10-30% influx of subducted component in the generation of the Mudigubba lamprophyres. A re-examination of the limited geochemical data available for the co-spatial Paleoproterozoic (2200-1600 Ma) alkaline plutons suggests this Neoarchaean subduction-event in this domain could in fact be a regional feature - all along the western margin of the Cuddapah basin and represents a hitherto unrecognised suture zone in the eastern Dharwar craton with the Paleoproterozoic (?) emplacement of Mudigubba lamprophyres post-dating this collisional event. Our findings provide significant geochemical support to the models invoking convergence towards the evolution of the Eastern Dharwar craton and impose important constraints on the geodynamics of the southern peninsular India.
Abstract: Kimberlites are volatile-rich deep mantle-derived rocks that often contain diamonds. Numerous Grenvillian (ca. 1.1 Gyr) diamondiferous kimberlites, ultramafic lamprophyres, and lamproites are exposed in the Eastern Dharwar Craton and the Bastar Craton, India, and are aligned almost parallel to the Eastern Ghats (granulite) Mobile Belt (EGMB). The trigger for these kimberlite and related magmatic events still remains an open question. We review the available geochronological and radiogenic isotopic data for the ~1.1 Gyr kimberlites, lamproites, and ultramafic lamprophyres from the Eastern Dharwar Craton and the Bastar Craton of the Indian shield. We show that kimberlites and associated magmas were emplaced for a longer duration (ca. 130 Myr) in the Indian shield during the Mesoproterozoic and sampled distinct mantle source regions. The kimberlites and ultramafic lamprophyre are characterized by slightly depleted to chondritic Nd isotopic ratios revealing their origin at deeper sub-lithospheric regions, whereas the lamproites essentially show an enriched Nd isotopic signature suggesting their derivation from enriched sub-continental lithospheric mantle. We argue that the absence of linear age progression, prolonged magmatic activity compared to the time span of coeval large igneous provinces (the Umkondo, the Keweenawan, and the Warakurna) and a cooler ambient mantle as revealed from the entrained xenoliths, constitute important limitations for a plume model earlier proposed for the genesis of these kimberlites and related magmas. These observations together with a geographical and temporal (Grenvillian) link to the EGMB points towards edge-driven convection as a trigger for kimberlite magmatism- similar to the model proposed for the Mid-Cretaceous kimberlite corridor in North America. However, this model can't explain the coeval formation of sub-continental lithospheric mantle-derived lamproites. As the timing of kimberlite and related magmatism coincides with that of the Grenvillian orogeny and succeeded a magmatic lull of ~360 Myr in the Dharwar Craton during the Mesoproterozoic, we instead, propose that small scale partial melting of heterogeneous mantle caused by plate reorganization during Columbia to Rodinia supercontinent extroversion served as a trigger for this ca. 1.1 Gyr magmatism in the southeastern Indian shield.
Proceedings Natural Science Academy, Vol. 86, 1, pp. 301-311.
India
kimberlite, lamproites
Abstract: Highlights of researches on kimberlites, lamproites and lamprophyres (and their entrained xenoliths) during 2016-2019 from the Indian context are presented. A few previously unknown occurrences have been brought to light, and a wealth of petrological, geochemical and isotopic data on these rocks became available. All these studies provided new insights into the nomenclatural as well as geodynamic aspects such as subduction-tectonics, mantle metasomatism, lithospheric thickness, supercontinent amalgamation, and break-up and nature of the sub-continental lithospheric mantle from the Indian shield.
Abstract: The Eastern Dharwar Craton (EDC) of the southern Indian shield host numerous Mesoproterozoic (~1100 Ma) kimberlite fields. Shoshonitic lamprophyre dykes synchronous to these kimberlites are exposed in the diamondiferous Wajrakarur kimberlite field (WKF). These lamprophyre dykes are characterized by the presence of olivine, biotite and zoned clinopyroxene phenocrysts set in a groundmass of feldspar, spinel and apatite. High K2O/Na2O (1.5-3.4) along with elevated Th content (2.2-8.6) in these dykes reveal their shoshonitic affinity. High Th/Yb (1.5-4.1), Nb/Yb (23.8-59.0) and superchondritic Zr/Hf ratios (40-50.4) similar to the oceanic island basalts rules out crustal assimilation and highlights the role of an enriched mantle source in their genesis. Age corrected bulk-rock ?Nd of the shoshonitic lamprophyres vary between -13.73 and -22.90, whereas the initial 87Sr/86Sr ranges between and 0.70533-0.71218 suggesting their derivation from an enriched lithospheric mantle. However, the coeval kimberlites of the WKF have distinct Sr-Nd isotopic composition with bulk-rock ?Nd and initial 87Sr/86Sr varying from 0.44 to 2.75 and 0.70209 to 0.70744, respectively, similar to that of the groundmass perovskite ?Nd (2.08-2.92) and initial 87Sr/86Sr (0.70234- 0.70255). Since, these kimberlites tap an isotopically depleted mantle source, unlike the shoshonitic lamprophyres, the geochemistry of these rocks points to a vertically heterogeneous lithospheric mantle beneath the EDC. We suggest that the Wajrakarur kimberlites generated from a deeper depleted mantle (>150 km) compared to the shallower enriched lithospheric mantle source for the shoshonitic lamprophyres. Neoarchean (2.6-2.8 Ga) Nd depleted mantle model ages for these lamprophyres suggest that the subcontinental lithospheric mantle was enriched during Neoarchean accretion-related evolution of the Dharwar Craton.
Journal of Mineralogical and petrological Sciences, Vol. 115, 2, pp. 202-215. pdf
India
deposit - Wajrakarur
Abstract: Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic-panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite-tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc-alkaline variety whereas their bulk-rock geochemistry portrays mixed signals of both alkaline as well as calc-alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic-anorogenic transitional type mantle source) from eastern Dharwar craton. Trace element ratios imply melt-derivation from an essentially the garnet bearing-enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and ‘enriched’ ?Ndinitial (?19.1 and ?24.2) values. The calculated TDM ages (2.7-2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co-spatial and co-eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc-alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence for highly heterogeneous nature of the sub-continental lithospheric mantle beneath the eastern Dharwar craton.
Journal of Earth System Science *** note date 2006 , Vol. 115, 1, Feb. pp. 161-183. pdf
India
deposit - Panna
Abstract: Amongst all the perceptible igneous manifestations (volcanic tuffs and agglomerates, minor rhyolitic flows and andesites, dolerite dykes and sills near the basin margins, etc.) in the Vindhyan basin, the two Mesoproterozoic diamondiferous ultramafic pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous but also well-known and have relatively deeper mantle origin. Hence, these pipes constitute the only yet available ‘direct’ mantle samples from this region and their petrology, geochemistry and isotope systematics are of profound significance in understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin. Their emplacement age (? 1100 Ma) also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks. The Majhgawan and Hinota pipes share the petrological, geochemical and isotope characteristics of kimberlite, orangeite (Group II kimberlite) and lamproite and hence are recognised as belonging to a ‘transitional kimberlite-orangeite-lamproite’ rock type. The namemajhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks. The parent magma of the Majhgawan and Hinota pipes is envisaged to have been derived by very small (<1%) degrees of partial melting of a phlogopite-garnet lherzolite source (rich in titanium and barium) that has been previously subjected to an episode of initial depletion (extensive melting during continent formation) and subsequent metasomatism (enrichment). There is absence of any subduction-related characteristics, such as large negative anomalies at Ta and Nb, and therefore, the source enrichment (metasomatism) of both these pipes is attributed to the volatile- and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere. Lithospheric/crustal extension, rather than decompression melting induced by a mantle plume, is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes. This paper is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar occurrences elsewhere.
Abstract: Lamprophyres are some of the oldest recognized alkaline rocks and have been studied for almost the last 150 years. Known for hosting economic minerals such as gold, diamond and base metals, they are also significant in our understanding of the deep-mantle processes (viz., mantle metasomatism and mantleplume-lithosphere interactions) as well as large-scale geodynamic processes (viz., subduction-tectonics, supercontinent amalgamation and break-up). The Indian shield is a collage of distinct cratonic blocks margined by the mobile belts and manifested by large igneous provinces (LIPs) such as the Deccan. A plethora of lamprophyres, varying in age from the Archaean to the Eocene, with diverse mineralogical and geochemical compositions, are recorded from the Indian shield and played a key role in clarifying the tectonic processes, especially during the Paleo- and Mesoproterozoic and the Late Cretaceous. A comprehensive review of the occurrence, petrology, geochemistry and origin of the Indian lamprophyres is provided here highlighting their tectonomagmatic significance. The relationship of the lamprophyres to the Kimberlite clan rocks (KCRs), focusing on the Indian examples, is also critically examined.
Abstract: We report a 40Ar/39Ar Mesoproterozoic radiometric age for a calc-alkaline lamprophyre dyke from the Mudigubba area towards the western margin of the Cuddapah Basin, Eastern Dharwar Craton (EDC), Southern India. Amphibole phenocryst separates from this lamprophyre yielded a plateau age of 1169 ± 8 Ma (2? ), which is almost 50 million years older than the majority of radiometric dates available for the Wajrakarur field kimberlites which are proximal to this dyke. Bulk-rock Sr-Nd isotopic analyses of the Mudigubba lamprophyre dykes (?Nd(t) between -13.3 and -12.4) reveal their derivation from an old, enriched, continental lithospheric mantle unlike the kimberlites (bulk-rock and perovskite in situ ?Nd(t) between -0.77 and +7.93), which originated either from a chondritic or depleted mantle source. This study provides further evidence for emplacement of compositionally distinct, mantle-derived Mesoproterozoic alkaline magmas in the EDC and highlights the extremely heterogeneous character of the lithospheric mantle beneath this craton.
Geological Society, London, Special Publication , 10.1144/SP513- 2020-247 53p. Pdf
India
lamproites
Abstract: We report Mesoproterozoic 40Ar-39Ar (whole-rock) ages of lamproites from (i) the Ramadugu field (R4 dyke : 1434 ± 19 Ma and R5 dyke: 1334 ± 12 Ma) and the Krishna field (Pochampalle dyke: 1439 ± 3 Ma and Tirumalgiri dyke: 1256 ± 12 Ma) from the Eastern Dharwar Craton (EDC) and (ii) the Garledinne (1433 ± 8 Ma) and the Chelima (1373 ± 6 Ma) dykes from within the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The ages reported for the Ramadugu and Tirumalgiri lamproites constitute their first radiometric dates. Ages of the Pochampalle and the Chelima lamproites from this study are broadly comparable to their previously reported 40Ar-39Ar (phlogopite) ages of c. 1500 Ma and 1418 ± 8 Ma, respectively. The ages of all these lamproites are much older than those of the (i) c. 1.1 Ga kimberlites from the Wajrakarur and Narayanpet fields of the EDC and (ii) c. 1.09 Ga lamproitic dykes at Zangamarajupalle which intrude the Cumbum Formation of the Cuddapah Basin. However, the age of the Tirumalgiri lamproite (c. 1256 Ma) is similar to that of the Ramannapeta lamproite (c. 1224 Ma) within the Krishna field. Our study provides evidence for protracted ultrapotassic (lamproitic) magmatism from c. 1.43 to 1.1 Ga over a widespread area (c. 2500 km2) in and around the Cuddapah Basin and the EDC. Implications of the obtained new ages for the diamond provenance of the Banganapalle Conglomerates, the age of the Kurnool Group and for the timing of break-up of the Paleo-Mesoproterozoic supercontinent of Columbia/Nuna are explored.
Raghuvanshi, S., Sharma, A., Talukdar, D., Chalapathi Rao, N.V.
Chrome-diopside xenocrysts entrained in a Neoproterozoic lamprophrye dyke from the Mysuru area: their origin and implications or lithospheric thickness beneath the Western Dharwar craton, southern India.
Journal of Geological Society of India, in press available 12p. Pdf
Geological Society of London Special Publication 513, pp. 1-16.
Global
lamproites
Abstract: Proterozoic to Cenozoic lamprophyres, lamproites and related rock types hold a unique potential for the investigation of processes affecting mantle reservoirs. They originated from primary mantle-derived melts that intruded both cratons and off-craton regions, which were parts of former supercontinents - Columbia, Rodinia and Gondwana-Pangaea. Well known for hosting economic minerals and elements such as diamonds, base metals, platinum-group elements and Au, they are also significant for our understanding of deep-mantle processes, such as mantle metasomatism and mantle plume-lithosphere interactions, as well as large-scale geodynamic processes, including subduction-related tectonics and supercontinent amalgamation and break-up. This Special Publication presents an overview of the state of the art and recent advances as achieved by individual research groups from different parts of the world, and outlines future research directions. Mineralogical, geochemical, geochronological and isotope analyses are used to decipher the complex petrogenetic and metallogenetic evolution of these extraordinary rocks and unravel a complete history of tectonic events related to individual supercontinent cycles. The Special Publication including this introductory chapter also deals with some issues related to the classification of these rocks.
Geological Society of London Special Publication 513, pp. 157-178.
India
lamproites
Abstract: We report Mesoproterozoic 40Ar-39Ar (whole-rock) ages of lamproites from (i) the Ramadugu field (R4 dyke : 1434 ± 19 Ma and R5 dyke: 1334 ± 12 Ma) and the Krishna field (Pochampalle dyke: 1439 ± 3 Ma and Tirumalgiri dyke: 1256 ± 12 Ma) from the Eastern Dharwar Craton (EDC) and (ii) the Garledinne (1433 ± 8 Ma) and the Chelima (1373 ± 6 Ma) dykes from within the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The ages reported for the Ramadugu and Tirumalgiri lamproites constitute their first radiometric dates. Ages of the Pochampalle and the Chelima lamproites from this study are broadly comparable to their previously reported 40Ar-39Ar (phlogopite) ages of c. 1500 Ma and 1418 ± 8 Ma, respectively. The ages of all these lamproites are much older than those of the (i) c. 1.1 Ga kimberlites from the Wajrakarur and Narayanpet fields of the EDC and (ii) c. 1.09 Ga lamproitic dykes at Zangamarajupalle which intrude the Cumbum Formation of the Cuddapah Basin. However, the age of the Tirumalgiri lamproite (c. 1256 Ma) is similar to that of the Ramannapeta lamproite (c. 1224 Ma) within the Krishna field. Our study provides evidence for protracted ultrapotassic (lamproitic) magmatism from c. 1.43 to 1.1 Ga over a widespread area (c. 2500 km2) in and around the Cuddapah Basin and the EDC. Implications of the obtained new ages for the diamond provenance of the Banganapalle Conglomerates, the age of the Kurnool Group and for the timing of break-up of the Paleo-Mesoproterozoic supercontinent of Columbia/Nuna are explored.
Journal of the Geological Society of India, Vol.. 98, 12p. Pdf
India
craton - Dharwar
Abstract: In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2-1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895-1026 °C (± 30 °C) and 32-38 kbar respectively and correspond to depth range of 106-127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ?115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
Abstract: Lamprophyres from the greenstone belts play a crucial role in deciphering tectonic and geodynamic processes operating during the Archean. This study presents a comprehensive mineralogical and geochemical study of three lamprophyre dykes with calc-alkaline to shoshonitic affinities from the Neoarchean Kadiri schist belt, eastern Dharwar craton, southern India. These rocks display porphyritic-panidiomorphic texture, typical of the lamprophyres with amphibole (magnesio-hornblende) as phenocrysts, biotite as microphenocrysts and feldspar, epidote, titanite and apatite confined to the groundmass. Alteration of biotite to chlorite is observed along with mild deformation in the amphibole phenocrysts. Based on mineralogy and major oxide geochemistry, these rocks are classified as the calc-alkaline lamprophyres. Higher Ba/Nb and low Nb/La points to their derivation from an enriched lithospheric mantle source and higher Th/Yb ratio along with negative TNT (Ti-Nb-Ta) and Zr-Hf anomalies on the primitive mantle (PM) normalized multi-element diagram indicates dehydrated fluids from the foundering slab could be the possible metasomatic agent. Fractionated HREE ratios (GdN/YbN >1.9) and higher SmN/YbN suggests that the source region lies in the garnet stability field. Higher than PM Rb/Sr along with positive correlation between K/La and Rb/La reveals presence of metasomatic phlogopite in the source region. Strong negative initial ?Nd along with radiogenic 87Sr/86Sr ratios further support an enriched mantle reservoir involved in their genesis. Non-modal batch melting (1-5%) of a mixed source (phlogopite-garnet peridotite) assuming 5% mixing of subducted sediment with ambient mantle wedge (depleted mantle) satisfies the multi-element concentration pattern shown by the Kadiri lamprophyres. The source enrichment can be linked to the accretion-related growth of Dharwar craton and its schist belts during Neoarchean. Our study shows that a majority of lamprophyres associated with the Archean greenstone belts display a shoshonitic character; this highlights the role of subduction-related processes in the growth and evolution of the greenstone belts .
Journal of the Geological Society of India, Vol. 98, 12p. Pdf
India
chrome-diopside
Abstract: In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2-1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895-1026 °C (± 30 °C) and 32-38 kbar respectively and correspond to depth range of 106-127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ?115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
Abstract: We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bol?´var mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E-W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative ( 20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.
A new model for the Paleogene motion of Greenland relative to North America: plate reconstructions of the Davis Strait and Nares Strait regions between Canada and Greenland.
Journal of Geophysical Research, Vol. 117, B 10, B10401.
Earth and Planetary Science Letters, Vol. 433, pp. 360-369.
Mantle
Geophysics - seismics
Abstract: In regions of the mantle where multi-phases coexist like at the olivine-wadsleyite-ringwoodite transitions, the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases, a possible source of dissipation. In such a situation, the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave. Here we report synchrotron-based measurements of the kinetics of the olivine to ringwoodite transformation at pressure-temperature conditions of the co-stability loop, for iron-rich olivine compositions. Both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries. Transformation-time data show an increase of reaction rates with temperature and iron content, and have been fitted to a rate equation for interface-controlled transformation: G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)]G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)], where XFaXFa is the fayalite fraction, the exponential factor n=9.7n=9.7, View the MathML sourceln?k0=?9.1 ms?1. View the MathML sourceXFa?1 and ?Ha=199 kJ/mol?Ha=199 kJ/mol, assuming V?=0 cm3/molV?=0 cm3/mol. Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict View the MathML sourceQK?1 and View the MathML sourceQ??1 significantly higher than the PREM values for both body waves and normal modes. This attests that the olivine-wadsleyite transition can significantly contribute to the attenuation of the Earth's mantle transition zone.
New uranium-lead (U-Pb) (U-Pb) geochronological and thermobarometric constraints on Proterozoic tectonic processes along the southeast margin of the Wyoming craton
Geological Society of America Annual Meeting Abstract Volume, Vol. 23, No. 5, San Diego, p. A 59
Abstract: Precambrian supercontinents Nuna-Columbia (1.7 to 1.3 billion years ago) and Rodinia (1.1 to 0.7 billion years ago) have been proposed. However, the arrangements of crustal blocks within these supercontinents are poorly known. Huge, dominantly basaltic magmatic outpourings and intrusions, covering up to millions of square kilometres, termed Large Igneous Provinces, typically accompany (super) continent breakup, or attempted breakup and offer an important tool for reconstructing supercontinents. Here we focus on the Large Igneous Province record for Siberia and Laurentia, whose relative position in Nuna-Columbia and Rodinia reconstructions is highly controversial. We present precise geochronology—nine U -Pb and six Ar -Ar ages—on dolerite dykes and sills, along with existing dates from the literature, that constrain the timing of emplacement of Large Igneous Province magmatism in southern Siberia and northern Laurentia between 1,900 and 720 million years ago. We identify four robust age matches between the continents 1,870, 1,750, 1,350 and 720 million years ago, as well as several additional approximate age correlations that indicate southern Siberia and northern Laurentia were probably near neighbours for this 1.2-billion-year interval. Our reconstructions provide a framework for evaluating the shared geological, tectonic and metallogenic histories of these continental blocks.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America’s assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Kilian, T.M., Bleeker, W., Chamberlain. K., Evans, D.A.D., Cousens, B.
Paleomagnetism, geochronology and geochemistry of the Paleoproterozoic Rabbit Creek and Powder River dyke swarms: implications for Wyoming in supercraton Superia.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 15-45.
Geochemistry, Geophysics, Geosystems, Vol. 20, 4, pp. 2091-2109.
Africa
geophysics
Abstract: In Ethiopia, the African Continent is rifting apart to slowly form a new ocean basin, which will expand the Red Sea and the Gulf of Aden. How and why this rifting is occurring remains an important unanswered question in earth science. We know tectonic forces are partly responsible, but magmatism also seems a key ingredient for breaking up Africa. Here we use seismic images obtained from signals pulled out of noise, to understand the crustal structure of the region; In particular, how and where magma is stored in the crust, and its relationship to the different stages of continental breakup visible in the region. We find evidence for long?term melt storage in places where rifting is just beginning in southern Ethiopia; whereas in regions where the crust is thinner due to extensive rifting, magma erupts more regularly. The long?term storage of magma in unrifted crust may help to heat and weaken it, allowing rifting to accelerate and propagate further south. We are also able to image regions with hydrothermal fluids in the shallow parts of the crust in inactive fault zones. These results provide insight into the breakup process and the role magma plays at different stages of rifting.
Van Kranendonk, M.J., Hugh Smithies, R., Hickman, A.H., Champion, D.C.
Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia.
Abstract: Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5?GPa (>50?km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40?km average thickness of modern continental crust.
Abstract: White diamonds color grading is one of the basic diamond evaluations. The color value based on a scale that ranges from D to Z, with D being the more colorless and more valuable, among other qualifications. As the diamond grade moves on this scale, its color appears more yellow progressively. This yellowish color, present only in Type I diamonds, is mainly due to the nitrogen related defects such as N3 center and C-center. The current color grading system is based on a visual method, where gemologist compares the sample with a Master Color set. However, this method is very subjective. Several defects responsible for light absorption in diamond are carrying electron spin and appear in Electron Paramagnetic Resonance (EPR) spectrum. In this study, we developed a new EPR based technique for a quantitative measurement of N3 center and C-center in diamond through quantitative EPR spectroscopy. The correlation between EPR spectra and color grades of diamond was established.
Abstract: Uncertainty exists in the configuration and extent of the Midcontinent Rift System (MRS) because of deficiencies in geophysical data and limited information from outcrops and basement drill holes. Additional ambiguity is caused by misunderstanding the definition of continental rifts. Six principal problematic regions in mapping the MRS are described. Gravity and magnetic data, supported by drill hole and seismic reflection data, show that the Eastern Lake Superior rift segment of the MRS continues south from Lake Superior and connects to a much narrower rift in northern Lake Michigan. The eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data, but is interpreted to occur near the U.S. - Canada border. The rift segment in southeastern Michigan intersects the Grenville Front and likely continues eastwards in modified form to near the boundary with Canada. Cross-cutting gravity and magnetic signatures may reflect Grenvillian overthrusts near the terminus of the MRS in Michigan. The proposed southerly extensions of both branches of the rift system into Oklahoma and Ohio are based primarily on positive gravity anomalies, but neither postulated extension appears to be associated with rifted troughs. Rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by Grenville orogenesis. Recent paleomagnetic investigations, in conjunction with high-resolution radiometric dating, imply that the MRS developed during the rapid southward movement of Laurentia during a quiescent period along its eastern continental margin. Massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin. The heated crust was made more ductile, fostering rifting due to extensional stresses. The Nipigon Embayment remains as a possible candidate for an early "third branch" of the MRS, but current evidence is insufficient to include the Fort Wayne "rift" as part of the MRS. Future studies of the MRS would be well-served by new age-dating and high-resolution seismic studies of the lithosphere.
Precambrian Research, in press available, 43p. Pdf
United States, Iowa
geophysics - seismics
Abstract: Large amplitude aeromagnetic and gravity anomalies over a ~9500 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.
Abstract: Large amplitude aeromagnetic and gravity anomalies over a ~9500 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.
Geological Society of London Special Publication, No. 463, pp. 137-169.
India
carbonatite
Abstract: The genetic connection between Large Igneous Province (LIP) and carbonatite is controversial. Here, we present new major and trace element data for carbonatites, nephelinites and Deccan basalts from Amba Dongar in western India, and probe the linkage between carbonatite and the Deccan LIP. Carbonatites are classified into calciocarbonatite (CaO, 39.5-55.9 wt%; BaO, 0.02-3.41 wt%; ?REE, 1025-12 317 ppm) and ferrocarbonatite (CaO, 15.6-31 wt%; BaO, 0.3-7 wt%; ?REE, 6839-31 117 ppm). Primitive-mantle-normalized trace element patterns of carbonatites show distinct negative Ti, Zr-Hf, Pb, K and U anomalies, similar to that observed in carbonatites globally. Chondrite-normalized REE patterns reveal high LREE/HREE fractionation; average (La/Yb)N values of 175 in carbonatites and approximately 50 in nephelinites suggest very-low-degree melting of the source. Trace element modelling indicates the possibility of primary carbonatite melt generated from a subcontinental lithospheric mantle (SCLM) source, although it does not explain the entire range of trace element enrichment observed in the Amba Dongar carbonatites. We suggest that CO2-rich fluids and heat from the Deccan plume contributed towards metasomatism of the SCLM source. Melting of this SCLM generated primary carbonated silicate magma that underwent liquid immiscibility at crustal depths, forming two compositionally distinct carbonatite and nephelinite magmas.
Journal of Petrology , Vol. 60, 6, pp. 1119--1134.
India
carbonatite
Abstract: There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr-Nd-Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan-Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (?2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.
Abstract: The alkaline rocks and carbonatites (ARCs) of the Great Indian Proterozoic belt bear the testimony of tectonic processes operating in the Proterozoic during the continental assembly and breakup of both Columbia and Rodinia. We present a comprehensive review, mainly focused on the petrology, geochemistry, and geochronology of 38 ARCs of Peninsular India, which are mostly concentrated within the Eastern Ghats Mobile Belt and Southern Granulite Terrain. Available geochronologic data reveals three distinct alkaline magmatic phases (2533-2340 Ma, 1510-1242 Ma, 833-572 Ma) and two metamorphic events (950-930 Ma and 570-485 Ma) that correlate with the Grenvillian and Pan-African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seems to have affected the nephelinite, nepheline syenite and syenite, carbonatite is affected by fractionation of calcite, dolomite, ankerite, pyroxene, apatite, magnetite, mica, and pyrochlore. Trace elements and Sr-Nd-Pb-C-O isotopic compositions of these ARCs strongly suggest a subcontinental lithospheric mantle source, that is enriched either by distribution of subducted crustal material or by metasomatism of mantle-derived fluids, for the generation of ARCs. Despite some isotopic variability that can result from crustal contamination, a trend showing enrichment in 87Sr/86Sri (0.702 to 0.708) and depletion in ?Nd(i) (-1.3 to -14.1) over a 2 Gyr duration indicates temporal changes in the lithospheric/asthenospheric source of ARCs, due to periodic enrichment of the source by mantle-derived fluids. ARC generation starts in an intracontinental rift setting (beginning of Wilson cycle). These early-formed ARCs are carriedto 100 km depths during continental collision (termination stage of Wilson cycle) and undergo extensive melting because of renewed rifting along suture zones to form new generation of ARCs.
Abstract: The extraction of geological lineaments from digital satellite data is a fundamental application in remote sensing. The location of geological lineaments such as faults and dykes are of interest for a range of applications, particularly because of their association with hydrothermal mineralization. Although a wide range of applications have utilized computer vision techniques, a standard workflow for application of these techniques to mineral exploration is lacking. We present a framework for extracting geological lineaments using computer vision techniques which is a combination of edge detection and line extraction algorithms for extracting geological lineaments using optical remote sensing data. It features ancillary computer vision techniques for reducing data dimensionality, removing noise and enhancing the expression of lineaments. We test the proposed framework on Landsat 8 data of a mineral-rich portion of the Gascoyne Province in Western Australia using different dimension reduction techniques and convolutional filters. To validate the results, the extracted lineaments are compared to our manual photointerpretation and geologically mapped structures by the Geological Survey of Western Australia (GSWA). The results show that the best correlation between our extracted geological lineaments and the GSWA geological lineament map is achieved by applying a minimum noise fraction transformation and a Laplacian filter. Application of a directional filter instead shows a stronger correlation with the output of our manual photointerpretation and known sites of hydrothermal mineralization. Hence, our framework using either filter can be used for mineral prospectivity mapping in other regions where faults are exposed and observable in optical remote sensing data.
Acta Geologica Sinica, Vol. 90, July abstract p. 191.
India
Deposit - Lattavaram
Abstract: Utility of geochemistry in mineral exploration is known since more than half-a-century. In reconnaissance diamond exploration, regolith geochemistry is a well known tool worldwide and helps in distinguishing bedrock geology in hard rock terrains. More than 100 kimberlite pipes were discovered so far in the Eastern Dharwar Craton (EDC) of South India by various public and private organizations. Within the EDC, majority of diamondiferous pipe clusters occur in Anantapur District of Andhra Pradesh state in India. Lattavaram kimberlite cluster (LKC) is one among them in this district and four pipes in this cluster are reported to be diamondiferous along with two kimberlite bodies at Muligiripalli. The kimberlite pipes at Lattavaram occur in semi-circular shape whereas a kimberlite body exposed at Muligiripalli village occurs as a dyke within the granitoid country. The pipe 4 at Lattavaram exhibits conspicuous difference in the soil color and texture i.e., greenish color and mottled texture when compared with the reddish brown compact residual soils of granitic country rock. In the Indian context, soil regolith geochemistry is not a popular subject in search of kimberlites. It is observed that little or no literature is available on the utility of regolith geochemistry applied to kimberlite or diamond exploration in India. An attempt is made in this study, to demonstrate and understand the spatial surface geochemical signatures using residual soil geochemistry on known kimberlite pipes viz., 3 and 4 of LKC. Spatial and statistical analysis of trace and rare earth elements revealed that certain elements show predominance in the vicinity of the kimberlite pipes which can act as an exploration guide in distinguishing kimberlitic rocks within a granitoid country. These elements show distinct variation in their dispersion in the soil which can be attributed to basement lithology. It is observed that elements like Cr, Ni, Co, Cu, Nb, Zr, Ti, Ba and rare earth elements (REE) are significantly enriched in the mottled zone and calcretized duricrust relative to the country rock granitoids. A suite of trace elements comprising those associated with ultramafic rocks (Cr, Co & Ni) and felsic rocks (Nb, La, Sm and P) can readily distinguish the ultramafic/kimberlitic regolith from that derived from granitoid or felsic rocks which can be used as an exploration path finder.
Cr-diopsides from Lattavaram and Kalyanadurgam kimberlites, Anatapur district, Andhra Praseh, southern India: inferences from loam sampling.
Department of Geophysics, University College of Science Osmania University, Hyderabad 500 007, March 16, 17, Role of Geophysics in Earth and Environmental studies: special emphasis on mineral exploration 1p. Abstract
National Seminar on Strategic trends and future perspectives in the development of natural resources of Telangana state, Kakatiya University, Abstract Volume, 1, March 30,31 pp. 18-19.
India, Telangana
Lamproites
Abstract: The authigenic carbonates which occur in arid and semi-arid regions of the world are commonly referred to as calcretes or caliche or kankar. These are pedogenic calcretes which occur in association with soil forming the residual regolith. Many rock types produce calcretes upon weathering and denudation, but calcrete derived from certain rocks acts as an exploration guide. Calcrete is a prominent sampling medium in countries like Australia and South Africa whereas it is not so popular in the Indian context. Kimberlites, being ultrapotassic in nature and owing to the enrichment of olivine, serpentine an calcite, often produce calcrete duricrust as a capping on the outcrops. The calcretes derived from kimberlites contain relict kimberlitic xenocrystic minerals like pyrope, ilmenite, Cr-diopside, pseudomorphs of olivine, phlogopite etc. unlike those derived from other rock types. The calcretes derived from granitoid rocks significantly contain minerals like chert, quartz, semi-weathered feldspar etc. Recently more than fifteen lamproites have been discovered at Vattikodu and Chintalapalli and one lamprophyre at Bayyaram of Telangana state, by the Geological Survey of India, unraveling new panorama that the state has a substantial potential for occurrence of more kimberlite clan rocks. Perhaps for the first time, an attempt has been made here to test the geochemical affinity of calcretes from various locations within Nalgonda district, which is endowed mostly with granitic terrain and Cuddapah sedeimentaries in the southern part. About sixteen samples have been collected from the in-situ regolith, spread in the granite-mafic dyke terrain, with an omission of calcretes occurring in transported black soil areas. The samples were geochemically analysed for major and trace elements for a preliminary study. The data has been compared with published geochemical data of lamproites of Ramadugu Field, to understand their geohchemical association. The calcretes are low in SiO2 (33.92-45.1wt %), high in K2O (1.07-2.21wt %) and CaO (0.78-13.61wt %). When compared to other major elements, MgO displays low concentration and K2O has a higher concentration than Na2O. The trace elements are found to be enriched in some of the samples collected in close vicinity of known lamproite occurrences. The samples show a high degree of chemical weathering, alteration and compositional variation indices. It is observed that enrichment of elements like Cr, Nb, Ni indicates, similar to parent kimberlite/lamproite rock, favourable targets for further ground exploration in virgin areas. In the present study, two samples, towards five kilometers southeast of Vattikodu Lamproite Field, possess higher Nb (>25ppm) concentration, which stand out as explorable targets for further ground investigations. Further field investigations such as geological mapping, pitting, petrography and geochemistry on these two locations are in progress to ascertain whether or not these two targets unveil new kimberlites/lamproites in the area.
Journal of Asian Earth Sciences, in press available, 11p.
India
geophysics - seismics
Abstract: The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60?km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500?°C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95?Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart from some recently discovered similar subsurface Gondwana occurrences in intracratonic parts, would indicate that Dharwar Craton was rifting even during Gondwana period, thereby challenging the long held view of cratonic stability.
Journal of Asian Earth Sciences, Vol. 157, pp. 302-316.
India
geophysics - seismics
Abstract: The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60?km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500?°C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95?Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart from some recently discovered similar subsurface Gondwana occurrences in intracratonic parts, would indicate that Dharwar Craton was rifting even during Gondwana period, thereby challenging the long held view of cratonic stability.
Geochemistry, Geophysics, Geosystems, Vol. 20, 2, pp. 1189-1201.
Mantle
subduction
Abstract: Tectonic plates plunge into the mantle at trenches, carrying water from the oceans. Some of this water may go down to the mantle transition zone between 410? and 660?km depth, where minerals have a large water storage capacity. In this study, we use observations of seismic anisotropy, the directional dependency of seismic wave speed, which is sensitive to the water content in the mantle transition zone. We find that the mantle transition zone beneath some subduction zones is drier than previously thought.
Geostandards and Geoanalytical Research, in press available, 21p.
Asia, Mongolia
olivine
Abstract: A new olivine reference material - MongOL Sh11?2 - for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn?Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5-2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA?ICP?MS, SIMS and bulk analytical methods (ID?ICP?MS for Mg and Fe, XRF, ICP?MS) for major, minor and trace elements at six institutions world?wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1-2). The presence of some mineral and fluid?melt micro?inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty?seven major, minor and trace elements.
Abstract: The Epembe Alkaline Carbonatite Complex (EACC) in northwestern Namibia was emplaced along a fault zone into medium- to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC), and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Nepheline syenite with minor syenite constitute the main lithologies, cross-cut by a calcite?carbonatite dyke. Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were studied using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189-44,100 ppm) with light rare-earth element (LREE) enrichment (LaN/YbN = 4-91) relative to heavy (H) REE consistent with a magmatic origin. Negative Eu anomalies (Eu/Eu* = 0.4-0.9) in syenite apatite are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite and carbonatite-hosted apatite is also commonly homogeneous in CL, while core-rim zoning and patchy textures are observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (LaN/YbN = 24-9) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by rim-ward increase in REE concentrations, which can be attributed to mineral fractionation. Patchy apatite is depleted in Na, Y and REE, particularly the LREE (LaN/YbN = 4-19) relative to other nepheline syenite apatite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the alteration fluids. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. An LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages are 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs <4.0, 2 SE). The Sr and Nd isotopic composition of apatite in syenite (87Sr/86Sr(i) = 0.7035-0.7048; ?Nd(t) = +2.5 to +3.2), nepheline syenites (87Sr/86Sr(i) = 0.7031-0.7037; ?Nd(t) = +1.5 to +4.4) and carbonatite (87Sr/86Sr(i) = 0.7031-0.7033; ?Nd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous source, located in the sub-lithospheric mantle.
Abstract: Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use three-dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660?km seismic discontinuity slabs are associated with fast SV anomalies up to about 3%, in the lower mantle fast SH anomalies of about 2% persist near slabs down to about 1,000-1,200?km. These observations are consistent with 3D numerical models of deformation from subducting slabs and the associated lattice-preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth’s lower mantle, providing new constraints on the debated nature of deformation in this key, but inaccessible, component of the deep Earth.
Journal of Geophysical Research,, Vol. 120, 12, pp. 8259-8280.
Mantle
Ringwoodite
Abstract: Review of recent mineral physics literature shows consistent trends for the influence of Fe and H2O on the bulk modulus (K0) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410-660?km). However, there is little consensus on the first pressure derivative, K0??=?(dK/dP)P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K0? in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H2O on K0? for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo90 wadsleyite containing 2.0 and 0.25?wt?% H2O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4?wt?% H2O. By measuring their pressure-volume evolution simultaneously up to 32?GPa, we constrain the difference in K0? independent of the pressure scale, finding that H2O has no effect on K0?, whereas the effect of H2O on K0 is significant. The fitted K0? values of hydrous wadsleyite (0.25 and 2.0?wt?% H2O) and hydrous ringwoodite (1.4?wt?% H2O) examined in this study were found to be identical within uncertainty, with K0? ~3.7(2). New secondary-ion mass spectrometry measurements of the H2O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5)?GPa/wt?% H2O, independent of Fe content for upper mantle compositions. Because K0? is unaffected by H2O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high-resolution, regional seismology studies.
Journal of African Earth Sciences, Vol. 113, pp. 35-50.
Africa, Cameroon
Geochemistry
Abstract: Trace element abundances in three indicator minerals (corundum, chrysoberyl, and zircon grains) from the western Mamfe gem placers, as determined by LA-ICP-MS analytical techniques, are shown to be sensitive to their crystallization conditions and source rock types. Corundum is dominantly composed of Al (standardized at 529,300 ppm), Fe (2496-12,899 ppm), and Ti (46-7070 ppm). Among element ratios, Fe/Mg (73-1107), Fe/Ti (0.5-245.0), Ti/Mg (1-175), and Ga/Mg (4-90) are generally higher whereas, Cr/Ga (<0.072) is low. The Fe (?12,899), Ga (?398), Mg (2-62), Cr (1.1-33.0), and V (3.0-93.0) contents (in ppm) mostly typify corundum grains formed in magmatic rocks, although some are metamorphic affiliated. A very higher Ti and significantly low Ga, Ta and Nb contents in some blue grains, suggest interesting concentrations of those high-tech metals in their source rocks. Chrysoberyl is dominantly composed of Al (standardized at 425,000 ppm) and Be (62701-64371 ppm). Iron (7605-9225 ppm), Sn (502-3394 ppm), and Ti (33-2251 ppm) contents are high, whereas Ga (333-608 ppm), Ta (<456.0 ppm), and Nb (<3.0 ppm) are significantly low. The high (Be and Sn) and significantly low Ga-Rb abundances, and Ta > Nb in the western Mamfe chrysoberyls show that they were crystallized in granitic pegmatites, with some of those source rocks being enriched in Ta and Sn. Zirconium oxide (ZrO2: standardized at 66.1 wt.%)) is the only major oxide in analysed coarse-grained zircons. Within the minor elementary suites: Hf (4576-12,565 ppm) and Y (48-2805 ppm) contents are significantly high. The trace element suites include: Th (7-1565 ppm), U (13-687 ppm), and ?REE (50-2161 ppm), whose values are significantly low. The (Yb/Sm)N, Ce/Ce*, and Eu/Eu* anomalies range from 1.0 to 227.0, 0 to 308, and 0.08 to 1.7 respectively. They are Hf-Y-HREE enriched and depleted zircons mainly crystallized in magmatic oxidized environments. They were mainly sorted from granitoids, syenites and kimberlites.
Geostandards and Geoanalytical Research, http://orchid.org/0000-0002-2701-4635 80p.
Asia, Sri Lanka
geochronology
Abstract: Here we document a detailed characterization of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U-Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean ²??Pb/²³?U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure?related parameters correspond well with the calculated alpha doses of 1.48 × 10¹? g?¹ (GZ7) and 2.53 × 10¹? g?¹ (GZ8), respectively, and the (U-Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U concentrations are 680 ?g g?¹ (GZ7) and 1305 ?g g?¹ (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U-Pb geochronology. In addition, GZ7 (Ti concentration 25.08 ?g g?¹ ± 0.18 ?g g?¹; 95% confidence uncertainty) may prove useful as reference material for Ti?in?zircon temperature estimates.
Mineralogy and Petrology, Vol. 115, 19p. Doi.org/101007 /s00710-020- 00723-x pdf
South America, Colombia
REE
Abstract: A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite-(Ce) with röntgenite-(Ce) and a phase which is assigned to B3S4 (i.e., bastnäsite-3-synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite-(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data - impeded by twinning, complex stacking patterns, sequential and compositional faults - indicates that the dominant parisite-(Ce) polytype M1 has space group Cc. Parisite-(Ce), the B3S4 phase and röntgenite-(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite-(Ce), the B3S4 phase and röntgenite-(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm?1. We propose to non-destructively differentiate parisite-(Ce) and röntgenite-(Ce) by their 1092 cm?1 / 1081 cm?1 ?1(CO3) band height ratio.
Abstract: A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite-(Ce) with röntgenite-(Ce) and a phase which is assigned to B3S4 (i.e., bastnäsite-3-synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite-(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data - impeded by twinning, complex stacking patterns, sequential and compositional faults - indicates that the dominant parisite-(Ce) polytype M1 has space group Cc. Parisite-(Ce), the B3S4 phase and röntgenite-(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite-(Ce), the B3S4 phase and röntgenite-(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm-1. We propose to non-destructively differentiate parisite-(Ce) and röntgenite-(Ce) by their 1092 cm-1 / 1081 cm-1 ?1(CO3) band height ratio.
Abstract: Experimental data reveal that Earth’s mantle melts more readily than previously thought, and may have remained mushy until two to three billion years ago.
Journal of Mining Science, Vol. 57, 2, pp. 286-297. pdf
Russia
IPKON RAS
Abstract: The thermodynamic analysis and tests of minerogenesis under higher temperatures determine conditions of thermochemical decomposition of hydrophilic attachments on diamond surface. It is found that hydrophilic mineral attachments can be removed from diamond surface by combining thermal treatment of slurry at the temperature of 80-85 ?C with electrochemical treatment of recirculated water, which enables required change in ion-molecule composition of water phase in the slurry. The hybrid conditioning technology ensures recovery of the natural hydrophobic behavior and floatability of diamonds and enhances performance of froth flotation of diamonds by 5.1%.
Journal of Mining Science, Vol. 51, 4, pp. 799-810.
Russia
Spectroscopy
Abstract: Using the Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), microscopy and microhardness test methods, the change in the crystalline and chemical properties and in microhardness of rock-forming minerals of kimberlites as a result of exposure to high-power nanosecond electromagnetic pulses (HPEM) has been studied. From FTIR and XPS data the non-thermal effect of HPEM results in damage of surface microstructure of dielectric minerals due to formation of microcracks, surface breakdowns and other defects, which ensure effective weakening of rock-forming minerals and reduction in their microhardness by 40-66%.
Journal of Mining Science , Vol. 51, 5, pp. 1235-1241.
Russia
Mineral processing ** in Russian
Abstract: The article presents new test results on structural and chemical properties of mineral formations on the surface of natural hydrophilic diamonds using Raman, X-ray phase and Auger spectroscopy methods. Analysis of morphological features of nano formations involved scanning electron microscope Jeol-5610 and analyzer INCA. Based on the studies into phase composition of diamonds non-recovered in the circuit of kimberlite ore processing, two types of mineral formations are discovered on their surface: microformations as silicate nature globules less than 1 ?m in size and silicate nano films more than 5 nm thick. The tests detect also presence of layered talc silicates that make diamond surface hydrophilic.
Journal of Mining Science, Vol. 52, no. 2, pp. 332-340.
Russia, Yakutia
Deposit - Mir
Abstract: The article gives new experimental data on spectral characteristics of photoluminescence of natural diamonds extracted from deep horizons of Mir and Internatsionalnaya Pipes, Republic of Sakha (Yakutia) depending on composition of basic and additional optically active structural defects in crystals and on temperature during spectrum recording, considering kinetics of luminescence. It is hypothesized on applicability of low-temperature effects to enhance efficiency of photoluminescence separation of diamond crystals.
Journal of Mining Science, Vol. 52, 2, pp. 332-340.
Technology
Spectroscopy
Abstract: The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (
Journal of Mining Science, Vol. 52, 3, pp. 535-540.
Russia
Diamond morphology
Abstract: The analytical research has yielded differences in composition of mineral species on the surface of natural diamonds of hyperaltered kimberlites under conditions of diamond ore occurrence and processing. The classification of the mineral species is based on the mineral origin, properties and attachment on the diamond crystal surface.
Journal of Mining Science, Vol. 51, 6, pp. 1235-1241.
Russia
Diamond morphology
Abstract: The article presents new test results on structural and chemical properties of mineral formations on the surface of natural hydrophilic diamonds using Raman, X-ray phase and Auger spectroscopy methods. Analysis of morphological features of nano formations involved scanning electron microscope Jeol-5610 and analyzer INCA. Based on the studies into phase composition of diamonds non-recovered in the circuit of kimberlite ore processing, two types of mineral formations are discovered on their surface: microformations as silicate nature globules less than 1 ?m in size and silicate nano films more than 5 nm thick. The tests detect also presence of layered talc silicates that make diamond surface hydrophilic.
Journal of Mining Science, Vol. 53, 1, pp. 126-132.
Russia
deposit - Mir
Abstract: The structural and chemical surface transformation of basic kimberlite-forming minerals (calcite, olivine, serpentine) under the contact with natural and waste mineralized water and products of electrochemical treatment of the water are studied using X-ray photoelectronic spectroscopy, scanning electron microscopy and X-ray spectral micro-analysis, and atomic force microscopy. It is found that contact with kimberlite extract and recycling water induces chemical modification of calcite surface, which consists in adsorption of hydrocarbon impurities, and chlorine- and silica-bearing compounds, majority of which are removed during interaction with the product of electrochemical treatment of recycling water. The change in the structural and chemical surface properties of rock-forming silicates, aside from adsorption-desorption of organic compound, is also connected with the distortion of nano-size layer structure after leaching of Mg, Fe and Si, and with the carbonatization of the surface.
Journal of Mineral Science, Vol. 54, 3, pp. 458-465.
Russia
luminescence
Abstract: Organic and inorganic luminophores of similar luminescence parameters as diamonds are selected. Indicators, based on the selected luminophores, are synthesized. Spectral and kinetic characteristics of luminophores are experimentally determined for making a decision on optimal compositions to ensure maximum extraction of diamonds in X-ray luminescence separation owing to extra recovery of non-luminescent diamond crystals. As the components of luminophore-bearing indicators, anthracene and K-35 luminophores are selected as their parameters conform luminescence parameters of diamonds detected using X-ray luminescence separator with standard settings.
Journal of Mining Science, Vol. 56, 1, pp. 96-103. pdf
Global
luminescence
Abstract: The authors present an efficient modification method of X-ray fluorescence separation with mineral and organic luminophores used to adjust spectral and kinetic characteristics of anomalously luminescent diamonds. The mechanism of attachment of luminophores at diamonds and hydrophobic minerals is proved, including interaction between the organic component of emulsions and the hydrophobic surface of a treated object and the concentration of insoluble luminophore grains at the organic and water interface. Selective attachment of the luminophore-bearing organic phase of emulsion at the diamond surface is achieved owing to phosphatic dispersing agents. Tri-sodium phosphate and sodium hexametaphosphate added to emulsion reduce attachment of the luminophore-bearing organic phase at the surface of kimberlite minerals. It is shown that phosphate concentration of 1.0-1.5 g/l modifies and stabilizes spectral and kinematic parameters of kimberlite mineral on the level of initial values. This mode maintains the spectral and kinematic characteristics of anomalously luminescent diamonds at the wanted level to ensure extraction of diamonds to concentrate.
Journal of Mining Science, Vol. 56, 1, pp. 96-103, 8p. Pdf
Russia
luminescence
Abstract: The authors present an efficient modification method of X-ray fluorescence separation with mineral and organic luminophores used to adjust spectral and kinetic characteristics of anomalously luminescent diamonds. The mechanism of attachment of luminophores at diamonds and hydrophobic minerals is proved, including interaction between the organic component of emulsions and the hydrophobic surface of a treated object and the concentration of insoluble luminophore grains at the organic and water interface. Selective attachment of the luminophore-bearing organic phase of emulsion at the diamond surface is achieved owing to phosphatic dispersing agents. Tri-sodium phosphate and sodium hexametaphosphate added to emulsion reduce attachment of the luminophore-bearing organic phase at the surface of kimberlite minerals. It is shown that phosphate concentration of 1.0-1.5 g/l modifies and stabilizes spectral and kinematic parameters of kimberlite mineral on the level of initial values. This mode maintains the spectral and kinematic characteristics of anomalously luminescent diamonds at the wanted level to ensure extraction of diamonds to concentrate.
Martirosyan, N., Yoshino, T., Shatskiy, A., Chanyshev, A., Litasov, K.
Kenetic study of Ca- carbonate - iron interaction. ( global geodynamic processes - diamond formation)
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept 22-26, 2p. Abstract
Martirosyan, N., Yoshino, T., Shatskiy, A., Chanyshev, A., Litasov, K.
Effect of water on the stability of magnesite in the mantle under reduced conditions.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Abstract: The subduction of carbonates beyond 250-300?km, where redox conditions favour the presence of metallic iron (Fe), will result in redox reactions with the Fe dispersed in the silicate rocks. Here, we studied the effect of water on the carbonate-Fe interaction in the hydromagnesite-Fe system at 6, 8 and 16?GPa and the peridotite-CO2-H2O-Fe system at 8?GPa, using a multianvil apparatus. In all of the studied samples, we observed the formation of magnesiowüstite, graphite and carbide. Additionally, in the peridotite-CO2-H2O-Fe system, magnesiowüstite reacted with pyroxenes, resulting in olivine enrichment. Kinetic calculations performed at 8?GPa showed that, at the pressure-temperature (P-T) parameters of the ‘hot’, ‘medium’ and ‘cold’ subduction, about 40, 12 and 4?vol% of carbonates, respectively, would be reduced in the hydrous system within 1 Myr, assuming direct contact with Fe. Based on the present results, it is suggested that carbonates will largely be consumed during the characteristic subduction time to the mantle transition zone by reaction with the reduced mantle in the presence of hydrous fluid.
Abstract: n recent years, nominal type IaAB and IIa diamonds with transient 2800 cm-1 FTIR absorption peaks arising from uncompensated boron produced under UV radiation have been reported (J. Li et al., A diamond with a transient 2804 cm-1 absorption peak, Journal of Gemmology, Vol. 35, 2016, pp. 248-252; Winter 2016 Lab Notes, pp. 412-413). The National Center of Supervision and Inspection on Quality of Gold and Silver Products recently examined a type IaB diamond that exhibited instantaneous 2803 cm-1 FTIR absorption shortly after exposure to an ultra-short-wave (< 230 nm) UV source.
Field and petrographic textural evidence for the epigenetic hydrothermalmetasomatic origin of the Bayan Obo rare earth ore deposit of inner Mongolia, China
International Mineralogical Association Meeting Held June, 1990 Beijing China, Vol. 2, extended abstract p. 930-931
Chao, E.C.T., Back, J.M., Minkin, J.A., en Yinchen
Host rock controlled epigenetic, hydrothermal metasomatic origin of the Bayan Obo rare earth elements (REE)-iron-Nb ore deposit, Inner Mongolia, P.R.C.
Rogermitchellite, a new mineral species from Mont Hilaire Quebec: description, structure, determination and relationship with HFSE bearing cyclosilicates.
Gold deposits estimation using indicator kriging.Sub-heading ..indicator kriging ideal for estimating the reserves of irregular mineralizations ie.diamonds
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin), Vol. 83, No. 934, February pp. 77-83
Earth and Planetary Science Letters, Vol. 500, pp. 156-167.
United States
geodynamics
Abstract: The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Note sur la Region Diamantifere de l'afrique Australe, Suivie D'un Tableau Resumant Les Etudes Faites Par M. Fouque et M. Michel-levy sur Les Roches Rapportees de l'afrique Australe Par l'auteur.
Note Accompagment la Presentation D'une Conference Faite Le30 Janvier, 1892 a la Association Scientific de France, Surles Mines de Diamant de l'afrique Australe.
Soc. Geol. France (paris) Bulletin., 3RD. SER. Vol. 20, PP. XXXI-XXXIII.
Deijanin, B., Simic, D., Zaitsev, A., Chapman, J., Dobrinets, I., Widemann, A., Del Re, N., Middleton, T., Dijanin, E., Se Stefano, A.
Characterization of pink diamonds of different origin: natural ( Argyle, non-Argyle), irradiated and annealed, treated with multi-process, coated and synthetic.
Diamond and Related Materials, Vol. 17, 7-10, pp. 1169-1178.
Abstract: At the Argyle diamond mine in Western Australia, an underground project is using block caving techniques to reach deeper portions of the diamondiferous lamproite. This program could extend the life of the mine to 2018. It entails a high level of automation, as well as measures to combat monsoonal downpours.
Abstract: Polycrystalline diamond aggregates (diamondites) are produced by rapid crystal nucleation caused by extreme carbon supersaturation in mantle fluids. They may form episodically and under variable chemical conditions, providing snapshots of diamond formation in the Earth’s mantle. Diamondites, thus, represent an extreme end member of diamond formation mechanisms, while forming via the same processes and ingredients as the gem-sized diamonds. We present results on a large suite of diamondites from the Venetia mine (South Africa), comprising a complete characterisation of the diamonds and their silicate inclusions and intergrowths. The highlighted characteristic of this sample suite is its heterogeneity in all aspects, from affiliated silicate to diamond composition and texture of the diamond aggregates. The diamond grains in the samples are intergrown with silicates (garnets, clinopyroxenes, phlogopites) comprising a websteritic-eclogitic and a peridotiticpyroxenitic suite of minerals. Diamonds, regardless of their affiliation based on their silicate phases, overlap in carbon and nitrogen composition and have ?13C values between -28 and -8 ‰, ?15N values of 0.8 to 16.3 ‰ and nitrogen contents of 4 to 2329 ppm. The entire range of carbon and nitrogen variability of the suite is also reflected in some individual samples. Cathodoluminescence imaging visualizes different zones in the samples that can be interpreted as different growth events with differing nitrogen contents and ?15N decoupled from ?13C values, in line with the variability off nitrogen aggregation states. Electron backscatter diffraction analyses identify an original texture of randomly intergrown diamond grains that is partly changed by deformation and newly grown smaller diamond grains. The large overall variability suggesting episodic formation of diamondite with nitrogen from crustal sources.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265-6.
Global
Fluoresence
Abstract: The effect of fluorescence on the appearance of diamonds has been a subject of debate for many years (Moses et al., 1997). In the trade, fluorescence is generally perceived as an undesirable characteristic. Nearly 80% of diamonds graded at HRD Antwerp receive a “nil” fluorescence grade, while the remainder are graded as “slight,” “medium,” and “strong,” their value decreasing with level of fluorescence. To understand how fluorescence might change diamond appearance, a selection of 160 round brilliant-cut diamonds were investigated in detail. This study focused on the effect of thetic samples, it is possible that some of the observed phosphorescence does not involve boron impurities. In this paper we report on the results of combined fluorescence, phosphorescence, thermoluminescence, and quantitative charge transfer investigations undertaken on both HPHT and CVD synthetic diamond, with the objective of identifying which defects are involved in the fluorescence and phosphorescence processes.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 305-6.
Global
synthetics
Abstract: Laboratory-grown diamonds are created using either high-pressure, high-temperature (HPHT) or chemical vapor deposition (CVD). With the influx of manmade diamonds on the market over the past few years, instrument producers and labs have launched screening and detection instruments to help dealers and jewelers spot HPHTor CVD-grown specimens. Most standard instruments are inaccurate testers or just type I and type II screening devices that do not give a definite answer about diamond genesis. Over the last four annual Mediterranean Gemmological and Jewellery Conferences and more than 30 workshops given in 17 countries, we have assembled a portable new Synthetic Diamond Identification Kit. The kit comprises two portable instruments and two booklets: 1) A PL inspector (mini UV lamp with magnifier) to inspect laboratory-grown, treated, and natural diamonds using long- and short-wave fluorescence and phosphorescence 2) A 2017 handbook with images and explanation of longand short-wave reactions of diamonds of all types 3) A mini foldable polariscope with portable light to separate natural diamonds using characteristic birefringence patterns from HPHT and CVD diamonds 4) A 2010 handbook with images and explanations of crosspolarized filter reactions of diamonds of all types The combination of this kit with professional training could identify all HPHT-grown diamonds and most CVD-grown diamonds on the market, loose or mounted. Also available are melee and jewelry inspectors consisting of larger UV lamps with magnifiers designed for identification of small loose or mounted diamonds. Different diamond types and subtypes can exhibit different birefringence under cross-polarized filters. A clear majority of natural diamonds exhibit some degree of internal strain, with type II natural diamonds showing a weak “tatami” pattern. HPHTgrown diamonds are free of such strain, and CVD-grown diamonds show mostly coarse columnar patterns. Most natural diamonds have a strong reaction to long-wave UV; this reaction is usually weaker (mostly blue) at shorter wavelengths. Laboratory-grown diamonds generally exhibit more intense fluorescence with short-wave UV compared to long-wave UV, with a chalky coloring tinged with green or yellow. Most HPHT-grown diamonds also phosphoresce. If a diamond is free of inclusions, fluorescence is a reliable screening test to flag suspicious stones that should be further checked under cross-polarized filters (figure 1). In the case of some rare near-colorless clean CVD-grown diamonds that do not show fluorescence or have a birefringence pattern that is coarse but resembling tatami in type IIa and weak patterns in natural Ia diamonds, additional tests using advanced spectroscopy and strong short-wave UV light to observe growth patterns are needed to confirm diamond genesis.
Gemmological Research Industries Inc. Vancouver B.C., isbn 978-1777369231 184p.
Global
Book - notice
Abstract: For those who have some portable and advanced instruments, this book will serve as a handbook with many useful spectra, cross polarised filters and fluorescence reactions to compare, plus an Appendix with results of tests conducted using 11 portable instruments on 64 samples, and suggestions as to what instruments to use depending on budget and needs. Even if you are not a diamond specialist but are merely interested in the science of diamond, or you trade in diamonds, the information in this book will make you more knowledgeable and confident to talk about this beautiful gem with friends and clients.
Abstract: Analysing the size frequency distributions (SFDs) of both micro diamonds and macro diamonds from primary deposits shows that the distributions are continuous across all sizes and that there are two regions of different character with a transition about 1-2 mm. Using log axes, the frequency curve is linear for the smaller sizes allowing slope and intercept parameters to be determined which are less ambiguous than stone counts and ratios of macro to micro populations that are generally reported. Modelling a diamond population that has undergone removal of a uniform thickness of the outer layer transforms a linear frequency curve into a quadratic form, which is also the form of the frequency curve for macro diamonds. Diamonds grown synthetically also display a linear distribution across a smaller fraction of their size distribution curve.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 169-190.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 237-252.
Abstract: Establishing relationships between the long-term landscape evolution of drainage basins and the fill of sedimentary basins benefits from analysis of bedrock river terrace deposits. These fragmented detrital archives help to constrain changes in river system character and provenance during sediment transfer from continents (source) to oceans (sink). Thick diamondiferous gravel terrace deposits along the lower Orange River, southern Namibia, provide a rare opportunity to investigate controls on the incision history of a continental-scale bedrock river. Clast assemblage and heavy mineral data from seven localities permit detailed characterisation of the lower Orange River gravel terrace deposits. Two distinct fining-upward gravel terrace deposits are recognised, primarily based on mapped stratigraphic relationships (cross-cutting relationships) and strath and terrace top elevations, and secondarily on the proportion of exotic clasts, referred to as Proto Orange River deposits and Meso Orange River deposits. The older early to middle Miocene Proto Orange River gravels are thick (up to 50 m) and characterised by a dominance of Karoo Supergroup shale and sandstone clasts, whereas the younger Plio-Pleistocene Meso Orange River gravels (6-23 m thick) are characterised by more banded iron formation clasts. Mapping of the downstepping terraces indicates that the Proto gravels were deposited by a higher sinuosity river, and are strongly discordant to the modern Orange River course, whereas the Meso deposits were deposited by a lower sinuosity river. The heavy minerals present in both units comprise magnetite, garnet, amphibole, epidote and ilmenite, with rare titanite and zircon grains. The concentration of amphibole-epidote in the heavy minerals fraction increases from the Proto to the Meso deposits. The decrease in incision depths, recorded by deposit thicknesses above strath terraces, and the differences in clast character (size and roundness) and type between the two units, are ascribed to a more powerful river system during Proto-Orange River time, rather than reworking of older deposits, changes in provenance or climatic variations. In addition, from Proto- to Meso-Orange River times there was an increase in the proportion of sediments supplied from local bedrock sources, including amphibole-epidote in the heavy mineral assemblages derived from the Namaqua Metamorphic Complex. This integrated study demonstrates that clast assemblages are not a proxy for the character of the matrix, and vice versa, because they are influenced by the interplay of different controls. Therefore, an integrated approach is needed to improve prediction of placer mineral deposits in river gravels, and their distribution in coeval deposits downstream.
Journal of Mineralogical and petrological Sciences, Vol. 115, 2, pp. 202-215. pdf
India
deposit - Wajrakarur
Abstract: Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic-panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite-tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc-alkaline variety whereas their bulk-rock geochemistry portrays mixed signals of both alkaline as well as calc-alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic-anorogenic transitional type mantle source) from eastern Dharwar craton. Trace element ratios imply melt-derivation from an essentially the garnet bearing-enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and ‘enriched’ ?Ndinitial (?19.1 and ?24.2) values. The calculated TDM ages (2.7-2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co-spatial and co-eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc-alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence for highly heterogeneous nature of the sub-continental lithospheric mantle beneath the eastern Dharwar craton.
Geochemistry and origin of the Proterozoic kimberlites, ultramafic and ultrapotassic magmatic rocks from Indravati Basin in Bastar Craton, central India.
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 94-97.
Mandal, N., Charavarty, K.H., Borah, K., Rai, S.S.
Is a cation ordering transition of the Mg-Fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the Indian craton?
Geochemistry, Geophysics, Geosystems, Vol. 19, 5, pp. 1690-1712.
Africa
geomorphology
Abstract: West African drainage reorganization during Cretaceous opening of the Atlantic Ocean is deciphered here from geochemical provenance studies of Central Atlantic sediments. Changes in the geochemical signature of marine sediments are reflected in major and trace element concentrations and strontium?neodymium radiogenic isotopic compositions of Cretaceous sedimentary rocks from eight Deep Sea Drilling Project (DSDP) sites and one exploration well. Homogeneous major and trace element compositions over time indicate sources with average upper (continental) crust signatures. However, detailed information on the ages of these sources is revealed by neodymium isotopes (expressed as ?Nd). The ?Nd(0) values from the DSDP sites show a three?step decrease during the Late Cretaceous: (1) the Albian?Middle Cenomanian ?Nd(0) values are heterogeneous (-5.5 to ?14.9) reflecting the existence of at least three subdrainage basins with distinct sedimentary sources (Hercynian/Paleozoic, Precambrian, and mixed Precambrian/Paleozoic); (2) during the Late Cenomanian?Turonian interval, ?Nd(0) values become homogeneous in the deepwater basin (-10.3 to ?12.4), showing a negative shift of 2 epsilon units interpreted as an increasing contribution of Precambrian inputs; (3) this negative shift continues in the Campanian?Maastrichtian (?Nd(0)?=??15), indicating that Precambrian sources became dominant. These provenance changes are hypothesized to be related to the opening of the South and Equatorial Atlantic Ocean, coincident with tectonic uplift of the continental margin triggered by Africa?Europe convergence. Finally, the difference between ?Nd(0)values of Cretaceous sediments from the Senegal continental shelf and from the deepwater basins suggests that ocean currents prevented detrital material from the Mauritanides reaching deepwater areas.
Abstract: The continental margin of the Guiana Shield formed at the intersection of the Central and Equatorial Atlantic Oceans that developed one after the other and, in doing so, achieved the break-up of the Gondwana supercontinent. To form these Ocean, the continent crust was stretched and broke but the way this thinning is actually varying along the margin and the causes are not known so we used offshore industrial data to map it. This allows us showing that the width of the continental margin depends primarily on the direction along which the crust was thinned such that the continental margin width is much wider (200-300 km) in domains where this direction is perpendicular to the margin than in domain where it is oblique (<100 km). This also allow us showing that the continental margin resulting from the opening of the Central Atlantic Ocean is systematically wider than the one resulting from the opening of the Equatorial Atlantic. Additionally, our observations suggest that Central Atlantic Ocean opened under warmer conditions than the Equatorial Atlantic. Finally, the area at the intersection of the Central and Equatorial Atlantic Oceans, individualized a promontory of continental crust that formed the present-day Demerara Plateau.
Abstract: Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I4¯2d structure indistinguishable from I4¯3d Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.
Abstract: The Earth’s crust-mantle boundary, the Mohorovi?i? discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (?-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure-temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth’s mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth’s virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth’s geomagnetic data9, and especially in studies of planetary bodies that no longer have a dynamo10, such as Mars.
Abstract: The formation of Iron Oxide-Apatite (IOA) systems has long been enigmatic. The compositions of both magnetite and apatite and the other component elements suggest derivation from high temperature (T) magmatic systems, with genetic models including iron oxide magmas or igneous magnetite and apatite flotation. Ideas related to the role of H2O and associated oxidative mechanisms have resurfaced from models of the late 1960s. As such, salt melts forming in open, differentially degassing systems could represent an end-member to the formation of IOA deposits. Another end-member involves autometasomatic decarbonation reactions involving ferroan carbonatites with co-genetic melts or host rocks generating CO2 capable of oxidizing carbonatites to enhance magnetite-apatite saturation. The syntectic decarbonation end-member presented here examines the reactions of carbonate melts of mantle origin or from syntectic reactions with limestone, with cogenetic silicate magmas. Although carbonate and silicate melts can coexist at magmatic pressure (P) and T, their compositions must be peralkalic. However, as P decreases, immiscibility or reactivity between these melts is such that CO2 is exsolved (decarbonation) to the point that at near surface conditions, decarbonation is complete. The addition of CO2 to silicate melt will drive the conversion of FeO to Fe2O3 in order to make carbon monoxide (CO), thus shifting the redox equilibria. For most silicate magmas, the amount of dissolved carbonate and CO2 is quite limited, and differential CO2 degassing results. These carbonate: silicate melt reactions then may result in oxidation of the silicate magma, to enhance immiscibility of IOA (liquation) and elemental partitioning associated with liquid-liquid immiscibility. This could be an oxidative mechanism for Fe-Ti tholeiites (ferrobasalts) and diorites to reach a two-liquid field and form IOA melts via liquation. Carbonates would typically be consumed in these reactions, although CO2 is an important degassing product that would substantially increase ?V of the reaction, which has implications during high-level emplacement.
Fedortchouk, Y., Matveev, S., Charnell, C., Carlson, J.A.
Kimberlitic fluid as recorded by dissolving diamonds and crystallizaing olivine phenocrysts in five Lac de Gras kimberlites, Northwest Territories, Canada.
Microscopic Textures and Structures in the Lake Ellen Kimberlite.
American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) MEETING HELD APRIL 24TH. HOUGHTON MICHIGAN., SKILLINGS MINING REVIEW, Vol. 74, No. 14, APRIL 6TH. P. 5. L
Kimberlite exploration techniques in glaciated terrains in Michigan
American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) Annual Section Meeting held April 24, 1991 Michigan Tech., 5p. courtesy of author
Implications of HFS/LIL trace element variations for the mantle sources of recent alkaline volcanism in northern British Columbia and the Yukon, Canada
Eos Transactions, Vol. 73, No. 14, April 7, supplement abstracts p.325
European Journal of Mineralogy, Vol. 30, 2, pp. 219-230.
Mantle
mineralogy
Abstract: The chemical diversity of minerals can be analysed in terms of the concept of mineral systems, defined by the set of chemical elements essential for the definition of a mineral species. Only species-defining elements are considered as essential. According to this approach, all minerals are classified into ten types of mineral systems with the number of essential components ranging from 1 to 10. For all the minerals known today, only 70 chemical elements act as essential species-defining constituents. The number of minerals of different chemical elements are calculated as follows (number of mineral species is given in parentheses): oxygen (4138), hydrogen (2814), silicon (1479), calcium (1182), sulfur (1064), aluminum (989), sodium (953), iron (953), copper (643), arsenic (601), phosphorus (599), and magnesium (576). The distribution of the majority of the species-defining elements among mineral systems submits to a normal distribution. Using the concept of mineral systems, different geological objects can be compared from the viewpoint of their mineral diversity as exemplified by alkaline massifs (Khibiny, Lovozero, Russia, and Mont Saint-Hilaire, Canada), evaporite deposits (Inder, Kazakhstan, and Searles Lake, USA) and fumaroles at active volcanoes (Tolbachik, Kamchatka, Russia, and Vulcano, Sicily, Italy). The concept of mineral systems can be applied to mineral evolution overall by calculating the mean number of elements for the first three stages in the evolution of minerals as proposed by R.M. Hazen and co-authors in 2008, plus a fourth period corresponding to Hazen's stages 4-10, as follows: 2.08?±?0.45 (I: ur-minerals); 2.68?±?0.13 (II: minerals of chondritic meteorites); 3.86?±?0.07 (III: Hadean minerals); 4.50?±?1.47 (IV: post-Hadean minerals).
Geology of Ore Deposits, Vol. 62,8, pp. 704-718. pdf
Russia, Canada
alkaline rocks
Abstract: The chemical diversity of minerals can be analyzed in terms of the concept of mineral systems based on the set of chemical elements that are essential for defining a mineral species. Only species-defining elements are considered to be essential. According to this approach, all minerals are classified into ten types of mineral systems with the number of essential components ranging from 1 to 10. For all known minerals, only 70 chemical elements act as essential species-defining constituents. Using this concept of mineral systems, various geological objects may be compared from the viewpoint of their mineral diversity: for example, alkali massifs (Khibiny and Lovozero in Russia; Mont Saint Hilaire in Canada), evaporite deposits (Inder in Kazakhstan and Searles Lake in the United States), fumaroles of active volcanoes (Tolbachik in Kamchatka and Vulcano in Sicily, Italy), and hydrothermal deposits (Otto Mountain in the United States and El Dragon in Bolivia). Correlations between chemical and structural complexities of the minerals were analyzed using a total of 5240 datasets on their chemical compositions and 3989 datasets on their crystal structures. The statistical analysis yields strong and positive correlations (R2 > 0.95) between chemical and structural complexities and the number of different chemical elements in a mineral. The analysis of relationships between chemical and structural complexities provides strong evidence for the overall trend of a greater structural complexity at a higher chemical complexity. Following R. Hazen, four groups of minerals representing four mineral evolution stages have been considered: (I) “Ur-minerals,” (II) minerals from chondrite meteorites, (III) Hadean minerals, and (IV) contemporary minerals. According to the obtained data, the number of species-defining elements in minerals and their average contents increase regularly and significantly from stage I to stage IV. The analyzed average chemical and structural complexities in these four groups demonstrate that both are gradually increasing in the course of mineral evolution. The increasing complexity follows an overall trend: the more complex minerals were formed in the course of geological time, without replacing the simpler ones. The observed correlations between chemical and structural complexities understood in terms of the Shannon information suggest that chemical differentiation is the major force that drives the increase of mineral complexity over the course of geological time.
Geology of Ore Deposits, Vol. 62, 8, pp. 704-718. pdf
Russia, Canada, Quebec
Mineralogy
Abstract: The chemical diversity of minerals can be analyzed in terms of the concept of mineral systems based on the set of chemical elements that are essential for defining a mineral species. Only species-defining elements are considered to be essential. According to this approach, all minerals are classified into ten types of mineral systems with the number of essential components ranging from 1 to 10. For all known minerals, only 70 chemical elements act as essential species-defining constituents. Using this concept of mineral systems, various geological objects may be compared from the viewpoint of their mineral diversity: for example, alkali massifs (Khibiny and Lovozero in Russia; Mont Saint Hilaire in Canada), evaporite deposits (Inder in Kazakhstan and Searles Lake in the United States), fumaroles of active volcanoes (Tolbachik in Kamchatka and Vulcano in Sicily, Italy), and hydrothermal deposits (Otto Mountain in the United States and El Dragon in Bolivia). Correlations between chemical and structural complexities of the minerals were analyzed using a total of 5240 datasets on their chemical compositions and 3989 datasets on their crystal structures. The statistical analysis yields strong and positive correlations (R2 > 0.95) between chemical and structural complexities and the number of different chemical elements in a mineral. The analysis of relationships between chemical and structural complexities provides strong evidence for the overall trend of a greater structural complexity at a higher chemical complexity. Following R. Hazen, four groups of minerals representing four mineral evolution stages have been considered: (I) “Ur-minerals,” (II) minerals from chondrite meteorites, (III) Hadean minerals, and (IV) contemporary minerals. According to the obtained data, the number of species-defining elements in minerals and their average contents increase regularly and significantly from stage I to stage IV. The analyzed average chemical and structural complexities in these four groups demonstrate that both are gradually increasing in the course of mineral evolution. The increasing complexity follows an overall trend: the more complex minerals were formed in the course of geological time, without replacing the simpler ones. The observed correlations between chemical and structural complexities understood in terms of the Shannon information suggest that chemical differentiation is the major force that drives the increase of mineral complexity over the course of geological time.
Abstract: he meta-analysis of about 13,000 analyses of scandium content in garnet grains shows that, below the spinel-garnet transition, this phase carries about three-quarters of the Sc budget of the mantle, indicating its control on Sc mobility. The Sc content of garnets in mafic rocks is low, due to a dilution effect resulting from their high modal content in garnet. Garnets from ultramafic rocks exhibit a wider range of Sc concentrations. We assess the relative influence of thermobarometry, crystal chemistry and fluid-related events on the distribution of Sc in garnet from such rocks to improve the tracking of geochemical processes in the mantle. Pressure and temperature of equilibration in the mantle are second-order factors influencing the Sc content of garnet, while crystal-chemistry, in particular and , is the main parameter controlling the compatibility of Sc. Scandium is incorporated in both X and Y sites of Cr-Ca-rich garnets, resulting in a behaviour intermediate between rare-earth elements, incorporated in the X site, and trivalent transition elements, occupying the Y site. This affinity for both sites results in a mild compatibility of Sc in the garnet stability field of the mantle; hence Sc concentration in garnet increases with melt extraction and can be reduced by silicate-melt metasomatism. In contrast, metasomatism by volatile-rich fluids increases the Sc concentration in garnet. The control of garnet on the compatibility of Sc in deep lithospheric rocks demonstrates the potential of using Sc to track the conditions of formation of magmas and their residual rocks, as well as the origin and nature of metasomatic fluids.
Abstract: Scandium is often considered as immobile during chemical weathering, based on its low solubility. In contrast to other conservative (i.e. relatively immobile) elements incorporated into accessory minerals resistant to weathering (e.g. zirconium, thorium or niobium), the scarcity of scandium minerals indicates that the processes accounting for scandium's immobilisation are distinctive. However, the evolution of scandium speciation during weathering is unknown, limiting the understanding of the processes controlling its dynamics in the critical zone. Exceptional scandium concentrations in east Australian laterites provide the possibility of unravelling these mechanisms. We follow scandium speciation through thick lateritic profiles (> 30 m) using a multiscale mineralogical and spectroscopic approach involving electron microprobe, laser-ablation--inductively coupled plasma mass spectrometry, selective leaching and X-ray absorption near-edge structure spectroscopy, complemented by mass-transfer calculations. We show that the initial reservoir of scandium contained in the parent rock is preserved under reducing conditions occurring in the lowest horizons of the profiles. The dissolution of scandium-bearing clinopyroxene generates smectitic clays that immobilise and concentrate scandium. It is subsequently trapped in the lateritic duricrust by goethite. Scandium mobilisation appears in this horizon and increases upward as a result of the dissolution of goethite, possibly assisted by dissolved organic matter, and the precipitation of hematite. Molecular-scale analyses demonstrate that changes in speciation govern scandium dynamics, with substitution in smectitic clays and adsorption on iron oxyhydroxides playing a crucial role in scandium immobility in the saprolite and lower lateritic duricrust. The higher affinity of scandium for goethite relative to hematite drives scandium mobilisation in the upper lateritic duricrust, leading to its concentration downward in the lower lateritic duricrust. These successive mechanisms illustrate how the unique complexity of the critical zone leads to scandium concentrations that may form new types of world-class scandium deposits. Comparison with conservative elements and with rare-earth elements, expected to have similar geochemical properties, emphasizes the unique behaviour of scandium in the critical zone. While scandium remains immobile during the early stages of weathering, intense and long-term alteration processes, observed in lateritic contexts, lead to scandium mobilisation. This study highlights the dependence of scandium mobility on weathering conditions.
American Mineralogist, Vol. 105, 7, 10.2138/am-2020-730
Global
scandium
Abstract: Scandium is often associated with iron oxides in the environment. Despite the use of scandium as a geochemical tracer and the existence of world-class supergene deposits, uncertainties on speciation obscure the processes governing its sequestration and concentration. Here, we use first-principles approaches to interpret experimental K-edge X-ray absorption near-edge structure spectra of scandium either incorporated in or adsorbed on goethite and hematite, at concentrations relevant for the environment. This modeling helps to interpret the characteristic spectral features, providing key information to determine scandium speciation when associated with iron oxides. We show that scandium is substituted into iron oxides at low concentration without modifying the crystal structure. When scandium is adsorbed onto iron oxide surfaces, the process occurs through outer-sphere complexation with a reduction in the coordination number of the hydration shell. Considering available X-ray absorption spectra from laterites, the present results confirm that scandium adsorption onto iron oxides is the dominant mechanism of sequestration in these geochemical conditions. This speciation explains efficient scandium recovery through mild metallurgical treatments of supergene lateritic ores. The specificities of scandium sorption mechanisms are related to the preservation of adsorbed scandium in million-years old laterites. These results demonstrate the emerging ability to precisely model fine X-ray absorption spectral features of trace metals associated with mineral phases relevant to the environment. It opens new perspectives to accurately determine trace metals speciation from high-resolution spatially-resolved X-ray absorption near-edge structure spectroscopy in order to constrain the molecular mechanisms controlling their dynamics.
Prospectors and Developers Association of Canada (PDAC) Meeting Workshop held March 27, Toronto, 379p. $ 55.00 plus postage $ 5.00 local $ 25.70 oversea
Gautam, I., Bhutani, R., Balakrishnan, S., Chatterjee, A., George, B.G., Ray, J.S.
142Nd/144Nd of alkaline magmas in Phenai Mat a complex, Chhota Udaipur, Deccan flood basalt province.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 14.
India
alkaline rocks
Abstract: The 65 million year old alkaline plug at Phenai Mata Complex, in Chota Udaipur sub province, is often linked to the last pulse of the Deccan volcanism. However, many believe that the Deccan-Reunion mantle plume that was responsible for the generation of flood basalts might not have been the source of Phenai Mata. It, however, could have acted as a heat source for these magmas derived from the subcontinentallithospheric- mantle (SCLM). Since the SCLM is generally considered to be a nonconvective mantle domain it has the potential to preserve some of the geochemical evidence of the early silicate Earth differentiation, e.g., 142Nd anomaly. In search of such signatures we analysed alkali basalts from the complex for their 142Nd/144Nd using high precision thermal ionization mass spectrometry. Whereas the geochemical characterization of these samples confirmed the lithospheric origin of their source magmas, their ? 142Nd compositions are found to be normal with respect to terrestrial standards. We infer that either the mantle source of Phenai Mata does not represent a true non-convective mantle or it is too young to retain any evidence of 146Sm decay.
Smith, C.B., Haggerty, S.E., Chatterjee, B., Beard, A., Townend, R.
Kimberlite, lamproite, ultramafic lamprophyre, and carbonatite relationships on the Dharwar Craton, India: an example from the Khaderpet pipe, a Diamondiferous ultramafic with associated carbonatite intrusion.
Smith, C.B., Haggerty, S.E., Chatterjee, B., Beard, A., Townend, R.
Kimberlite, lamproite, ultramafic lamprophyre, carbonatite relationships on the Dharwar Craton, India; and example from the Khaderpet pipe, a Diamondiferous ultramafic with associated carbonatite intrusion.
Metamorphic evolution of the Naga Hills eclogite and blueschist northeast India: implications for early subduction of the Indian Plate under Burma microplate
Journal of Metamorphic Geology, Vol. 28, 2, pp. 209-225.
Contributions to Mineralogy and Petrology, 176, 21p. Pdf
India
alkaline rocks
Abstract: Strongly SiO2-undersaturated alkalic rocks (Mg#?>?50, SiO2???45 wt%, Na2O?+?K2O???3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes in the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath the Moho at?~?15 kbar/1270 °C to?~?11-12 kbar/1115-1156 °C pressures and temperatures. Primary magma compositions in equilibrium with lherzolite were estimated through reverse fractionation calculations by incrementally adding equilibrium phases to the rocks in olivine:clinopyroxene:spinel:phlogopite?=?12:68:20:15 proportions at low temperatures followed by olivine:clinopyroxene:spinel?=?12:68:20 proportions at higher temperatures. A comparison of the primary magmas with experimentally generated melts shows that their compositions are consistent with an origin from garnet lherzolite sources with?
Abstract: 186Os/188Os variations in mantle peridotites provide constraints on the long-term Pt/Os evolution of the depleted mantle and the Pt/Os ratio of the primitive upper mantle (PUM). We report new 186Os/188Os data for mantle peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ? 0.67-4.42 wt.%) and 187Os/188Os ratios (0.1138-0.1305). Although peridotite 186Os/188Os values span only a narrow range (from 0.1198345 to 0.1198384), 186Os/188Os broadly correlates with indices of melt depletion including bulk rock Al2O3, spinel Cr#, and clinopyroxene Cr#, consistent with Pt depletion in residual peridotites. PUM 186Os/188Os is estimated to be 0.1198378 ± 23 (2 SD) based on extrapolation of 186Os/188Os-fertility trends, which is very slightly lower than H-chondrites [? 0.1198398 ± 16 (2 SD); Brandon et al., 2006]. This value is consistent with a PUM Pt/Os of 1.7 ± 0.2, similar to average Pt/Os ratios of fertile continental peridotites. The inferred PUM Pt/Os is slightly lower than but within error of Pt/Os values measured in several classes of chondrites [Carbonaceous ? 1.8 ± 0.2, Ordinary ? 1.9 ± 0.1, and Enstatite ? 1.9 ± 0.1 (Brandon et al., 2006)] indicating that PUM Pt/Os is broadly chondritic. In contrast, estimates for PUM Ru/Ir and Pd/Ir (cf. Becker et al., 2006) are suprachondritic. The addition of a chondritic late veneer alone cannot create a combination of chondritic and suprachondritic HSE ratios for the PUM. Instead, minor core segregation occurring concurrently with the addition of a late veneer may explain the observed mantle HSE abundances and ratios. Combined 186Os/188Os-187Os/188Os isotopic and Pt/Os and Re/Os variability in peridotites suggest an average mantle homogenization timescale of ~ 1.2 Ga. In contrast, combined Hf-Nd isotopic and Lu/Hf and Sm/Nd variability in peridotites indicate much shorter homogenization timescales (< 0.4 Ga), potentially reflecting enhanced homogenization by melt-rock interaction to which the Pt-Os and Re-Os systems are relatively immune. The mechanical mixing timescale inferred from Os isotopes is consistent with timescales predicted for whole mantle convection.
Abstract: The Central Indian Tectonic Zone (CITZ) is a major E-W striking mobile belt dissecting the Indian Craton along which the northern and southern Indian cratonic blocks have joined to make the Greater Indian Landmass (GIL). CITZ has a long evolutionary history spanning over 1000 Myrs (2.1-0.9 Ga), overlapping with the assembly and dispersal of two supercontinents - Columbia and Rodinia. Despite a lot of recent work carried out on the CITZ, several outstanding issues remain, especially on the nature and timing of different orogenic events identified in the southern part of this mobile belt. The present contribution attempts to summarize the major petrological, structural and geochronological studies carried out in the CITZ and reappraise the tectonic models in the context of the current database. It is surmised that, while the northern part of CITZ records Paleoproterozoic (ca. 1.8 Ga) orogenic events, the southern part is dominated by a late Palaeoproterozoic-early Mesoproterozoic (ca.1.6-1.5 Ga) collision, followed by crustal extension, and finally a late Mesoproterozoic to early Neoproterozoic (ca. 1.04-0.93 Ga) collision that led to the final stitching of the North and South Indian cratonic blocks. Tectonic evolution of the CITZ is discussed in the context of the Proterozoic supercontinent cycle.
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.
Journal of African Earth Sciences, Vol. 129, pp. 366-379.
Africa, Mozambique
Craton, Zimbabwe
Abstract: The eastern margin of the Zimbabwe Craton, along the Mozambique-Zimbabwe border, includes the oldest rocks of west-central Mozambique constituting a large terrain of granite-greenstone type dated between 3000 and 2500 Ma. These rocks consist mainly of gneisses and granitoid rocks of tonalitic-trondhjemitic-granodioritic composition belonging to the Mudzi Metamorphic Complex in the northern part and to the Mavonde Complex in the southern part. The latter is associated with a granite-greenstone terrain, which includes the eastern part of Mutare-Odzi-Manica greenstone belt. A volcano-sedimentary sequences cover, belonging to the apparently Mesoproterozoic and Paleoproterozoic Umkondo and Gairezi groups respectively was deposited along the eastern margin of the craton and is exposed in the territory of Mozambique. The Umkondo minimum age is marked by intrusive dolerite in Zimbabwe dated at 1100 Ma while for the Ghairezi it is still not well established. The Gairezi Group was subjected to progressive metamorphism of Pan-African age. At the margin of the Zimbabwe Craton, in its northern part, metasedimentary units occur representing a passive margin of Neoproterozoic age. They make up the Rushinga Group, which includes felsic metavolcanic rocks dated at ca.800 Ma. Granulites and medium- to high-grade paragneisses, and migmatites of the Chimoio, Macossa and Mungari Groups of Neoproterozoic metamorphic age, overly the ortho-metamorphic pre-existing rock of ca. 1100 Ma, which belongs to the Báruè Magmatic Arc. They characterize the N-S trend Mozambique Belt, which appears to the east of the craton tectonically juxtaposed on the Archean rocks. The maximum age of deposition of these rocks, indicated by U-Pb dating of detrital zircons, is ca. 700 Ma and their minimum age is limited by a few monzonitic Cambrian intrusions dated at ca. 500 Ma. The Neoproterozoic bimodal Guro Suite, dated at ca. 850 Ma and composed of felsic and mafic members characterizes the east-dipping outer rim of the craton margin in the north. The felsic member comprises the Serra Banguatere aplitic granite gneiss-migmatite and the mafic member consists of the Magasso metagabbro and mafic gneiss-migmatite. The geochemical signature and bimodality are all characteristics of anorogenic, A-type granites. The tectono-thermal effects of the Pan-African orogenic event, of approximately 500 Ma, are visible along the margin of the Zimbabwe Craton. Deformation and metamorphism are progressive from the craton towards the belt, from greenschist facies to granulite facies. The main suture in the study area shall be placed along the frontal thrusts of the Mungari and Macossa/Chimoio nappes of Neoproterozoic to Cambrian age. To the west of the suture the rejuvenated margin of the craton occurs, indicated by K-Ar dating. To the east, the Mozambique Belt occurs with its paragneisses of the Neoproterozoic overlaying the Mesoproterozoic granitoids of the Báruè magmatic arc.
Transactions Indian Institute of Metallurgy, Vol. 74, 8, pp. 1969-1977. 9p. Pdf
India
deposit - Panna
Abstract: During diamond mining at Panna Mines [India], a huge amount of tailing (Kimberlite) is generated. About 6 Million Tonnes is accumulated and stored near mine site and about 0.9 Million Tonnes of tailings are further being generated each year. Till now, no established method is available for utilization of this material, effective utilization is essential for sustainable diamond mining. The Kimberlite contains about 32% [MgO?+?CaO] and 2% Al2O3, and rest are primarily silica, iron oxide [Fe2O3] and LOI. Initially, attempts were made to utilize Kimberlite as flux in iron ore pellet making; however, during green pellet making, it was observed that strength of pellets improved with higher dosages of Kimberlite. Accordingly, further studies were taken to replace conventional binder bentonite with binder prepared from Kimberlite. The iron ore pellet produced with Kimberlite-based binder have exhibited better physical and metallurgical characteristics than pellets produced with conventional binders (Bentonite).
Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: a record of pervasive, multi stage metasomatism in shallow refractory mantle.
Geochimica et Cosmochimica Acta, Vol. 70, 5, pp. 1231-1260.
Geostandards and Geoanalytical Research, in press available, 21p.
Asia, Mongolia
olivine
Abstract: A new olivine reference material - MongOL Sh11?2 - for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn?Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5-2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA?ICP?MS, SIMS and bulk analytical methods (ID?ICP?MS for Mg and Fe, XRF, ICP?MS) for major, minor and trace elements at six institutions world?wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1-2). The presence of some mineral and fluid?melt micro?inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty?seven major, minor and trace elements.
Abstract: Kimberlites are volcanic rocks that derive from deep in Earth’s mantle, but the nature of their source is uncertain. A study of this source’s evolution over two billion years provides valuable information about its properties.
Abstract: Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinópolis and Campo Belo/Bonfim Complexes in the southern São Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (TDM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17-20 kbar and 600-700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550-650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.
Abstract: We estimate density and P-wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P-wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P-wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250???300 km. These values correlate with high-velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean São Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high-density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re-Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P-wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere. The high-density PMP lithosphere is presently gravitationally unstable and prone to delamination.
REM, Int. Journal Ouro Preto, Vol. 73, 1, pp. 51-58. pdf
South America, Brazil, Minas Gerais
deposit - Canastra-3
Abstract: The study identifies the Canastra-3 Kimberlite magnetic anomaly as the likely primary source of the alluvial diamonds recovered by "garimpeiros" in the Santo Antônio River basin (Delfinópolis, southwestern Minas Gerais). This conclusion is based on cumulative geophysical, hydrographic, metallogenical and mineral geochemistry evidences. The study area is located within fertile ground in the border of the São Francisco craton, close to other diamond primary sources and secondary deposits. This kimberlitic target is the only known in the Santo Antônio River basin. In addition, the known mineralized gravels of this river, worked in the past by "garimpeiros", have evidence of a short transport (angular pebbles and blocks), further evidence of a nearby source. The original data collected in the "Minas Gerais Aerogeophysical Survey Program" was processed and analyzed with the Euler Deconvolution method, implemented in software Oasis Montaj. With the exception of the Canastra-3 body anomaly, all others in the study were classified as non-kimberlitic. Recent sampling work on the weathered top of the Canastra-3 Kimberlite recovered indicator minerals, notably a high proportion of pyrope garnets of the G-10 type, which is unusual among the kimberlites of the region.
Excursai de campo do 6 simposio Brasileiro de geologia do diamante - regiao diamantifera do Alto Paranaiba, Minas Gerais ( outline and historical documents for field excursion)
6 Simposio Brasileiro de Geologia do Diamante, Aug. 3-7, 27p.
Abstract: Almost half of the scholarly papers that people attempt to access online are now freely and legally available, according to a huge study that tracked 100,000 online requests for journal papers in June. The work, published on 2 August in PeerJ Preprints1, examined reader data from a web-browser extension called Unpaywall, which trawls the Internet to find free-to-read versions of paywalled papers. The tool, which launched in April, was developed by two authors of the study, Jason Priem and Heather Piwowar, who co-founded the non-profit company Impactstory in Vancouver, Canada. It has been installed by more than 80,000 people worldwide and is used around 50,000 times a day, says Priem.
Abstract: Lamproites and lamprophyres from Ryabinovoye gold deposit (Aldan Shield, Siberia) were studied. We demonstrate that these rocks, varying from Ol-Di-Phl-lamproites to syenite-porphyries, form a continuous series of lamproite magma differentiation. At the stage of phlogopite and clinopyroxene crystallization, silicate-carbonate and then carbonate-salt immiscibilities occur. A suggestion is that during these processes LREE, Y, U, Sr and Ba distribute to a phosphate-fluoride fraction and probably accumulate in apatite-fluorite gangues. Based on our results and considering existing data onore-bearing massifs within Central Aldan (lnagli, Ryabinoviy) and also of the Nam-Xe ore-bearing province (Vietnam), we concluded that Au, PGE and Th-U-Ba-REE deposits can be genetically connected with low-titanium lamproite magmas.
Abstract: The Guli and Bor-Uryakh massifs, a part of the Siberian Large igneous province (LIP) are mafic-ultramafic intrusive complexes, withstrongalkaline affinity. They contain deposits of apatite and arealsoknown to be source rocks ofOs-Ir-Ruplacers.These massifs are of great interest for petrologists worldwide, as they are composed of an unusual variety of rocks (dunites/olivinites, shonkinites, melilitites, alkali syenites and carbonatites) and being coeval with Siberian trap volcanic rocks, includingdiamondiferous kimberlites. Since mineralogical approaches based on spinel-group minerals have been proved to be efficient in constraining origin of the ultramafics, we present the first descriptive study of chromite and magnetite mineralization, observed in olivine-dominated rocks of the Guli and Bor-Uryakh intrusions. In dunites of Guli massif spinel-group minerals are dominated by Mg-poor chromite (FeMg)Cr2O4and Cr-Ti-rich magnetiteFeFe2O4, while in Bor-Uryakh massif spinel-group minerals are predominantly magnetite with only minor Mg-poor chromite.These minerals form either small euhedral inclusions in olivine or largesubhedral to anhedral grains in serpentinized fractures and interstitial space. The lattertype of grainscan have intricated irregular shapeand contain inclusions. We also observed abundant Cr-magnetite lamellae in olivine and chromite/magnetite micro-grains within olivine-hosted multiphase inclusions.Spinel (MgAl2O4) is occasionally found in intergrowths with chromite and magnetite.The obtained data show that spinel-group minerals in the massifsdo not correspond to primary-magmatic varieties and suggestextensive alteration during post-magmatic processes. Textural and chemical evidenceof substantial modification of initially-cumulative lithologies of Guli and Bor-Uryakh massifsfavorsmeta-magmatic origin for these massifs.
Doklady Earth Sciences, Vol. 481, 2, pp. 1008-1012.
Russia, Aldan shield
lamproite
Abstract: Obtained data shows that high-potassic dyke rocks of the Ryabinoviy massif (Central Aldan) belong to low-titanium lamproite series (Mediterranean type) and are distinct with “classic” high-titanium lamproites. Based on Al-in-olivine thermometer, temperature of olivine-chrome-spinel pair crystallization varies in range between 1100 and 1250°C. This suggests lithospheric mantle source for the parental melt and makes role of mantle plume insignificant. High-precision data on olivine composition and bulk rock traceelement composition imply mixed source for the parental melt, consisted of depleted peridotite and enriched domains, originated during ancient subduction.
Abstract: Mesozoic (125-135 Ma) cratonic low-Ti lamproites from the northern part of the Aldan Shield do not conform to typical classification schemes of ultrapotassic anorogenic rocks. Here we investigate their origins by analyzing olivine and olivine-hosted inclusions from the Ryabinoviy pipe, a well preserved lamproite intrusion within the Aldan Shield. Four types of olivine are identified: (1) zoned phenocrysts, (2) high-Mg, high-Ni homogeneous macrocrysts, (3) high-Ca and low-Ni olivine and (4) mantle xenocrysts. Olivine compositions are comparable to those from the Mediterranean Belt lamproites (Olivine-1 and -2), kamafugites (Olivine-3) and leucitites. Homogenized melt inclusions (MIs) within olivine-1 phenocrysts have lamproitic compositions and are similar to the host rocks, whereas kamafugite-like compositions are obtained for melt inclusions within olivine-3. Estimates of redox conditions indicate that “lamproitic” olivine crystallized from anomalously oxidized magma (?NNO +3 to +4 log units.). Crystallization of "kamafugitic" olivine occurred under even more oxidized conditions, supported by low V/Sc ratios. We consider high-Ca olivine (3) to be a fingerprint of kamafugite-like magmatism, which also occurred during the Mesozoic and slightly preceded lamproitic magmatism. Our preliminary genetic model suggests that low-temperature, extension-triggered melting of mica- and carbonate-rich veined subcontitental lithospheric mantle (SCLM) generated the kamafugite-like melts. This process exhausted carbonate and affected the silicate assemblage of the veins. Subsequent and more extensive melting of the modified SCLM produced volumetrically larger lamproitic magmas. This newly recognized kamafugitic "fingerprint" further highlights similarities between the Aldan Shield potassic province and the Mediterranean Belt, and provides evidence of an overlap between "orogenic" and "anorogenic" varieties of low-Ti potassic magmatism. Moreover, our study also demonstrates that recycled subduction components are not an essential factor in the petrogenesis of low-Ti lamproites, kamafugites and leucitites.
Baker, J., Chazot, G., Menzies, M.A., Thirlwall, M.
Lithospheric mantle beneath Arabia: a Pan-African protolith modified by the Afar and older plumes, rather than a source for continental flood volcanism?
Geological Society of America Special Paper, No. 362, pp. 65-80.
Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: a record of pervasive, multi stage metasomatism in shallow refractory mantle.
Geochimica et Cosmochimica Acta, Vol. 70, 5, pp. 1231-1260.
El Atrassi, F., Brunet, F., Chazot, G., Bouybaouene, M., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif ( northern Morocco): petrography, mineral chemistry and thermobarometry.
El Atrassi, F., Brunet, F., Chazot, G., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera Massif ( northern Morocco): mineralogical, chemical and textural records.
Abstract: Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.
Mollex, G., France, L., Furi, E., Bonnet, R., Botcharnikov, R.E., Zimmermann, L., Wilke, S., Deloule, E., Chazot, G., Kazimoto. E.O., Marty, B., Burnard, P.
Abstract: Cognate xenoliths that have been emitted during the last sub-plinian eruption in 2007-08 at Oldoinyo Lengai (OL) represent a unique opportunity to document the igneous processes occuring within the active magma chamber. Detailed petrographic descriptions coupled to a thermobarometric approach, and to the determination of volatile solubility models, allow us to identify the melt evolution at magma chamber conditions, and the storage parameters (P, T). Results indicate that a fresh phonolite melt (~1060°C) was injected into a crustal magma chamber at 11.5 ±3.5 km depth, in agreement with geophysical surveys performed during the eruption. The phonolite contains high volatile contents: 3.2 wt.% H2O and 1.4 wt.% CO2. The liquid line of descent highlights an evolution to nephelinite compositions by cooling down to 880°C. Our results support previous results related to this eruption, and are similar to the historical products emitted during the whole volcano history, allowing us to suggest that no major modification in the plumbing system has occured during the OL evolution. New noble gas results show that: i. fumaroles display constant He isotopic signature since 1988; ii. Cognate xenoliths documenting the active magma chamber and fumaroles display similar He isotopic values (6.58±0.46RA, and 7.31±0.40RA, respectively); iii. OL He isotopic composition is similar to that of other silicate volcanoes of the Arusha region, and comparable to the typical subcontinental lithospheric mantle (SCLM) range (5.2 to 7.0 RA); iv. Ne isotopic ratio of OL is following the MORB signature. Those results are interpreted as showing that 1/ no major modification in the hydrothermal system architecture has occured since 1988 despite major modification of the summit crater morphology, 2/ no contamination by either the atmospheric gases, or crustal material assimilation has occured between the magma chamber and the surface, and 3/ the source of OL and of the other silicate volcanoes in the Arusha region is a SCLM metasomatized by asthenospheric fluids.
Abstract: The article focuses on the study of composition of garnets of the lherzolitic and harzburgitic parageneses and the conditions of peridotite. As per the study, reconstruction of the conditions of metasomatism of peridotitic sources of kimberlite is possible in the evolution of garnet. It mentions the importance of dry and hydrous carbonatitic melt upon alteration of peridotitic sources of kimberlite as it acted as an another heat source.
Doklady Earth Sciences, Vol. 465, 2, pp. 1262-1267.
Russia
Deposit - Udachnaya
Abstract: Study of the mechanism of carbonation and wehrlitization of harzburgite upon metasomatism by carbonatitic melts of various genesis was carried out. Experiments with durations of 60-150 h were performed at 6.3 GPa and 1200°C. The data showed that carbonatite with MgO/CaO > 0.3 percolating into the peridotitic lithosphere may provide crystallization of magnesite in it. The influence of all studied carbonatites results in wehrlitization of peridotite. The compositions of melts formed by interaction with harzburgite (?2 wt % SiO2, Ca# = 36-47) practically do not depend on the composition of the initial carbonatite. Based on the data obtained, we conclude that the formation of magnesite-bearing and magnesite-free metasomatized peridotites may have a significant influence on the CO2 regime in the further generation of kimberlitic magmas of groups I and II.
Abstract: Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite-carbonatite-peridotite in graphite containers at 1200-1350 °C and 5.5-7.0 GPa in a split-sphere multianvil apparatus. Starting compositions are lherzolite and harzburgite, as well as carbonatite which may form in the upper part of a slab or in a plume-related source. Most experimental runs were of 150 h duration in order for equilibrium to be achieved. The interaction produced carbonatitic melts with low SiO2 (? 7 wt.%) and high alkalis. At 1200 °C, melt-peridotite interaction occurs through Mg-Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350 °C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO ? 4.3. The resulting peridotite-saturated melt has Ca# (37-50) depending on primary carbonatite composition. Compositions of silicate phases are similar to those of high-temperature peridotite but are different from megacrysts in kimberlites. CaO and Cr2O3 changes in garnet produced from the melt-harzburgite interaction at 1200 and 1350 °C perfectly match the observed trend in garnet from metasomatized peridotite of the Siberian subcontinental lithospheric mantle. K-rich carbonatite melts equilibrated with peridotite at 5.5-7.0 GPa and 1200-1350 °C correspond to high-Mg inclusions in fibrous diamond. Carbonatite melt is a weak solvent of entrained xenoliths and therefore cannot produce kimberlitic magma if temperatures are ~ 1350 °C on separation from the lithospheric peridotite source and ~ 1000 °C on eruption.
Abstract: Generation of ultra-alkaline melts by the interaction of lherzolite with cardonatites of various genesis was simulated at the P-T parameters typical of the base of the subcratonic lithosphere. Experiments with a duration of 150 h were performed at 5.5 and 6.3 GPa and 1350°C. The concentrations of CaO and MgO in melts are buffered by the phases of peridotite, and the concentrations of alkalis and FeO depend on the composition of the starting carbonatite. Melts are characterized by a low (<7 wt %) concentration of SiO2 and Ca# from 0.40 to 0.47. It is demonstrated that only high-Mg groups of carbonatitic inclusions in fibrous diamonds have a composition close to that of carbonatitic melts in equilibrium with lherzolite. Most likely, the formation of kimberlite-like melts relatively enriched in SiO2 requires an additional source of heat from mantle plumes and probably H2O fluid.
Russian Geology and Geophysics, Vol. 58, pp. 1222-1231.
Russia
carbonatite
Abstract: We present results of U-Pb (SHRIMP II) and Ar-Ar geochronological study of the rocks of the Chuktukon massif, which is part of the Chadobets alkaline-carbonatite complex, and of the weathering crust developed after them. Perovskite from picrites and monazite from the weathering crust were dated by the U-Pb (SHRIMP II) method, and rippite from carbonatites, by the Ar-Ar method. Rippite has first been used as a geochronometer. The estimated ages (252 ± 12 and 231 ± 2.7 Ma) testify to two magmatism pulses close in time (within the estimation error) to the stages of alkaline magmatism in the Siberian Platform (250-245 and 238-234 Ma). These pulses characterize, most likely, the processes accompanying and completing the activity of the mantle superplume that formed the Siberian Igneous Province at 250-248 Ma. The monazite-estimated age (102.6 ± 2.9 Ma) reflects the time of formation of the ore-bearing weathering crust on the massif rocks.
Contributions to Mineralogy and Petrology, Vol. 174, 4, doi.org/10. 1007/s00410-018-1530-x 13p.
Mantle
carbonatite
Abstract: Here we present an experimental study of the distribution of a broad range of trace elements between carbonatite melt, calcite and fluorite. The experiments were performed in the CaCO3 + CaF2 + Na2CO3 ± Ca3(PO4)2 synthetic system at 650-900 °C and 100 MPa using rapid-quench cold-seal pressure vessels. Starting mixtures were composed of reagent-grade oxides, carbonates, Ca3(PO4)2 and CaF2 doped with 1 wt% REE-HFSE mixture. The results show that the distribution coefficients of all the analyzed trace elements for calcite and fluorite are below 1, with the highest values observed for Sr (0.48-0.8 for calcite and 0.14-0.3 for fluorite) and Y (0.18-0.3). The partition coefficients of REE gradually increase with increasing atomic number from La to Lu. The solubility of Zr, Hf, Nb and Ta in the synthetic F-rich carbonatitic melts, which were used in our experiments, is low and limited by crystallization of baddeleyite and Nb-bearing perovskite.
Abstract: The petrogenesis of temporally and spatially associated carbonatitic and deeply derived carbonated alkaline silicate magmas provides an opportunity to gain insights into the nature of the deepest lithospheric mantle. The Chuktukon massif, which is part of the Chadobets alkaline ultramafic carbonatite complex (Chadobets upland, Siberian craton) is a carbonatite-melilitite-damtjernite intrusion, whose emplacement was coeval with the Siberian Traps large igneous province (LIP). In this study, the sources of the primary melts are examined, the petrogenetic evolution of the complex is reconstructed and the relationship with the Siberian LIP is also discussed. Isotopic and geochemical information indicate that the source for the Chuktukon primary melts was isotopically moderately depleted and the primarymelts were formed by lowdegree partial melting of garnet carbonated peridotite. Hydrothermal processes caused 18O- and 13C- enrichment. The weathering process was accompanied by trace element re-distribution and enrichment of the weathering crust in Zn, Th, U, Nb, Pb and REE, relative to the Chuktukon rocks and a change in radiogenic (Sr, Nd) isotope compositions.
Petrography, mineralogy and SIMS U-Pb geochronology of 1.0 - 1.8 Ga carbonatites and associated alkaline rocks of the Central Aldan magnesiocarbonatite province ( South Yakutia, Russia).
Mineralogy and Petrology, Doi.org/a0.1007/ s00710-019-00661-3 24p.
Abstract: The petrogenesis of temporally and spatially associated carbonatitic and deeply derived carbonated alkaline silicate magmas provides an opportunity to gain insights into the nature of the deepest lithospheric mantle. The Chuktukon massif, which is part of the Chadobets alkaline ultramafic carbonatite complex (Chadobets upland, Siberian craton) is a carbonatite-melilitite-damtjernite intrusion, whose emplacement was coeval with the Siberian Traps large igneous province (LIP). In this study, the sources of the primary melts are examined, the petrogenetic evolution of the complex is reconstructed and the relationship with the Siberian LIP is also discussed. Isotopic and geochemical information indicate that the source for the Chuktukon primary melts was isotopically moderately depleted and the primary melts were formed by low degree partial melting of garnet carbonated peridotite. Hydrothermal processes caused 18 O- and 13 C- enrichment. The weathering process was accompanied by trace element re-distribution and enrichment of the weathering crust in Zn, Th, U, Nb, Pb and REE, relative to the Chuktukon rocks and a change in radiogenic (Sr, Nd) isotope compositions.
Petrography, mineralogy and SIMS U-Pb geochronology of 1.9-1.8 Ha carbonatites and associated alkaline rocks of the Central-Aldan magnesiocarbonatite province ( South Yakutia, Russia).
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.
A New Occurrence of Garnet-ultra basite in the Caledonides; A Chromium Rich Chromite Garnet Lherzolite from Tvaerdalen, Liverpool Land, East Greenland.
Terra Cognita., Vol. 1, No. 1, P. 74. (abstract.).
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...
2015 IEEE 10th International Conference on Nan/Micro Engineered and Molecular Systems, NEMS 2015, pp. 372-374.
Technology
X-ray dectector
Abstract: Development synthesizing diamond film on silicon substrate was processed by Hot Filament Chemical Vapor Deposition (HFCVD). The gas processes using of H2 and CH4 to produce intrinsic diamond and MSM device constructed on the diaphragm of diamond film. Schottky junctions on the top and the lower diaphragm were produced by aluminum metal. After that, the result of detecting a direct and indirect X-ray of MSM diamond was satisfactory because it could respond along with increasing of X-ray intensity. The X-ray expose time of indirect expose by BaF2 scintillator faster more than direct expose.
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: In the central part of the European part of Russia in the southeastern part of the Kursk tectonic block, some deposits and occurrences of apatite genetically related to the alkaline-carbonatite complex have been revealed. The results of U-Pb analysis of titanite provided the first confident age estimate of silicate-carbonate (phoscorite) rocks in the Dubravin alkaline-ultramafic-carbonatite massif: they formed no later than 2080 ±13 Ma, which indicates their crystallization in the pre-Oskol time during the final stage of the Early Paleoproterozoic (post-Kursk time) stabilization phase of the Kursk block of Sarmatia (about 2.3-2.1 Ga).
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104284 11p. Pdf
China
REE
Abstract: The carbonatites in the southern margin of the North China Craton are distinguishable by containing abundant quartz and are closely spatially associated with Mo-(REE) deposits. Unveiling the nature of ore-forming fluids is key to understand the genesis of these Mo-(REE) deposits and to explore their potential genetic relationships with the quartz-rich carbonatites, but such issues were currently not convincingly addressed. Here, we provide detailed petrographic, microthermometric and LA-ICP-MS analyses of the fluid inclusions hosted in the primary quartz from the carbonatites in the Huangshuian Mo-(REE) deposit which is the largest Mo-(REE) one in the region, containing 0.4 million tons of Mo metal with REEs as the major by-product. Our results show that the fluid inclusions in the quartz of the carbonatites are two- and three-phase CO2-bearing types with high homogenization temperatures (average at 396 °C) and low salinities (average at 3.88 wt% NaCl equiv). The LA-ICP-MS analyses of these inclusions reveal that the primary fluids contain high concentrations of La, Ce, Pr, Nd, Sr, and Ba, similar to typical carbonatite-related fluids. In addition, they are characterized by high Y, Cu, Pb, and Zn. Such a metal association is broadly consistent with the mineral assemblages of the Huangshuian Mo-(REE) deposit, such as the widespread barite, bastnäsite, xenotime, chalcopyrite, galena, and sphalerite, strongly supporting the close genetic relation of the deposit with the quartz-rich carbonatites. Although the concentrations of Mo are extremely low in these inclusions (below the detect limit), it was constrained to be gradually enriched in evolved fluids. Considering that the recorded fluids in quartz represent earliest generation of fluids exsolved from carbonatite magmas, our new results highlight that quantifying metal budgets of fluid inclusions could be a robust way to evaluate fertility of carbonatites that are widespread in the southern margin of the North China Craton.
Isotopic constraints on age and duration of fluid assisted high pressure eclogite facies recrystallization during exhumation of deeply subducted continental crurs
Journal of Metamorphic Geology, Vol. 24, 8, pp. 687-702.
Tian, W., Chen, B., Ireland, T.R., Green, D.H., Suzuki, K., Chu, Z.
Petrology and geochemistry of dunites, chromitites and mineral inclusions from the Gaositai Alaskan type complex, North Chin a craton: mantle source characters
Abstract: Carbonates are common rock-forming minerals in the Earth’s crust and act as sinks of atmospheric carbon dioxide. Subduction of hydrothermally altered oceanic lithosphere returns carbon to the interior, where more than three quarters of Earth’s carbon is stored. The contribution of subducted carbonates to the Earth's long-term deep carbon cycle is uncertain and has recently emerged as a topic of intense debate [1]. Moreover, mantle-slab interaction has been proposed as a mechanism to produce super-deep diamonds, thus questioning the use of certain mineral inclusions to infer lower-mantle origin [2]. Here we report new data on the chemical stability and reaction kinetics of carbonates in the mantle from multianvil and diamond-anvil-cell experiments. Our results suggest that carbon can be sequestered into deep Earth through reaction freezing and that the index minerals for super-deep diamonds are not reliable indicators for their formation depths.
Abstract: Magnesite is proposed to be a major oxidized carbon storage phase in the mantle due to its wide P-T range of stability [1-2]. The presence of magnesite in the Earth's interior will depend on the redox state of the Earth's interior. Large part of the deep mantel is considered to be significantly reduced with considerable amount of FeO dispersed in rocks [3]. During slab-mantle interaction, subducted carbonates in the slab will undergo redox reactions with metallic Fe. However, the mechanism of this interaction is not well understood. In order to understand diamond genesis during the slabmantle interactions, we have conducted high-pressure and high-temperature experiments in a 2000-ton multi-anvil highpressure press on samples containing MgCO3 and iron foils (50 ?m thick) in BN capsules. The samples under pressures from 10 to 16 GPa were heated to 1200-1700 K. The samples were quenched under pressure and the quenched samples were polished and then analyzed with multi-wavelength micro-Raman spectrometers using 785, 514.5 and 532 nm laser excitations. Micro-Raman investigations show that the iron foils reduce MgCO3 to various sp2 carbon phases, mainly graphite, followed by the transformation to diamond upon long-duration heating. The transformation to diamond is driven by the temperature. For example, in the Run number PL066 with staring material containing magnesite and two Fe foils heated to 1400 K at 10 GPa for 24 hrs, and quenched, the run products were [Mg,Fe]O, and diamond and graphite. The sample PL044 with staring material containing magnesite and three Fe foils heated to 1600 K at 14 GPa for 12 hrs, the run products were larger size (~10 ?m) diamonds, iron carbide and small amount of graphite. Our results indicate that in slow subduction (T~1500 K) all carbonates will be converted in diamond and iron carbide. Under rapid subduction of the slab, the carbonate will survive and be carried to greater depth. The inclusions of [Mg,Fe]O in diamonds, however, do not necessarily indicate that this phase is of lower mantle origin.
Abstract: Carbon has been suggested as one of the light elements existing in the Earth's core. Under core conditions, iron carbide Fe7C3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe7C3, however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe7C3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V0 = 745.2(2) Å3, bulk modulus K0 = 167(4) GPa, its first pressure derivative K0? = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation ?T = 4.7(9) × 10-5 + 3(5) × 10-8 × (T - 300) K-1. We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth's inner core.
Geophysical Research Letters, Vol. 46, 4, pp. 1984-1992.
Mantle
diamond genesis
Abstract: Superdeep diamonds originate from great depths inside Earth, carrying samples from inaccessible mantle to the surface. The reaction between carbonate and iron may be an important mechanism to form diamond through interactions between subducting slabs and surrounding mantle. Interestingly, most superdeep diamonds formed in two narrow zones, at 250-450 and 600-800 km depths within the ~2,700?km?deep mantle. No satisfactory hypothesis explains these preferred depths of diamond formation. We measured the rate of a diamond forming reaction between magnesite and iron. Our data show that high temperature promotes the reaction, while high pressure does the opposite. Particularly, the reaction slows down drastically at about 475(±55) km depth, which may explain the rarity of diamond formation below 450 km depth. The only exception is the second zone at 600-800 km, where carbonate accumulates and warms up due to the stagnation of subducting slabs at the top of lower mantle, providing more reactants and higher temperature for diamond formation. Our study demonstrates that the depth distribution of superdeep diamonds may be controlled by reaction rates.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
Abstract: Diamonds are key messenger from the deep Earth because someare sourced from the longest isolated and deepest accessible regions of the Earth’s mantle. They are prime recorders of the carbon isotopic compositionof the Earth. The C isotope composition (d13C) of natural diamonds showsa widevariationfrom -41‰ to +3‰ with the primary mode at -5 ± 3‰ [1]. In comparison, the d13C values of chondrites and other planetary bodies range between -26‰ and -15‰ [2]. It is possible that some of the low d13C values were inherited from the Earth’s building blocks,but this is unlikely to be the sole explanation for all low d13C values that can reach as low as -41‰. Organic matter at the Earth’s surface that has low d13C values[3] has been regarded as a possible origin for low d13C values. However, organic carbon is usually accompanied by carbonate with higher d13C values (~0 ‰),and it is not clear why this d13C value does not appear frequently in diamonds. Low d13C diamonds were also formed by deposition from C-O-H fluids,but the equilibrium fractionationinvolved between diamonds and fluids issmall at mantle temperatures [1] and the low d13C values of diamonds can only be achieved after extensive Rayleigh distillation. One unique feature of the Earth isactive plate tectonics driven by mantle convection. Relatively oxidized iron and carbon species at the surface, such as carbonate, Fe2+-and Fe3+-bearing silicatesand oxides, are transported to the deep mantle by subducted slabs and strongly involved inthe redox reactions that generatediamonds [4]. The extent to which the isotopic compositionof C duringdiamond formation recordsredox processes that shaped the Earth is still controversial. Here we report onvibration properties of C andFe at high pressure in carbonates, diamond and Fe3C,based on nuclear resonant inelastic X-ray scattering measurements and density functional theory calculationsand further calculate equilibrium C isotope fractionations among these C-bearing species. Our results demonstrate that redox reactions in subducted slabs could generate eclogitic diamonds with d13C values as low as -41‰ if C in diamonds was sourced from the oxidation of a Fe-C liquid. The large C isotopic fractionation and potentially fast separation between diamonds and a Fe-C melt could enable diamond formation as high as 2%with d13C lower than -40‰.
Earth and planetary Science Letters, Vol. 579, 117359, 11p. Pdf
Mantle
ringwoodite
Abstract: The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances the Vp (P-wave velocity) jump, yet decreases the Vs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases both Vp and Vs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ?1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence of Vs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.
Geochemical Perspectives Letters, Vol. 17, pp. 11-15. pdf
Mantle
carbonatites
Abstract: Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer. To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report ?44/42Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol. Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to ?44/42Ca values of basalts, while non-primary carbonatites show detectable ?44/42Ca variations that are correlated to ?13C values. Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution. The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
Abstract: SiC and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a SiC-dominated ultra-reduced mineral assemblage, including SiC, TiC, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. SiC is 20-50 ?m in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of SiC polytypes were identified, which are dominated by ?-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These SiC are featured by 13C-depleted isotopic compositions (?13C = ?13.2 to ?22.8‰, average = ?17.7‰) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low ?13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
Abstract: Across much of North America, the contact between Precambrian basement and Paleozoic strata is the Great Unconformity, a surface that represents a >0.4 b.y.-long hiatus. A digital elevation model (DEM) of this surface visually highlights regional-scale variability in the character of basement topography across the United States cratonic platform. Specifically, it delineates Phanerozoic tectonic domains, each characterized by a distinct structural wavelength (horizontal distance between adjacent highs) and/or structural amplitude (vertical distance between adjacent lows and highs). The largest domain, the Midcontinent domain, includes long-wavelength epeirogenic basins and domes, as well as fault-controlled steps. The pronounced change in land-surface elevation at the Rocky Mountain Front coincides with the western edge of the Midcontinent domain on the basement DEM. In the Rocky Mountain and Colorado Plateau domains, west of the Rocky Mountain Front, structural wavelength is significantly shorter and structural amplitude significantly higher than in the Midcontinent domain. The Bordering Basins domain outlines the southern and eastern edges of the Midcontinent domain. As emphasized by the basement DEM, several kilometers of structural relief occur across the boundary between these two domains, even though this boundary does not stand out on ground-surface topography. A plot of epicenters on the basement DEM supports models associating intraplate seismicity with the Midcontinent domain edge. Notably, certain changes in crustal thickness also coincide with distinct changes in basement depth.
Earth and Planetary Science Letters, Vol. 481, pp. 223-235.
United States, Illinois, Indiana, Kentucky
geophysics - seismics Reelfoot Rift
Abstract: Seismic discontinuities between the Moho and the inferred lithosphere-asthenosphere boundary (LAB) are known as mid-lithospheric discontinuities (MLDs) and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution. In this study, we used S-to-P converted waves recorded by the USArray Transportable Array and the OIINK (Ozarks-Illinois-Indiana-Kentucky) Flexible Array to investigate lithospheric structure beneath the central U.S. This region, a portion of North America's cratonic platform, provides an opportunity to explore how terrane accretion, cratonization, and subsequent rifting may have influenced lithospheric structure. The 3D common conversion point (CCP) volume produced by stacking back-projected Sp receiver functions reveals a general absence of negative converted phases at the depths of the LAB across much of the central U.S. This observation suggests a gradual velocity decrease between the lithosphere and asthenosphere. Within the lithosphere, the CCP stacks display negative arrivals at depths between 65 km and 125 km. We interpret these as MLDs resulting from the top of a layer of crystallized melts (sill-like igneous intrusions) or otherwise chemically modified lithosphere that is enriched in water and/or hydrous minerals. Chemical modification in this manner would cause a weak layer in the lithosphere that marks the MLDs. The depth and amplitude of negative MLD phases vary significantly both within and between the physiographic provinces of the midcontinent. Double, or overlapping, MLDs can be seen along Precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks. A prominent negative Sp phase can be clearly identified at 80 km depth within the Reelfoot Rift. This arrival aligns with the top of a zone of low shear-wave velocities, which suggests that it marks an unusually shallow seismic LAB for the midcontinent. This boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late Precambrian/early Cambrian rifting. These observations suggest that the lithospheric structure beneath the Reelfoot Rift may be an example of a global phenomenon in which MLDs act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: A new data compilation shows that in intermediate to felsic rocks, zircon Eu/Eu* [chondrite normalized Eu/ graphic] correlates with whole rock La/Yb, which has been be used to infer crustal thickness. The resultant positive correlation between zircon Eu/Eu* and crustal thickness can be explained by two processes favored during high-pressure differentiation: (1) supression of plagioclase and (2) endogenic oxidation of Eu2+ due to garnet fractionation. Here we calibrate a crustal thickness proxy based on Eu anomalies in zircons. The Eu/Eu*-in-zircon proxy makes it possible to reconstruct crustal thickness evolution in magmatic arcs and orogens using detrital zircons. To evaluate this new proxy, we analyzed detrital zircons separated from modern river sands in the Gangdese belt, southern Tibet. Our results reveal two episodes of crustal thickening (to 60-70 km) since the Cretaceous. The first thickening event occurred at 90-70 Ma, and the second at 50-30 Ma following Eurasia-India collision. These findings are temporally consistent with contractional deformation of sedimentary strata in southern Tibet.
Journal of Petrology, Vol. 63, 4, 10.1093/perology/egac021
Mantle
peridotite
Abstract: The Co, Mn, Fe, and Ni contents of olivine phenocrysts and host basalts are sensitive to source mantle lithology, which suggests they may be used to constrain the processes of mantle melting and identify basalts formed from non-peridotitic (i.e. pyroxenitic) mantle sources. Here, we use a new comprehensive, forward model involving multiple parameters to simulate partitioning of Co and Mn during partial melting of the mantle in different tectonic settings: (1) polybaric continuous melting of peridotite mantle in mid-ocean ridges can generate melts that show decreasing Co and Mn with increasing degrees of melting so that the mid-ocean ridge basalts (MORBs) contain ~39-84 ?g/g Co and?~900-1600 ?g/g Mn; (2) flux-melting of the mantle wedge in subduction zones tends to produce a melt that has Co increasing from ~24 to 55 ?g/g and Mn from ~500 to 1110 ?g/g with increasing temperature; (3) melts produced by isobaric melting of the subcontinental lithospheric mantle are also sensitive to increasing temperature and have ~35-160 ?g/g Co and ~800-2600 ?g/g Mn; (4) decompression melting of peridotite related to the mantle plume generates melts containing ~45-140 ?g/g Co and?~1000-2000 ?g/g Mn, and the abundances of these metals decrease with increasing degrees of melting; and (5) partitioning behaviors of Co, Mn, and Ni during decompression melting of MORB-like pyroxenite contrast with those during decompression melting of peridotite due to the different mineralogy and compositions in mantle lithologies, and the MORB-like pyroxenite-derived melt is metal-poor with ~25-60 ?g/g Co, ~290-1600 ?g/g Mn, and?~160-340 ?g/g Ni. Although high-Ni, low-Mn forsteritic olivine phenocrysts and high melt Fe/Mn ratio have been proposed as diagnostic indicators of pyroxenitic components in the mantle, our models show that these features can be also generated by melting of peridotite at greater depth (i.e. a high pressure and temperature). To quantify the effect of high-pressure melting of peridotite on these diagnostic indicators, we modeled the correlations of melt Fe/Mn and olivine Co, Mn, and Ni contents with melting depth along the decompression melting path of a thermal plume. When Fe/Mn ratios of basalts and/or compositions of olivine phenocrysts deviate significantly from our modeled correlation lines, high-pressure melting of peridotite cannot explain these data, and the existence of pyroxenitic component in the mantle source is likely required. The pyroxenite-derived melt is modeled to be Ni-poor, but mixing with a peridotite-derived melt can strongly increase the partition coefficient of Ni between olivine and mixed melt, resulting in the generation of high-Ni olivine phenocrysts in plume-associated magmatic suites.
Isotopic constraints on age and duration of fluid assisted high pressure eclogite facies recrystallization during exhumation of deeply subducted continental crurs
Journal of Metamorphic Geology, Vol. 24, 8, pp. 687-702.
Journal of Petrology, 10.1093/petrology/egaa079 110p. Pdf
China
xenoliths
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ? 90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880 ? 970 oC), and low-Mg# (70 ? 80) plagioclase-bearing websterites with low equilibration temperatures (550 ? 835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd?=?+11.41, 87Sr/86Sr = ?0.7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0.703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14.20 ? -16.74, 87Sr/86Sr?=?0.7070 ? 0.7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr?=?0.706-0.711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian ocean and the Late Mesozoic lithospheric extension of eastern Asia.
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ?90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880-970 oC), and low-Mg# (70-80) plagioclase-bearing websterites with low equilibration temperatures (550-835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd = +11•41, 87Sr/86Sr = ?0•7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0•703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14•20- -16•74, 87Sr/86Sr = 0•7070-0•7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0•706-0•711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia.
Yoneda, A., Chen, G., Spetzler, H.A., Getting, I.G.
The effect of composition, temperature and pressure on the elasticity of olivine and garnet: implications for interpreting seismic velocity variations in mantle.
Abstract: We have found >10 in situ microdiamonds in thin sections of eclogites from the Dabie and Su-Lu regions of central eastern China since the first occurrence of microdiamond in eclogites from the Dabie Mountains (DMT) reported in 1992. The microdiamonds are found not only in the central part but also in the northern part of the DMT. Several free crystals have been recovered from the crushed eclogites from the central DMT. Most in situ microdiamonds are inclusions in garnets but a few larger ones are intergranular. Most of the diamondiferous eclogites in the central part of the DMT are associated with coesite. Most importantly, the observation of microdiamonds in northern Dabie lead us to question the supposition that this is a low-P metamorphic terrane. All the diamondiferous eclogites from both the north and central DMT are of continental affinity as demonstrated by their negative ?Nd values. Therefore, both the north and central eclogite belts in the DMT are considered to be from the deep subducted terrane. Five in situ microdiamonds and two free crystals are first reported in this paper. The dimensions of the in situ microdiamonds are 30-80 ?m and the free crystals are up to 400–-00 ?m across. All the microdiamonds are confirmed as such by Raman spectroscopy. The results of an infrared spectroscopic investigation on two larger free crystals and two in situ microdiamonds show that all the microdiamonds from both the Dabie and Su-Lu regions are mixed types IaA and IaB diamonds and there is no indication of any synthetic microdiamonds in our samples because such synthetic microdiamonds are always rich in type Ib.
Abstract: A modeling approach combining the entropy method and color difference formula is proposed in order to quantitatively evaluate diamond scintillation. The images of 66 diamonds were captured from 0° to 105° rotation at 15° intervals. The color difference of corresponding pixels in adjacent rotation angle images was calculated using a MatLab r2014a program, which indicated the diamond’s color change due to its scintillation. A threshold (10) was determined to divide the color difference into seven color difference intervals, the percentage of which indicated the color-change area. The color difference and the percentage were comprehensively analyzed using the entropy method to evaluate diamond scintillation objectively and quantitatively. Lightness was the main factor affecting the diamond scintillation while chroma and hue also significantly affected it.
Abstract: While mineral phases stable in the mantle transition zone (such as wadsleyite and ringwoodite) can store up to 3 wt% H2O, those in the lower mantle such as bridgmanite and ferropericlase can contain a very small amount (<50 ppm). While such dramatic differences can lead to dehydration/hydration and hydrous melting at 660-km depth in the mantle [1,2] it is uncertain how much water can be transported and stored at these depths. In order to answer this question, we have conducted a series of high pressure experiments in laser-heated diamondanvil cell and multi-anvil press combined with X-ray diffraction, infrared spectroscopy, laser Raman spectroscopy, and secondary ion mass spectrometry. Initially we examined the water storage capacity of dense (Al free) silica polymorphs at high pressure and temperature. We found that water can dramatically reduce the rutile-type to CaCl2-type phase transition from 55 GPa to 25 GPa and stabilize a new "disordered inverse" inverse NiAs-type phase at pressures above 50 GPa, which is not stable in dry SiO2 system. The CaCl2-type and NiAs-type silica polymorphs contain up to 8 wt% of H2O at 1400-2100 K up to at least 110 GPa. We next explored the effects of water on the mineralogy of the lower mantle and found that hydrous Mg2SiO4 ringwoodite (1 wt% H2O) breaks down to silica + bridgmanite + ferropericlase at pressures up to 60 GPa and 2100 K. The recovered silica samples contain 0.3-1.1 wt% H2O, suggesting that water stabilizes silica even under Si-undersaturated systems because of their large water storage capacity. Therefore, our observations support the stability of silica in hydrous regions in the pyrolitic lower mantle. In the subducting oceanic crust (basalt and sediment), silica represents 20-80% of the mineralogy. Because its stability range spans the mantle transition zone to the deep mantle, hydrous silica is expected to play a major role in the transport and storage of water in the deep mantle.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 268.
China
synthetics
Abstract: China is the world’s largest producer of HPHT-grown industrial diamonds. Its 2016 production of about 20 billion carats accounted for 98% of the global supply. Since the beginning of 2015, meleesized colorless HPHT synthetic diamonds have been tested at the National Gemstone Testing Center’s (NGTC) Shenzhen and Beijing laboratories in parcels submitted by different clients, which means that colorless HPHT synthetic diamonds have entered the Chinese jewelry market and may be mistaken for natural diamonds. CVD synthesis technology has grown rapidly in recent years. Large colorless and colored (blue, pink) CVD-grown diamonds have been entering the market, and a few have been fraudulently sold as natural diamonds. China has independently developed gem-grade HPHT synthetic diamond production technology since 2002, and can grow gem-grade type Ib, IIa, and IIb and high-nitrogen-content synthetic diamonds in volume, depending on market needs. Gemgrade type Ib, IIa, and IIb HPHT synthetic diamonds have been grown using the temperature gradient method, under a cubic press at high pressure (e.g., 5.4 GPa) and high temperature (1300-1600°C). Driven by a specific temperature gradient, the carbon source from high-purity graphite (>99.9%) located at the high-temperature zone can diffuse into the seed crystals in the cubic press, resulting in the crystallization of synthetic diamonds. Chinese production of melee-sized colorless to near-colorless HPHT synthetic diamonds accounts for about 90% of the global output. Gem-grade type IIa and IIb CVD synthetic diamonds are grown using the microwave plasma chemical vapor deposition (MPCVD) and direct current (DC) arc plasma methods. Faceted colorless CVD diamonds can be grown in sizes up to 6 ct by at least two Chinese companies (table 1). After testing and analyzing thousands of natural and synthetic diamonds collected directly from the Chinese companies, NGTC independently developed the GV5000, PL5000, DS5000, and ADD6000 instruments for rapidly screening and identifying the diamonds based on the gemological characteristics obtained. Besides HPHT and CVD synthetic diamonds, a thickly layered hybrid diamond consisting of both natural and CVD material was identified at the NGTC Beijing laboratory (figure 1). The identification features and properties of regrown CVD synthetic diamonds using natural type Ia diamond crystals as seeds will be reported. The current status and features of colored stones examined at NGTC laboratories, including several cases studies, will be discussed.
Abstract: The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the Qinling Orogen, Central China. The deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs, some of which (e.g., U, Se, and Sr) reaching super-large scale. Pyrochlore, allanite, monazite, barite-celestite and galena are the major ore minerals at Huayangchuan. Uranium is mainly hosted in the primary mineral of pyrochlore, and the mineralization is mainly hosted in or associated with carbonatite dikes. According to the mineral assemblages and crosscutting relationships, the alteration/mineralization at Huayangchuan comprises four stages, i.e., pegmatite REE mineralization (I), main mineralization (II), skarn mineralization (III) and post-ore alteration (IV). Coarse-grained euhedral allanite is the main Stage I REE mineral, and the pegmatite-hosted REE mineralization (ca. 1.8 Ga) occurs mostly in the shallow-level of northwestern Huayangchuan, corresponding to the Paleoproterozoic Xiong'er Group volcanic rocks (1.80-1.75 Ga) in the southern margin of North China Block. Carbonatite-hosted Stage II mineralization contributes to the majority of U-Nb-REE-Ba-Sr resources, and is controlled by the Huayangchuan Fault. Stage II mineralization can be further divided into the sulfate mineralization (barite-celestite) (II-A), alkali-rich U mineralization (aegirine-augite + pyrochlore + uraninite + uranothorite) (II-B) and REE (allanite + monazite + chevkinite)-U (pyrochlore + uraninite) mineralization (II-C) substages. Stage II mineralization may have occurred during the Late Triassic Mianlue Ocean closure. Skarn mineralization contributed to the majority of Pb and minor U-REE (uraninite-allanite) resources at Huayangchuan, and is spatially associated with the Late Cretaceous-Early Jurassic (Yanshanian) Huashan and Laoniushan granites. We suggested that hydrothermal fluids derived from the Laoniushan and Huashan granites may have reacted with the Triassic carbonatites, and formed the Huayangchuan Pb skarn mineralization. The mantle-derived Triassic carbonatites may have been fertilized by the U-rich subducting oceanic sediments, and were further enriched through reacting with the Proterozoic U-REE-rich pegmatite wallrocks at Huayangchuan. Ore-forming elements were likely transported in metal complexes (F?, and ), and deposited with the dilution of the complex concentration. This may have formed the distinct vertical mineralization zoning, i.e., sodic fenite-related alkali-U mineralization at depths and potassic fenite-related REE-U mineralization at shallow level.
American Mineralogist, Vol. 105, pp. 1342-1348. pdf
Mantle
bridgmanite
Abstract: To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.
Abstract: The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the Qinling Orogen, Central China. The deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs, some of which (e.g., U, Se, and Sr) reaching super-large scale. Pyrochlore, allanite, monazite, barite-celestite and galena are the major ore minerals at Huayangchuan. Uranium is mainly hosted in the primary mineral of pyrochlore, and the mineralization is mainly hosted in or associated with carbonatite dikes. According to the mineral assemblages and crosscutting relationships, the alteration/mineralization at Huayangchuan comprises four stages, i.e., pegmatite REE mineralization (I), main mineralization (II), skarn mineralization (III) and post-ore alteration (IV). Coarse-grained euhedral allanite is the main Stage I REE mineral, and the pegmatite-hosted REE mineralization (ca. 1.8 Ga) occurs mostly in the shallow-level of northwestern Huayangchuan, corresponding to the Paleoproterozoic Xiong'er Group volcanic rocks (1.80-1.75 Ga) in the southern margin of North China Block. Carbonatite-hosted Stage II mineralization contributes to the majority of U-Nb-REE-Ba-Sr resources, and is controlled by the Huayangchuan Fault. Stage II mineralization can be further divided into the sulfate mineralization (barite-celestite) (II-A), alkali-rich U mineralization (aegirine-augite + pyrochlore + uraninite + uranothorite) (II-B) and REE (allanite + monazite + chevkinite)-U (pyrochlore + uraninite) mineralization (II-C) substages. Stage II mineralization may have occurred during the Late Triassic Mianlue Ocean closure. Skarn mineralization contributed to the majority of Pb and minor U-REE (uraninite-allanite) resources at Huayangchuan, and is spatially associated with the Late Cretaceous-Early Jurassic (Yanshanian) Huashan and Laoniushan granites. We suggested that hydrothermal fluids derived from the Laoniushan and Huashan granites may have reacted with the Triassic carbonatites, and formed the Huayangchuan Pb skarn mineralization. The mantle-derived Triassic carbonatites may have been fertilized by the U-rich subducting oceanic sediments, and were further enriched through reacting with the Proterozoic U-REE-rich pegmatite wallrocks at Huayangchuan. Ore-forming elements were likely transported in metal complexes (F?, and ), and deposited with the dilution of the complex concentration. This may have formed the distinct vertical mineralization zoning, i.e., sodic fenite-related alkali-U mineralization at depths and potassic fenite-related REE-U mineralization at shallow level.
Song, S., Zhang, L., Chen, J., Liou, J.G., Niu, Y.
Sodic amphibole exsolutions in garnet from garnet-peridotite, North Qaidam UHP belt, NW China: implications for ultradeep origin and hydroxyl defects in mantle garnets.
Journal of Volcanology and Geothermal Research, Vol. 310, pp. 1-11.
China
LiDAR
Abstract: Dyke geometries are useful indicators of the palaeostress field during magma emplacement. In this paper, we present a multi-scale extraction method of dyke geometries by integrating WorldView-2 (WV2) imagery and terrestrial light detection and ranging (LiDAR) data. Color composite and fusion WV2 images with 0.5-m resolution were generated by using the Gramm-Schmidt Spectral Sharpening approach, which facilitates the discrimination of dyke swarms and provides the ability to measure the orientation, exposed length, and thickness of dykes in sub-horizontal topographic exposures. A terrestrial laser scanning survey was performed on a sub-vertical exposure of dykes to obtain LiDAR data with point spacing of ~ 0.02 m at 30 m. The LiDAR data were transformed to images for extracting dyke margins based on image segmentation, then the dyke attitudes, thicknesses, and irregularity of dyke margins were measured according to the points on dyke margins. This method was applied at Sijiao Island, Zhejiang, China where late Cretaceous mafic dyke swarms are widespread. The results show that integrating WV2 imagery and terrestrial LiDAR improves the accuracy, efficiency, and objectivity in determining dyke geometries in two and three dimensions. The ENE striking dykes are dominant, and intruded the host rock (mainly granite) with sub-vertical dips. Based on the aspect ratios of the dykes, the magmatic overpressure was estimated to be less than 11.5 MPa, corresponding to a magma chamber within 6.6 km in the lithosphere.
Abstract: Modern high-pressure experimental techniques have enabled us to achieve the pressure and temperature at the center of Earth (about 360 GPa and 6000 K) in laboratories. However, studies of rheological properties of minerals under controlled strain rate (creep experiments) have been limited to the pressure equivalent to that in Earth's transition zone, a depth only about one-tenth of Earth's radius. Determinations of rheological laws that govern the flows and viscosities of minerals in Earth's deep mantle have been far beyond our reach. In the absence of such critical data, the nature of mantle dynamics-such as whether the convection involves the entire lower mantle, yielding a chemically homogeneous deep mantle-remains controversial. Discovery of the breakdown of ringwoodite into the denser bridgmanite and magnesiowüstite phases at 24 GPa (1) removed the need for a major chemical discontinuity in Earth inferred from observations of a strong seismic reflector at 660 km depth. On page 144 of this issue, Girard et al. (2) report on the detailed rheological nature of this bridgmanite plus magnesiowüstite mineral aggregate, shedding more light on the mantle convection. The integration of brilliant synchrotron radiations and rotating apposed anvils enables creep experiments for large strain at pressures equivalent to that in Earth's lower mantle.
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.
Abstract: We report the first occurrence of magmatic haggertyite (BaFe6Ti5MgO19) from the Miocene lamproites of the West Kimberley region of Western Australia. This contrasts with the metasomatic formation reported in an olivine lamproite host at the type locality, Prairie Creek, Arkansas. Haggertyite occurs in the groundmass of a diamondiferous olivine lamproite pipe in the Ellendale field, and within the large zoned Walgidee Hills lamproite where it forms part of an extensive suite of Ba- and K-bearing titanate and Ti-rich silicate minerals. The haggertyite co-exists with chromian spinel, perovskite, and ilmenite in the Ellendale lamproite, and with priderite and perovskite and, in one locality, with priderite, jeppeite, ilmenite, and perovskite, in the Walgidee Hills lamproite. Unlike priderite and perovskite, which are common groundmass phases in the Ellendale olivine lamproites and present throughout the Walgidee Hills lamproite, haggertyite appears restricted in its occurrence and crystallization interval, with sparse ilmenite apparently mostly crystallizing as an alternative phase. In the Walgidee Hills lamproite the haggertyite-bearing assemblage is succeeded by the Ba-titanate assemblage priderite plus jeppeite in the evolved central part of the body. The haggertyite in the main zone of the Walgidee Hills lamproite has an average composition of (Ba0.7K0.3)1.0(Ti5.0Fe3+2.1Cr0.1Fe2+3.8Mn0.2Mg0.6Na0.1)12O19 and is thus very similar to the original haggertyite described from xenoliths in the Prairie Creek lamproite apart from being poorer in Cr and Ni. Haggertyite in the groundmass of the Ellendale olivine lamproite and the central zone of the Walgidee Hills lamproite, in addition to variations in Mg and Cr, show significant variation in Ti and Fe contents and in calculated Fe3+ and Fe2+. A linear inverse relationship between Ti and Fe, and Ti and Fe3+, indicates that Fe3+ is accommodated by the coupled substitution Ti4+ + Fe2+ ? 2 Fe3+. A marked trend to higher Fe3+ in the haggertyite in Ellendale 9 olivine lamproite is ascribed to increasing oxidation during crystallization, with fO2 estimated from the olivine-spinel thermometer and oxygen barometer at Dlog FMQ = -1 to +3 at temperatures of 790-660 °C. The haggertyite in the central zone of the Walgidee Hills lamproite, in contrast, shows a marked trend to Fe2+ enrichment, which is associated with decreasing Fe in perovskite. This is inferred to indicate formation under more reducing conditions, but sufficiently oxidized to permit Fe3+ in co-existing priderite and jeppeite. Trace-element analysis by LA-ICP-MS shows the Walgidee Hills haggertyite contains minor amounts of Na, Si, Ca, V, Co, Zn, Sr, Zr, Nb, and Pb, and only traces of Al, P, Sc, Rb, REE, Hf, and Ta. Moreover, the haggertyite is preferentially enriched in certain lithophile (Ba, Sr), siderophile (Mn, Fe, Co, Ni), and chalcophile (Zn, Pb) elements relative to co-existing priderite. Haggertyite crystallization appears to be a consequence not only of the very high Ba, Ti, and K contents of the lamproite, but of relatively high-Fe concentrations and low temperatures in evolved olivine lamproite magma with the Fe3+/Fe2+ ratio determined by the prevailing fO2. The new data suggest that haggertyite might also be present but previously unrecognized in the evolved groundmass of other olivine lamproites. Haggertyite is one of an increasing number of new minerals in upper mantle rocks and volcanics derived from the upper mantle hosting large-ion-lithophile and high field strength cations.
Contributions to Mineralogy and Petrology, 176, 16p. Pdf
Mantle
geothermobarometry
Abstract: The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25-4.5 GPa and 1100-1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ? 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10-100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule (lnDNigrt/olv;NigrtNiolv), the Ca mole fraction in garnet (XCagrt; Ca/(Ca?+?Fe?+?Mg)), and the Cr mole fraction in garnet (XCrgrt; Cr/(Cr?+?Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T(?C)=?8254.568((XCagrt×3.023)+(XCrgrt×2.307)+(lnDNigrtolv?2.639))?273±55 where DNigrt/olv=NigrtNiolv, Ni is in ppm, XCagrt = Ca/(Ca?+?Fe?+?Mg) in garnet, and XCrgrt= Cr/(Cr?+?Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541-554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.
Episodic growth of zircon in UHP orthogneisses from the North Dabie Terrane of east central China: implications for crustal architecture of a collisional orogen.
Journal of Metamorphic Geology, In press available,
Abstract: The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago.
Chen, Y.X., Zheng, Y-F., Chen, R-X., Zhang, S-B., Li, Q., Dai, M., Chen, L.
Metamorphic growth and recrystallization of zircons in extremely 18 O depleted rocks during eclogite facies metamorphism: evidence from U-Pb ages, trace elements and O-Hf isotopes.
Geochimica et Cosmochimica Acta, Vol. 75, 17, pp. 4877-4898.
Earth and Planetary Science Letters, Vol. 570, 117088
Mantle
bridgemanite
Abstract: Water solubility in the dominant lower-mantle bridgmanite phase remains controversial. Discrepancies between previous results highlight the importance of the growth high-quality single crystals of bridgmanite under high-pressure and high-temperature conditions corresponding to the top of the lower mantle. Here we synthesized high-quality single crystals of aluminous bridgmanite up to 300 ?m in size that were saturated with hydrous melt at 24-26 GPa and 1700-1900 K using both stoichiometric and MgO-rich non-stoichiometric hydrous starting materials in a multi-anvil press. Fourier-transform infrared spectroscopy measurements on clear and pure spots of the single-crystal bridgmanites did not detect any pronounced OH-stretching bands, which were prominent in some earlier studies. The present results support that the lower-mantle dominated bridgmanite is nearly dry, at least at the top of the lower mantle, and that Al3+ and Fe3+ cannot enhance water incorporation into the crystal structure even in the presence of oxygen vacancies. Large partition coefficients of water between transition-zone minerals and dry lower-mantle dominated bridgmanite further support dehydration melting at the top of the lower mantle. We suggest that the majority of the top of a pyrolitic lower mantle is nearly dry based on the dry principal minerals and stability of hydrous phases in this region.
Abstract: The detailed phase composition and characteristics of diamond crystals grown in the metal-silicate-H2O-C system at 5.5 GPa and 1385 °C are reported in this paper. The conversion efficiency of the graphite-to-diamond in the metal-silicate-C system is lower than that in the metal-C system, which significantly decreases the growth rate of crystal. As the Mg2Si3O8•5H2O content increases to 1.5 wt%, growth pits and {110} related features of trigonal pyramids, skeletal structure, rhombic dodecahedron, and {110} dendrites exhibit in sequence. Simultaneously, the content of graphite and metal inclusions inside the crystal increases. These systematic changes are accompanied by the appearance of Csingle bondH, Csingle bondO, and Cdouble bondO bonds and a decrease of nitrogen content from ?210 ppm to ?60 ppm. It is speculated that H2O will further decompose and bond with carbon atoms and finally enter the diamond structure. The formation of Csingle bondH and Cdouble bondO bonds will terminate the extension of the three-dimensional network of Csingle bondC bonds. These defects will accumulate along the [111] direction and form {110} related characteristics. These chemical bonds also compete with the nitrogen in the system during entering into the diamond lattice. Our experimental model may provide implications for the morphology and formation environment of natural diamonds.
Abstract: Removal and thinning of cratonic lithosphere is believed to have occurred under different tectonic settings, for example, near subduction zones and above mantle plumes. Subduction-induced cratonic modification has been widely discussed; however, the mechanisms and dynamic processes of plume-induced lithospheric removal remain elusive and require further systematic investigation. In this study, we conduct a series of 2-D thermo-mechanical models to explore the dynamics of the removal and thinning of cratonic lithosphere due to the interaction between a mantle plume and a weak mid-lithosphere discontinuity (MLD) layer. Our modeling results suggest that the interaction between a mantle plume and weak MLD layer can lead to a large-scale removal of the cratonic lithosphere as long as the connection between the hot upwelling and weak MLD layer is satisfied. The presence of a vertical lithospheric weak zone and its closeness to the plume center play critical roles in creating a connection between the weak MLD and hot plume/asthenosphere. Furthermore, delamination of cratonic lithosphere is favored by a larger plume radius/volume, a higher plume temperature anomaly, and a lower viscosity of the MLD layer. A systematic comparison between subduction-induced and plume-induced lithospheric thinning patterns is further conducted. We summarize their significant differences on the origin and migration of melt generation, the water content in melts, and topographic evolution. The combination of numerical models and geological/geophysical observations indicates that mantle plume-MLD interaction may have played a crucial role in lithospheric removal beneath South Indian, South American and North Siberian Cratons.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U-Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Lithospheric and asthenospheric sources of lamprophyres in the Jiadong Peninsula: a consequence of rapid lithospheric thinning beneath the North Chin a craton?
Geochimica et Cosmochimica Acta, Vol. 124, pp. 250-271.
Abstract: The accretionary mobile belts surrounding ancient cratonic cores are an important facet of the growth and preservation of continental landmasses. Peridotites from Nuominhe in the Xing'an Mongolia Orogenic Belt (XMOB) provide an additional opportunity to examine the age, structure and evolution of mantle lithosphere separating two of the largest existing ancient continental nuclei: the North China Craton and the Siberian Craton. This suite of mantle rocks comprises fertile to refractory garnet- and spinel-facies harzburgites and lherzolites. Their lithophile element systematics show that the peridotites were metasomatized to variable extent by silicate?carbonate melts. Despite this, the highly siderophile element and Os isotope systematics appear to have been largely undisturbed. The Nuominhe peridotites have Re-depletion Os model ages (TRD) that range from 0.5 Ga to 2.4 Ga, with three peaks/major ranges at ~2.0-2.4 Ga, ~1.4-1.5 Ga and ~ 0.8 Ga, of which the latter two are closely similar to those data from other XMOB localities reported in the literature. The only section of the mantle that appears to have ages which correlate with crust formation is the suite with Neoproterozoic (~0.8 Ga) depletion ages, while the older mantle domains document older episodes of mantle depletion. Given the lack of correlation between equilibrium temperatures and bulk composition or TRD ages, the Nuominhe peridotites were inter-mixed in the mantle column, most likely as a result of incorporation of recycled older continental mantle fragments into juvenile Neoproterozoic mantle during the orogenic processes responsible for new lithosphere formation. Geothermobarometry of the Nuominhe peridotites indicates a conductive geotherm of ~60 mWm?2 and therefore a lithosphere thickness of ~125 km, which is thicker than most Phanerozoic continental terranes, and even thicker than Proterozoic regions that comprise the larger cratonic unit of the Siberian craton. This thick Proterozoic lithosphere sandwiched between the converging North China and Siberian cratons was evidently partly constructed from recycled refractory continental mantle fragments, perhaps extant in the convecting mantle, or in-part derived from the surrounding cratons, leading to a composite nature of the mantle in this re-healed continental suture. Re-accretion of recycled refractory old continental mantle fragments plays a significant role in affecting mantle composition and controlling the thickness of circum-cratonic landmasses between cratonic blocks.
Geochimica et Cosmochimica Acta, Vol. 295, pp. 207-223.
China
nephelinites, basanites
Abstract: Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM?=?1.1?±?0.8, 1?), relatively high total PGE contents (e.g., Ir?=?0.25?±?0.14?ppb) and modern mantle-like 187Os/188Os (0.142?±?0.020). These characteristics are coupled with lighter Mg isotope (?26Mg?=??0.48?±?0.07‰) and heavier Zn isotope (?66Zn = +0.46?±?0.06‰) compositions compared to the mantle values (?26Mg: ?0.25?±?0.07‰; ?66Zn: +0.18?±?0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN?=?4.4?±?3.3; Ir?=?0.037?±?0.027?ppb) and radiogenic 187Os/188Os (0.279?±?0.115) coupled with less fractionated Mg-Zn isotope compositions (?26Mg: ?0.39?±?0.05‰; ?66Zn: +0.35?±?0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U–Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Economic Geology Research Institute 2015, Vol. 17,, # 1147, 1p. Abstract
China
Peridotite, xenoliths
Abstract: The mantle process during the Early Permian Tarim plume event is revealed by flood basalt and mantle xenoliths. Permian Tarim flood basalts have typical two pulses' eruption. The first pulse of the Tarim flood basalt was erupted at 291-290Ma, characterized by OIB-like Zr/Nb (~5.83), Nb/La and Ce/Pb ratios, and PUM-like initial 187Os/188Os ratios (0.1308-0.1329). They're plotted along a 290±11Ma isochron, implying a pristine "plume mantle" source. The second pulse of the Tarim flood basalt was erupted at 283-281 Ma, with Zr/Nb (~13.6), Nb/La and Ce/Pb ratios similar or close to the lower crust and initial 187Os/188Os ratios (0.1743~19.6740) that deviated from the ~290 Ma isochron line, indicative of significant crustal assimilation. Mantle-derived peridotite and pyroxenite xenoliths hosted in Cenozoic alkali basalts (~20 Ma) are found in the Xikeer, western Tarim Block. Based on their petrographic and geochemical characteristics, peridotite xenoliths can be divided into three groups. Group 1 peridotites, with the presence of the high Mg-number of olivines (91-93) and spinel-pyroxenes clusters, experienced high-degree melt extraction (~17% fractional melting) from garnet- to spinel-stable field. Groups 2 and 3 peridotites, characterized by the clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively, underwent extensive silicate melt metasomatism at low melt/rock ratios (15) and that the host basanite is incapable of being the metasomatic agent. The Re-Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an isochron of 290±11 Ma, virtually identical to the age of Tarim flood basalts. Their PUM-like Os initial ratios and convecting mantle-like É?Nd(t=290 Ma) strongly suggest that the Xikeer mantle xenoliths derive from the plume mantle. We propose that the Xikeer xenolith suite recorded mantle 'auto-refertilization' process, i.e., they may have been initially formed by melt extraction from the convecting mantle and, shortly after, was refertilized by plume melts during the Early Permian.
Abstract: Mineral inclusions in diamonds have been used to track potential information on the Earth's deep mantle. Here we report results from a detailed study on the mineral inclusions in a ca. 0.28 ct diamond from the Shengli No. 1 kimberlite in Mengyin County, Shandong Province, eastern China. Our study reveals the presence of ?-quartz, albite and olivine in the diamond. At an inferred depth of ca. 165 km for the diamond crystallization, the inclusions of ?-quartz and albite suggest the possible involvement of deep subducted crustal material, traces of which were captured during the diamond growth and magma migration.
Abstract: Maohokite, a post?spinel polymorph of MgFe2O4, was found in shocked gneiss from the Xiuyan crater in China. Maohokite in shocked gneiss coexists with diamond, reidite, TiO2?II, as well as diaplectic glasses of quartz and feldspar. Maohokite occurs as nano?sized crystallites. The empirical formula is (Mg0.62Fe0.35Mn0.03)2+Fe3+2O4. In situ synchrotron X?ray microdiffraction established maohokite to be orthorhombic with the CaFe2O4?type structure. The cell parameters are a = 8.907 (1) Å, b = 9.937(8) Å, c = 2.981(1) Å; V = 263.8 (3) Å3; space group Pnma. The calculated density of maohokite is 5.33 g cm?3. Maohokite was formed from subsolidus decomposition of ankerite Ca(Fe2+,Mg)(CO3)2 via a self?oxidation?reduction reaction at impact pressure and temperature of 25-45 GPa and 800-900 °C. The formation of maohokite provides a unique example for decomposition of Fe?Mg carbonate under shock?induced high pressure and high temperature. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017?047). The mineral was named maohokite after Hokwang Mao, a staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, for his great contribution to high pressure research.
Abstract: Magnetic monopoles play a central role in various areas of fundamental physics, ranging from electromagnetism to topological states of matter. While their observation is elusive in high-energy physics, monopole sources of artificial gauge fields have been recently identified in synthetic matter. String theory, a potentially unifying framework that encompasses quantum mechanics, promotes the conventional \emph{vector} gauge fields of electrodynamics to \emph{tensor} gauge fields, and predicts the existence of more exotic \emph{tensor monopoles} in 4D space. Here we report on the characterization of a tensor monopole synthesized in a 4D parameter space by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we characterize the tensor monopole by measuring its quantized topological charge and its emanating Kalb-Ramond field. By introducing a fictitious external field that breaks chiral symmetry, we further observe an intriguing transition in the spectrum, characterized by spectral rings protected by mirror symmetries. Our work represents the first detection of tensor monopoles in a solid-state system and opens up the possibility of emulating exotic topological structures inspired by string theory.
Journal of Volcanology and Geothermal Research, Vol. 310, pp. 1-11.
China
LiDAR
Abstract: Dyke geometries are useful indicators of the palaeostress field during magma emplacement. In this paper, we present a multi-scale extraction method of dyke geometries by integrating WorldView-2 (WV2) imagery and terrestrial light detection and ranging (LiDAR) data. Color composite and fusion WV2 images with 0.5-m resolution were generated by using the Gramm-Schmidt Spectral Sharpening approach, which facilitates the discrimination of dyke swarms and provides the ability to measure the orientation, exposed length, and thickness of dykes in sub-horizontal topographic exposures. A terrestrial laser scanning survey was performed on a sub-vertical exposure of dykes to obtain LiDAR data with point spacing of ~ 0.02 m at 30 m. The LiDAR data were transformed to images for extracting dyke margins based on image segmentation, then the dyke attitudes, thicknesses, and irregularity of dyke margins were measured according to the points on dyke margins. This method was applied at Sijiao Island, Zhejiang, China where late Cretaceous mafic dyke swarms are widespread. The results show that integrating WV2 imagery and terrestrial LiDAR improves the accuracy, efficiency, and objectivity in determining dyke geometries in two and three dimensions. The ENE striking dykes are dominant, and intruded the host rock (mainly granite) with sub-vertical dips. Based on the aspect ratios of the dykes, the magmatic overpressure was estimated to be less than 11.5 MPa, corresponding to a magma chamber within 6.6 km in the lithosphere.
Journal of Asian Earth Sciences, Vol. 214, 104772, 8p. Pdf
Mantle
carbon
Abstract: The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.
Chen, Y.X., Zheng, Y-F., Chen, R-X., Zhang, S-B., Li, Q., Dai, M., Chen, L.
Metamorphic growth and recrystallization of zircons in extremely 18 O depleted rocks during eclogite facies metamorphism: evidence from U-Pb ages, trace elements and O-Hf isotopes.
Geochimica et Cosmochimica Acta, Vol. 75, 17, pp. 4877-4898.
Metamorphic growth and recrystallization of zircons in negative delta 18 O metamorphic rocks: a combined study of U-Pb dating, trace elements and O-Hf isotopes.
Multistage growth of garnet in ultrahigh pressure eclogite during continental collision in the Dabie Orogen: constrained by trace elements and U Pb ages.
Abstract: Mineral water contents, together with the major and trace element compositions of minerals and whole-rock, were determined for garnet pyroxenites enclosed by ultrahigh-pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The garnet pyroxenites have low SiO2 contents of 40.25 to 46.68 wt% and MgO contents of 10.99 to 14.79 wt%. They are characterized by enrichment in LREE and LILE (Ba, Sr, Pb) but depletion in HFSE (Nb, Zr) and HREE. They were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424-660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water-rich clinopyroxene. Garnet also has the high water contents, suggesting its importance in hosting water at mantle depths. Calculated whole-rock H2O/Ce ratios are 63-145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene-rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the presence of ultramafic metasomatites in the mantle wedge would enhance its water storage and thus reduce the water transport into deeper mantle by subduction.
Abstract: The compound [CH3 CH2 NH3 ][Cu(HCOO)3 ] undergoes a phase transition at 357 K, from a perovskite to a diamond structure, by heating. The backward transition can be driven by pressure at room temperature but not cooling under ambient or lower pressure. The rearrangement of one long copper-formate bond, the switch of bridging-chelating mode of the formate, the alternation of N-H???O H-bonds, and the flipping of ethylammonium are involved in the transition. The strong N-H???O H-bonding probably locks the metastable diamond phase. The two phases display magnetic and electric orderings of different characters.
Yang, J.S., Wirth, R., Wiedenbeck, M., Griffin, W.L., Meng, F.C., Chen, S.Y., Bai, W.J., Xu, X.X., Makeeyev, A.B., Bryanchaniniova, N.I.
Diamonds and highly reduced minerals from chromitite of the Ray-Iz ophiolite of the Polar Urals: deep origin of podiform chromitites and ophiolitic diamonds.
Earth and Planetary Science Letters, Vol. 412, pp. 42-51.
Mantle
Coesite
Abstract: Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded KS0=103.6(4) GPaKS0=103.6(4) GPa, G0=61.6(2) GPaG0=61.6(2) GPa and View the MathML sourceK0?=2.9(1), View the MathML sourceG0?=0.3(1) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite–stishovite transition, reaching ?39% and ?48% for P and S wave velocity contrasts, and ?70% and 78% for P and S wave impedance contrasts, respectively, at pressure ?8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., View the MathML sourcedln?V(PorS)/dh?0) within 8–12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO2 content, d(ln?VP)/d(SiO2)?0.43 (wt%)?1d(ln?VP)/d(SiO2)?0.43 (wt%)?1 and d(ln?VS)/d(SiO2)?0.60 (wt%)?1d(ln?VS)/d(SiO2)?0.60 (wt%)?1, can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle.
Geophysical Prospecting, Vol. 67, 6, pp. 1626-1636.
Global, Canada, Northwest Territories
geophysics - graviometry
Abstract: For airborne gravity gradiometry in rugged terrain, helicopters offer a significant advantage over fixed?wing aircraft: their ability to maintain much lower ground clearances. Crucially, this provides both better signal?to?noise and better spatial resolution than is possible with a fixed?wing survey in the same terrain. Comparing surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan, demonstrates that there is some loss of spatial resolution in the more rugged terrain. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components is done by a Fourier method over gentle terrain and an equivalent source method for rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth model inversion, and the source distribution must be designed to suit the survey design.
In situ determination of major and trace elements in calcite and apatite, and U-Pb ages of apatite from the Oka carbonatite complex: insights into a complex crystallization history.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 24.
Canada, Northwest Territories
Deposit - Ekati
Abstract: How would a large open pit mine on caribou range (e.g., the Ekati Diamond Mine in the Bathurst caribou’s summer range) have influenced caribou? A traditional knowledge study on the cumulative impacts on the Bathurst caribou herd qualitatively described how mining activities might have influenced the herd (Mackenzie et al. 2013): caribou migration routes deflected away from the mines probably due to seeing mining activities or hearing the noises; and skinny caribou or abnormal smells and materials in caribou meat, liver, or the hide linings probably related to changes in caribou forage and quality of water and air. In other words, the potential influences of mining operations on caribou were most likely through altering what caribou can see, hear, smell (e.g., dusts and fine particle matter < 2.5 ?m (PM2.5) in the air, and from acidity in the soil), and taste (e.g., dust on foliage, vegetation composition change). Boulanger et al. (2012) estimated the size of a zone of influence (ZOI) of the Ekati-Diavik mining complex in the Bathurst caribou summer range, using caribou presence dataset. They also explored the mechanisms of ZOI using the spatial distribution of the total suspended particles, which was simulated with an atmospheric transport and dispersion model (Rescan, 2006). While these studies have added to our understanding of the possible impacts of mining operations on caribou, knowledge gaps remain. One outstanding gap is the lack of direct measurements about the caribou relevant environmental changes caused by mining operations. For example, exactly from how far away can caribou clearly see the vehicles driving on a mining road, or the buildings and the elevated waste piles in a camp? From how far away might caribou hear the noise caused by mining operations? To what spatial extent had the dusts and PM2.5 from mining operations influenced the tundra ecosystems? And how the dusts and PM2.5 from mining operations might have influenced caribou forage quality? Potentially these questions can be answered by in-situ measurements and satellite remote sensing. For example, studies have showed that it is possible to remotely sense PM2.5 distribution using twice-daily MODIS data at a spatial resolution of 1 km (Lyapustin et al., 2011; Chudnovsky et al., 2013; Hu et al., 2014). The objective of this study is thus to quantitatively measure these changes around the Ekati Diamond Mine, by means of in-situ surveys and satellite remote sensing. We conducted field surveys at more than 100 sites around the Ekati Diamond Mine during August 14-23, 2015, a collaborative effort of the NWT CIMP project entitled “Satellite Monitoring for Assessing Resource Development’s Impact on Bathurst Caribou (SMART)”, and the Dominion Diamond Ekati Corporation. In this presentation, we will report preliminary results and lessons learned from our first year’s study.
Abstract: The Miaoya carbonatite complex in the South Qinling orogenic belt hosts one of the largest rare earth element (REE)-Nb deposits in China that is composed of carbonatite and syenite. The emplacement age of the complex and the geochronological relationship between the carbonatite and syenite have long been debated. In this study, in situ U-Th-Pb ages have been obtained for the constituent minerals zircon, monazite and columbite from carbonatite and syenite of the Miaoya complex, together with their chemical and isotopic compositions. In situ trace element compositions for zircon from carbonatite and syenite are highly variable. The zircon displays slightly heavy REE (HREE)-enriched chondrite-normalized patterns with no Eu anomaly and various light REE (LREE) contents. In situ Th-Pb dating for zircon from the Miaoya complex by laser ablation ICP-MS yields ages of 442.6 ± 4.0 Ma (n = 53) for syenite and 426.5 ± 8.0 Ma (n = 23) for carbonatite. Monazite from carbonatite and syenite shows similar chondrite-normalized REE patterns and yields a consistent Th-Pb age of ~ 240 Ma. Based on petrographic and chemical composition, columbite from the carbonatite can be identified into two groups. The columbite dispersed within carbonatite is characterized by slightly LREE-enriched chondrite-normalized REE patterns, whereas columbite associated with apatite is characterized by LREE-depleted trends. Columbite has been further determined to have a weighted mean 206Pb/238U age of 232.8 ± 4.5 Ma (n = 9) using LA-ICP-MS. Detailed geochronological and chemical investigations suggest that there were two major episodes of magmatic/metasomatic activities in the formational history of the Miaoya carbonatite complex. The early alkaline magmatism emplaced in the Silurian was related to the opening of the Mianlue Ocean, whereas the late metasomatism or hydrothermal overprint occurred during the Triassic South Qinling orogeny. The latter serves as the major ore formation period for both REE (e.g., monazite) and Nb (e.g., columbite).
Abstract: The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the world’s largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361-913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent ?Nd(1.3Ga) values (0.3 ± 0.6) close to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower 87Sr/86Sr ratios (0.7024-0.7030) than the recrystallized dolomite (0.7038-0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks.
Abstract: Carbonatite and related alkaline silicate rocks contain one of the most significant rare earth element (REE) reserves in the world. It is well-known that these REE deposits are characterized by a strong light REE enrichment with a steep fractionation from La to Lu in the chondrite-normalized diagram. However, the origin of their REE enrichment remains debatable. The Shaxiongdong (SXD) carbonatite in the South Qinling orogenic belt hosts one of the most important REE deposits in central China. In this study, in situ chemical and isotopic data have been obtained for carbonate minerals from the complex. Our results show that calcite has variable trace element abundances, especially REEs. In situ Pb isotope data for calcite reveal extreme variations of 206Pb/204Pb (18.05-31.71) and 207Pb/204Pb (15.49-16.36) ratios. Interestingly, Pb isotope variations display positive correlations with REE enrichments [i.e., (La/Yb)N and (La/Nd)N]. Calcite with extreme radiogenic Pb isotopic compositions displays upper mantle C and O isotopic compositions (?13Cavg?=??5.74‰, ?18Oavg?=?7.13‰) and depleted 87Sr/86Sr isotopic ratios (~0.7030). The observed various REE enrichments accompanying the variable Pb isotopic composition within SXD calcite possibly result from a closed-system metasomatic event. The U-bearing mineral (i.e., pyrochlore) accumulating abundant uranogenic lead since their Silurian formation serves as the radiogenic Pb and LREE source for the metasomatism. Alternatively, the chemical and isotopic composition observed might suggest involvement of two mantle sources (PREMA and the distinct radiogenic Pb mantle reservoir).
Abstract: The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440?Ma), and was subsequently impacted by a late-stage hydrothermal event (~232?Ma) involving REE-rich fluids/melt. This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by ?11B values that vary between ?7 (typical asthenospheric mantle) and?+?4‰; ?11B values and B abundances (~0.2 to ~1?ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300?Ma old) carbonatites worldwide.
Abstract: Magnetite (Fe3O4) is one of the most common accessory minerals in magmatic rocks, and it can accommodate a wide variety of major, minor and trace elements that can be measured by laser ablation ICP-MS. In this study, we investigate the chemical compositions of magnetite from four carbonatite complexes (Oka, Mushgai Khudag, Hongcheon and Bayan Obo). The minor elements (Mg, Ti, Al, Mn) in magnetite vary significantly both within and between different complexes. High field strength elements (Zr, Hf, Nb, Ta, U, Th) are generally depleted in magnetite from carbonatite complexes, whereas K, Rb, Cs, Ca and P are commonly below detection limits. V and Zn display significant variations from tens to thousands of ppm. Co, Ni and Ga are present in ppm or tens of ppm, whereas Cu, Sr, Y, Ba and Pb are characterized by sub-ppm levels. Mo and Ge are identified at the ppm level, whereas a consistent concentration of 2-5?ppm is observed for Ge. The determined chemical compositions of magnetite from carbonatite complexes are quite distinguishable compared to those formed in silicate and sulfide melts. This is clearly shown using multielement variation diagrams, and the distinct signatures of carbonatite-related magnetite include strong positive anomalies of Mn and Zn and negative anomalies of Cu, Co and Ga. The discriminant diagrams of Ti vs. Zr?+?Hf, Ti vs. Nb?+?Ta and Ni/Cr vs. Ti are applicable for distinguishing magmatic and hydrothermal magnetite in carbonatite-related environments. In addition, the discriminant diagram of Zn/Co vs. Cu/Mo and Cu vs. Zr?+?Hf can be used to distinguish carbonatite-related magnetite from magnetite that formed in other environments.
Abstract: Methods for a spherical harmonic analysis and synthesis of global gravitational and lithospheric structure models are applied to compile the mantle and sub-lithospheric mantle gravity maps. Both gravity maps are then interpreted and assessed by means of their accuracy. The mantle gravity map exhibits a gravitational signature that mainly reflects a thermal state of the lithospheric mantle. This is particularly evident over the oceanic lithosphere, with gravity lows along mid-oceanic spreading ridges. The increasing gravity signal with the ocean-floor age is attributed to conductive cooling of the oceanic lithosphere. Gravity lows extend along continental rift systems. Gravity lows also mark active convergent tectonic margins (in Pacific, Mediterranean, and Caribbean). The old, cold and tectonically stable cratonic mantle is typically characterized by gravity highs. A thermal signature of upwelling mantle under mid-oceanic spreading ridges clearly manifests (by gravity lows) also in the sub-lithosphere mantle gravity map. Nevertheless, the overall signature of conductive cooling is less pronounced in this gravity map, and a thermal signature of the asthenosphere under most of the continental lithosphere is weak. This indicates that a lateral thermal gradient within the asthenosphere tends to be weaker than within the overlying lithospheric mantle. The most pronounced feature in this gravity map is the signature of subducted slabs in West Pacific, marked by gravity highs. An antipodal signature of two large low shear-velocity provinces in both mantle gravity maps is absent, while its long-wavelength pattern could clearly be recognized in the free-air gravity map. We explain this finding by the fact that gravity-stripping procedures applied in this study superpose a gravitational signature of an intermediate layer, in this case the lithospheric mantle and the asthenosphere, over a much weaker signature of deeper mantle density heterogeneities. Moreover, the interpretational quality of both mantle gravity maps is considerably worsen by the LITHO1.0 lithospheric model uncertainties, especially within a more complex structure of the continental lithosphere. As a result, some spatial features in presented gravity maps could be artefacts rather than a real gravity signal. Despite accuracy limitations of currently available lithospheric density models, such types of gravity maps provide a useful information for various purposes in geophysics, among others gravimetric interpretations of Earth's inner structure or a separation of gravitational signals from different sources. In geodesy, a primary motivation is related to a compilation of Earth's synthetic density model based on the condition of fulfilling the total mass budget for testing numerical techniques applied in gravimetric forward modelling by means of solving Newton's volume integral.
Abstract: The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (>?0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnäsite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr?=?0.70292-0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr?=?0.70295-0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.
Evolution of the carbonatite Mo-HREE deposits in the Lesser Qinling orogen: insights from in situ geochemical investigation of the calcite and sulfate. Huanglongpu, Huangshuian
Geochimica et Cosmochimica Acta, in press available 45p. Pdf
China
deposit - Miaoya
Abstract: A majority of carbonatite-related rare earth element (REE) deposits are found in cratonic margins and orogenic belts, and metasomatic/hydrothermal reworking is common in these deposits; however, the role of metasomatic processes involved in their formation remains unclear. Here, we present a comprehensive in situ chemical and isotopic (C-Sr) investigation of calcite and fluorapatite within the Miaoya carbonatite complex located in the South Qinling orogenic belt, with the aim to better define the role of late-stage metasomatic processes. Carbonatite at Miaoya commonly occurs as stocks and dykes intruded into associated syenite, and can be subdivided into equigranular (Type I) and inequigranular (Type II) calcite carbonatites. Calcite in Type I carbonatite is characterized by the highest Sr (up to ?22,000?ppm) and REE (195-542?ppm) concentrations with slight LREE-enriched chondrite normalized patterns [(La/Yb)N?=?2.1-5.2]. In situ C and Sr isotopic compositions of calcite in Type I carbonatite define a limited range (87Sr/86Sr?=?0.70344-0.70365; ?13C?=??7.1 to ?4.2 ‰) that are consistent with a mantle origin. Calcite in Type II carbonatite has lower Sr (1708-16322?ppm) and REEs (67-311?ppm) and displays variable LREE-depleted chondrite normalized REE patterns [(La/Yb)N?=?0.2-3.3; (La/Sm)N?=?0.2-2.0]. In situ 87Sr/86Sr and d13C isotopic compositions of Type II calcite are highly variable and range from 0.70350 to 0.70524 and ?7.0 to ?2.2 ‰, respectively. Fluorapatite in Type I and Type II carbonatites is characterized by similar trace-element and isotopic compositions. Both types of fluorapatite display variable trace element concentrations, especially LREE contents, whereas they exhibit relatively consistent near-chondritic Y/Ho ratios. Fluorapatite is characterized by consistent Sr isotopic compositions with a corresponding average 87Sr/86Sr ratio of 0.70359, which suggests that fluorapatite remained relatively closed in relation to contamination. The combined geochemical and isotopic data for calcite and fluorapatite from the Miaoya complex suggest that carbonatite-exsolved fluids together with possible syenite assimilation during the Mesozoic metasomatism overprinted the original trace-element and isotopic signatures acquired in the early Paleozoic magmatism. Hydrothermal reworking resulted in dissolution-reprecipitation of calcite and fluorapatite, which served as the dominant source of REE mineralization during the much younger metasomatic activity. The results from this study also suggest that carbonatites located in orogenic belts and cratonic edges possess a great potential for forming economic REE deposits, especially those that have undergone late-stage metasomatic reworking.
Geochemical Perspectives Letters, Vol. 17, pp. 11-15. pdf
Mantle
carbonatites
Abstract: Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer. To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report ?44/42Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol. Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to ?44/42Ca values of basalts, while non-primary carbonatites show detectable ?44/42Ca variations that are correlated to ?13C values. Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution. The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
Abstract: Apatite is a ubiquitous mineral in carbonatites, and incorporates a variety of trace elements including rare earth elements (REEs). In this study, the textural and chemical variations of apatite were examined in order to trace the magmatic and hydrothermal petrogenesis of three carbonatite-related REE deposits: Shaxiongdong, Miaoya, and Bayan Obo. Various apatite textures were revealed by cathodoluminescence and back-scattered electron imaging. Magmatic apatite, which occurs predominantly in samples from Shaxiongdong, is euhedral, and commonly shows oscillatory or growth zonation with a yellow-green luminescent core and a violet luminescent rim. Euhedral to subhedral metasomatic apatite from Miaoya and Bayan Obo has a turbid texture, with the majority of grains associated with exsolved monazite. Hydrothermal apatite from Bayan Obo, typically occurring as aggregates in close association with fluorite and barite, is anhedral, with green or light violet luminescence. The different apatite textures are characterised by distinct trace element compositions. Magmatic apatite contains the highest concentrations of Mn (avg. 457 ppm) and Sr (avg. 18,285 ppm) and is characterised by a steeply inclined REE chondrite-normalised pattern. Metasomatic apatite, which has undergone in situ dissolution-reprecipitation, contains lower Mn (avg. 272 ppm) and Sr (avg. 9945 ppm) concentrations. It is characterised by highly variable REE trends with an La/SmN ratio varying from 0.13 to 5.61, and lower average La/YbN, La/SmN, and Sr/Y ratios (46, 2.2, and 18, respectively) than magmatic apatite. Hydrothermal apatite that was precipitated from a fluid is characterised by convex upward chondrite-normalised REE distributions with the lowest La/YbN, La/SmN, and Sr/Y ratios (13, 0.69, and 5.8, respectively). The average concentrations of Mn and Sr in this apatite are 270 and 6610 ppm, respectively. There are no Eu anomalies (Eu/Eu* = 0.97) in the chondrite-normalised REE plots for any of the analysed apatite samples. The combined textural and compositional variations of apatite in the three deposits reflect diverse magmatic and hydrothermal processes, including: 1) mineral fractionation contributing to core-rim zoning within the Shaxiongdong magmatic apatite; 2) dissolution-reprecipitation inducing monazite precipitation in Miaoya and Bayan Obo metasomatic apatite; and 3) coprecipitation with fluorite and barite from fluids generating the Bayan Obo hydrothermal apatite. A compilation of published apatite compositions from other rock types demonstrates that trace element compositions of apatite can be used to differentiate crystallisation environments and differentiate apatite from other rock types. Apatite from carbonatite has high Sr, REEs, La/YbN, Th/U, and Sr/Y, and no Eu anomaly, compared with apatite from igneous silicate rocks (except ultramafic rocks), and iron-oxide copper gold (IOCG) or iron-oxide apatite (IOA) deposits.
Australian Journal of Earth Sciences, Vol. 66, 7, pp. 1041-1062.
China
carbonatite
Abstract: During the past 50 years, many geological and ore-deposit investigations have led to the discovery of the Fe-P-(Ti)-oxide deposits associated with mafic-ultramafic-carbonatite complexes in the Kuluketage block, northeastern Tarim Craton. In this paper, we discuss the genetic and ore-forming ages, tectonic setting, and the genesis of these deposits (Kawuliuke, Qieganbulake and Duosike). LA-ICP-MS zircon U-Pb dating yielded a weighted mean 206Pb/238U ages of 811?±?5?Ma, 811?±?4?Ma, and 840?±?5?Ma for Kawuliuke ore-bearing pyroxenite, Qieganbulake gabbro and Duosike ore-bearing pyroxenite, respectively. The CL images of the Kawuliuke apatite grains show core-rim structure, suggesting multi-phase crystallisation, whereas the apatite grains from Qieganbulake and Dusike deposits do not show any core-rim texture, suggesting a single-stage crystallisation. LA-ICP-MS apatite 207Pb-corrected U-Pb dating provided weighted mean 206Pb/238U ages of 814?±?21?Ma and 771?±?8?Ma for the Kawuliuke ores, and 810?±?7?Ma and 841?±?7?Ma for Qieganbulake and Duosike ores, respectively. The core-rim texture in apatite by CL imaging as well as two different ore-forming ages in the core and rim of the apatite indicate two metallogenic events for the Kawuliuke deposit. The first metallogenic period was magmatic in origin, and the second period was hydrothermal in origin. The initial ore-forming age of the Kawuliuke Fe-P-Ti mineralisation was ca 814?Ma and the second one was ca 771?Ma. On the other hand, the ore-forming ages of the Qieganbulake and Duosike deposits were ca 810?Ma and ca 841?Ma, respectively. Qieganbulake and Duosike deposits were of magmatic origin. Combined with previous geochronological data and the research on the tectonic background, we infer that the Kawuliuke, Qieganbulake and Duosike Fe-P-(Ti)-oxide deposits were formed in a subduction-related tectonic setting and were the product of subduction-related magmatism.
Abstract: Carbonatites and related rocks are the premier source for light rare earth element (LREE) deposits. Here, we outline an ore formation model for LREE-mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. The LREEs can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and LREEs concentrate in the residual melt due to their incompatibility in early crystal-lising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution of the residual liquid into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These primary carbonates are rarely preserved owing to dissolution by later fluids, and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.-
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104310, 50p. Pdf
China
carbonatite, REE
Abstract: The Huayangchuan deposit in the North Qinling alkaline province of Central China is a unique carbonatite-hosted giant U-Nb-REE polymetallic deposit. The mineralization is characterized by the presence of betafite, monazite, and allanite as the main ore minerals, but also exhibit relatively high budgets of heavy rare earth elements (HREE = Gd-Lu and Y). The origin of carbonatites has long been controversial, thus hindering our understanding of the genesis of the deposit. Here, we conducted an in-situ trace elemental, Sr-Nd isotopic, and bulk C-O isotopic analyses of multi-type calcites in the deposit. Two principal types (Cal-I and Cal-II), including three sub-types (Cal-I-1, Cal-I-2 and Cal-I-3) of calcites were identified based on crosscutting relationships and calcite textures. Texturally, Cal-I calcites in carbonatites display cumulates with the grain size decreasing from early coarse- (Cal-I-1) to medium- (Cal-I-2) and late fine-grained (Cal-I-3), whereas Cal-II calcites coexist with zeolite displaying zeolite-calcite veinlets. Geochemically, Cal-I calcites contain relatively high REE(Y) (151-2296 ppm), Sr (4947-9566 ppm) and Na (28.6-390 ppm) contents, characterized by right- to left-inclined flat distribution patterns [(La/Yb)N=0.2-4.2] with enrichment of HREE(Y) (136-774 ppm), whereas Cal-II calcites display low REE, Sr and undetectable Na contents, characterized by a right-inclined distribution pattern [(La/Yb)N=13.5, n=16]. The U-Nb-REE mineralization, accompanied with intense and extensive fenitization and biotitization, is mainly associated with the Cal-I-3 calcites which show flat to relatively left-inclined flat REE distribution patterns [(La/Yb)N=0.2-1.0]. Isotopic results show that Cal-I calcites with mantle signatures are primarily igneous in origin, whereas Cal-II are hydrothermal, postdating the U-Nb-REE mineralization. Cal-I calcites (Cal-I-1, Cal-I-2 and Cal-I-3) from mineralized and unmineralized carbonatites, displayed regular changes in REE, Na and Sr contents, but similar trace element distribution patterns and Sr-Nd-C-O isotopic signatures, indicating that these carbonatites originated from the same enriched mantle (EM1) source by low-degree partial melting of HREE-rich carbonated eclogites related to recycled marine sediments. The combination of trace elements and Sr-Nd isotopic composition of calcites further revealed that these carbonatites have undergone highly differentiated evolution. Such differentiation is conducive to the enrichment of ore-forming elements (U-Nb-REE) in the late magmatic-hydrothermal stages owing to extensive ore-forming fluids exsolved from carbonatitic melts. The massive precipitation of the U-Nb-REE minerals from ore-forming hydrothermal fluids may have been triggered by intense fluid-rock reactions indicated by extensive and intense fenitization and biotitization. Therefore, the Huayangchuan carbonatite-related U-Nb-REE deposit may have formed by a combination of processes involving recycled U-Nb-REE-rich marine sediments in the source, differentiation of the produced carbonatitic magmas, and subsequent exsolution of U-Nb-REE-rich fluids that precipitated ore minerals through reactions with wall rocks under the transitional tectonic regime from compression to extension at the end of Late Triassic.
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104284 11p. Pdf
China
REE
Abstract: The carbonatites in the southern margin of the North China Craton are distinguishable by containing abundant quartz and are closely spatially associated with Mo-(REE) deposits. Unveiling the nature of ore-forming fluids is key to understand the genesis of these Mo-(REE) deposits and to explore their potential genetic relationships with the quartz-rich carbonatites, but such issues were currently not convincingly addressed. Here, we provide detailed petrographic, microthermometric and LA-ICP-MS analyses of the fluid inclusions hosted in the primary quartz from the carbonatites in the Huangshuian Mo-(REE) deposit which is the largest Mo-(REE) one in the region, containing 0.4 million tons of Mo metal with REEs as the major by-product. Our results show that the fluid inclusions in the quartz of the carbonatites are two- and three-phase CO2-bearing types with high homogenization temperatures (average at 396 °C) and low salinities (average at 3.88 wt% NaCl equiv). The LA-ICP-MS analyses of these inclusions reveal that the primary fluids contain high concentrations of La, Ce, Pr, Nd, Sr, and Ba, similar to typical carbonatite-related fluids. In addition, they are characterized by high Y, Cu, Pb, and Zn. Such a metal association is broadly consistent with the mineral assemblages of the Huangshuian Mo-(REE) deposit, such as the widespread barite, bastnäsite, xenotime, chalcopyrite, galena, and sphalerite, strongly supporting the close genetic relation of the deposit with the quartz-rich carbonatites. Although the concentrations of Mo are extremely low in these inclusions (below the detect limit), it was constrained to be gradually enriched in evolved fluids. Considering that the recorded fluids in quartz represent earliest generation of fluids exsolved from carbonatite magmas, our new results highlight that quantifying metal budgets of fluid inclusions could be a robust way to evaluate fertility of carbonatites that are widespread in the southern margin of the North China Craton.
Abstract: Secondary hydrothermal reworking of REEs has been widely documented in carbonatites/alkaline rocks, but its potential role in the REE mineralization associated with these rocks is currently poorly understood. This study conducted a combined textural and in situ chemical investigation on the REE mineralization in the ~430 Ma Miaoya carbonatite-syenite complex, central China. Our study shows that the REE mineralization, dated at ~220 Ma, is characterized by a close association of REE minerals (monazite and/or bastnäsite) with pervasive carbonatization overprinting the carbonatites and syenites. In these carbonatites and syenites, both the apatite and calcite, which are the dominant magmatic REE-bearing minerals, exhibit complicated internal textures that are generally composed of BSE-bright and BSE-dark domains. Under BSE imaging, the former domains are homogeneous and free of pores or mineral inclusions, whereas the latter have a high porosity and inclusions of monazite and/or bastnäsite. In situ chemical analyses show that the BSE-dark domains of the apatite and calcite have light REE concentrations and (La/Yb)N values much lower than the BSE-bright areas. These features are similar to those observed in metasomatized apatite from mineral-fluid reaction experiments, thus indicating that the BSE-dark domains formed from primary precursors (i.e., represented by the BSE-bright domains) through a fluid-aided, dissolution-reprecipitation process during which the primary light REEs are hydrothermally remobilized. New, in situ Sr-Nd isotopic results of apatite and various REE minerals, in combination with mass balance calculations, strongly suggest that the remobilized REEs are responsible for the subsequent hydrothermal REE mineralization in the Miaoya complex. Investigations of fluid inclusions show that the fluids responsible for the REE mobilization and mineralization are CO2-rich, with medium temperatures (227-340 °C) and low salinities (1.42-8.82 wt‰). Such a feature, in combination with C-O isotopic data, indicates that the causative fluids are likely co-genetic with fluids from coeval orogenic Au-Ag deposits (220-200 Ma) in the same tectonic unit. Our new findings provide strong evidence that the late hydrothermal upgrading of early cumulated REEs under certain conditions could also be an important tool for REE mineralization in carbonatites, particularly for those present in convergent belts where faults (facilitating fluid migration) and hydrothermal fluids are extensively developed.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Possible density segregation of subducted oceanic lithosphere along a weak serpeninite layer and implications for composition stratification of mantle.
Earth and Planetary Science Letters, Vol. 255, 3-4, March 30, pp. 357-366.
Yin, A., Manning, C.E., Lovera, O., Menold, C.A., Chen, X., Gehrels, G.
Early Paleozoic tectonic and thermomechanical evolution of ultrahigh pressure (UHP) metamorphic rocks in the northern Tibetan Plateau, northwest China.
International Geology Review, Vol. 49, 8, pp. 681-716.
Geochemistry, Geophysics, Geosystems, 10.1029/2021GC009710 20p. Pdf
United States, Canada
geophysics- seismic
Abstract: We probe the properties of upper mantle rocks beneath northeastern North America using the observations of seismic waves from distant earthquakes. We examine signals of converted P-S waves that originate from locations of rapid vertical or directional changes in seismic velocities. These abrupt velocity boundaries are thought to originate from rock deformation, variations in composition, temperature, or melt content. The sharp transitions detectable by this method are compositionally more plausible within the cold tectonic plate than within the hot convecting asthenosphere. Previous studies in this region that analyzed the same type of seismic data report boundaries with sharp downward reduction in seismic velocities between the depths of 60 and 100 km. Their widespread distribution and local consistency with seismic velocity models was used as evidence of them marking the transition between the cold tectonic plate and the hot convecting asthenosphere. Here we expand our search to other types of boundaries and find numerous examples at much greater depths (down to ?185 km). These deeper boundaries primarily reflect changes in directional variation of seismic velocities (anisotropy). The distribution of our deep boundaries broadly agrees with lithospheric thickness estimates in global upper mantle models that consider seismic, gravity, and heat flow data.
Contributions to Mineralogy and Petrology, Vol. 177, 2, 21p.pdf
Mantle
metasomatism
Abstract: Melting experiments on a carbonated pelite were performed at 5.5-15.5 GPa, 800-1875 °C using multi-anvil apparatuses to determine the melting phase relations and the P-T stability fields of various phases, which may shed some light on the source of silica-undersaturated magmas and the deep Earth carbon and water cycles. The subsolidus assemblages contain garnet, clinopyroxene, coesite/stishovite at all investigated pressures. Phengite, aragonite or magnesite, and topaz-OH occur below 9.5 GPa. Phase egg, K-hollandite, Ti-oxide, and CAS phase appear at 12-15.5 GPa. Phengite is stable up to 6 GPa and 800 °C, with the phengite-out boundary overlapping with the carbonate-out curve. Thus, the initial melt is carbonatitic and extremely potassium-rich, with K2O/Na2O weight-ratios larger than 40 at fluid-present conditions. The melting reaction phase egg?+?magnesite?+?aragonite?+?(clinopyroxene)?+?stishovite???melt?+?garnet?+?kyanite defines the solidus at 9.5 GPa, 1000-1100 °C. With increasing pressure, the composition of the near-solidus melts gradually evolves from potassium-rich to sodium-rich due to the formation of K-hollandite and the destabilization of clinopyroxene, and as a result of the clinopyroxene-out, the near-solidus melt has the lowest K2O/Na2O value and partitioning coefficient of sodium between clinopyroxene and melt (Dcpx/meltNa) at 15.5 GPa. In addition, phase egg remains stable up to 1400 °C at 15.5 GPa. Thus, phase egg is a good candidate as a deep-water carrier during subduction of pelitic sediments. This study concludes that low degree partial melting of carbonated pelite produces alkali-rich carbonatite melts evolving from potassium-rich (6-12 GPa) to sodium-rich (above 12 GPa) with increasing pressure, and if a slab stagnates at depth, and/or subduction slows down, the produced carbonatite melts will be more silicate-rich with increasing temperature. Moreover, the produced melts generally evolve from relatively silicate-rich to carbonatite-rich with increasing pressure. These alkali-rich carbonatite melts are compositionally similar to those in diamond inclusions, which provides strong evidence for the origin of deep-seated silica-undersaturated carbonatitic magma. Such magma is an ideal metasomatic agent that can give rise to mantle heterogeneity.
Guo, S., Ye, K., Wu, Y., Chen, Y., Yang, Y., Zhang, L., Liu, J., Mao, Q., Ma, Y.
A potential method to confirm the previous existence of lawsonite in eclogite: the mass imbalance of Sr and LREEs in multi stage epidote ( Ganghe, Dabie UHP terrane).
Journal of Metamorphic Gology, Vol. 31, 4, pp. 415-435.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Mantle
NAMs Nominally Anhydrous Minerals
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Journal of Geophysical Research, Vol. 120, 10, pp. 6993-7012.
Canada, Alberta
Geophysics - seismics LVZs
Abstract: Crustal low-velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern-central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200?km wide, over 10?km thick midcrustal LVZ is well resolved by shear velocity inversions using P-to-S receiver functions from more than 4600 earthquakes. It is generally overlain by a high-velocity upper crust in the depth range of 8-15?km, especially in western-central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mass transfer from the subducted crust to the mantle wedge through orogenic peridotites.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mas
Journal of Asian Earth Sciences, Vol. 164, pp. 179-193.
Asia, Myanmar
peridotites
Abstract: Peridotites from the Myitkyina ophiolite are mainly composed of lherzolite and harzburgite. The lherzolites have relatively fertile compositions, with 39.40-43.40?wt% MgO, 1.90-3.17?wt% Al2O3 and 1.75-2.84?wt% CaO. They contain spinel and olivine with lower Cr# (12.6-18.2) and Fo values (88.7-91.6) than those of the harzburgites (24.5-59.7 and 89.6-91.6 respectively). The harzburgites have more refractory compositions, containing 42.40-46.23?wt% MgO, 0.50-1.64?wt% Al2O3 and 0.40-1.92?wt% CaO. PGE contents of the peridotites show an affinity to the residual mantle. Evaluation of petrological and geochemical characteristics of these peridotites suggests that the lherzolites and harzburgites represent residual mantle after low to moderate degrees of partial melting, respectively, in the spinel stability field. The U-shaped, primitive mantle-normalized REE patterns and strong positive Ta and Pb anomalies of the harzburgites suggest melt/fluid refertilization in either a MOR or SSZ setting after their formation at a MOR. Mineral separation of the peridotites has yield a range of exotic minerals, including diamond, moissanite, native Si, rutile and zircon, a collection similar to that reported for ophiolites of Tibet and the Polar Urals. The discovery of these exotic minerals in the Myitkyina ophiolite supports the view that they occur widely in the upper oceanic mantle.
Abstract: The Western Canada Sedimentary Basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. Its crystalline basement has documented more than 3 billion years of evolution history of western Laurentia. Here we conduct a high?resolution survey of the mantle P and S wave velocities using finite?frequency tomography. Our models show pronounced eastward increases of 4% P and 6% S wave velocities beneath the foreland region, which define a sharp seismic Cordillera?Craton boundary. In the cratonic region, distinctive high? (>2%) velocity anomalies representing depleted mantle lithospheres are well correlated with major Precambrian crustal domains. The largest lithosphere thickness contrast coincides with the Snowbird Tectonic Zone, where the Hearne province extends down to ~300 km, nearly 100 km deeper than the Proterozoic terranes in northern Alberta. In the latter region, a pronounced cylindrical negative velocity anomaly extends subvertically from 75 to ~300?km depth, which potentially results from significant tectonothermal modifications during subduction and/or plume activities. At the basin scale, mantle velocities show no apparent correlations with surface heat flux, suggesting a minimum mantle contribution to the regional thermal variability. Furthermore, the long?wavelength isostatic gravity correlates negatively with the velocities, which confirms that the melt extraction from Precambrian cratons is responsible for the formation of highly depleted mantle lithospheres. Moreover, our model reveals the increased concentrations of kimberlites and lamproites near the zones of high horizontal velocity gradients. The distinct spatial pattern may reflect either preferential formation or eruption of potentially diamondiferous rocks at lithospheric weak zones near the western margin of Laurentia.
Journal of Geophysical Research: Solid Earth, Vol. 124, 7, pp. 6504-6522.
Mantle
subduction
Abstract: Mantle wedge hybridization by crust?derived melt is a crucial mechanism responsible for arc lavas. However, how the melt?rock reactions proceed in the mantle wedge and affect melt compositions is poorly understood. Garnet peridotites from Jiangzhuang in the Sulu orogen (eastern China) host garnetite and pyroxenite veins formed by slab?mantle interactions at different melt/rock ratios. The Jiangzhuang peridotites consist mainly of garnet lherzolites and minor harzburgites and represent a fragment of the mantle wedge influenced by ultrahigh?pressure metamorphism (5.2-6.1 GPa) in the subduction channel. Petrography, major and trace element geochemistry, and in situ clinopyroxene Sr isotope values of the garnetite and pyroxenite veins reveal their derivation from interactions between mantle wedge peridotites and deeply subducted crust?derived melts. The two veins share a common metamorphic and metasomatic history and have similar mineral assemblages and compositions, enriched isotope signatures, and formation P?T conditions, indicating the same source for their reacting melts. The different mineral proportions and microtextures between the garnetite and pyroxenite veins are ascribed to different melt/rock ratios. The garnetite vein formed at relatively high melt/rock ratios (>1:1), which would likely produce hybrid slab melts with Mg?rich, high?silica adakitic signatures. In contrast, the pyroxenite vein formed at low melt/rock ratios (<1:1), and the expected hybrid slab melts would evolve into high?Mg andesites. Moreover, recycled heterogeneous garnetite and pyroxenite could contribute to the mantle sources of intraplate magmas. Therefore, slab?mantle interactions at different melt/rock ratios could be an important crustal input to lithological and geochemical heterogeneities in the mantle.
Abstract: The Late Cretaceous kimberlites in northern Alberta, Canada, intruded into the Paleoproterozoic crust and represent a nonconventional setting for the discovery of diamonds. Here, we examined the origin of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (-1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicate a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Taken together, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta.
Dating the cratonic lower crust by the ion microprobe SHRIMP: an U-Th-lead isotopic study on zircons from lower crustal xenoliths from kimberlite pipes
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 45-48
Chen, Y.X., Zheng, Y-F., Chen, R-X., Zhang, S-B., Li, Q., Dai, M., Chen, L.
Metamorphic growth and recrystallization of zircons in extremely 18 O depleted rocks during eclogite facies metamorphism: evidence from U-Pb ages, trace elements and O-Hf isotopes.
Geochimica et Cosmochimica Acta, Vol. 75, 17, pp. 4877-4898.
Trace element composition of continentally subducted slab-derived melt: insight from multiphase solid inclusions in ultrahigh pressure eclogite in the Dabie Orogen.
Journal of Metamorphic Geology, Vol. 31, 4, pp. 453-468.
Ore Geology Reviews, doi.org/10.1016/j.oregeorev.2021.104310, 50p. Pdf
China
carbonatite, REE
Abstract: The Huayangchuan deposit in the North Qinling alkaline province of Central China is a unique carbonatite-hosted giant U-Nb-REE polymetallic deposit. The mineralization is characterized by the presence of betafite, monazite, and allanite as the main ore minerals, but also exhibit relatively high budgets of heavy rare earth elements (HREE = Gd-Lu and Y). The origin of carbonatites has long been controversial, thus hindering our understanding of the genesis of the deposit. Here, we conducted an in-situ trace elemental, Sr-Nd isotopic, and bulk C-O isotopic analyses of multi-type calcites in the deposit. Two principal types (Cal-I and Cal-II), including three sub-types (Cal-I-1, Cal-I-2 and Cal-I-3) of calcites were identified based on crosscutting relationships and calcite textures. Texturally, Cal-I calcites in carbonatites display cumulates with the grain size decreasing from early coarse- (Cal-I-1) to medium- (Cal-I-2) and late fine-grained (Cal-I-3), whereas Cal-II calcites coexist with zeolite displaying zeolite-calcite veinlets. Geochemically, Cal-I calcites contain relatively high REE(Y) (151-2296 ppm), Sr (4947-9566 ppm) and Na (28.6-390 ppm) contents, characterized by right- to left-inclined flat distribution patterns [(La/Yb)N=0.2-4.2] with enrichment of HREE(Y) (136-774 ppm), whereas Cal-II calcites display low REE, Sr and undetectable Na contents, characterized by a right-inclined distribution pattern [(La/Yb)N=13.5, n=16]. The U-Nb-REE mineralization, accompanied with intense and extensive fenitization and biotitization, is mainly associated with the Cal-I-3 calcites which show flat to relatively left-inclined flat REE distribution patterns [(La/Yb)N=0.2-1.0]. Isotopic results show that Cal-I calcites with mantle signatures are primarily igneous in origin, whereas Cal-II are hydrothermal, postdating the U-Nb-REE mineralization. Cal-I calcites (Cal-I-1, Cal-I-2 and Cal-I-3) from mineralized and unmineralized carbonatites, displayed regular changes in REE, Na and Sr contents, but similar trace element distribution patterns and Sr-Nd-C-O isotopic signatures, indicating that these carbonatites originated from the same enriched mantle (EM1) source by low-degree partial melting of HREE-rich carbonated eclogites related to recycled marine sediments. The combination of trace elements and Sr-Nd isotopic composition of calcites further revealed that these carbonatites have undergone highly differentiated evolution. Such differentiation is conducive to the enrichment of ore-forming elements (U-Nb-REE) in the late magmatic-hydrothermal stages owing to extensive ore-forming fluids exsolved from carbonatitic melts. The massive precipitation of the U-Nb-REE minerals from ore-forming hydrothermal fluids may have been triggered by intense fluid-rock reactions indicated by extensive and intense fenitization and biotitization. Therefore, the Huayangchuan carbonatite-related U-Nb-REE deposit may have formed by a combination of processes involving recycled U-Nb-REE-rich marine sediments in the source, differentiation of the produced carbonatitic magmas, and subsequent exsolution of U-Nb-REE-rich fluids that precipitated ore minerals through reactions with wall rocks under the transitional tectonic regime from compression to extension at the end of Late Triassic.
Abstract: Secondary hydrothermal reworking of REEs has been widely documented in carbonatites/alkaline rocks, but its potential role in the REE mineralization associated with these rocks is currently poorly understood. This study conducted a combined textural and in situ chemical investigation on the REE mineralization in the ~430 Ma Miaoya carbonatite-syenite complex, central China. Our study shows that the REE mineralization, dated at ~220 Ma, is characterized by a close association of REE minerals (monazite and/or bastnäsite) with pervasive carbonatization overprinting the carbonatites and syenites. In these carbonatites and syenites, both the apatite and calcite, which are the dominant magmatic REE-bearing minerals, exhibit complicated internal textures that are generally composed of BSE-bright and BSE-dark domains. Under BSE imaging, the former domains are homogeneous and free of pores or mineral inclusions, whereas the latter have a high porosity and inclusions of monazite and/or bastnäsite. In situ chemical analyses show that the BSE-dark domains of the apatite and calcite have light REE concentrations and (La/Yb)N values much lower than the BSE-bright areas. These features are similar to those observed in metasomatized apatite from mineral-fluid reaction experiments, thus indicating that the BSE-dark domains formed from primary precursors (i.e., represented by the BSE-bright domains) through a fluid-aided, dissolution-reprecipitation process during which the primary light REEs are hydrothermally remobilized. New, in situ Sr-Nd isotopic results of apatite and various REE minerals, in combination with mass balance calculations, strongly suggest that the remobilized REEs are responsible for the subsequent hydrothermal REE mineralization in the Miaoya complex. Investigations of fluid inclusions show that the fluids responsible for the REE mobilization and mineralization are CO2-rich, with medium temperatures (227-340 °C) and low salinities (1.42-8.82 wt‰). Such a feature, in combination with C-O isotopic data, indicates that the causative fluids are likely co-genetic with fluids from coeval orogenic Au-Ag deposits (220-200 Ma) in the same tectonic unit. Our new findings provide strong evidence that the late hydrothermal upgrading of early cumulated REEs under certain conditions could also be an important tool for REE mineralization in carbonatites, particularly for those present in convergent belts where faults (facilitating fluid migration) and hydrothermal fluids are extensively developed.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Metamorphic growth and recrystallization of zircons in negative delta 18 O metamorphic rocks: a combined study of U-Pb dating, trace elements and O-Hf isotopes.
Multiphase solid inclusions in zoisite bearing eclogite: evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision.
Multiphase solid inclusions in zoisite bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision.
Abstract: Zircon provides one of the best records of the formation and reworking of continental crust in the early Earth. However, Hadean to Eoarchean zircons are relatively scarce worldwide. Here we present the first report of relict Eoarchean magmatic zircons in granitic gneisses from the Sulu Orogen, eastern China. Based on internal structures, trace element contents, and U-Pb ages, we identified four groups of zircon domains with U-Pb ages of?~?3.7?Ga (Group I), ~2.1?Ga (Group II), ~790?Ma (Group III), and?~720?Ma (Group IV). Group I domains exhibit variable Th/U ratios, steep HREE patterns, and negative Eu anomalies. They yield lower intercept U-Pb ages of 1.82-1.95?Ga and discordia upper intercept ages of 3.65-3.69?Ga that are similar to the oldest concordant spot age of 3680?±?29?Ma. This indicates their growth from an Eoarchean magma and reworking during the Paleoproterozoic. The oldest Eoarchean domains with U-Pb ages of 3606?±?28 to 3680?±?29?Ma have low P contents of 216-563?ppm and high (Y?+?REE)/P molar ratios of 1.13-3.34, consistent with an igneous source. They show ?Hf(t) values of -2.8 to -0.9 at 3.67?Ga and TCHUR2 ages of 3.7-4.0?Ga, suggesting the growth of juvenile crust during the early Eoarchean. Group II to IV domains have consistent TDM2 ages of 2.6-3.0?Ga, suggesting that they grew during multiple reworkings of the Archean crust. Group II domains have variable Th/U ratios and steep to flattened HREE patterns that suggest growth during Paleoproterozoic crustal anatexis. Groups III and IV zircon domains have Th/U ratios and trace element contents that indicate growth from magmas that formed during Neoproterozoic continental rifting. In view of the unique feature of Neoproterozoic rifting magmatism in South China, the relict Eoarchean magmatic zircons would have originated in the Yangtze Craton and then undergone multiple phases of reworking during the Paleoproterozoic and Neoproterozoic. The results indicate the presence of?~3.7?Ga relict magmatic zircons in the Sulu Orogen, and they represent the oldest remnants of crustal material in the Yangtze Craton.
Journal of Petrology, 10.1093/petrology /egab070 98p. Pdf
Africa, Angola
deposit - Catoca
Abstract: Reconstructed whole-rock and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03-1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). Reconstructed whole-rock compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and ?18O of 5.5-7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07-0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ?30%. The narrow ?18O range of 5.5-7.4‰ near the ‘gabbro-basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths — plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites) — can provide similar and overlapping ?18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8-12 wt% of Nb2O5) enriched with HFSE (Zr/Hf of 72.6-75.6) and multiple trace-element signatures (including reconstructed whole-rock NMORB-normalised Ce/Yb of 3.9-10.6 and Sr/Y of 5.8-9.6, MgO contents of 15.7-17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800-3740 ppm), garnet (430-1400 ppm), and accessory rutile (700-2530 ppm), together with extremely low Li contents of 1.0-2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145-160 km (4.5-4.8 GPa).
Abstract: Reconstructed whole-rock (RWR) and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03-1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). RWR compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and ?18O of 5.5-7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07-0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ?30%. The narrow ?18O range of 5.5-7.4‰ near the ‘gabbro-basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths—plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites)—can provide similar and overlapping ?18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8-12 wt% of Nb2O5) enriched with high field strength elements (HFSE) (Zr/Hf of 72.6-75.6) and multiple trace-element signatures (including RWR, NMORB-normalised Ce/Yb of 3.9-10.6 and Sr/Y of 5.8-9.6, MgO contents of 15.7-17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800-3740 ppm), garnet (430-1400 ppm), and accessory rutile (700-2530 ppm), together with extremely low Li contents of 1.0-2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145-160 km (4.5-4.8 GPa).
Earth and Planetary Science Letters, Vol. 441, pp. 200-210.
Mantle
Subduction
Abstract: Subduction zones are one of the most striking feature on Earth. They represent one of the two types of convergent plate boundaries, in which one tectonic plates sinks underneath another into the Earth’s mantle. Soon after the advent of the theory of plate tectonics scientists recognized that subduction zones are one of the main drivers of plate motion and mantle convection [Elsasser, 1971]. With trench motion during progressive subduction, overriding plates incorporated in subduction zones may follow the trench and/or deform internally. Such deformation is often characterized by backarc extension, which leads to opening of backarc basins, such as the Tyrrhenian Sea, the Scotia Sea, the Aegean Sea, the North Fiji Basin, and the Lau Basin.
Ultra high pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone: evidence from mineral inclusions -texture
Journal of Asian Earth Sciences, Vol. 42, 4, pp. 607-617.
Journal of Geophysical Research: Solid Earth, Vol. 124, 7, pp. 6504-6522.
Mantle
subduction
Abstract: Mantle wedge hybridization by crust?derived melt is a crucial mechanism responsible for arc lavas. However, how the melt?rock reactions proceed in the mantle wedge and affect melt compositions is poorly understood. Garnet peridotites from Jiangzhuang in the Sulu orogen (eastern China) host garnetite and pyroxenite veins formed by slab?mantle interactions at different melt/rock ratios. The Jiangzhuang peridotites consist mainly of garnet lherzolites and minor harzburgites and represent a fragment of the mantle wedge influenced by ultrahigh?pressure metamorphism (5.2-6.1 GPa) in the subduction channel. Petrography, major and trace element geochemistry, and in situ clinopyroxene Sr isotope values of the garnetite and pyroxenite veins reveal their derivation from interactions between mantle wedge peridotites and deeply subducted crust?derived melts. The two veins share a common metamorphic and metasomatic history and have similar mineral assemblages and compositions, enriched isotope signatures, and formation P?T conditions, indicating the same source for their reacting melts. The different mineral proportions and microtextures between the garnetite and pyroxenite veins are ascribed to different melt/rock ratios. The garnetite vein formed at relatively high melt/rock ratios (>1:1), which would likely produce hybrid slab melts with Mg?rich, high?silica adakitic signatures. In contrast, the pyroxenite vein formed at low melt/rock ratios (<1:1), and the expected hybrid slab melts would evolve into high?Mg andesites. Moreover, recycled heterogeneous garnetite and pyroxenite could contribute to the mantle sources of intraplate magmas. Therefore, slab?mantle interactions at different melt/rock ratios could be an important crustal input to lithological and geochemical heterogeneities in the mantle.
Geochimica et Cosmochimica Acta, in press available 21p.
Africa, South Africa
deposit - Bultfontein
Abstract: Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new ?15N and ?18O data have been collected for thirteen mantle xenoliths - harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks - from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The ?18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (?18Oolivine?=?+5.2?±?0.3‰; ?18Oorthopyroxene?=?+5.7?±?0.3‰; 2?s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (?18O?=?+0.2‰) in the wehrlites of this study. In contrast with broadly mantle-like ?18O values, the ?15N value of phlogopite in a wehrlite sample (+5.9‰) differs from the mantle composition (?15N?=??5?±?2‰). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle. Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene ?18O values (as low as +4.4‰) extend below those reported in mantle peridotites (i.e. ?18Oclinopyroxene?=?+5.6?±?0.3‰; 2?s.d.), and strong negative correlations are found between mineral ?18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite ?15N values (+4 to +7‰) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low ?18O-high ?15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.
Abstract: White diamonds color grading is one of the basic diamond evaluations. The color value based on a scale that ranges from D to Z, with D being the more colorless and more valuable, among other qualifications. As the diamond grade moves on this scale, its color appears more yellow progressively. This yellowish color, present only in Type I diamonds, is mainly due to the nitrogen related defects such as N3 center and C-center. The current color grading system is based on a visual method, where gemologist compares the sample with a Master Color set. However, this method is very subjective. Several defects responsible for light absorption in diamond are carrying electron spin and appear in Electron Paramagnetic Resonance (EPR) spectrum. In this study, we developed a new EPR based technique for a quantitative measurement of N3 center and C-center in diamond through quantitative EPR spectroscopy. The correlation between EPR spectra and color grades of diamond was established.
Abstract: White diamonds color grading is one of the basic diamond evaluations. The color value based on a scale that ranges from D to Z, with D being the more colorless and more valuable, among other qualifications. As the diamond grade moves on this scale, its color appears more yellow progressively. This yellowish color, present only in Type I diamonds, is mainly due to the nitrogen related defects such as N3 center and C-center. The current color grading system is based on a visual method, where gemologist compares the sample with a Master Color set. However, this method is very subjective. Several defects responsible for light absorption in diamond are carrying electron spin and appear in Electron Paramagnetic Resonance (EPR) spectrum. In this study, we developed a new EPR based technique for a quantitative measurement of N3 center and C-center in diamond through quantitative EPR spectroscopy. The correlation between EPR spectra and color grades of diamond was established.
Abstract: The beginning of plate tectonics on Earth remains the subject of fundamental debate. Also, future evolution of plate tectonics has not yet been addressed adequately in the literature. Here I develop models to extrapolate secular trends of plate tectonics closely associated with mantle potential temperature; intensity of magmatic activities to estimate the lifetime of plate tectonics. First a fractal model is utilized to analyze global igneous and detrital zircon U-Pb age datasets to characterize nonlinear intensity and periodicity of peaks in magmatic activities associated with deep-seated avalanche-type events (slab break off, lithospheric root detachments and mantle plumes). The results show descending trends of both nonlinear intensity and duration of peaks in magmatism observed from 3?Ga to the present day, suggesting a general trend of mantle cooling. This relation when extrapolated yields 1.45?Gyr of time for when nonlinear intensity of major magmatic activity would vanish. Further it is demonstrated by an independent polynomial model fitted to the relation between mantle potential temperature and future age of the earth that this result conforms to the time estimated for when mantle potential temperature is reduced to mantle solidus temperature.
Abstract: Multifractal features of element concentrations in the Earth’s crust have demonstrated to be closely associated with multiple probability distributions such as normal, lognormal and power law. However, traditional understanding of geochemical distribution satisfying normal, lognormal or power-law models still faces a serious problem in adjusting theoretical statistics with the empirical distribution. Given that the differences among different geochemical distribution populations may have considerable effects on the target estimation, a new perspective from the singularity of fractal density is adopted to investigate mixed geochemical distribution patterns within frequency and space domains. In the framework of fractal geometry, ordinary density such as volume density (e.g., g/cm3 and kg/m3) described in Euclidean space can be considered as a special case of the fractal density (e.g., g/cm? and kg/m?). According to the nature of fractal density, geochemical information obtained from Euclidean geometry may not sufficiently reflect inherent geochemical features, because some information might be hidden within fractal geometry that can be only revealed by means of a set of fractional dimensions. In the present study, stream sediment geochemical data collected from west Tianshan region, Xinjiang (China), were used to explore element distribution patterns in the Earth’s crust based on a fractal density model. Four elements Cu, Zn, K and Na were selected to study the differences between minor and major elements in terms of their geochemical distribution patterns. The results strongly suggest that element distribution patterns can be well revealed and interpreted by means of a fractal density model and related statistical and multifractal parameters.
Cheng, Z., Zhang, Z., Santosh, M., Hou, T., Zhang, D.
Carbonate and silicate rich globules in the kimberlitic rocks of northwestern Tarim large igneous province, NW China: evidence for carbonated mantle source.
Journal of Asian Earth Sciences, Vol. 95, pp. 114-135.
Abstract: The contribution of magmatic and hydrothermal processes to rare earth element (REE) mineralization of carbonatites remains an area of considerable interest. With the aim of better understanding REE mineralization mechanisms, we conducted a detailed study on the petrology, mineralogy and C-O isotopes of the Bachu carbonatites, NW China. The Bachu carbonatites are composed predominantly of magnesiocarbonatite with minor calciocarbonatite. The two types of carbonatite have primarily holocrystalline textures dominated by dolomite and calcite, respectively. Monazite-(Ce) and bastnäsite-(Ce), the major REE minerals, occur as euhedral grains and interstitial phases in the carbonatites. Melt inclusions in the dolomite partially rehomogenize at temperatures above 800?°C, and those in apatite have homogenization temperatures (Th) ranging from 645 to 785?°C. Oxygen isotope ratios of the calciocarbonatite intrusions (?18OV-SMOW?=?6.4‰ to 8.3‰), similar to the magnesiocarbonatites, indicate the parental magma is mantle-derived, and that they may derive from a more evolved stage of carbonatite fractionation. The magnesiocarbonatites are slightly enriched in LREE whereas calciocarbonatites have higher HREE concentrations. Both dolomite and calcite have low total REE (TREE) contents ranging from 112 to 436?ppm and 88 to 336?ppm, respectively, much lower than the bulk rock composition of the carbonatites (371 to 36,965?ppm). Hence, the fractional crystallization of carbonates is expected to elevate REE concentrations in the residual magma. Rocks from the Bachu deposit with the highest TREE concentration (up to 20?wt%) occur as small size (2?mm to 3 cm) red rare earth-rich veins (RRV) with barite + celestine + fluorapatite + monazite-(Ce) associations. These rocks are interpreted to have a hydrothermal origin, confirmed by the fluid inclusions in barite with Th in the range 198-267?°C. Hydrothermal processes may also explain the existence of interstitial textures in the carbonatites with similar mineral assemblages. The C-O isotopic compositions of the RRV (?13CV-PDB?=??3.6 to ?4.3‰, ?18OV-SMOW?=?7.6 to 9.8‰) are consistent with an origin resulting from fluid exsolution at the end of the high temperature fractionation trend. A two-stage model involving fractional crystallization and hydrothermal fluids is proposed for the mineralization of the Bachu REE deposit.
Abstract: Aillikites are carbonate-rich ultramafic lamprophyres often associated with carbonatites. Despite their common field relationships, the petrogenetic links, if any, between aillikites and carbonatites remain controversial. To address this question, this study reports the results of a detailed geochemical and isotopic examination of the Permian Wajilitag aillikites in the northwestern Tarim large igneous province, including bulk-rock major-, trace-element and Sr-Nd isotope compositions, olivine major- and trace-element and (in-situ secondary ion mass spectrometry) oxygen isotope compositions, oxygen isotope data for clinopyroxene separates, and bulk-carbonate C-O isotopic analyses. Olivine in the aillikites occurs in two textural types: (i) microcrysts, 0.3-5?mm; and (ii) macrocrysts, 0.5-2.5?cm. The microcrysts exhibit well-defined linear correlations between Fo (79-89), minor and trace elements (e.g., Ni?=?1304-3764??g/g and Mn?=?1363-3042??g/g). In contrast, the olivine macrocrysts show low Fo79-81, Ni (5.3-442??g/g) and Ca (477-1018??g/g) and very high Mn (3418-5123??g/g) contents, and are displaced from the compositional trend of the microcrysts. The microcrysts are phenocrysts crystallized from the host aillikite magmas. Conversely, the lack of mantle-derived xenoliths in these aillikites suggests that the macrocrysts probably represent cognate crystals (i.e., antecrysts) that formed from earlier, evolved aillikite melts. Olivine phenocrysts in the more primitive aillikite dykes (Dyke 1) have relatively higher Fo82-89 and mantle-like oxygen isotope values, whereas those in the more evolved dykes (Dyke 2 and 3) exhibit lower Fo79-86 and oxygen isotope values that trend toward lower than mantle ?18O values. The decreasing ?13C values of carbonate from Dyke 1 through to Dyke 2 and 3, coupled with the indistinguishable Sr-Nd isotopes of these dykes, suggest that the low ?18O values of olivine phenocrysts in Dyke 2 and 3 resulted from carbonate melt/fluid exsolution from a common progenitor melt. These lines of evidence combined with the overlapping emplacement ages and Sr-Nd isotope compositions of the aillikites and carbonatites in this area suggest that these exsolved carbonate melts probably contributed to the formation of the Tarim carbonatites thus supporting a close petrogenetic relationship between aillikites and carbonatites.
Abstract: Aillikites are carbonate-rich ultramafic lamprophyres, and although they are volumetrically minor components of large igneous province (LIP), these rocks provide important clues to melting and meta-somatism in the deep mantle domain during the initial stages of LIPs. In this study, we investigate the Wajilitag “kimberlites” in the northwestern part of the Tarim LIP that we redefine as hypabyssal aillikites based on the following features: (1) micro-phenocrystic clinopyroxene and Ti-rich andradite garnet occurring in abundance in the carbonate-rich matrix; (2) Cr-spinel exhibiting typical Fe-Ti enrichment trend also known as titanomagnetite trend; and (3) olivine showing dominantly low Mg values (Fo < 90). To constrain the magma source and evolution, the major, minor, and trace element abundance in olivine grains from these rocks were analyzed using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Olivine in the aillikites occurs as two textural types: (1) groundmass olivines, as sub-rounded grains in matrix, and (2) macrocrysts, as euhedral-anhedral crystals in nodules. The groundmass olivines show varying Mg (Fo89-80) with high-Ni (1606-3418 ppm) and Mn (1424-2860 ppm) and low-Ca (571-896 ppm) contents. In contrast, the macrocrysts exhibit a restricted Fo range but a wide range in Ni and Mn. The former occurs as phenocrysts, whereas the latter are cognate cumulates that formed from earlier, evolved aillikite melt. The two olivine populations can be further divided into sub-groups, indicating a multi-stage crystallization history of the aillikite melt. The crystallization temperatures of groundmass olivines and macrocrysts in dunite nodules as computed from the spinel-olivine thermometers are 1005-1136 and 906-1041 °C, respectively. The coupled enrichment of Ca and Ti and lack of correlation between Ni and Sc and Co in the olivine grains suggest a carbonate-silicate metasomatized mantle source. Moreover, the high 100•Mn/Fe (average 1.67) at high Ni (up to 3418 ppm), overlapping with OIB olivine, and the 100•Ni/Mg (~1) of primitive Mg-Ni-rich groundmass olivines suggest a mixed source that involved phlogopite- and carbonate-rich metasomatic veins within mantle peridotite.
Geochimica et Cosmochimica Acta, Vol. 326, pp. 77-96.
China
allikites
Abstract: Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186-0.313) with age-corrected ?Os values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr(i) = 0.70349-0.70384; ?Nd(i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr(i) = 0.70340-0.70390; ?Nd(i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (?Nd(i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr(i) = 0.70362-0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume.
Geochemical Perspectives Letters, Vol. 17, pp. 11-15. pdf
Mantle
carbonatites
Abstract: Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer. To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report ?44/42Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol. Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to ?44/42Ca values of basalts, while non-primary carbonatites show detectable ?44/42Ca variations that are correlated to ?13C values. Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution. The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
Origin of clinopyroxene and amphibole megacrysts in the alkali basaltic rocks from western Taiwan as constrained by rare earth elements (REE) geochemistry
Proceedings of the Geological Society of China, Vol. 33, No. 3, July pp. 177-204
Geological Society of London, Special Publication, Vol. 470, doi:10.1144 /SP470.10
Mantle
wilson cycle
Abstract: Although the Wilson cycle is usually considered in terms of wide oceans floored with normal oceanic crust, numerous orogens result from the closure of embryonic oceans. We discuss how orogenic and post-orogenic processes may be controlled by the size/maturity of the inverted basin. We focus on the role of lithospheric mantle in controlling deformation and the magmatic budget. We describe the physical properties (composition, density, rheology) of three types of mantle: inherited, fertilized and depleted oceanic mantle. By comparing these, we highlight that fertilized mantle underlying embryonic oceans is mechanically weaker, less dense and more fertile than other types of mantle. We suggest that orogens resulting from the closure of a narrow, immature extensional system are essentially controlled by mechanical processes without significant thermal and lithological modification. The underlying mantle is fertile and thus has a high potential for magma generation during subsequent tectonic events. Conversely, the thermal state and lithology of orogens resulting from the closure of a wide, mature ocean are largely modified by subduction-related arc magmatism. The underlying mantle wedge is depleted, which may inhibit magma generation during post-orogenic extension. These end-member considerations are supported by observations derived from the Western Europe-North Atlantic region.
Journal of Mineralogical and Petrological Sciences, Vol. 115, pp. 236-246. pdf
Mantle
diamond inclusions
Abstract: The genesis of CLIPPIR diamonds (Cullinan-like, large, inclusion-poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P-T parameters in Fe-S melt containing large amounts of silicate components (5-20 wt%). The experiments were performed using a split-sphere multi-anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal-sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.
Abstract: The first results on the interaction between an Fe-Ni melt and anthracene (?14?10) in the presence of olivine at 3 GPa and 1500°? and on the study of the component composition of the fluid generated in this process are presented. The stability of aliphatic hydrocarbons in the implemented conditions is confirmed experimentally. It is established that, under these conditions, crystallization of high-magnesian olivines occurs (Fo = 97-98 mol %). The composition of the fluid is similar to the composition of the fluid from inclusions in synthetic diamonds. The conditions implemented in the experiment might have occurred at the early stages of the Earth’s evolution.
Journal of Mineralogical and Petrological Sciences, Vol. 115, pp. 236-246. pdf
Mantle
diamond inclusions
Abstract: The genesis of CLIPPIR diamonds (Cullinan-like, large, inclusion-poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P-T parameters in Fe-S melt containing large amounts of silicate components (5-20 wt%). The experiments were performed using a split-sphere multi-anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal-sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.
Doklady Earth Sciences, Vol. 471, 2, pp. 1277-1279.
Mantle
Diamond morphology
Abstract: The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P-T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.
European Journal of Mineralogy, Vol. 28, pp. 329-336.
Russia
Deposit - Udachnaya
Abstract: An experimental simulation of serpentine and chromite interaction was conducted at the pressure (P) and temperature (T) conditions of garnet-peridotite stability in order to clarify the potential role of serpentinite as a source for the crystallization of subcalcic garnet in the depleted subcratonic mantle. The experiments were performed at 4 GPa and 1100 C and 5.5 GPa and 1200 C using the high-pressure apparatus BARS. Natural antigorite from ophiolites of the Eastern Sayan (Russia) was used as a starting material. Two groups of chromite grains with different Cr# ¼ 100Cr/(Cr þ Al) ratios (from spinel peridotite xenoliths from the Udachnaya kimberlite pipe, Yakutia) were added to the antigorite. Newly formed garnet, spinel, olivine and orthopyroxene were observed as the products in the experiments. Garnet formed only around chromite grains with the lower Cr# value (46.4). Garnet has low CaO contents (,0.05 -1.10 wt.%) with chromium contents showing wide intra- and inter-grain variations (Cr# ¼ 0.7 -33.5). The Cr content increases from core to rim with the outer zones corresponding most closely to the equilibrium composition of the relevant bulk composition. The garnet total FeO content is in the range 3.4 -5.8 wt.%. The experiments demonstrate that serpentinite decomposed at a temperature of 700 C to olivine þ orthopyroxene þ water. If mingled mechanically with spinel-bearing mantlewedge peridotite upon subduction, it could react to form the range of subcalcic garnet compositions found as inclusions in diamonds.
From subcalcic pyropes to uvarovites: experimental crystalllization of Cr-rich garnets in ultramafic systems with presence of Ca bearing hydrous fluid.
Abstract: This paper focuses on formation of subcalcic Cr-rich garnet (up to 14.25?wt% Cr2O3) in the model ultramafic system corresponding to natural harzburgite with the presence of REE-bearing fluid phase. The experiments were carried out using a “split-sphere” type multi-anvil high-pressure apparatus (BARS) at a pressure of 5?GPa and a temperature of 1300?°C. Natural serpentine, chromite, corundum and REE-carbonatite were used as starting components. Crystallization of garnet occurred in subsolidus conditions by the reaction of orthopyroxene and spinel in the presence of fluid phase. Composition of fluid was controlled by interaction of water released by decomposition of serpentine with carbonate. By using different amounts of carbonatite (0.5 and 1.5?wt%) as a source of calcium and REE, subcalcic Cr-rich garnets with up to 3.5?wt% CaO were crystallized, which are typical for inclusions of harzburgitic paragenesis in natural diamonds. The experiments demonstrated that the rare earth elements (REE) released from the initial carbonatite were transported by the fluid and were incorporated into the newly formed garnet. The distribution of REE in garnet revealed a vivid enrichment toward the heavy REE (HREE), showing the pattern with a very steep slope. These results confirmed high partitioning of HREE into garnet. The present study indicates that the mantle carbonatites, which contain very high proportions of light REE (LREE) to HREE, can play an important role as source material in formation of REE-rich fluids to crystallize garnets with typical REE patterns in mantle peridotites.
International Geology Review, Vol. 61, 4, pp. 504-519.
Russia, China
coesite, UHP
Abstract: The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2-3 mm synthetic diamond monocrystals at 2-4 GPa in ????3 (1550°?) and ?2??3 (1450°?) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450-1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.
International Geology Review, Vol. 61, 4, pp. 504-519.
Russia, China
coesite, UHP
Abstract: The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2-3 mm synthetic diamond monocrystals at 2-4 GPa in ????3 (1550°?) and ?2??3 (1450°?) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450-1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.
Abstract: Recently it was found that large natural diamonds can grow from a metal liquid. One of the principal issues of the proposed hypothesis is the formation of so-called “pockets” filled with Fe-Ni melt and hydrocarbons in the Earth's mantle. The existing models of Fe migration imply percolation of liquid melt through interconnected interstices between silicate minerals, although these models face several fundamental problems in explaining the process of penetration of Fe melt between solid crystalline phases like silicate and oxide minerals. The aim of the present study is to contribute to the mechanism of Fe-Ni melt migration, and to elucidate the evolution of the "pockets" in the presence of hydrocarbons. The experiments were performed using a high-pressure apparatus "BARS" at pressures 3 and 5?GPa, and temperature 1600?°C. A silicate matrix consisting of natural olivine grains was used. The interstices in olivine were filled with anthracene that decomposes under high P-T into a complex hydrocarbon fluid. Percolation of Fe-Ni (64/36?wt%) melt through the interstices was demonstrated which occurred at relatively high rates. The basis of the proposed mechanism is "solubility-enhanced infiltration": Fe-Ni occupies the space filled with light elements or substances that are soluble in the melt. It is suggested that the following simple, but efficient mechanism supports the growth of large diamonds as well as their resorption and storage within silicate mantle of the Earth for a long time.
Abstract: Recently it was found that large natural diamonds can grow from a metal liquid. One of the principal issues of the proposed hypothesis is the formation of so-called “pockets” filled with Fe-Ni melt and hydrocarbons in the Earth's mantle. The existing models of Fe migration imply percolation of liquid melt through interconnected interstices between silicate minerals, although these models face several fundamental problems in explaining the process of penetration of Fe melt between solid crystalline phases like silicate and oxide minerals. The aim of the present study is to contribute to the mechanism of Fe-Ni melt migration, and to elucidate the evolution of the "pockets" in the presence of hydrocarbons. The experiments were performed using a high-pressure apparatus "BARS" at pressures 3 and 5?GPa, and temperature 1600?°C. A silicate matrix consisting of natural olivine grains was used. The interstices in olivine were filled with anthracene that decomposes under high P-T into a complex hydrocarbon fluid. Percolation of Fe-Ni (64/36?wt%) melt through the interstices was demonstrated which occurred at relatively high rates. The basis of the proposed mechanism is "solubility-enhanced infiltration": Fe-Ni occupies the space filled with light elements or substances that are soluble in the melt. It is suggested that the following simple, but efficient mechanism supports the growth of large diamonds as well as their resorption and storage within silicate mantle of the Earth for a long time.
Abstract: The study of diamond surfaces is traditionally undertaken in geology and materials science. As a sample material, two natural diamond crystals of type Ia were selected, and their luminescence and nitrogen state was characterized. In order to etch the surface catalytic hydrogenation was performed using Fe particles as an etchant. Micromorphology of the surface was investigated by scanning electron and laser confocal microscopy. It was demonstrated that etching occurred perpendicular to the crystal surface, with no signs of tangential etching. The average depth of caverns did not exceed 20-25 ?m with a maximal depth of 40 ?m. It is concluded that catalytic hydrogenation of natural type Ia diamonds is effective to produce a porous surface that can be used in composites or as a substrate material. Additionally, the comparison of results with porous microsculptures observed on natural impact diamond crystals from the Popigai astrobleme revealed a strong resemblance.
Abstract: The study of diamond surfaces is traditionally undertaken in geology and materials science. As a sample material, two natural diamond crystals of type Ia were selected, and their luminescence and nitrogen state was characterized. In order to etch the surface catalytic hydrogenation was performed using Fe particles as an etchant. Micromorphology of the surface was investigated by scanning electron and laser confocal microscopy. It was demonstrated that etching occurred perpendicular to the crystal surface, with no signs of tangential etching. The average depth of caverns did not exceed 20-25 ?m with a maximal depth of 40 ?m. It is concluded that catalytic hydrogenation of natural type Ia diamonds is effective to produce a porous surface that can be used in composites or as a substrate material. Additionally, the comparison of results with porous microsculptures observed on natural impact diamond crystals from the Popigai astrobleme revealed a strong resemblance.
Abstract: Diamond etching in high-temperature ambient-pressure experiments has been performed aimed to assess possible postimpact effects on diamonds in impact craters, for the case of the Popigai crater in Yakutia (Russia). The experiments with different etchants, including various combinations of silicate melts, air, and inert gases, demonstrated the diversity of microstructures on {111} diamond faces: negative or positive trigons, as well as hexagonal, round, or irregularly shaped etch pits and striation. The surface features obtained after etching experiments with kimberlitic diamonds are similar to those observed on natural impact diamonds with some difference due to the origin of the latter as a result of a martensitic transformation of graphite in target rocks. Extrapolated to natural impact diamonds, the experimental results lead to several inferences: (1) Diamond crystals experienced natural oxidation and surface graphitization during the pressure decrease after the impact event, while the molten target rocks remained at high temperatures. (2) Natural etching of diamonds in silicate melts is possible in a large range of oxidation states controlled by O2 diffusion. (3) Impact diamonds near the surface of molten target rocks oxidized at the highest rates, whereas those within the melt were shielded from the oxidizing agents and remained unchanged.
Abstract: Diamond etching in high-temperature ambient-pressure experiments has been performed aimed to assess possible postimpact effects on diamonds in impact craters, for the case of the Popigai crater in Yakutia (Russia). The experiments with different etchants, including various combinations of silicate melts, air, and inert gases, demonstrated the diversity of microstructures on {111} diamond faces: negative or positive trigons, as well as hexagonal, round, or irregularly shaped etch pits and striation. The surface features obtained after etching experiments with kimberlitic diamonds are similar to those observed on natural impact diamonds with some difference due to the origin of the latter as a result of a martensitic transformation of graphite in target rocks. Extrapolated to natural impact diamonds, the experimental results lead to several inferences: (1) Diamond crystals experienced natural oxidation and surface graphitization during the pressure decrease after the impact event, while the molten target rocks remained at high temperatures. (2) Natural etching of diamonds in silicate melts is possible in a large range of oxidation states controlled by O2 diffusion. (3) Impact diamonds near the surface of molten target rocks oxidized at the highest rates, whereas those within the melt were shielded from the oxidizing agents and remained unchanged.
Abstract: The results of a study using scanning electron microscopy and atomic force microscopy comprising the micromorphology of the ditrigonal and trigonal layers on surfaces near the edges of octahedral diamond crystals from the Udachnaya-Eastern kimberlite pipe in Yakutia are presented. The studied surface sculptures are elongated parallel to the direction ?111? and have similar morphological features, characterized by a wavy profile across the lamination, the absence of flat areas at the micro- and nanolevel. It is proposed that both sculpture types were formed as a result of dissolution under natural conditions. This suggestion is corroborated by the revelation of negative trigons on the octahedral facets of the studied diamonds.
Abstract: An experimental study of the dissolution of natural and synthetic diamonds in a sulfur-bearing iron melt (Fe0.7S0.3) with high P-T parameters (4 GPa, 1400°?) was performed. The results demonstrated that under these conditions, octahedral crystals with flat faces and rounded tetrahexahedral diamond crystals are transformed into rounded octahedroids, which have morphological characteristics similar to those of natural diamonds from kimberlite. It was suggested that, taking into account the complex history of individual natural diamond crystals, including the dissolution stages, sulfur-bearing metal melts up to sulfide melts were not only diamond-forming media during the early evolution of the Earth, but also natural solvents of diamond in the mantle environment before the formation of kimberlitic melts.
Journal of Volcanology and Seismology, Vol. 12, 2, pp. 140-149.
Russia
deposit- Nyurbinskaya
Abstract: The study of viscosity in sub-liquidus heterogeneous media, which includes kimberlite magma at the pressures and temperatures that prevail in the mantle, is an urgent task. We have conducted experiments in the serpentine-olivine, serpentine-CaCO3?olivine, and native kimberlite-olivine systems at a pressure of 4 GPa and temperatures of 1400?1600°? in a BARS high-pressure device using the technique of a falling Pt pellet. The samples were examined after experiments to find fine-grained chilled mass of crystals where the Pt pellet was observed at the time of chilling. The concentration of the solid phase was varied in the experiments between 10 and 50 wt %. We showed that when 50 wt % of olivine grains has been introduced, it was not possible to detect the motion of the Pt pellet, while when the concentration of olivine xenocrysts reached 10 wt %, the Pt pellet very rapidly descended to the bottom of the reaction volume. Viscosity was calculated using the Stokes method. We found that the viscosity of a homogeneous kimberlite melt at 4 GPa and 1600°? is below 2 Pa s, with the viscosity of a melt that contained up to 10 wt % of the solid phase being approximately constant. A kimberlite melt that contained 30 wt % of the solid phase had a viscosity on the order of 100 Pa s, while with 50 wt % of the solid phase the relative viscosity of an ultrabasic system increased to reach values over 1000 Pa s.
Abstract: The results of experiments on dissolution of diamond in a Fe melt with variable concentrations of S at high P-T parameters are presented. It is established that the maximal degree of diamond dissolution occurs at a sulfur concentration of 15 wt %. With decreasing or increasing S content, dissolution of diamond slows down and almost does not occur during the period of the experiment (60 min), when the "eutectic" composition is gained. In contrast to a pure Fe melt, the presence of S decreases the carbon solubility and, therefore, reduces the aggressiveness of metal melt in relation to diamonds, thus, stimulating their preservation in the Earth’s mantle, especially if the concentration of S exceeds that in the "eutectic" composition.
Doklady Earth Sciences, Vol. 489, 2, pp. 1449-1452 .pdf
Russia
diamond morphology, CLIPPIR
Abstract: The primary results are presented on the dissolution of plane-faced diamond crystals of octahedral habit in a Fe-Ni-S melt under 3.5 GPa and 1400°C. It was found that the dissolution resulted in the transformation of plane-faced into curve-faced individuals of morphological features characteristic for kimberlite diamonds. It was concluded that the diamond forms as such might have formed in reduced domains of the Earth’s mantle before becoming involved in the kimberlite magma.
Geology of Ore Deposits, Vol. 62, 6, pp. 497-507. pdf
Russia, Yakutia
deposit Yubileinaya
Abstract: An experimental study was carried out on the dissolution of natural octahedral diamonds from the Internatsionalnaya and Yubileinaya kimberlite pipes (Yakutia) in an Fe-S melt at 4 GPa and 1450-1500°C with different sulfur contents (10-25 wt %). It was found that with an increase in sulfur content in the iron melt, the degree of diamond dissolution sharply decreases. The stationary (final) shape of diamond crystal dissolution under the achieved conditions corresponds to an octahedroid with trigonal etching layers, which is confirmed by photogoniometry. Diamonds with similar morphology are common in kimberlite pipes, especially in mantle xenoliths from kimberlites. It was concluded that diamonds with this shape did not undergo natural dissolution in a kimberlite magma, but, similar to flat-faced octahedra, were probably isolated from it in xenoliths. Therefore, the higher the content of octahedroid-shaped diamonds with trigonal layers in a deposit, the smaller the direct influence of an aggressive kimberlite magma on the diamond content.
Geochemistry International, Vol. 54, 10, pp. 992-927.
Technology
Synthetics
Abstract: The IR-peak 1450 cm-1 (H1a-center) associated with nitrogen interstitials have been studied in nitrogen-bearing diamonds synthesized at high P-T parameters in the Fe-Ni-C system. FTIR study shows that manifestation of this nitrogen form is restricted to the regions of active transformation of C-defects into A-defects, which confirms the connection of its formation with C => A aggregation process. An examination of the dependence of the 1450 cm-1 peak on the degree of nitrogen aggregation indicates that H1a-centers are not only formed during C/A aggregation but also disappear simultaneously with the end of C => A transformation. Established facts suggest direct involving of nitrogen as interstitials in the C => A aggregation and serve as strong experimental argument in support of the "interstitial" mechanism of nitrogen migration during aggregation in diamonds containing transition metals.
Doklady Earth Sciences, Vol. 471, 2, pp. 1277-1279.
Mantle
Diamond morphology
Abstract: The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P-T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.
Abstract: The results of a study using scanning electron microscopy and atomic force microscopy comprising the micromorphology of the ditrigonal and trigonal layers on surfaces near the edges of octahedral diamond crystals from the Udachnaya-Eastern kimberlite pipe in Yakutia are presented. The studied surface sculptures are elongated parallel to the direction ?111? and have similar morphological features, characterized by a wavy profile across the lamination, the absence of flat areas at the micro- and nanolevel. It is proposed that both sculpture types were formed as a result of dissolution under natural conditions. This suggestion is corroborated by the revelation of negative trigons on the octahedral facets of the studied diamonds.
Abstract: An experimental study of the dissolution of natural and synthetic diamonds in a sulfur-bearing iron melt (Fe0.7S0.3) with high P-T parameters (4 GPa, 1400°?) was performed. The results demonstrated that under these conditions, octahedral crystals with flat faces and rounded tetrahexahedral diamond crystals are transformed into rounded octahedroids, which have morphological characteristics similar to those of natural diamonds from kimberlite. It was suggested that, taking into account the complex history of individual natural diamond crystals, including the dissolution stages, sulfur-bearing metal melts up to sulfide melts were not only diamond-forming media during the early evolution of the Earth, but also natural solvents of diamond in the mantle environment before the formation of kimberlitic melts.
Journal of Volcanology and Seismology, Vol. 12, 2, pp. 140-149.
Russia
deposit- Nyurbinskaya
Abstract: The study of viscosity in sub-liquidus heterogeneous media, which includes kimberlite magma at the pressures and temperatures that prevail in the mantle, is an urgent task. We have conducted experiments in the serpentine-olivine, serpentine-CaCO3?olivine, and native kimberlite-olivine systems at a pressure of 4 GPa and temperatures of 1400?1600°? in a BARS high-pressure device using the technique of a falling Pt pellet. The samples were examined after experiments to find fine-grained chilled mass of crystals where the Pt pellet was observed at the time of chilling. The concentration of the solid phase was varied in the experiments between 10 and 50 wt %. We showed that when 50 wt % of olivine grains has been introduced, it was not possible to detect the motion of the Pt pellet, while when the concentration of olivine xenocrysts reached 10 wt %, the Pt pellet very rapidly descended to the bottom of the reaction volume. Viscosity was calculated using the Stokes method. We found that the viscosity of a homogeneous kimberlite melt at 4 GPa and 1600°? is below 2 Pa s, with the viscosity of a melt that contained up to 10 wt % of the solid phase being approximately constant. A kimberlite melt that contained 30 wt % of the solid phase had a viscosity on the order of 100 Pa s, while with 50 wt % of the solid phase the relative viscosity of an ultrabasic system increased to reach values over 1000 Pa s.
Abstract: The results of experiments on dissolution of diamond in a Fe melt with variable concentrations of S at high P-T parameters are presented. It is established that the maximal degree of diamond dissolution occurs at a sulfur concentration of 15 wt %. With decreasing or increasing S content, dissolution of diamond slows down and almost does not occur during the period of the experiment (60 min), when the "eutectic" composition is gained. In contrast to a pure Fe melt, the presence of S decreases the carbon solubility and, therefore, reduces the aggressiveness of metal melt in relation to diamonds, thus, stimulating their preservation in the Earth’s mantle, especially if the concentration of S exceeds that in the "eutectic" composition.
Doklady Earth Sciences, Vol. 482, 1, pp. 1207-1211.
Russia
spectrometry
Abstract: The first chromatography-mass spectroscopy data on volatiles in diamonds synthesized in the Fe-S-C system with 5 wt % S at 1400-1450°C and 5.0-5.5 GPa indicate the evolution of volatile composition during the diamond growth and, correspondingly, the variation in redox conditions of the reaction cell. A significant role is played by various hydrocarbons (HCs) and their derivatives, the content of which can reach 87%. Our data on possible abiogenic synthesis of HCs (components of natural gas and oil) can result in global recalculations (including climate) related to the global C cycle.
Doklady earth Sciences, Vol. 483, 1, pp. 1473-1474.
Mantle
melting
Abstract: The experimental data on diamond growth in the Fe-Ni-S-C and Fe-S-C systems with a sulfur content of 5-14 wt % at 5.5 GPa and 1300-1350°C are reported. Colorless and light yellow diamond crystals with a weight of 0.1-0.8 ct were synthesized. It is shown in the Fe-S-C system that at 5.5. GPa diamond may crystallize in a very narrow temperature range, from 1300 to 1370°C. Based on comparative analysis of the experimental data and the results of the study of native iron inclusions in natural diamonds from kimberlite pipes, it is suggested that diamond genesis may be partly controlled by the pre-eutectic (by the concentration of sulfur in relation to metal) metal-sulfide melt.
Abstract: The first results on the interaction between an Fe-Ni melt and anthracene (?14?10) in the presence of olivine at 3 GPa and 1500°? and on the study of the component composition of the fluid generated in this process are presented. The stability of aliphatic hydrocarbons in the implemented conditions is confirmed experimentally. It is established that, under these conditions, crystallization of high-magnesian olivines occurs (Fo = 97-98 mol %). The composition of the fluid is similar to the composition of the fluid from inclusions in synthetic diamonds. The conditions implemented in the experiment might have occurred at the early stages of the Earth’s evolution.
Abstract: Data on the interaction of the Fe-Ni melt with CaCO3 and graphite at 5 GPa and 1400°? under the thermogradient conditions used in experiments on the growth of diamond on the BARS high-pressure apparatus are presented. The phase composition and component composition of the fluid captured by diamonds in the form of inclusions were studied by gas chromatography-mass spectrometry (GC-MS). Diamonds were synthesized from graphite. During the interaction of the Fe-Ni melt with CaCO3, Ca-Fe oxides and (Fe, Ni)3C carbide were formed. The stability of heavy hydrocarbons under the experimental conditions was confirmed. It was established that the composition of the fluid in synthesized diamonds is close to the composition of the fluid from inclusions in some natural diamonds. Nevertheless, it was concluded that crystallization of large diamonds under natural conditions is hardly possible due to the filling of the main crystallization volume with refractory oxide phases.
Abstract: The first results on diamond growth in the Fe-?-S system with 1 wt % S (relative to Fe) at 6 GPa and 1450°C have been reported. The diamonds obtained contain about 30 ppm N, on average, and belong to the low-N transition diamond group Ib-IIa. It has been suggested that the reduction conditions formed by certain active elements such as S can play an important role in the formation of natural low-N diamonds.
Geology of Ore Deposits, Vol. 62, 6, pp. 497-507. pdf
Russia, Yakutia
deposit Yubileinaya
Abstract: An experimental study was carried out on the dissolution of natural octahedral diamonds from the Internatsionalnaya and Yubileinaya kimberlite pipes (Yakutia) in an Fe-S melt at 4 GPa and 1450-1500°C with different sulfur contents (10-25 wt %). It was found that with an increase in sulfur content in the iron melt, the degree of diamond dissolution sharply decreases. The stationary (final) shape of diamond crystal dissolution under the achieved conditions corresponds to an octahedroid with trigonal etching layers, which is confirmed by photogoniometry. Diamonds with similar morphology are common in kimberlite pipes, especially in mantle xenoliths from kimberlites. It was concluded that diamonds with this shape did not undergo natural dissolution in a kimberlite magma, but, similar to flat-faced octahedra, were probably isolated from it in xenoliths. Therefore, the higher the content of octahedroid-shaped diamonds with trigonal layers in a deposit, the smaller the direct influence of an aggressive kimberlite magma on the diamond content.
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.
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.
Metasomatism in lithospheric mantle roots: constraints from whole rock and mineral chemical composition of deformed peridotite xenoliths from kimberlite pipe Udachnaya.
Geochemical evolution of rocks at the base of the lithospheric mantle: evidence from study of xenoliths of deformed peridotites from kimberlite of Udachnaya
Geochimica et Cosmochimica Acta, Vol. 232 pp. 206-224.
Technology
diamond - inclusions DFT
Abstract: Diffusivities of helium, deuterium and hydrogen have been characterized in diamond. Polished CVD diamond was implanted with either 3He, 2H, or 1H. Implanted samples were sealed under vacuum in silica glass capsules, and annealed in 1-atm furnaces. 3He, 2H and 1H distributions were measured with Nuclear Reaction Analysis. We obtain these Arrhenius relations: DHe = 4.00?×?10?15 exp(?138?±?14?kJ?mol?1/RT) m2?s?1. D2H = 1.02?×?10?4 exp(?262?±?17?kJ?mol?1/RT) m2?s?1. D1H = 2.60?×?10?4 exp(?267?±?15?kJ?mol?1/RT) m2?s?1. Diffusivities of 1H and 2H agree within experimental uncertainties, indicating little diffusive mass fractionation of hydrogen in diamond. To complement the experimental measurements, we performed calculations using a first-principles quantum mechanical description of diffusion in diamond within the Density Functional Theory (DFT). Differences in 1H and 2H diffusivities from calculations are found to be ?4.5%, reflected in differences in the pre-exponential factor. This small difference in diffusivities, despite the large relative mass difference between these isotopes, is due to the fact that the atomistic process involved in the transition along the diffusion pathway is dictated by local changes to the diamond structures rather than to vibrations involving 1H/2H. This finding is consistent with the experimental results given experimental uncertainties. In contrast, calculations for helium diffusion in diamond indicate a difference of 15% between diffusivities of 3He and 4He. Calculations of diffusion distances for hydrogen using our data yield a distance of 50??m in diamond in 300,000?years at 500?°C and ?30?min at 1400?°C. Diffusion distances for He in diamond are shorter than for H at all temperatures above ?350?°C, but differences increase dramatically with temperature because of the higher activation energy for H diffusion. For example, a 50??m diffusion distance for He would be attained in ?40 Myr at 500?°C and 400?yr at 1400?°C. For comparison, a 50??m diffusion distance for N in diamond would require nearly 1 billion years at 1400?°C. The experimental data indicate that diamonds equilibrate with ambient H and He in the mantle on timescales brief relative to most geological processes and events. However, He diffusion in diamond is slower than in any other mineral measured to date, including other kimberlite-hosted minerals. Under some circumstances, diamond may provide information about mantle He not recoverable from other minerals. One possibility is diamonds entrained in kimberlites. Since the ascent of kimberlite from the mantle to near-surface is very rapid, entrained diamonds may retain most or all of the H and He acquired in mantle environments. Calculations using reasonable ascent rates and T-t paths indicate that He diffusive loss from kimberlite-hosted diamonds is negligible for grains of 1.0-0.2?mm radius, with fractional losses <0.15% for all ascent rates considered. If the host kimberlite magma is effectively quenched in the near-surface (or is erupted), diamonds should contain a faithful record of [He] and He isotopes from the mantle source region. Preservation of H in kimberlite-hosted diamonds is less clear-cut, with model outcomes depending critically upon rates of ascent and cooling.
Mathematical modeling of the stress strain state in rock and artificial masses during slice chamber mining of underpit reserves in Internationnapa kimberlite.
Journal of Mining Science, Vol. 41, 3, pp. 215-224.
Physicsa Status Solidi , doi:10.1002/pssa.201900888
Global
HPHT
Abstract: Various samples of multisectoral high?pressure high?temperature (HPHT) single?crystal diamond plate (IIa type) (4?×?4?×?0.53?mm) are tested for particle detection applications. The samples are investigated by X?ray diffractometry, photoluminescence spectroscopy, Raman spectroscopy, Fourier?transform infrared, and visible/ultraviolet (UV) absorption spectroscopy. High crystalline perfection and low impurity concentration (in the {100} growth sector) are observed. To investigate detector parameters, circular 1.0 and 1.5?mm diameter Pt Schottky barrier contacts are created on {111} and {100} growth sectors. On the backside, a Pt contact (3.5?×?3.5?mm) is produced. The {100} growth sector is proved to be a high?quality detector: the full width at half maximum energy resolution is 0.94% for the 5.489?MeV 226Ra ??line at an operational bias of +500?V. Therefore, it is concluded that the HPHT material {100} growth sector is used for radiation detector production, whose quality is not worse than the chemical vapor deposition method or specially selected natural diamond detectors.
Russian Geology and Geophysics, Vol. 63, pp. 166-183.
Russia, Siberia
deposit - Udachnaya
Abstract: Results of a geochemical study of brines and oil occurrences in the Udachnaya kimberlite pipe are presented. Like other intrusions in the Daldyn-Alakit diamondiferous region, this diamond deposit is a unique cryohydrogeologic microstructure differing from the host sedimentary rocks and other diamond pipes of the Yakutian diamond-bearing province. Two waterlogged zones distinguished in the section of orebodies at the explored depths of the deposit correspond to the upper and middle Cambrian aquifers. Predominantly acidic (average pH = 5.5) Cl-Ca and Cl-Ca-Na brines with TDS from 94.3 to 391.3 g/dm3 are widespread within the orebodies and host rocks. The brine mineralization and contents of major salt-forming components increase with depth, to the horizon at the -365 m elevation, where TDS reaches 391 g/dm3, while below, at the -650 m level with noted hydrogeochemical-field inversion, TDS is 253 g/dm3. The mineralization of Cl-Ca, Cl-Ca-Na, Cl-Ca-Mg, and Cl-Ca-Mg-Na brines in the upper Cambrian rocks varies from 102.9 to 192.9 g/dm3, and the pH values, from 4.9 to 6.2, averaging 5.6. Among the microcomponents, the highest average concentrations (mg/dm3) are found for Br1292.8 > S875.7 > Sr453.7 > Fe79.7 > Li53.4 > B32.7 > I13.3 > Si10.8 > Mn6.4 > Se3.6 > Rb2.3. The values of genetic coefficients vary widely: The rNa/rCl coefficient ranges from 0.18 to 0.31; rCa/rMg, from 1.03 to 3.60; Ca/Cl, from 0.2 to 0.3; and the integrated metamorphism index S (according to S.L. Shvartsev) varies from 193 to 277. The middle Cambrian rock complex, containing more saline brines, has been examined in much more detail. It hosts Cl-Ca, Cl-Ca-Na, Cl-Ca-Mg, and Cl-Na-Mg brines with TDS from 94.3 to 391.3 g/dm3 and high average concentrations (mg/dm3) of microcomponents: Br2224.9 > Sr1024.9 >S500.1 > B202.9 > Li147.1 > Fe97.0 > I33.2 > Rb11.4 > Si9.6 > Se9.5 > Mn3.6 > Ni1.7. As compared with brines in the overlying rocks, the middle Cambrian brines show a wider variation in element ratios: rNa/rCl from 0.14 to 0.34, rCa/rMg from 0.66 to 9.71, and Ca/Cl from 0.03 to 0.45. These brines are also characterized by a significantly higher metamorphism grade, which is indicated not only by the rNa/rCl and rCa/rMg ratios but also by the S index varying from 278 to 316. The composition of stable isotopes ?D and ?18O) and dissolved inorganic carbon ?13C) of the brines was investigated. The studied waters are assumed to be of sedimentary-metamorphic origin. Their isotopic composition reflects the climatic conditions existing at the time of their burial, which were probably aggravated by the contribution of the oxygen isotope exchange with water-bearing rocks. The ?13C values of carbon dioxide dissolved in water allow an inference about its biogenic origin. The biogenic carbon isotope exchange is governed by the relationship between methanogenic and SMT processes. Analysis of the 87Rb/86Sr and 87Sr/86Sr isotope ratios of the studied brines has revealed affinity between the isotopic compositions of waters in the Cambrian deposits and in ancient seawaters. The mass chromatograms of saturated-hydrocarbon (HC) fractions show at least two individual types of oils and malthas (naphthides). The third variety resulted from their mixing at different stages of migration. The fourth is from the contact zone; it changed during the explosion of kimberlites. The first, most common, type of naphthides (“postexplosive”) is similar in all geochemical parameters to oils from the Nepa-Botuobiya anteclise, in particular, to those from the Mirnyi arch. Oils of the second (pre-explosive) type are found only in the Udachnaya Formation, within the depth range 1130-1430 m.
Physicsa Status Solidi , doi:10.1002/pssa.201900888
Global
HPHT
Abstract: Various samples of multisectoral high?pressure high?temperature (HPHT) single?crystal diamond plate (IIa type) (4?×?4?×?0.53?mm) are tested for particle detection applications. The samples are investigated by X?ray diffractometry, photoluminescence spectroscopy, Raman spectroscopy, Fourier?transform infrared, and visible/ultraviolet (UV) absorption spectroscopy. High crystalline perfection and low impurity concentration (in the {100} growth sector) are observed. To investigate detector parameters, circular 1.0 and 1.5?mm diameter Pt Schottky barrier contacts are created on {111} and {100} growth sectors. On the backside, a Pt contact (3.5?×?3.5?mm) is produced. The {100} growth sector is proved to be a high?quality detector: the full width at half maximum energy resolution is 0.94% for the 5.489?MeV 226Ra ??line at an operational bias of +500?V. Therefore, it is concluded that the HPHT material {100} growth sector is used for radiation detector production, whose quality is not worse than the chemical vapor deposition method or specially selected natural diamond detectors.
Journal of Mining Science, Vol. 57, 2, pp. 286-297. pdf
Russia
IPKON RAS
Abstract: The thermodynamic analysis and tests of minerogenesis under higher temperatures determine conditions of thermochemical decomposition of hydrophilic attachments on diamond surface. It is found that hydrophilic mineral attachments can be removed from diamond surface by combining thermal treatment of slurry at the temperature of 80-85 ?C with electrochemical treatment of recirculated water, which enables required change in ion-molecule composition of water phase in the slurry. The hybrid conditioning technology ensures recovery of the natural hydrophobic behavior and floatability of diamonds and enhances performance of froth flotation of diamonds by 5.1%.
Journal of Mining Science, Vol. 52, 5, pp. 835-841.
Russia
deposit - Yubilieny
Abstract: The article focuses on seismic monitoring of causes of landslides. Such studies are of great importance in open pit mining in permafrost rocks. Extensive mining-induced impact in combination with natural thawing of permafrost as a consequence of the planet warming may end in catastrophe. The authors describe a procedure for plotting velocity profiles of seismic waves along slopes in the presence of extremely contrast discontinuities conditioned by permafrost rocks. The presented approach enables studying slip surfaces of landslides and detecting potential failure zones where wave velocities are lower due to extensive jointing. The processed field data obtained in the area near Chagan-Uzun settlement in Kosh-Agach district of the Republic of Altai are reported.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Abstract: The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5?m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7?±?4.4?Ma and 86.5?±?2.6?Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster. The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections. The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel - magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration. Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn?/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.
Earth and Planteray Science Letters, Vol. 489, pp. 258-266.
Mantle
subduction
Abstract: The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ?3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10-50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100-200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10-50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10-50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.
Abstract: We have studied a suite of mantle zircons from several differently aged pipes of the Siberian kimberlite province via UPb and LuHf isotope analyses and trace element compositions. The UPb ages we obtained confirmed four main episodes (Silurian, Devonian, Triassic and Jurassic) of kimberlite activity on the Siberian craton. The Druzhba pipe had two populations of zircons dating from the Silurian and Devonian, respectively. The geochemical features of our suite of mantle zircons show low concentrations of U, Th and heavy rare earth elements (REEs), positive Ce anomalies, and weak or absent Eu anomalies, which is in accord with the mantle-derived nature of the zircon. Despite having broadly similar geochemistry, zircons from differently aged kimberlites had some clear differences arising from variations in the composition of the protokimberlite metasomatic melt and from peculiarities of fractional crystallization. The Th/U ratios were highest in the Silurian zircons and sharply decreased toward the Devonian. The Triassic zircons had elevated and highly variable Ce/Nb ratios with low and nearly constant Th/U ratios. Zircons from Siberian kimberlites with different UPb ages showed systematic variations in their initial Hf isotope compositions. The oldest Silurian kimberlite field, Chomurdakh, had two zircon populations: Silurian zircons, with ?Hft values in the range of +2.8 to +5.9 units, and Devonian zircons, with ?Hft values in the range of +1.6 to +2.0 units. Zircons from the Devonian field kimberlites were in the range of +5.6 to +9.6 ?Hft units. The Triassic kimberlitic zircons had the most juvenile Hf isotope composition, at +9.3 to +11.2 ?Hft units, while the Jurassic zircons had +6.9 ?Hft units. The combination of the UPb and LuHf isotope data suggests a periodic rejuvenation of the lithospheric mantle roots by low-volume melts from the asthenospheric mantle, resulting shortly after in kimberlite emplacements. Some Devonian and Jurassic kimberlites may have been melted by re-heating the Silurian and Triassic age sources, respectively, about 60 Myr after they were formed.
Abstract: Mesozoic (125-135 Ma) cratonic low-Ti lamproites from the northern part of the Aldan Shield do not conform to typical classification schemes of ultrapotassic anorogenic rocks. Here we investigate their origins by analyzing olivine and olivine-hosted inclusions from the Ryabinoviy pipe, a well preserved lamproite intrusion within the Aldan Shield. Four types of olivine are identified: (1) zoned phenocrysts, (2) high-Mg, high-Ni homogeneous macrocrysts, (3) high-Ca and low-Ni olivine and (4) mantle xenocrysts. Olivine compositions are comparable to those from the Mediterranean Belt lamproites (Olivine-1 and -2), kamafugites (Olivine-3) and leucitites. Homogenized melt inclusions (MIs) within olivine-1 phenocrysts have lamproitic compositions and are similar to the host rocks, whereas kamafugite-like compositions are obtained for melt inclusions within olivine-3. Estimates of redox conditions indicate that “lamproitic” olivine crystallized from anomalously oxidized magma (?NNO +3 to +4 log units.). Crystallization of "kamafugitic" olivine occurred under even more oxidized conditions, supported by low V/Sc ratios. We consider high-Ca olivine (3) to be a fingerprint of kamafugite-like magmatism, which also occurred during the Mesozoic and slightly preceded lamproitic magmatism. Our preliminary genetic model suggests that low-temperature, extension-triggered melting of mica- and carbonate-rich veined subcontitental lithospheric mantle (SCLM) generated the kamafugite-like melts. This process exhausted carbonate and affected the silicate assemblage of the veins. Subsequent and more extensive melting of the modified SCLM produced volumetrically larger lamproitic magmas. This newly recognized kamafugitic "fingerprint" further highlights similarities between the Aldan Shield potassic province and the Mediterranean Belt, and provides evidence of an overlap between "orogenic" and "anorogenic" varieties of low-Ti potassic magmatism. Moreover, our study also demonstrates that recycled subduction components are not an essential factor in the petrogenesis of low-Ti lamproites, kamafugites and leucitites.
Abstract: Mesozoic (125-135 Ma) cratonic low-Ti lamproites from the northern part of the Aldan Shield do not conform to typical classification schemes of ultrapotassic anorogenic rocks. Here we investigate their origins by analyzing olivine and olivine-hosted inclusions from the Ryabinoviy pipe, a well preserved lamproite intrusion within the Aldan Shield. Four types of olivine are identified: (1) zoned phenocrysts, (2) high-Mg, high-Ni homogeneous macrocrysts, (3) high-Ca and low-Ni olivine and (4) mantle xenocrysts. Olivine compositions are comparable to those from the Mediterranean Belt lamproites (Olivine-1 and -2), kamafugites (Olivine-3) and leucitites. Homogenized melt inclusions (MIs) within olivine-1 phenocrysts have lamproitic compositions and are similar to the host rocks, whereas kamafugite-like compositions are obtained for melt inclusions within olivine-3. Estimates of redox conditions indicate that “lamproitic” olivine crystallized from anomalously oxidized magma (?NNO +3 to +4 log units.). Crystallization of "kamafugitic" olivine occurred under even more oxidized conditions, supported by low V/Sc ratios. We consider high-Ca olivine (3) to be a fingerprint of kamafugite-like magmatism, which also occurred during the Mesozoic and slightly preceded lamproitic magmatism. Our preliminary genetic model suggests that low-temperature, extension-triggered melting of mica- and carbonate-rich veined subcontitental lithospheric mantle (SCLM) generated the kamafugite-like melts. This process exhausted carbonate and affected the silicate assemblage of the veins. Subsequent and more extensive melting of the modified SCLM produced volumetrically larger lamproitic magmas. This newly recognized kamafugitic "fingerprint" further highlights similarities between the Aldan Shield potassic province and the Mediterranean Belt, and provides evidence of an overlap between "orogenic" and "anorogenic" varieties of low-Ti potassic magmatism. Moreover, our study also demonstrates that recycled subduction components are not an essential factor in the petrogenesis of low-Ti lamproites, kamafugites and leucitites.
Abstract: Pyrope xenocrysts (N = 52) with associated inclusions of Ti- and/or Cr-rich oxide minerals from the Aldanskaya dyke and Ogonek diatreme (Chompolo field, southeastern Siberian craton) have been investigated. The majority of xenocrysts are of lherzolitic paragenesis and have concave-upwards (normal) rare earth element (REEN) patterns that increase in concentration from light REE to medium-heavy REE (Group 1). Four Ca-rich (5.7-7.4 wt.% CaO) pyropes are extremely low in Ti, Na and Y and have sinusoidal REEN spectra, thus exhibiting distinct geochemical signatures (Group 2). A peculiar xenocryst, s165, is the only sample to show harzburgitic derivation, whilst demonstrating a normal-to-weakly sinusoidal REEN pattern and the highest Zr (93 ppm) and Sc (471 ppm). Chromite-magnesiochromite, rutile, Mg-ilmenite and crichtonite-group minerals comprise a suite of oxide mineral inclusions in the pyrope xenocrysts. These minerals are characteristically enriched in Cr with 0.6-7.2 wt.% Cr2O3 in rutile, 0.7-3.6 wt.% in Mg-ilmenite and 7.1-18.0 wt.% in the crichtonite-group minerals. Complex titanates of the crichtonite group enriched in large ion lithophile elements (LILE) are high in Al2O3 (0.9-2.2 wt.%), ZrO2 (1.5-5.4 wt.%) and display a trend of compositions from the Ca-Sr-specific varieties to the Ba-dominant species (e.g. lindsleyite). In the pyrope xenocrysts the oxides coexist with silicates (clino- and orthopyroxene and olivine), hydrous silicates (talc, phlogopite and amphibole), carbonate (magnesite), sulfides (pentlandite, chalcopyrite, breakdown products of monosulfide and bornite solid solutions), apatite and graphite. P-T estimates imply the inclusion-bearing pyrope xenocrysts have been derived from low-temperature peridotite assemblages that resided at temperatures of ~600-800°C and a pressure range of ~25-35 kbar in the graphite stability field. Pyrope genesis is linked to the metasomatic enrichment of peridotite protoliths by Ca-Zr-LILE-bearing percolating fluid-melt phases containing significant volatile components. These metasomatic agents are probably volatile-rich melts or supercritical C-O-H-S fluids that were released from a Palaeo-subduction slab.
Journal of Mining Science, Vol. 54, 4, pp. 628-637.
Russia, Yakutia
mining
Abstract: Potential ranges of use of draglines at steeply dipping diamond ore deposits in Yakutia are discussed. Technology of stripping with direct dumping and rehandling by draglines is substantiated for upper overburden layers. A variant of increasing height of stripping benches on haulage horizons through the use of draglines and crane lines is discussed. A resource saving technology is proposed for mining roundish and extended ore bodies with alternating advance of mining front and with internal dumping. The method of estimating efficient thickness of overburden in case of direct dumping is developed using the layer coefficient of overburden rehandling. Expediency of using blasting for displacement of broken overburden to internal dump is specified.
Abstract: The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt–rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle–crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt–rock reactions associated with lithospheric thinning.
Comptes Rendus Geoscience, in press available 11p.
Africa, Morocco
Peridotite
Abstract: The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt-rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle-crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt-rock reactions associated with lithospheric thinning.
Journal of African Earth Sciences, Vol. 158, available 14p. pdf
Africa, South Africa
magmatism
Abstract: Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof. The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180?Ma), mainly sill complexes to later (182-174?Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.
Abstract: North America provides an important test for assessing the coupling of large continents with heterogeneous Archean- to Cenozoic-aged lithospheric provinces to the mantle flow. We use the unprecedented spatial coverage of the USArray seismic network to obtain an extensive and consistent data set of shear wave splitting intensity measurements at 1436 stations. Overall, the measurements are consistent with simple shear deformation in the asthenosphere due to viscous coupling to the overriding lithosphere. The fast directions agree with the absolute plate motion direction with a mean difference of 2° with 27° standard deviation. There are, however, deviations from this simple pattern, including a band along the Rocky Mountain front, indicative of flow complication due to gradients in lithospheric thickness, and variations in amplitude through the central United States, which can be explained through varying contributions of lithospheric anisotropy. Thus, seismic anisotropy may be sourced in both the asthenosphere and lithosphere, and variations in splitting intensity are due to lithospheric anisotropy developed during deformation over long time scales.
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.
Basin Research, doi.org/10.111 /bre.12527 23p. Pdf
Africa, Namibia
Thermochron, temperature logging
Abstract: Products of onshore passive continental margin erosion are best preserved in offshore sedimentary basins. Therefore, these basins potentially hold a recoverable record of the onshore erosion history. Here, we present apatite fission track (AFT) data for 13 samples from a borehole in the southern Walvis basin, offshore Namibia. All samples show AFT central ages older or similar to their respective stratigraphic ages, while many single grain ages are older, implying none of the samples has been totally annealed post?deposition. Furthermore, large dispersion in single grain ages in some samples suggests multiple age components related to separate source regions. Using Bayesian mixture modelling we classify single grain ages from a given sample to particular age components to create ‘subsamples’ and then jointly invert the entire dataset to obtain a thermal history. For each sample, the post?depositional thermal history is required to be the same for all age components, but each component (‘subsample’) has an independent pre?depositional thermal history. With this approach we can resolve pre? and post?depositional thermal events and identify changes in sediment provenance in response to the syn? and post?rift tectonic evolution of Namibia and southern Africa. Apatite U?Pb and compositional data obtained during the acquisition of LA?ICP?MS FT data are also presented to help track changes in provenance with time. We constrain multiple thermal events linked to the exhumation and burial history of the continental and offshore sectors of the margin over a longer timescale than has been possible using only onshore AFT thermochronological data.
Earth and Planetary Science Letters, Vol. 584, doi 10.1016/j.espl.2022.117480
Africa, Mali, Mauritania
craton
Abstract: Large Igneous Provinces (LIPs) are exceptionally voluminous magmatic events frequently related to continental break-up, global climate changes and mass extinctions. One interesting aspect of many LIPs is their spatial proximity to cratons, begging the question of a potential control of thick lithosphere on their emplacement. In this study, we investigate the relationship between the emplacement of the Central Atlantic Magmatic Province (CAMP) and the thick lithospheric mantle of the Precambrian cratons that formed the central portion of Pangea and are currently located on the continents surrounding the Central Atlantic Ocean. CAMP outcrops are frequently located over the margins of the thick cratonic keels, as imaged by recent tomographic studies, suggesting a role of lithosphere architecture in controlling magma genesis and emplacement. Here we focus on CAMP dykes and sills from the Hank, Hodh, and Kaarta basins in North-Western Africa (NW-Africa, Mali and Mauritania) emplaced at the edge of the Reguibat and Leo-Man Shields. The investigated intrusive rocks show compositions similar to most CAMP magmas, in particular those of the Tiourjdal geochemical group, limited to NW-Africa, and of the Prevalent group, occurring all over the CAMP. Geochemical modelling of CAMP basalts requires a Depleted MORB Mantle (DMM) source enriched by recycled continental crust (1-4%) and melting beneath a lithosphere of ca. 80 km in thickness. On the contrary, melting under a significantly thicker lithosphere (>110 km) does not produce magmas with compositions similar to those of CAMP basalts. This suggests that CAMP magmatism was likely favoured by decompression-induced partial melting of the upwelling asthenospheric mantle along the steep lithospheric boundaries of stable cratons. The architecture of the pre-existing lithosphere (i.e., the presence of stable thick cratonic keels juxtaposed to relatively thinner lithosphere) appears to have been a critical factor for localizing mantle upwelling and partial melting during extensive magmatic events such as in the CAMP.
Abstract: The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In contrast, oxygen fugacity (graphic) data of upper mantle under orogens worldwide are rare, and the mechanism responsible for the mantle graphic condition under orogens is not well constrained. In this study, we investigated the graphic of mantle xenoliths derived from the southern Tibetan lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic volcanic rocks hosting the xenoliths. The graphic of mantle xenoliths ranges from ?FMQ = +0.5 to +1.2 (where ?FMQ is the deviation of log graphic from the fayalite-magnetite-quartz buffer), indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and oceanic mantle, and it falls within the typical range of mantle wedge graphic values. Mineralogical evidence suggests that water-rich fluids and sediment melts liberated from both the subducting Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized the southern Tibetan lithospheric mantle. The graphic conditions of ultrapotassic magmas show a shift toward more oxidized conditions during ascent (from ?FMQ = +0.8 to +3.0). Crustal evolution processes (e.g., fractionation) could influence magmatic graphic, and thus the redox state of mantle-derived magma may not simply represent its mantle source.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 77.
Canada, Northwest Territories
Deposit - Ekati
Abstract: The Ekati Waste Rock Storage Piles (WRSA) water balance model was developed in Goldsim as a module designed to support water quality estimates. The water balance module accounts for direct precipitation, snowmelt, seepage, runoff, and delays to flow within the WRSA. As the Ekati Mine is located within a climate zone of continuous permafrost, a portion of the water infiltrating the WRSA’s becomes trapped within the waste rock as ice when it encounters sub-freezing internal temperatures. Seepage leaching from the WRSAs is thus limited to the outer surface of the WRSAs (active layer) where water produced by melting of seasonal surficial ice and snow. The active layer was modeled by detaining all water in the WRSA from October to the end of June. After June the water was released from the layer using a delay function, which is described below. The model divides seepage and runoff into three physically-based flow paths. ? The primary flow path is of water that falls infiltrates vertically through the waste rock until it encounters an impermeable lens of ice-saturate rock, and travels horizontally, to ultimately emerge at the toe of the WRSAs. ? The secondary flow path is water that falls on the outer slopes of the WRSAs and seeps under the outer slopes to the toe. ? The third flow path is also of water that falls on the outer slopes of the WRSAs and travels along the surface of the WRSA to the toe as runoff. Water losses were accounted at the surface of the pile prior to infiltration and within the pile as follows; ? water losses from evaporation is represented by a runoff coefficients; and ? water loss to the pile is modeled based on a percentage of volume of waste rock. As each WRSA is saturation flows exiting the pile increase. Flat infiltration is the slowest flow path and creates base flows that maintain flows out of the WRSAs during late summer and early winter periods. The slopes seepage is released more slowly over several days or weeks. While slopes runoff is the fastest flow path creating storm peaks during rainfall events. Results of the total WRSA discharge are a constant slow outflow at the toe with small increases due to precipitation events and the freshet, which is consistent with observations of waste rock drainage. Flows are attenuated using a time delay, which was simulated for each flow path using an Erlang function. The Erlang function refers to a two-parameter Gaussian distribution, where the shape parameter n is an integer. Hydrologically, the parameter n corresponds to the number of hypothetical linear reservoirs (Nash 1957). For the slopes runoff, n = 1 is assumed, which gives an exponential distribution. For the flats infiltration and slope seepage n = 2 is assumed, which gives a typical unit hydrograph shape with a delayed peak flow. The value of the lag parameter for each component was determined through model calibration.
Abstract: Sedimentary carbonate rocks, which exist extensively in the oceanic realm, are subducted to differing degrees during the closure of oceanic basins. However, very few observational data exist to provide details on the mechanisms of transport of carbonate materials from the surface to mantle depths and back to the Earth’s surface. Here we presented a series of diamond-bearing carbonatite xenoliths, carbonatite intrusions and carbonatite veins along the northern margin of the North China Craton (NCC). These carbonatites show geochemical features of recycled limestone (similar trace element patterns and high 87Sr/86Sr ratios of 0.705-0.709), indicating that they had a sedimentary limestone precursor. However, the presence of diamond, reduced minerals (e.g., moissanite), mantle-derived silicate minerals (eg., Cpx and Opx), and high Ni content and 143Nd/144Nd ratio indicate their staying for a time in the mantle. Combining with the zircon age spectrums of the carbonatite xenoliths and intrusions and the extensive high-87Sr/86Sr (up to 0.708) carbonatite metasomatism in the lithospheric mantle along the northern margin of NCC, we suggest that the limestone precursor could have been derived from the Paleo-Asian Ocean, and these carbonatites mark the subduction of a carbonate platform of the Paleo-Asian Oceanic slab to mantle depths beneath the NCC. Extensive mantle recycling of sedimentary carbonate could have contributed to the modification of the lithospheric mantle along the northern margin of the North China Craton.
Abstract: The Os-isotope compositions of sulphides in mantle xenoliths hosted by Late Miocene alkali basalts from the Sviyaginsky volcano, Russian Far East, reveal the presence of Archaean-Proterozoic subcontinental lithospheric mantle beneath the Khanka massif. Their TMA and TRD model ages reveal similar peaks at 1.1 and 0.8 Ga suggesting later thermotectonic events in the subcontinental lithospheric mantle, whereas TRD model ages range back to 2.8 ± 0.5 (2?) Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those recorded in the crust of the Khanka massif. The sulphide Os-isotope data show that the subcontinental lithospheric mantle beneath the Khanka massif had formed at least by the Mesoproterozoic, and was subsequently metasomatized by juvenile crustal-growth events related to the evolution of the Altaids. The Khanka massif is further proposed to have tectonic affinity to the Siberia Craton and should originate from it accordingly.
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: 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: 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: Xenoliths play a crucial role in interpretation of mantle deformation and geochemistry. The classic work of Mercier and Nicolas (1975) introduced the concept of the peridotite deformation cycle, which connected observed microstructures to a physical sequence of deformation. We revisit Mercier and Nicolas' original concept, bringing in new constraints using large area EBSD maps and associated microstructural datasets, analysis of water contents in nominally anhydrous minerals, and trace element chemistry of pyroxenes and garnets. We apply these techniques to a well-characterized suite of peridotite xenoliths from the Eocene-age Homestead and Williams kimberlites in the northwestern Wyoming Craton. Pyroxene water content and trace element mineral chemistries reveal ubiquitous hydrous metasomatism beneath the craton, most likely linked to the Cenozoic Laramide Orogeny. Homestead xenoliths primarily exhibit coarse protogranular and equigranular textures, B-type olivine fabrics, and generally elevated mineral water contents compared to Williams. Xenoliths from Williams are strongly deformed, with porphyroclastic and transitional textures containing annealed olivine tablets, mostly A-type olivine fabrics, and generally lower mineral water contents. As a whole, mantle from Homestead to Williams reflects a cratonic scale deformation cycle that likely initiated in Laramide times and lasted until the end of orogeny in the Eocene. At Williams, evidence for a rapid deformation “sub-cycle” within the main deformation cycle is preserved in the tablet-bearing xenoliths, corresponding to the enigmatic “transitional” texture of Mercier and Nicolas (1975). Our results suggest that this texture reflects interruption of the main deformation cycle by processes possibly related to a rapidly forming lithospheric instability and generation of the kimberlite magma - offering a new interpretation of this ambiguous peridotite texture. Collectively, our results incorporate typically disparate geochemical and textural datasets on xenoliths to shed new insights into how metasomatism, volatiles, and deformation are connected in the deep cratonic lithosphere.
The Story of de Beers. with Some Notes on the Company's Financial, Farming, Railway and Industrial Activities in Africa and Some Introductory Chapters on the River Diggings and Earlt Kimberley.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 379-402.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 359-378.
Contributions to Mineralogy and Petrology, Vol. 173, 22p. Doi.org/10.1007/s00410-018-1512-z
Africa, Tanzania
magmatism
Abstract: Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine?+?clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.
Reviews in Advance *** NOTE DATE, Vol. 39, pp. 59-90. pdf
Mantle
cratons
Abstract: Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became "continental" after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.
United States, Montana, Wyoming, Utah, Canada, Alberta, Saskatchewan
craton
Abstract: Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep.
Abstract: Xenoliths play a crucial role in interpretation of mantle deformation and geochemistry. The classic work of Mercier and Nicolas (1975) introduced the concept of the peridotite deformation cycle, which connected observed microstructures to a physical sequence of deformation. We revisit Mercier and Nicolas' original concept, bringing in new constraints using large area EBSD maps and associated microstructural datasets, analysis of water contents in nominally anhydrous minerals, and trace element chemistry of pyroxenes and garnets. We apply these techniques to a well-characterized suite of peridotite xenoliths from the Eocene-age Homestead and Williams kimberlites in the northwestern Wyoming Craton. Pyroxene water content and trace element mineral chemistries reveal ubiquitous hydrous metasomatism beneath the craton, most likely linked to the Cenozoic Laramide Orogeny. Homestead xenoliths primarily exhibit coarse protogranular and equigranular textures, B-type olivine fabrics, and generally elevated mineral water contents compared to Williams. Xenoliths from Williams are strongly deformed, with porphyroclastic and transitional textures containing annealed olivine tablets, mostly A-type olivine fabrics, and generally lower mineral water contents. As a whole, mantle from Homestead to Williams reflects a cratonic scale deformation cycle that likely initiated in Laramide times and lasted until the end of orogeny in the Eocene. At Williams, evidence for a rapid deformation “sub-cycle” within the main deformation cycle is preserved in the tablet-bearing xenoliths, corresponding to the enigmatic “transitional” texture of Mercier and Nicolas (1975). Our results suggest that this texture reflects interruption of the main deformation cycle by processes possibly related to a rapidly forming lithospheric instability and generation of the kimberlite magma - offering a new interpretation of this ambiguous peridotite texture. Collectively, our results incorporate typically disparate geochemical and textural datasets on xenoliths to shed new insights into how metasomatism, volatiles, and deformation are connected in the deep cratonic lithosphere.
Journal of Geophysical Research, Vol. 123, 7, pp. 5644-5660.
Africa, east Africa
geophysics - seismic
Abstract: Although the East African rift system formed in cratonic lithosphere above a large?scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small?volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake?bottom seismometers and 67 land stations in the Tanganyika?Rukwa?Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift?perpendicular strain, rift?parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N?S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS?splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large?scale asthenospheric flow or passive rifting.
Partition of oxygen isotopes and trace elements between carbonate and silicate melts at 1 kilobar, 800 degrees C and its bearing on the origin ofcarbonatite
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.
GAC MAC Meeting Special Session SS11: Cratons, kimberlites and diamonds., Keynote abstract
Technology
Emplacement
Abstract: The study develops a new approach utilizing parameters of trigonal etch pits on diamond crystals to infer the conditions of diamond residence in kimberlite magma. Diamond crystals from dissolution experiments conducted at 1 GPa and 1150-1350 °C in the presence of H2O-rich or CO2-rich fluid were studied with atomic force microscopy (AFM). The AFM data of resorbed diamond surfaces show that much deeper surface relief was produced in CO2 fluid. It also clearly distinguishes the profiles of the trigonal etch pits forming regular flat-bottomed trigons in H2O fluid, and round- or pointed-bottomed trigons in CO2 fluid. The relationship between the diameter and the depth of the trigonal pits is found to be another important indicator of the fluid composition. Dissolution in H2O fluid develops trigons with constant diameter and variable depth where the diameter increases with temperature. Trigons developed in CO2 fluid have a large range of diameters showing a strong positive correlation with the depth. The developed criteria applied to the natural diamond crystals from three Ekati Mine kimberlites indicate significant variation in CO2-H2O ratio and temperature of their magmatic fluid. This conclusion based on diamond resorption agrees with the mineralogy of microphenocrysts and groundmass of the studied kimberlites offering new method to study crystallization conditions of kimberlite magma.
van den Heuvel, Q., Matveev, S., Drury, M., Gress, M., Chinn, I., Davies, G.
Genesis of diamond inclusions: an integrated cathodluminescence ( CL) and electron backscatter diffraction (EBSD) study on eclogitic and peridotitic inclusions and their diamond host.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 6564 Abstract
Abstract: Roots of continental cratons keep a long record of multiple metasomatic events, but their trace is complicated due to the mixed signals left by these events in the composition of mantle silicate minerals. Simple composition helps diamonds to provide a more robust record of the latest metasomatic events which they witnessed. Growth and dissolution features on the diamond surface are sensitive to the composition of the reacting media. In this study we use mantle-derived resorption features on natural diamonds to examine the nature of metasomatic events in diamondiferous mantle lithologies. We use experiments at mantle conditions to examine how the composition of fluids and melts affect diamond resorption. We then compare these results to the features of natural diamonds to determine which of the tested compositions could have acted as metasomatic agents in Earth’s cratonic roots. Diamond dissolution experiments conducted at 6 GPa, 1200 - 1500oC using synthetic MgO-CaO-SiO2-CO2-H2O system examined the effect of CHO fluid, silica-saturated CHO fluid, aqueous and “dry” silica-carbonate and carbonate melts. Results show that the main control of diamond resorption morphology is the state of the reacting media: fluid vs. melt. We compared the experimental results to diamonds with mantle-derived resorption features from two kimberlites from the Orapa kimberlite cluster (Botswana). We identified twelve mantle-derived resorption types, none of which resembled the products of resorption in fluids. Most of the observed resorption types could be produced by dissolution in mantle melts with variable proportions of carbonate and silicate components and in the range of temperatures. The most abundant resorption type resembles the product of diamond dissolution in carbonate melts at temperatures above 1450oC. Our results suggest that fluid-metasomatism is not destructive for diamonds while melt-metasomatism is. The lower hydrous carbonated solidus of lherzolite compared to harzburgite can result in the shift the process from diamond growth in fluids to diamond dissolution in melts due to metasomatic transformation of harzburgite into lherzolite.
Bulbuc, K.M., Galarneau, M., Stachel, T., Stern, R.A., Kong, J., Chinn, I.
Contrasting growth conditions for sulphide-and garnet-included diamonds from the Victor mine ( Ontario).
2018 Yellowknife Geoscience Forum , p. 97-98. abstract
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Victor Diamond Mine, located in the Attawapiskat kimberlite field (Superior Craton), is known for its exceptional diamond quality. Here we study the chemical environment of formation of Victor diamonds. We imaged eight sulphide-included diamond plates from Victor using cathodoluminescence (CL). Then, along core-rim transects, we measured nitrogen content and aggregation state utilizing Fourier Transform Infrared (FTIR) spectroscopy, and the stable isotope compositions of carbon (?13C) and nitrogen (?15N), using a multi-collector ion microprobe (MC-SIMS). We compare the internal growth features and chemical characteristics of these sulphide inclusion-bearing diamonds with similar data on garnet inclusion-bearing diamonds from Victor (BSc thesis Galarneau). Using this information, possible fractionation processes during diamond precipitation are considered and inferences on the speciation of the diamond forming fluid(s) are explored. Sulphide inclusion-bearing diamonds show much greater overall complexity in their internal growth features than garnet inclusion-bearing diamonds. Two of the sulphide-included samples have cores that represent an older generation of diamond growth. Compared to garnet inclusion-bearing diamonds, the sulphide-included diamonds show very little intra-sample variation in both carbon and nitrogen isotopic composition; the inter-sample variations in carbon isotopic composition, however, are higher than in garnet included diamonds. For sulphide-included diamonds, ?13C ranges from -3.4 to -17.5 and ?15N ranges from -0.2 to -9.2. Garnet inclusion-bearing diamonds showed ?13C values ranging from -4.6 to -6.0 and ?15N ranging from -2.8 to -10.8. The observation of some 13C depleted samples indicates that, unlike the lherzolitic garnet inclusion-bearing diamonds, the sulphide inclusion-bearing diamonds are likely both peridotitic and eclogitic in origin. The total range in N content across sulphide inclusion-bearing diamonds was 2 to 981 at ppm, similar to the garnet-included samples with a range of 5 to 944 at ppm. The very limited variations in carbon and nitrogen isotopic signatures across growth layers indicate that sulphide-included Victor diamonds grew at comparatively high fluid:rock ratios. This is contrasted by the garnet inclusion-bearing diamonds that commonly show the effects of Rayleigh fractionation and hence grew under fluid-limited conditions.
Canada, Ontario, Attawapiskat, Africa, South Africa
deposit - Victor, Finsch, Newlands
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. “Planed” and “ribbed” trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Siva-Jothy, W., Chinn, I., Stachel, T., Pearson, D.G.
Resorption features of macro and micro diamonds from Gahcho Kue.
2018 Yellowknife Geoscience Forum , p. 120. abstract
Canada, Northwest Territories
deposit - Gahcho Kue
Abstract: Studies into the relationship between oxygen fugacity of mantle fluids/melts and etch features on diamond surfaces have shown specific fluid/melt compositions correspond to associated etch features. A classification scheme has been proposed to determine the fluid composition within a kimberlite by examining etch features associated with diamond surfaces as a proxy for fluid composition in an ascending diamondiferous kimberlite. A suite of 388 microdiamonds (defined as diamonds which pass through a 0.5 mm square mesh screen) and 88 macrodiamonds taken from various drill hole depths in the Hearne kimberlite and 88 inclusion-bearing macrodiamonds from the Gahcho Kué mine (NWT) were viewed under a secondary electron microscope for their surface features in accordance with this scheme. Two hundred and thirty specimens show shallow-depth etch features that can be easily classified: the main features observed were trigons and truncated trigons on the {111} faces and/or tetragons on the {100} faces (indicating etching by fluids of variable CO2:H2O ratios). Thirty-four specimens show deeper etched features that represent either extreme degrees of regular etching (such as deeply-etched tetragons), or corrosion type etching, wherein the diamond lattice is etched in a fluid-free melt. Variability between crystal habits exists between the size fractions studied, with cubic habits only being observed in the microdiamond population. This implies variable formation conditions for the two different diamond size fractions studied from Gahcho Kué. Among microdiamonds, surface textures associated with fluid-related etching are markedly more variable, with truncated trigons, tetragons, and both positive and negative trigons being observed. However, these often occur in combination with features showing a large variability in their depth to size ratio between samples, which is typically caused by mantle-related etching. These observations suggest repeated interaction of fluids/melts with the Gahcho Kué diamond population, with at least some of the fluids affecting the microdiamonds being more CO2-rich than those that etched the macrodiamond fraction.
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. "Planed" and "ribbed" trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Abstract: Diamonds are the deepest accessible “fragments” of Earth, providing records of deep geological processes. Absolute ages for diamond formation are crucial to place these records in the correct time context. Diamond ages are typically determined by dating inclusions, assuming that they were formed simultaneously with their hosts. One of the most widely used mineral inclusions for dating diamond is garnet, which is amenable to Sm-Nd geochronology and is common in lithospheric diamonds. By investigating worldwide garnet-bearing diamonds, we provide crystallographic evidence that garnet inclusions that were previously considered to be syngenetic may instead be protogenetic, i.e., they were formed before the host diamond, raising doubts about the real significance of many reported diamond “ages.” Diffusion modeling at relevant pressures and temperatures, however, demonstrates that isotopic resetting would generally occur over geologically short time scales. Therefore, despite protogenicity, the majority of garnet-based ages should effectively correspond to the time of diamond formation. On the other hand, our results indicate that use of large garnet inclusions (e.g., >100 ?m) and diamond hosts formed at temperatures lower than ?1000 °C is not recommended for diamond age determinations.
Abstract: The thermobarometric analysis of inclusions in lithospheric diamonds indicates that they originated from a wide range of depths, with a global mode at ca. 170±15 km [1]. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by rising kimberlites, or even a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness, and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. P-T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm. They may record old mantle thermal regimes or local thermal perturbations related to infiltration of parent fluids or melts. Nonetheless, the diamonds show similar depth distributions for different localities, with a distinct mode at ?175 ?? 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions at both local and global scale and on thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ?160 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially the 160-190 km interval) may correspond to significant diamond potential.
Abstract: Kimberlites are mantle-derived igneous rocks emplaced in the upper crust. Class 1 kimberlite are multi-phase bodies consisting of coherent kimberlite (CK) and different pyroclastic facies, including diatreme Kimberley-type pyroclastic kimberlite (KPK). The composition, crystallisation conditions and emplacement processes of these multiphase kimberlites are poorly understood, especially the formation of KPK. CK facies include hypabyssal kimberlite (HK) and ambiguous partially fragmented CK. Ilmenite macrocrysts from some Orapa kimberlites show reaction rims, the composition of which correlates with kimberlite facies. The goal of this study is to document the reaction products on ilmenite from different kimberlite facies and to use them to determine crystallisation temperature (T) and oxygen fugacity (fO2). Obtaining a better understanding of fO2 is important not only scientifically, but also for economic reasons, because highly oxidising conditions would have promoted resorption of diamonds in the kimberlite. This study used thin sections taken in well constrained depth intervals from drillholes in AK15 and BK1 kimberlites from the Orapa kimberlite cluster (Botswana). The AK15 intrusion consists of a single phase of CK facies. The BK1 pipe consists of two CK facies (CK-A and CK-B) and one KPK facies. CK-B is a HK and CK-A shows areas of partial fragmentation. Kimberlite textures were examined with a petrographic microscope. Ilmenite reaction rims were identified with SEM. EMP analyses were performed on perovskite, ilmenite and magnetite grains for T and fO2 calculation. We found that ilmenite macrocrysts in CK-A develop rims composed of magnetite and rutile. The reaction rims on ilmenite macrocrysts in KPK are highly variable and are distinguished by the presence of titanite. In CK-B, ilmenite macrocrysts are replaced by a symplectic intergrowth of magnetite and perovskite. In AK15, ilmenite macrocrysts consist of magnetite rims. fO2 estimated using ferric iron content in CaTiO3 perovskite varies from NNO -5.74 to -1.30 showing progressive oxidation upwards and within KPK facies. Such fO2 conditions require T during perovskite crystallisation between 560 and 700 °C. The observed textures suggest that BK1 ilmenite macrocrysts reacted with the melt to produce magnetite and perovskite rims followed by full ilmenite replacement by symplectic intergrowth of perovskite and magnetite in CK-B and replacement of perovskite with TiO2 oxide in CK-A. Development of titanite in KPK indicates assimilation of crustal xenoliths, while variability of reaction rims and fO2 estimates within the same sample confirm the high degree of material mixing in KPK. Similarities of ilmenite rims in CK-A and KPK indicate similarity in the process of their formation.
Abstract: Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds. Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions. Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate?carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 8p.
Africa, Botswana
Deposit - AK6
Abstract: The AK6 kimberlite is situated 25 km south of the Debswana Orapa Mine in Botswana and was discovered by De Beers geologists in 1969 during the follow-up of geophysical targets in the Orapa area. The kimberlite was not extensively pursued at the time as the initial bulk sampling indicated it to be of limited size and low grade, factors largely contributed to by the basalt breccia capping. Completion of high resolution integrated geophysical techniques and drill bulk sampling to depth recovered 97 tons of kimberlite during 2003 and 2004, which led to the increased size and grade estimates. Bulk sampling by Large Diameter Drilling (LDD, 23 inch diameter) commenced in 2005; 13 holes were drilled to a cumulative depth of 3,699 m and 689 carats of diamonds were recovered. In July 2006 the De Beers Mineral Resource Classification Committee classified these Phase I LOO results at a High Inferred level with an average grade of 24 carats per hundred tonnes (cpht) at a bottom cut-off of +1 mm, and a modeled average diamond value of 150 dollars per carat. A second phase of LDO drilling was initiated in 2006, and bulk sampling by trenching commenced in 2007 in order to deliver a resource estimate at indicated level. An Indicated Resource of 11.1 million carats at an average grade of 22 cpht was declared for the deposit mining lease application lodged in 2007.
Earth and Planetary Science Letters, Vol. 463, pp. 178-188.
Africa, Botswana
Deposit - Lethakane
Abstract: Monocrystalline diamonds commonly record complex internal structures reflecting episodic growth linked to changing carbon-bearing fluids in the mantle. Using diamonds to trace the evolution of the deep carbon cycle therefore requires dating of individual diamond growth zones. To this end Rb-Sr and Sm-Nd isotope data are presented from individual eclogitic silicate inclusions from the Orapa and Letlhakane diamond mines, Botswana. ?13C?13C values are reported from the host diamond growth zones. Heterogeneous 87Sr/86Sr ratios (0.7033-0.7097) suggest inclusion formation in multiple and distinct tectono-magmatic environments. Sm-Nd isochron ages were determined based on groups of inclusions with similar trace element chemistry, Sr isotope ratios, and nitrogen aggregation of the host diamond growth zone. Diamond growth events at 0.14±0.090.14±0.09, 0.25±0.040.25±0.04, 1.1±0.091.1±0.09, 1.70±0.341.70±0.34 and 2.33±0.022.33±0.02 Ga can be directly related to regional tectono-magmatic events. Individual diamonds record episodic growth with age differences of up to 2 Ga. Dated diamond zones have variable ?13C?13C values (?5.0 to ?33.6‰ vs PDB) and appear to imply changes in subducted material over time. The studied Botswanan diamonds are interpreted to have formed in different tectono-magmatic environments that involve mixing of carbon from three sources that represent: i) subducted biogenic sediments (lightest ?13C?13C, low 87Sr/86Sr); ii) subducted carbonate-rich sediments (heavy ?13C?13C, high 87Sr/86Sr) and iii) depleted upper mantle (heavy ?13C?13C, low 87Sr/86Sr). We infer that older diamonds from these two localities are more likely to have light ?13C?13C due to greater subduction of biogenic sediments that may be related to hotter and more reduced conditions in the Archaean before the Great Oxidation Event at 2.3 Ga. These findings imply a marked temporal change in the nature of subducted carbon beneath Botswana and warrant further study to establish if this is a global phenomenon.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 5540 Abstract
Africa, Botswana
Deposit - Letlhakane
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
Canadian Institute of Mining and Metallurgy, Vol. 7, 4, pp. 216-239.
Global
microdiamonds
Abstract: Concerns around the use of micro-diamonds for resource estimation have been raised by some workers because: 1) multiple diamond populations are present in many parts of the mantle source region, 2) small diamonds in kimberlite could be exposed to proportionately greater levels of resorption and modification, and 3) euhedral micro-diamonds could crystallize immediately prior to kimberlite eruption. This paper addresses these concerns and discusses the geology of the mantle and the principal diamond host rocks, the impact of mantle processes, compares micro- and macro-diamond properties and features, and outlines several steps that can be undertaken to identify and mitigate the risk of resorption of diamond and its impact on the diamond grade size relationship.
Abstract: Kimberlite magmas, the primary source of diamonds, have many features indicative of explosive eruptions and high volatile contents. The main approaches used to establish exsolution of fluid during magma ascent include theoretical modeling and experimental estimates of volatile solubility in kimberlite-like melts. Both approaches are hampered by the poorly constrained composition of kimberlite melts. Resorption features on diamonds are very sensitive to the presence and composition of the kimberlite fluid as well as to temperature and pressure. Here, we use direct evidence from diamond resorption features as a new method for investigating the parameters of fluid exsolution. The method is based on experimental reproduction of diamond resorption in kimberlite melts with and without an exsolved fluid phase. We studied 802 diamonds from two kimberlites (BK1 and AK15) from the Orapa cluster, Botswana. Samples from the BK1 pipe include three lithologies: two coherent kimberlites (CK-A and CK-B) and a pyroclastic kimberlite (massive volcaniclastic kimberlite, MVK). The known depth of diamond samples in each kimberlite lithology allows us to demonstrate an increase in the intensity of kimberlite-induced resorption with depth of diamond recovery in the drill holes. Each kimberlite lithology has a different proportion of diamonds with kimberlite-induced resorption, which is unique in style in each lithology: glossy surfaces in MVK due to reaction with C-O-H fluid, rough corroded surfaces in CK-B due to reaction with volatile-undersaturated melt, and a combination of glossy surfaces with corroded features in CK-A due to an overprint of melt resorption after fluid resorption. Both diamond resorption and kimberlite textures in the BK1 kimberlite show evidence of fluid exsolution only in CK-A and MVK lithologies, but no fluid presence in CK-B. The observed diamond resorption features may be controlled by (1) a temporary separation of the rising magma column into a bubble-rich head and bubble-poor volatile-depleted tail and (2) fluid exsolution at depths greater than decompressional degassing. We discuss how the depth of fluid exsolution from kimberlite melt may affect the diamond grade and the resorption of diamond populations in a kimberlite.
Abstract: Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95?Ga) and one Proterozoic (1.15?Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1?Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.
Mineralogy and Petrology, in press available, 11p.
Africa, Botswana
deposit - Orapa
Abstract: Spatially resolved analyses reveal considerable isotopic heterogeneity within and among diamonds ranging in size from 0.15 to 4.75 mm from the Orapa Mine, Botswana. The isotopic data are interpreted in conjunction with nitrogen aggregation state data and growth zone relationships from cathodoluminescence images. The integrated information confirms that a distinct diamond growth event (with low IaAB nitrogen aggregation states, moderately high nitrogen contents and ?13C and ?15N values compatible with average mantle values) is younger than the dominant population(s) of Type IaAB diamonds and cores of composite diamonds with more negative and positive ?13C and ?15N values, respectively. A significant proportion of the older diamond generation has high nitrogen contents, well outside the limit sector relationship, and these diamonds are likely to reflect derivation from subducted organic matter. Diamonds with low ?13C values combined with high nitrogen contents and positive ?15N values have not been previously widely recognised, even in studies of diamonds from Orapa. This may have been caused by prior analytical bias towards inclusion-bearing diamonds that are not necessarily representative of the entire range of diamond populations, and because of average measurements from heterogeneous diamonds measured by bulk combustion methods. Two distinct low nitrogen/Type II microdiamond populations were recognised that do not appear to continue into the macrodiamond sizes in the samples studied. Other populations, e.g. those containing residual singly-substituted nitrogen defects, range in size from small microdiamonds to large macrodiamonds. The total diamond content of the Orapa kimberlite thus reflects a complex assortment of multiple diamond populations.
Geophysical Research Abstracts www.researchgate.net, Vol. 20, EGU2018-128291p. Abstract
Africa, South Africa
diamond inclusions
Abstract: Changing recycling budgets of surface materials and volatiles by subduction of tectonic plates influence the compositions of Earth’s major reservoirs and affect climate throughout geological time. Fluids play a key role in processes governing subduction recycling, but quantifying the exact fate of volatiles introduced into the mantle at ancient and recent destructive plate boundaries remains difficult. Here, we report on the role of fluids and the fate of volatiles and other elements at two very different tectonic settings: 1) at subduction settings, and 2) within the subcontinental lithospheric mantle (SCLM). We will show how olivine-hosted melt inclusions from subduction zones and mineral inclusions in diamond from the SCLM are used to reveal how changing tectonic settings influence volatile cycles with time. Melt inclusions from the complex Italian post-collisional tectonic setting are used to identify changing subduction recycling through time. The use of CO2 in deeply trapped melt inclusions instead of in lavas or volcanic gases provides a direct estimate of deep recycling, minimizing possible effects of contamination during transfer through the crust. The aim is to distinguish if increased recycling of sediments from the down-going plate at continental subduction settings results in increased deep CO2 recycling or if the increased CO2 flux results from crustal degassing of the overriding plate. Both processes likely affected climate through Earth history but could thus far not be discriminated. The study of mineral inclusions and their host diamonds from the SCLM can link changes in the cycling of carbon-rich fluids and the time and process through which the carbon redistribution took place. We use Sm-Nd isotope techniques to date the mineral inclusions and use the carbon isotope data of the host diamonds to investigate the growth conditions. I will present case-studies of peridotitic and eclogitic diamonds from three mines in Southern Africa.
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.
Abstract: The diamondiferous Jwaneng kimberlite cluster (~240 Ma) is located on the NW rim of the Archaean Kaapvaal Craton in central Botswana. Previous studies report eclogitic diamond formation in the late Archean (2.9 Ga) and in the Middle Proterozoic (1.5 Ga) involving different mantle and sedimentary components [1;2;3]. Here we report newly acquired Sm- Nd ages of individual eclogitic pyrope-almandine and omphacite inclusions along with their major element data and nitrogen data from the diamond hosts to re-examine Jwaneng’s diamond formation ages. The Sm-Nd isotope analyses were performed via TIMS using 1013? resistors [4]. An initial suite of three pyropealmandine and 14 omphacite inclusions yield 143Nd/144Nd from 0.51102±7 to 0.5155±5. 147Sm/144Nd vary from 0.024 to 0.469. Major element data defines two inclusion populations: (1) seven omphacites with high Mg#, high Cr# and one pyropealmandine with low-Ca define an isochron age of 1.93±0.16 Ga with ?Ndi= +3.5; (2) seven omphacites with low Mg#, low Cr# and two pyrope-almandines with low-Ca define an isochron age of 0.82±0.06 Ga with ?Ndi= +3.7. Nitrogen contents of corresponding diamond host growth zones in Group (1) are ? 50 at.ppm whereas Group (2) range between 50 to 700 at.ppm with N-aggregation > 70 %B. Additional data used to define “co-genetic” inclusion suites include Sr-isotopes and trace elements of the inclusions and carbon isotopes of the diamond hosts. Re-Os data of coexisting sulphide inclusions from the same silicate-bearing diamonds further validates the ages and indicates more periods of diamond formation at Jwaneng than previously assumed. The integrated data indicate the possibility of an extensive Paleoproterozoic diamond-forming event in southern Africa.
Mineralogy and Petrology, 10.1007/ s710-018-0592 -9, 10p.
Africa, Botswana
deposit - Orapa, Damtshaa, BK09, BK12,AK01
Abstract: Twenty eclogitic diamonds from Orapa Mine (Botswana) with an unusual yellow colour are characterised for their growth structure, N systematics, and C isotope composition, and the major element composition of their silicate inclusions. The diamonds show complex luminescence with green, blue and non-luminescent zones and occasional sector zonation. All parts of the diamonds have low total N concentrations (<50 at.ppm, with one exception of <125 at.ppm) and a limited range in C isotope composition (?5.7 to ?10.6‰). Fourier Transform Infrared spectra show bands at 1334, 1332, 1282, and 1240 cm?1 typical for Ib-IaA diamonds. Relict unaggregated N defects (Nso and Ns+) are present and the preservation is likely caused by the low N concentrations and possible low mantle residence temperatures rather than young diamond formation (inclusion ages of 140, 1096, 1699 Ma; Timmerman et al. Earth Planet Sc Lett 463:178-188, 2017). Garnet and clinopyroxene inclusions extracted from 14 diamonds have an eclogitic composition with relatively low Ca contents and based on all characteristics, these diamonds form a distinct population from Orapa.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0634-3 12p.
Africa, Botswana
deposit - Jwaneng
Abstract: To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Abstract: The role of fluid(s) in the formation of different lithological facies of kimberlites is still poorly understood. The uncertainty in the composition of kimberlite melts hampers understanding the composition of volatiles, the depth of exsolution, and the effect on magma ascent and fragmentation. Recent estimates of H2O and CO2 solubility in kimberlite-like magmas suggest very shallow exsolution of fluid, while many features of kimberlites indicate the presence of significant fluid fraction at depth. Deep magmatic fluid produces negative trigonal etch pits on natural diamonds, the characteristics of which depend on the temperature and composition of the fluid. Positively oriented trigonal etch pits are very rare on natural diamonds and are likely a feature of resorption events unique to only some kimberlite magmas. Here we present the first systematic study of positively oriented trigonal etch pits on natural diamonds from Snap Lake kimberlite dike, Northwest Territories, Canada. The study used 91 micro-diamonds selected from a population of 251 diamonds representative of all six kimberlite litho-facies identified in the Snap Lake dike. We established that unlike the majority of diamonds from kimberlite pipes in the Northwest Territories, every studied Snap Lake diamond shows positively oriented trigons. These trigons cover the whole diamond surface starting from the {111} faces and continuing over the resorbed edges. They overprint negatively oriented trigons and modify them into hexagons. Atomic force microscopy obtained detailed geometry of 154 positive trigons on 14 diamonds. Three distinct trigon morphologies dependent on the type of the crystal lattice defect were recognized. The point-bottomed shape and positive correlation between the depth and diameter of the individual pits suggest a high CO2 content in the fluid. Comparison with the existing experimental data on positive trigons implies resorption at low-pressure conditions in the 800-1000 °C temperature range by trapped magmatic fluid after the dike emplacement. The intensity of this late resorption event (and the size of the positive trigons) increases from the dike contact with the country rock into the interior of the dike. Such a late resorption event is absent in the majority of kimberlites, which form pipes, and might be a specific feature of hypabyssal kimberlite bodies (dikes). The absence of positive trigons on diamonds from the majority of kimberlites suggests very quick magma cooling below ?800 °C after the pipe emplacement, precluding the development of any late resorption features. Our study shows that for kimberlitic magmas, for which mineral chemistry is unable to provide a robust record of magmatic fluid, morphological details of dissolution features on the surface of diamond and other mantle-derived minerals can serve as a fluid proxy. Better constraints of the pressure, temperature, and oxygen fugacity of the reversal in the trigon orientation on diamond may help to reconstruct the emplacement path of geologically diverse kimberlite bodies.
Abstract: A study of the morphology, mineral inclusions, nitrogen content, and nitrogen aggregation state of diamonds from the Voorspoed kimberlite, was conducted. The diamonds are characterised by a highly unusual inclusion mineral paragenesis dominated by eclogitic and transitional websteritic-lherzolitic inclusions along with related websteritic inclusions, while a comparatively minor harzburgitic diamond inclusion suite account for the remainder. This differs substantially from many of the diamond populations in kimberlites on the Kaapvaal Craton where a harzburgitic inclusion paragenesis predominates. Only in the case of the Orapa kimberlite in Botswana has a similar diamond inclusion suite been encountered before, although in that instance an eclogitic and websteritic inclusion suite predominate. Calculated garnet-clinopyroxene equilibration temperatures, at an assumed pressure of 50?kbar, range from 1040 °C to 1296?°C. Temperatures of 1114?°C to 1348?°C, at 50?kbar, are calculated for lamellar orthopyroxene-clinopyroxene intergrowths. ‘Reconstituted’ pyroxenes mathematically reconstructed from the lamellar orthopyroxene-clinopyroxene intergrowths produce temperatures of 1238?°C to 1416?°C, suggesting that the lithosphere cooled by at least 100?°C after diamond crystallisation. Nitrogen contents and nitrogen aggregation states of whole diamonds range from below the detection limit (~10?ppm) to 1442 atomic ppm (average 167 atomic ppm), and up to 97% of the highly aggregated ‘B' component (average 65%), respectively. The elevated aggregation state of the nitrogen in the diamonds from Voorspoed, coupled with a high level of platelet degradation in many of the diamonds analysed, relative to a world-wide database, is unusual, but broadly comparable to diamonds from kimberlites occurring in craton margin settings, such as Argyle and Venetia. Diamond inclusion thermobarometry, as well as the elevated nitrogen aggregation states of the diamonds (coupled with the evidence for platelet degradation in the diamonds) are indicative of a diamond crystallisation event associated with a transient thermal pulse, as well as associated deformation of the mantle containing the diamonds. The websteritic and the transitional websteritic-lherzolitic mantle source rocks of the Voorspoed diamonds likely formed through melt infiltration into mantle peridotite, possibly in the reaction envelope surrounding an ascending mantle plume. In order to account for the unusually low abundance of harzburgitic diamonds at Voorspoed, it is postulated that a primary, pre-existing suite of harzburgitic diamonds which have crystallised in the Mesoarchaean, was destroyed through heating and oxidation during the passage of magmas associated with the Ventersdorp large igneous province at 2.72?Ga, and which was subsequently followed by the crystallisation of a younger eclogitic and transitional websteritic-lherzolitic suite of diamonds.
Abstract: Experiments on diamond crystallization in kimberlite melt were performed for 40 h at 6.3 GPa in the temperature range of 1300-1570 °C and at 7.5 GPa in the temperature range of 1450-1570 °C, using a multianvil high-pressure apparatus of split-sphere type. Group I kimberlite from the Udachnaya-East pipe and a synthetic multicomponent mixture modeling the average composition of group II kimberlites were used as starting materials. The experiments have shown that diamond growth on seed crystals in the kimberlite melt in equilibrium with olivine, pyroxene, and garnet starts from 1400 °C at 7.5 GPa and from 1520 °C at 6.3 GPa. Diamond nucleation requires higher temperature and pressure, 1570 °C and 7.5 GPa. The alkali-enriched and silicate-depleted derivates of kimberlite melts ensure the growth and nucleation of diamond at lower P and T values: 1400 °C at 7.5 GPa and 1520 °C at 6.3 GPa. The results obtained evidence that temperature, pressure, and the composition of crystallization medium are the main factors controlling diamond formation processes in the kimberlite melts and their derivates.
Abstract: The thermobarometric analysis of inclusions in lithospheric diamonds has shown that these diamonds may originate from a wide range of depths, with a global mode at ~175 ± 15 km. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by ascending kimberlites, or simply a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. The revised P-T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm recorded by the xenocrysts. These conditions may represent old mantle thermal regimes or local thermal perturbations related to infiltration of parental fluids or melts. Nonetheless, the studied diamonds show similar depth distributions for the different localities, with a distinct mode at ?180 ± 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions and thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ?165 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts or fluids after diamond formation. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially from the 160-190 km interval) may correspond to significant diamond potential.
Geochimica et Cosmochimica Acta, 10.1016/j.gca.2020.10.010 35p. Pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (> 100’s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (< 10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C < -21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.
Geochimica et Cosmochimica Acta, Vol. 293, pp. 1-17. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8?Ga) and one Neoproterozoic (0.85?Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100?s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10?km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C?
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich "fibrous" and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Appendix to previous Lithos article in March 2021, 11p. Pdf
Africa, Botswana
deposit - Damtshaa, Orapa
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Abstract: Thermal events and metasomatic processes have influenced the Kaapvaal craton in South Africa. High-density fluids (HDFs) trapped as microinclusions in diamond are main metasomatic agents which provide an insight to these processes in the Earth's mantle. Here we present data for 15 fibrous, HDF-bearing diamonds from the Voorspoed mine, South Africa, reflecting multiple diamond-forming events in a cooling lithosphere. Analyzed by FTIR and EPMA, the Voorspoed fibrous diamonds reveal three populations that differ in their nitrogen aggregation and HDF composition. A silicic?carbonatitic group containing 11-30% B-centers, a saline group containing 5-16% B-centers, and a single high-Mg carbonatitic diamond with 0% B-centers. The distinct nitrogen aggregation of the fibrous diamond groups in Voorspoed and the lack of clear major element evolutionary trends for each HDF type or intermediate compositions between the different types suggest different time-temperature formation histories. Thermobarometry of mineral inclusions in non-fibrous monocrystalline Voorspoed diamonds (Viljoen et al., 2018) indicates that the Voorspoed lithosphere cooled by 100-200 °C since their host diamonds crystallized at high initial temperatures. High temperatures in Voorspoed lithosphere can be correlated with the eruption of the Ventersdorp flood basalts at the central Kaapvaal (ca. 2.7 Ga) or the Bushveld complex (ca. 2.06 Ga), and cooling rates of the lithosphere provide a time frame for a cooling process that originated ~2-3 Ga. Combining these data with the nitrogen aggregation systematics of fibrous and monocrystalline Voorspoed diamonds, we suggest that most Voorspoed diamonds formed during 4 metasomatic events: the oldest one recorded took place between 2 and 3 Gyr as a result of a major thermal perturbation, whereas the following three occurred between 200 and 600 Myr, 30-90 and < 30 Myr before kimberlite eruption in a cooling lithosphere. An even older (or deeper) event is hinted by a few diamonds where all nitrogen is in B-centers. The sequence of events implied by Voorspoed HDF compositional and nitrogen aggregation differences show affinities with other occurrences in South Africa (e.g. Kimberley, Finsch and Koffiefontein) and may reflect thermal and lithological variation between the central and southwest Kaapvaal lithosphere.
Abstract: South Africa, and attended by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that rough, exported diamonds were free of conflictual concerns. This meeting was supported later in 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by diamond-producing and diamond-importing countries. Over 70 countries were included as members of the KPCS at the end of 2007. To prevent trade in "conflict diamonds" while protecting legitimate trade, the KPCS requires that each country set up an internal system of controls to prevent conflict diamonds from entering any imported or exported shipments of rough diamonds. Every diamond or diamond shipment must be accompanied by a Kimberley Process (KP) certificate and be contained in tamper-proof packaging. The objective of this study was (1) to assess the naturally occurring endowment of diamonds in Mali (potential resources) based on geological evidence, previous studies, and recent field data and (2) to assess the diamond-production capacity and measure the intensity of mining activity. Several possible methods can be used to estimate the potential diamond resource. However, because there is generally a lack of sufficient and consistent data recording all diamond mining in Mali and because time to conduct fieldwork and accessibility to the diamond mining areas are limited, four different methodologies were used: the cylindrical calculation of the primary kimberlitic deposits, the surface area methodology, the volume and grade approach, and the content per kilometer approach. Approximately 700,000 carats are estimated to be in the alluvial deposits of the Kenieba region, with 540,000 carats calculated to lie within the concentration grade deposits. Additionally, 580,000 carats are estimated to have been released from the primary kimberlites in the region. Therefore, the total estimated diamond resources in the Kenieba region are thought to be nearly 1,300,000 carats. The Bougouni zones are estimated to have 1,000,000 carats with more than half, 630,000 carats, contained in concentrated deposits. When combined, the Kenieba and Bougouni regions of Mali are estimated to be host to 2,300,000 carats of diamonds.
Abstract: In May of 2000, a meeting was convened in Kimberley, South Africa, and attended by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that rough, exported diamonds were free of conflictual concerns. This meeting was supported later in 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by both diamond-producing and diamond-importing countries. Over 70 countries were included as members at the end of 2007. To prevent trade in "conflict" diamonds while protecting legitimate trade, the KPCS requires that each country set up an internal system of controls to prevent conflict diamonds from entering any imported or exported shipments of rough diamonds. Every diamond or diamond shipment must be accompanied by a Kimberley Process (KP) certificate and be contained in tamper-proof packaging. The objective of this study was to assess the alluvial diamond resource endowment and current production capacity of the alluvial diamond-mining sector in Ghana. A modified volume and grade methodology was used to estimate the remaining diamond reserves within the Birim and Bonsa diamond fields. The production capacity of the sector was estimated using a formulaic expression of the number of workers reported in the sector, their productivity, and the average grade of deposits mined. This study estimates that there are approximately 91,600,000 carats of alluvial diamonds remaining in both the Birim and Bonsa diamond fields: 89,000,000 carats in the Birim and 2,600,000 carats in the Bonsa. Production capacity is calculated to be 765,000 carats per year, based on the formula used and available data on the number of workers and worker productivity. Annual production is highly dependent on the international diamond market and prices, the numbers of seasonal workers actively mining in the sector, and environmental conditions, which influence seasonal farming.
Abstract: In May of 2000, a meeting was convened in Kimberley, South Africa, by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that export shipments of rough diamonds were free of conflict concerns. Outcomes of the meeting were formally supported later in December of 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by diamond-producing and diamond-importing countries. The goal of this study was to estimate the alluvial diamond resource endowment and the current production capacity of the alluvial diamond mining sector of Guinea. A modified volume and grade methodology was used to estimate the remaining diamond reserves within Guinea’s diamondiferous regions, while the diamond-production capacity of these zones was estimated by inputting the number of artisanal miners, the number of days artisans work per year, and the average grade of the deposits into a formulaic expression. Guinea’s resource potential was estimated to be approximately 40 million carats, while the production capacity was estimated to lie within a range of 480,000 to 720,000 carats per year. While preliminary results have been produced by integrating historical documents, five fieldwork campaigns, and remote sensing and GIS analysis, significant data gaps remain. The artisanal mining sector is dynamic and is affected by a variety of internal and external factors. Estimates of the number of artisans and deposit variables, such as grade, vary from site to site and from zone to zone. This report has been developed on the basis of the most detailed information available at this time. However, continued fieldwork and evaluation of artisanally mined deposits would increase the accuracy of the results.
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.
Mine Water and the Environment, in press available, 19p.
Canada, Northwest Territories
Deposit - Gahcho Kue
Abstract: Eight water models were used to assess potential aquatic environmental effects of the proposed Gahcho Kué diamond mine on groundwater and surface water flow and quality in the Northwest Territories, Canada. This sequence of models was required to cover different spatial and temporal domains, as well as specific physico-chemical processes that could not be simulated by a single model. Where their domains overlapped, the models were interlinked. Feedback mechanisms amongst models were addressed through iterative simulations of linked models. The models were used to test and refine mitigation plans, and in the development of aquatic component monitoring programs. Key findings generated by each model are presented here as testable hypotheses that can be evaluated after the mine is operational. This paper therefore offers a record of assumptions and predictions that can be used as a basis for post-validation.
Mine Water and the Environment, Vol. 35, pp. 350-368.
Canada, Northwest Territories
Deposit - Gahcho Kue
Abstract: Eight water models were used to assess potential aquatic environmental effects of the proposed Gahcho Kué diamond mine on groundwater and surface water flow and quality in the Northwest Territories, Canada. This sequence of models was required to cover different spatial and temporal domains, as well as specific physico-chemical processes that could not be simulated by a single model. Where their domains overlapped, the models were interlinked. Feedback mechanisms amongst models were addressed through iterative simulations of linked models. The models were used to test and refine mitigation plans, and in the development of aquatic component monitoring programs. Key findings generated by each model are presented here as testable hypotheses that can be evaluated after the mine is operational. This paper therefore offers a record of assumptions and predictions that can be used as a basis for post-validation.
Abstract: Bolide impact is a ubiquitous geological process in the Solar System, which produced craters and basins filled with impact melt sheets on the terrestrial planets. However, it remains controversial whether these sheets were able to undergo large-scale igneous differentiation, or not. Here, we report on the discovery of large discrete bodies of melanorites that occur throughout almost the entire stratigraphy of the 1.85-billion-year-old Sudbury Igneous Complex (SIC) - the best exposed impact melt sheet on Earth - and use them to reaffirm that conspicuous norite-gabbro-granophyre stratigraphy of the SIC is produced by fractional crystallization of an originally homogeneous impact melt of granodioritic composition. This implies that more ancient and compositionally primitive Hadean impact melt sheets on the Earth and other terrestrial planets also underwent large-volume igneous differentiation. The near-surface differentiation of these giant impact melt sheets may therefore have contributed to the evolution and lithological diversity of the proto-crust on terrestrial planets.
Economic Geology Research Institute 2015, Vol. 17,, #3510, 1p. Abstract
Russia
Plume geodynamics
Abstract: The Early Paleoproterozoic stage in the Earth's evolution was marked by the initiation of global rift systems, the tectonic nature of which was determined by plume geodynamics. These processes caused the voluminous emplacement of mantle melts with the formation of dike swarms, mafic-ultramafic layered intrusions, and volcanic rocks. All these rocks are usually considered as derivatives of SHMS (siliceous high-magnesian series). Within the Eastern Baltic Shield, the SHMS volcanic rocks are localized in the domains with different crustal history: in the Vodlozero block of the Karelian craton with the oldest (Middle Archean) crust, in the Central Block of the same craton with the Neoarchean crust, and in the Kola Craton with a heterogeneous crust. At the same time, these rocks are characterized by sufficiently close geochemical characteristics: high REE fractionation ((La/Yb)N = 4.9-11.7, (La/Sm)N=2.3-3.6, (Gd/Yb)N =1.66-2.74)), LILE enrichment, negative Nb anomaly, low to moderate Ti content, and sufficiently narrow variations in Nd isotope composition from -2.0 to -0.4 epsilon units. The tectonomagmatic interpretation of these rocks was ambiguous, because such characteristics may be produced by both crustal contamination of depleted mantle melts, and by generation from a mantle source metasomatized during previous subduction event. Similar REE patterns and overlapping Nd isotope compositions indicate that the studied basaltic rocks were formed from similar sources. If crustal contamination en route to the surface would play a significant role in the formation of the studied basalts, then almost equal amounts of contaminant of similar composition are required to produce the mafic rocks with similar geochemical signatures and close Nd isotopic compositions, which is hardly possible for the rocks spaced far apart in a heterogeneous crust. This conclusion is consistent with analysis of some relations between incompatible elements and their ratios. In particular, the rocks show no correlation between Th/Ta and La/Yb, (Nb/La)pm ratio and Th content, and eNd and (Nb/La)N ratio. At the same time, some correlation observed in the eNd-Mg# and (La/Sm)N-(Nb/La)N diagrams in combination with the presence of inherited zircons in the rocks does not allow us to discard completely the crustal contamination. Examination of Sm/Yb-La/Sm relations and the comparison with model melting curves for garnet and spinel lherzolites showed that the parental melts of the rocks were derived by 10-30% mantle melting at garnet-spinel facies transition. Two stage model can be proposed to explain such remarkable isotope-geochemical homogeneity of the mafic volcanic rocks over a large area: (1) ubiquitous emplacement of large volumes of sanukitoid melts in the lower crust of the shield at 2.7 Ga; (2) underplating of plume-derived DM melts at the crust-mantle boundary, melting of the lower crust of sanukitoid composition, and subsequent mixing of these melts with formation of SHMS melts at 2.4 Ga. A simple mixing model showed that in this case the Nd isotope composition of obtained melts remained practically unchanged at variable amounts of contaminant (up to 30%). This work was supported by the RFBR no. 14-05-00458.
Abstract: Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian-Ludicovian LIP in the Fennoscandian Shield, the Permian-Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ?20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.
Abstract: The paper reports first comprehensive geological, petrographic, mineralogical, and geochemical data on one of the world’s oldest Tiksheozero ultramafic?alkaline?carbonatite complex (~1.99 Ga), which belongs to the Mid-Paleoproterozoic igneous province of the Baltic Shield. The complex was formed in three intrusive phases. The first phase is composed of the low-alkali mafic?ultramafic rocks: dunites, wehrlites, clinopyroxenites, and gabbro. The rocks of the second phase are alkaline ultramafic rocks represented mainly by jacupirangites (alkaline clinopyroxenites) and foidolites (melteigites, ijoliltes, and urtites), with subordinate olivinites, alkaline gabbro, and nepheline syenites. The third intrusive phase is made up of carbonatites. Geochemical and mineralogical data indicate that all three phases were derived from different primary melts. It is shown that the nepheline syenites were obtained by fractionation of foidolites. A model of formation of such complexes through decompressional melting of mantle plume head enriched in carbonate fluid is proposed.
Abstract: The Tiksheozero ultramafic-alkaline-carbonatite intrusive complex, like numerous carbonatite-bearing complexes of similar composition, is a part of a large igneous province related to the ascent of a thermochemical mantle plume. The geochemical and isotopic data indicate that the formation of the ultramafic and alkaline rocks was related to crystallization differentiation of a primary alkali picritic melt, whereas carbonatite magmas were derived from an independent mantle source. We suggest that the origin of parental magmas of the Tiksheozero Complex, as well as other ultramafic-alkaline-carbonatite complexes, was provided by two-stage melting of the mantle-plume head: (1) adiabatic melting of its inner part generated moderately alkaline picrites, the subsequent fractional crystallization of which led to the appearance of alkaline magmas, and (2) incongruent melting of the upper cooled margin of the plume head under the influence of CO2-rich fluids, which arrived from underlying adiabatic melting zone, gave rise to carbonatite magmas.
Kovach, V.,Salnikova, E., Wang, K-L., Jahn, B-M., Chiu, H-Y., Reznitskiy, L., Kotov, A., Lizuka, Y., Chung, S-L.
Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high grade complex, southeastern Siberia: implication for continental growth and evolution of the Central Asian orogenic belt.
Journal of Asian Earth Sciences, Vol. 62, pp. 18-36.
Abstract: While mineral phases stable in the mantle transition zone (such as wadsleyite and ringwoodite) can store up to 3 wt% H2O, those in the lower mantle such as bridgmanite and ferropericlase can contain a very small amount (<50 ppm). While such dramatic differences can lead to dehydration/hydration and hydrous melting at 660-km depth in the mantle [1,2] it is uncertain how much water can be transported and stored at these depths. In order to answer this question, we have conducted a series of high pressure experiments in laser-heated diamondanvil cell and multi-anvil press combined with X-ray diffraction, infrared spectroscopy, laser Raman spectroscopy, and secondary ion mass spectrometry. Initially we examined the water storage capacity of dense (Al free) silica polymorphs at high pressure and temperature. We found that water can dramatically reduce the rutile-type to CaCl2-type phase transition from 55 GPa to 25 GPa and stabilize a new "disordered inverse" inverse NiAs-type phase at pressures above 50 GPa, which is not stable in dry SiO2 system. The CaCl2-type and NiAs-type silica polymorphs contain up to 8 wt% of H2O at 1400-2100 K up to at least 110 GPa. We next explored the effects of water on the mineralogy of the lower mantle and found that hydrous Mg2SiO4 ringwoodite (1 wt% H2O) breaks down to silica + bridgmanite + ferropericlase at pressures up to 60 GPa and 2100 K. The recovered silica samples contain 0.3-1.1 wt% H2O, suggesting that water stabilizes silica even under Si-undersaturated systems because of their large water storage capacity. Therefore, our observations support the stability of silica in hydrous regions in the pyrolitic lower mantle. In the subducting oceanic crust (basalt and sediment), silica represents 20-80% of the mineralogy. Because its stability range spans the mantle transition zone to the deep mantle, hydrous silica is expected to play a major role in the transport and storage of water in the deep mantle.
Abstract: While the water storage capacities of the upper 700 km depths of the mantle have been constrained by high-pressure experiments and diamond inclusion studies, the storage capacity of the lower mantle remains controversial. A recent high-pressure experimental study on CaSiO3 perovskite, which is the third most abundant mineral in the lower mantle, reported possible storage of H2O up to a few weight percent. However, the substitution mechanism for H in this phase remains unknown. We have conducted a series of density functional theory calculations under static-lattice conditions and high pressures to elucidate hydration mechanisms at the atomic scale. All of the possible dodecahedral (Ca2+ ? 2H+) and octahedral (Si4+ ? 4H+) substitution configurations for a tetragonal perovskite lattice have very small energy differences, suggesting the coexistence of multiples of H configurations in CaSiO3 perovskite at mantle pressures and temperatures. The dodecahedral substitutions decrease the bulk modulus, resulting in a smaller unit-cell volume of hydrous CaSiO3 perovskite under pressure, consistent with the experimental observations. Although the octahedral substitutions also decrease the bulk modulus, they increase the unit-cell volume at 1 bar. The H atoms substituted in the dodecahedral sites develop much less hydrogen bonding with O atoms, leading to a large distortion in the neighboring SiO6 octahedra. Such distortion may be responsible for the non-cubic peak splittings observed in experiments on hydrous CaSiO3 perovskite. Our calculated infrared spectra suggest that the observed broad OH modes in CaSiO3 perovskite can result from the existence of multiples of H configurations in the phase. Combined with the recent experimental results, our study suggests that CaSiO3 can be an important mineral phase to consider for the H2O storage in the lower mantle.
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.
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: We present a comprehensive petrographic, mineralogical and geochemical study of calc-alkaline lamprophyres (CAL) from the Archean Yilgarn Craton, Western Australia. Previous studies have shown that the emplacement age of CAL from the Eastern Goldfields Superterrane of the Yilgarn Craton is ~2684 to ~2640 Ma. A new Rb/Sr mica age for a CAL sample in the Western Yilgarn is ~2070 Ma. Both Archean and Proterozoic CAL analysed in this study display porphyritic textures and contain phenocrysts of amphibole, minor clinopyroxene and biotite in a fine-grained groundmass dominated by feldspar. High MgO, Ni and Cr abundances (up to 11.9 wt%, 373 and 993 ppm. respectively) are consistent with derivation of primitive magmas from a mantle source. Enrichment in H2O, reflected in the abundance of magmatic amphibole and mica, combined with high whole-rock LILE, Th/Yb ratios and negative Nb-Ta anomalies in trace element patterns are consistent with a source that was metasomatised by hydrous fluids analogous to those generated by Phanerozoic subduction-related processes. Chondritic ?Nd and ?Hf signatures and Archean mantle-like Sr isotope signatures of the Late Archean CAL indicate that the fluid metasomatism required to explain their volatile and trace-element enriched composition shortly preceded partial melting (i.e. there was insufficient time to develop enriched radiogenic isotopic signatures). The concurrence of apparently juvenile radiogenic isotopes and fluid-related trace element compositions requires a geodynamic scenario whereby dehydration of a subducted slab triggered metasomatism of the overlying mantle wedge. Our findings therefore support a subduction setting at ~2.6-2.7 Ga along the eastern margin of the Yilgarn Craton. The CAL from the Western Yilgarn have similar compositions but enriched Sr-Nd-Hf isotopes compared to those in the Eastern Goldfields Superterrane. This signature is consistent with melting of lithospheric mantle domains previously enriched by subduction-related metasomatism. Hence, our study suggests the presence of a subduction setting in the Western Yilgarn during the Archean, which is consistent with previous geodynamic reconstructions. However, the geodynamic trigger for the early Proterozoic event that generated CAL magmatism in the Western Yilgarn is currently unclear.
Nature Scientific Reports, doi.org/10.1038/ s41598-020-76800-0 10p. Pdf
Australia, Africa, South Africa
alkaline magmatism
Abstract: Large-scale mantle convective processes are commonly reflected in the emplacement of Large Igneous Provinces (LIPs). These are high-volume, short-duration magmatic events consisting mainly of extensive flood basalts and their associated plumbing systems. One of the most voluminous LIPs in the geological record is the ~?2.06 billion-year-old Bushveld Igneous Complex of South Africa (BIC), one of the most mineralised magmatic complexes on Earth. Surprisingly, the known geographic envelope of magmatism related to the BIC is limited to a series of satellite intrusions in southern Africa and has not been traced further afield. This appears inconsistent with the inferred large size of the BIC event. Here, we present new radiometric ages for alkaline magmatism in the Archean Yilgarn Craton (Western Australia), which overlap the emplacement age of the BIC and indicate a much more extensive geographic footprint of the BIC magmatic event. To assess plume involvement at this distance, we present numerical simulations of mantle plume impingement at the base of the lithosphere, and constrain a relationship between the radial extent of volcanism versus time, excess temperature and plume size. These simulations suggest that the thermal influence of large plume events could extend for thousands of km within a few million years, and produce widespread alkaline magmatism, crustal extension potentially leading to continental break-up, and large ore deposits in distal sectors. Our results imply that superplumes may produce very extensive and diverse magmatic and metallogenic provinces, which may now be preserved in widely-dispersed continental blocks.
Abstract: The Yilgarn Craton and its northern margin contain a variety of petrogenetically poorly defined small-volume alkaline ultramafic rocks of Proterozoic age. This study documents the petrography, mineral and bulk-rock geochemistry and Nd-Hf-Sr-Pb isotope compositions of a selected suite of these rocks. They comprise ~2.03-2.06 Ga ultramafic lamprophyres (UML) and carbonatites from the Eastern Goldfields Superterrane (EGS), ~0.86 Ga UML from Norseman, and orangeites from the Earaheedy Basin, including samples from Jewill (~1.3 Ga), Bulljah (~1.4 Ga) and Nabberu (~1.8-1.9 Ga). The Proterozoic UML and carbonatites from the EGS and Norseman display very consistent chondritic to superchondritic Nd-Hf isotope compositions and trace-element ratios similar to modern OIBs, which are indicative of a common mantle source across this wide alkaline province. These Nd-Hf isotope compositions overlap with the evolution trends of global kimberlites through time, thus suggesting that this mantle source could be deep and ancient as that proposed for kimberlites. Conversely, the orangeites located in the Earaheedy Basin along the northern margin of the Yilgarn Craton display trace element signatures similar to subduction-related calc-alkaline magmas. Taken together with their highly enriched Sr-Nd-Hf isotope compositions, these characteristics indicate an ancient lithospheric mantle source, which was probably metasomatised by subduction-related fluids. As the ages of the Bulljah and Jewill orangeites overlap with the breakup of the Columbia supercontinent, it is proposed that orangeite magmatism was triggered by changes in plate stress conditions associated with this event. This study provides a comprehensive picture of the genesis of Proterozoic alkaline magmatism in the Yilgarn Craton, highlighting the complex tectono-magmatic evolution of this lithospheric block after its assembly in the Archean.
Abstract: We present a comprehensive petrographic, mineralogical and geochemical study of calc-alkaline lamprophyres (CAL) from the Archean Yilgarn Craton, Western Australia. Previous studies have shown that the emplacement age of CAL from the Eastern Goldfields Superterrane of the Yilgarn Craton is ~2684 to ~2640 Ma. A new Rb/Sr mica age for a CAL sample in the Western Yilgarn is ~2070 Ma. Both Archean and Proterozoic CAL analysed in this study display porphyritic textures and contain phenocrysts of amphibole, minor clinopyroxene and biotite in a fine-grained groundmass dominated by feldspar. High MgO, Ni and Cr abundances (up to 11.9 wt%, 373 and 993 ppm. respectively) are consistent with derivation of primitive magmas from a mantle source. Enrichment in H2O, reflected in the abundance of magmatic amphibole and mica, combined with high whole-rock LILE, Th/Yb ratios and negative Nb-Ta anomalies in trace element patterns are consistent with a source that was metasomatised by hydrous fluids analogous to those generated by Phanerozoic subduction-related processes. Chondritic ?Nd and ?Hf signatures and Archean mantle-like Sr isotope signatures of the Late Archean CAL indicate that the fluid metasomatism required to explain their volatile and trace-element enriched composition shortly preceded partial melting (i.e. there was insufficient time to develop enriched radiogenic isotopic signatures). The concurrence of apparently juvenile radiogenic isotopes and fluid-related trace element compositions requires a geodynamic scenario whereby dehydration of a subducted slab triggered metasomatism of the overlying mantle wedge. Our findings therefore support a subduction setting at ~2.6-2.7 Ga along the eastern margin of the Yilgarn Craton. The CAL from the Western Yilgarn have similar compositions but enriched Sr-Nd-Hf isotopes compared to those in the Eastern Goldfields Superterrane. This signature is consistent with melting of lithospheric mantle domains previously enriched by subduction-related metasomatism. Hence, our study suggests the presence of a subduction setting in the Western Yilgarn during the Archean, which is consistent with previous geodynamic reconstructions. However, the geodynamic trigger for the early Proterozoic event that generated CAL magmatism in the Western Yilgarn is currently unclear.
Mineral chemistry of spinel peridotite xenoliths from Baengnyeong Island, South Korea, and its applications for the paleogeotherm of the uppermost mantle.
Earth and Planetary Science Letters, Vol. 433, pp. 360-369.
Mantle
Geophysics - seismics
Abstract: In regions of the mantle where multi-phases coexist like at the olivine-wadsleyite-ringwoodite transitions, the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases, a possible source of dissipation. In such a situation, the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave. Here we report synchrotron-based measurements of the kinetics of the olivine to ringwoodite transformation at pressure-temperature conditions of the co-stability loop, for iron-rich olivine compositions. Both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries. Transformation-time data show an increase of reaction rates with temperature and iron content, and have been fitted to a rate equation for interface-controlled transformation: G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)]G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)], where XFaXFa is the fayalite fraction, the exponential factor n=9.7n=9.7, View the MathML sourceln?k0=?9.1 ms?1. View the MathML sourceXFa?1 and ?Ha=199 kJ/mol?Ha=199 kJ/mol, assuming V?=0 cm3/molV?=0 cm3/mol. Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict View the MathML sourceQK?1 and View the MathML sourceQ??1 significantly higher than the PREM values for both body waves and normal modes. This attests that the olivine-wadsleyite transition can significantly contribute to the attenuation of the Earth's mantle transition zone.
El Atrassi, F., Brunet, F., Chazot, G., Bouybaouene, M., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif ( northern Morocco): petrography, mineral chemistry and thermobarometry.
El Atrassi, F., Brunet, F., Chazot, G., Chopin, C.
Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera Massif ( northern Morocco): mineralogical, chemical and textural records.
Proceedings of the Royal Society B: Biological sciences, Vol. 284. no 1856, pp.
Mantle
pangea
Abstract: The break-up of the supercontinent Pangaea around 180 Ma has left its imprint on the global distribution of species and resulted in vicariance-driven speciation. Here, we test the idea that the molecular clock dates, for the divergences of species whose geographical ranges were divided, should agree with the palaeomagnetic dates for the continental separations. Our analysis of recently available phylogenetic divergence dates of 42 pairs of vertebrate taxa, selected for their reduced ability to disperse, demonstrates that the divergence dates in phylogenetic trees of continent-bound terrestrial and freshwater vertebrates are consistent with the palaeomagnetic dates of continental separation.
Chorowicz, J., Guezlane, M., Rudant, J., Vidal, G.
Use of MOMS-1 dat a for geological mapping of the Aswa lineament(East African Rift)
National Technical Information Service, In ESA Proceedings 4th. International Colloquium in Spectral Signatures in Remote, N89 10382/4 4p. April 1988 Entire Conference $ 49.95
South African Journal of Geology, Vol. 123, 4, pp. 597-614. pdf
Africa, South Africa
alkaline rocks
Abstract: Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world’s largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2 200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1 400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative ?Nd and ?Hf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.
Journal of Earth System Science, Vol. 127, 6, pp. 76- doi:10.1007/s12040-018-0988-2
India
cratons
Abstract: The northern part of the Nellore-Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U-Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at ?1900 and ?1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita-Godavari (P-G) valley basin at ?1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 135-163.
India
Geotectonics
Abstract: Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites-rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66-0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites-rhyolites are silicic rocks (SiO2 = 61-72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5-1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U-Pb age of 2353 ± 32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers.
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.
Abstract: We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034-0.710582) and sub-chondritic initial ?Nd (?8.6 to 2.1) are akin to those of the (i) ca. 65?Ma Ambadongar carbonatite, NW India, and (ii) ca. 65?Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564-961?Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100?km at ca. 65?Ma.
Abstract: EPMA data are obtained from the P-5 kimberlite from the Wajrakarur field in the Eastern Dharwar craton of southern India (EDC). The studied sample consists of xenocrysts and xenoliths set in a variable grain size groundmass of olivine (with two textures: rounded-anhedral and subhedraleuhedral), phlogopite, perovskite, spinel, pyroxene, spinel and spinel containing Ti-garnet core. Ti-rich garnet associated with spinel is a rare occurrence in kimberlites. Two types of spinel have been identified (a) fine grained (<80 ?m) and compositionally non titaniferous, and (b) large macrocrysts (>100 ?m) having replacement cores having distinctly Ti-rich (TiO2 up to 28.51 wt %) compositions. Spinel is an abundant phase varying from <20 to >300 ?m in size, mostly subhedral to euhedral in shape. Pipe-5 has atolland necklace-textured spinels in addition to the euhedral groundmass spinels. Apart from individual grains in ground mass spinel there are also spinel intergrowths with perovskite (no apparent reaction texture observed), and sieve-like intergrowths. The composition of groundmass spinel is extensively used as petrogenetic indicator mineral (Roeder and Schulze 2008). Ti-garnets contain significant Ti (21.25-28.51wt.% TiO2), Ca (15.45-27.69 wt.% CaO), Fe (2.62-24.46 wt.% FeO) and low Cr (0.08-1.52 wt.% Cr2O3) and low Al (1.40-3.87 wt.% Al2O3). Ti- garnets and their paragenetic relationships to spinel are considered here as vital petrogenetic indicators of metasomatic fluids (Dongre et al., 2016; Cheng et al., 2014), and textural association with spinel shows that Ti-garnet formed when early crystallizing spinel interacted with residual melt during magma crystallization.
Abstract: P-5 and Kl-4 Mesoproterozoic (ca. 1110 Ma) kimberlites from the Wajrakarur and Kalyandurg clusters, Eastern Dharwar craton (EDC), southern India are intruded into the diamondiferous cratonic roots. The spinel compositions is straddling between magnesian ulvöspinel (Group-1 kimberlite) and titanomagnetite (Group-2 kimberlite), comparable with orangeite and lamproites. These Ti-rich minerals have orangeitic affinity, as in the Kaapvaal craton of South Africa, and reflect the high Ti-, high Ca- and the low Al-bearing nature of the parent magma (Group II kimberlites). Larger chrome spinel macrocrysts/xenocrysts show >500 ?m of size with distinctly high chromium (Cr2O3 up to 59.62 wt%), and TiO2-poor (<1.19 wt%). The high chromium spinel macrocrysts represent fragments of mantle xenocrysts and their composition falls within the diamond stability field. The groundmass spinel has been replaced by Ti- schorlomite. The schorlomite garnet represents solid solution of schorlomite -pyrope -almandine-grossular and Crrich schorlomite -pyrope -almandine- uvarovite solid solution. These associations recommend that the schorlomite formed through the replacement of spinel through interaction of late residual fluids/melts in the final stages of crystallization of the kimberlite magma and enrichment in Fe and Ti in schorlomite suggests the involvement of metasomatized sub-continental lithospheric mantle. Present study may have useful application in diamond prospectivity.
Abstract: Kl-4 and P-5 mesoproterozoic kimberlite pipes along with several other well-known diamondiferous (ca. 1110 Ma) kimberlites in the Wajrakarur kimberlite field (WKF) intruded into the cratonic roots of Eastern Dharwar craton (EDC) in southern India. The groundmass minerals of the kimberlites exhibit inequigranular texture contain spinel, Ti-rich schorlomite garnet, two generations of olivine (macrocrysts and groundmass microphenocrysts), phlogopite, perovskite, clinopyroxene (diopside), ilmenite (low Mn) and rare apatite. We identified three distinct spinel associations in Kl-4 and P-5: (i) fine-grained (<50 ?m) microcrysts in the groundmass; (ii) resorbed euhedral atoll spinel, consisting of titanomagnetite (magnesian-ulvospinel-magnetite to titanian-chrome-magnetite) which is isolated from the rim of magnetite by spongy lagoon phase of schorlomite, and (iii) larger chrome spinel macrocrysts/xenocrysts (>500 ?m). The schorlomite garnet in both P-5 and Kl-4 represents solid solution of schorlomite-pyrope-almandine-grossular. Additionally, Kl-4 contains another Cr-rich schorlomite-pyrope-almandine-uvarovite solid solution. Macrocrystic spinel exhibits distinct composition of chromium (Cr2O3 up to 59.62 wt%), and poor in TiO2 (<1.19 wt%). The high chromium spinel macrocrysts from Kl-4 are confirmed to be fragments of mantle xenocrysts and their composition falls within the diamond stability field. Atoll-textured epitaxial mantled resorbed spinel associated with schorlomite suggests that they formed through the replacement of spinel possibly through interaction of late residual fluids/melts in the final stages of crystallization of the kimberlite magma. The significant enrichment of Fe and Ti in schorlomite suggests the involvement of metasomatized sub-continental lithospheric mantle. It is also inferred that spinel immiscibility played an important role in the metasomatic replacement. The Ti-rich minerals have orangeitic affinity, similar to those in the Kaapvaal craton of South Africa, and suggest the high Ti-, high Ca- and the low Al-bearing nature of the parent magma (Group II kimberlites). The groundmass tetraferriphlogopite is Al- and Ba-poor and spinel show compositions straddling between magnesian ulvöspinel (Group I kimberlite) and titanomagnetite (Group II kimberlite) comparable with orangeite and lamproites. The results presented in this study suggest that the P-5 and Kl-4 has orengeitic or lamproitic affinity. Our findings can be useful as an indicator mineral in diamond prospecting.
Abstract:
Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Abstract: Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Abstract: Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Abstract: Rio Tinto had been developing a diamond mining project in Madhya Pradesh for a decade when in 2017 it hastily abandoned the project. We analyse this counterintuitive exit through an ethnographic approach nested within a qualitative case study framework. We argue that the exit was caused by multi-scalar politics. Local protests over livelihood and labour issues -pre-emptively rearticulated by regional civil society groups through an ecological ‘framing’ - led to litigation. The national forest bureaucracy posed regulatory hurdles, and a change in the national political regime in 2014 brought to power a party that leveraged national capital of a certain variety, which weakened Rio Tinto’s political position. Lastly, a slump in the global diamond market created economic uncertainties, finally leading to its exit. It has not, however, deterred the government from facilitating investment by Indian mega-corporate houses in mining diamonds, once again ignoring local dissent. Under the current regime in India, the space for activism is increasingly restricted, and that restriction, we contend, can lead to the disarray in strategising alliances and goals between ecological and social justice concerns.
PNAS, Vol. 118 (46) e2105746118 doi.org/10.73/pnas.210746118 pdf
Mantle
tectonics
Abstract: When and how Earth's earliest continents—the cratons—first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure-temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ?3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary-magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth’s earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (?50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision-driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric-oceanic oxygenation (O2-whiffs) and glaciation on Archean Earth.
Abstract: This paper presents a computer-vision-based methodology for automatic image-based classification of 2042 training images and 284 unseen (test) images divided into 68 categories of gemstones. A series of feature extraction techniques (33 including colour histograms in the RGB, HSV and CIELAB space, local binary pattern, Haralick texture and grey-level co-occurrence matrix properties) were used in combination with different machine-learning algorithms (Logistic Regression, Linear Discriminant Analysis, K-Nearest Neighbour, Decision Tree, Random Forest, Naive Bayes and Support Vector Machine). Deep-learning classification with ResNet-18 and ResNet-50 was also investigated. The optimal combination was provided by a Random Forest algorithm with the RGB eight-bin colour histogram and local binary pattern features, with an accuracy of 69.4% on unseen images; the algorithms required 0.0165 s to process the 284 test images. These results were compared against three expert gemmologists with at least 5 years of experience in gemstone identification, who obtained accuracies between 42.6% and 66.9% and took 42-175 min to classify the test images. As expected, the human experts took much longer than the computer vision algorithms, which in addition provided, albeit marginal, higher accuracy. Although these experiments included a relatively low number of images, the superiority of computer vision over humans is in line with what has been reported in other areas of study, and it is encouraging to further explore the application in gemmology and related areas.
Abstract: A drastically altered chemistry was recently discovered in the Fe-O-H system under deep Earth conditions, involving the formation of iron superoxide (FeO2Hx with x?=?0 to 1), but the puzzling crystal chemistry of this system at high pressures is largely unknown. Here we present evidence that despite the high O/Fe ratio in FeO2Hx, iron remains in the ferrous, spin-paired and non-magnetic state at 60-133?GPa, while the presence of hydrogen has minimal effects on the valence of iron. The reduced iron is accompanied by oxidized oxygen due to oxygen-oxygen interactions. The valence of oxygen is not -2 as in all other major mantle minerals, instead it varies around -1. This result indicates that like iron, oxygen may have multiple valence states in our planet’s interior. Our study suggests a possible change in the chemical paradigm of how oxygen, iron, and hydrogen behave under deep Earth conditions.
Abstract: Primary melt composition of kimberlites remains poorly constrained due to the contamination from mantle and crustal rocks, loss of volatiles during emplacement, significant alteration, and the lack of any quenched melts. Additionally, kimberlite bodies have multiple morphologies of which their emplacement mechanism remains elusive. Apatite is a common accessory mineral in kimberlite. Its structure incorporates many trace elements of which partitioning depends on the composition of the melt. Concentrations of trace elements in kimberlitic apatite can help to assess the content of carbonate and silicate components in kimberlite melt. Apatite is also often used as an indicator mineral of magma degassing in igneous systems. As such, it should be applied to kimberlitic systems to study the volatile behaviour during emplacement. However, the existing estimates for the trace elements partitioning in apatite provide controversial estimates for carbonatitic melts and estimates for silicate melts use compositions very different than the composition of kimberlites. This study experimentally determines partition coefficients of trace elements and kimberlite-like melts. The experiments were done in piston cylinder apparatus at 1250-1350 °C and 1-2 Gpa. Partition coefficients for Nb, Sr, Rb, Zr, Sm, Cs, Hf, La, Yb, and Eu were examined in synthetic compositions representing evolved kimberlite melts: three lamproitic compositions (17-23 wt % SiO2 and 9-33 wt % CO2) and a composition modelled after a magmatic kimberlite (14-29 wt % SiO2 and 7-33 wt % CO2). The effects of melt composition, temperature, pressure, water, and oxygen fugacity have been tested. The obtained partition coefficients were applied to natural kimberlitic apatites from Ekati Mine (Canada) and Orapa cluster (Botswana) to model kimberlite melt composition. Observed variation in the presence, textures, and composition of natural apatites relative to depth in kimberlite pipes of differing lithologies is compared to experimental run products to infer crystallization conditions of different kimberlite bodies.
Yellowknife Forum NWTgeoscience.ca, abstract volume poster p.103.
Mantle
magmatism
Abstract: Kimberlites are the deepest mantle magmas to reach the surface of the Earth and the hosts of the major primary diamond deposits. The enigmatic nature of kimberlites owning to poorly constrained triggers of kimberlite magmatism, melt composition, and crystallization conditions, are mostly limited by unknown content and composition of volatiles. The content and ratio of H2O and CO2 are important parameters of magma emplacement. They are essential for the mantle melting and rapid ascent of kimberlites. Volatiles also greatly affect diamond preservation in kimberlites during the ascent to the surface. Apatite has been used as an indicator of volatiles in different magmatic systems. Partitioning of trace elements between apatite and a melt is also sensitive to the carbonate component in the melt. This makes apatite potentially very useful indicator of evolution of melt composition and fluid during kimberlite ascent and emplacement. However, trace element partition coefficients between apatite and melts have been determined for compositions much more SiO2-rich than kimberlites, whereas coefficients in carbonate melts show large discrepancies. This study presents experimentally determined trace element partition coefficients (D) between apatite and synthetic analogues of kimberlite (SiO2 content ranging from 11 to 23 wt%) and carbonatite melts (SiO2 content <1 wt% and 4.5 wt%). The experiments were conducted in piston-cylinder apparatus at 1150o, 1250o, 1350o, at 1 and 2 GPa, and at oxygen fugacity -3.97, 0.27, 4.83 log units relative to fayalite-magnetite-quartz (FMQ) buffer. We demonstrate the increase of DREE for Rare Earth Elements (REE) with increase in SiO2 of the melt and the effect of apatite composition (natural Durango apatite vs. synthetic apatite) on D and substitution mechanism. We found no effect of temperature, pressure, water content, and oxygen fugacity on D in these low SiO2 melts. This allows us to use trace element content of natural apatite to examine the composition of kimberlite magma. We apply the results to apatite from two kimberlite classes: Class 1 composite kimberlite pipe from Orapa cluster (Botswana) with hypabyssal coherent and volcaniclastic kimberlite facies and Class 3 kimberlite pipes from Ekati Mine (Northwest Territories, Canada) filled with effusive coherent kimberlites in Leslie pipe and both volcaniclastic and coherent kimberlites in Boa pipe, and discuss how changes in SiO2 content of kimberlite melt affect apatite saturation. We further apply our experimentally determined DREE to model the composition of evolved kimberlite melt using published bulk rock and apatite compositions in kimberlites.
Geology, petrology, geochemistry and mineral chemistry of new kimberlite fields in the Wajrakarur kimberlite field, Anantapur district, Andhra Pradesh.
National Seminar on Exploration Survey, Geological Society of India Special Publication, No. 58, pp. 593-602.
Abstract: The rock record and geochemical evidence indicate that continental recycling has been occurring since the early history of the Earth. The stabilization of felsic continents in place of Earth’s early mafic crust about 3.0 to 2.0 billion years ago, perhaps due to the initiation of plate tectonics, implies widespread destruction of mafic crust during this time interval. However, the physical mechanisms of such intense recycling on a hotter, (late) Archaean and presumably plate-tectonic Earth remain largely unknown. Here we use thermomechanical modelling to show that extensive recycling via lower crustal peeling-off (delamination but not eclogitic dripping) during continent-continent convergence was near ubiquitous during the late Archaean to early Proterozoic. We propose that such destruction of the early mafic crust, together with felsic magmatism, may have caused both the emergence of silicic continents and their subsequent isostatic rise, possibly above the sea level. Such changes in the continental character have been proposed to influence the Great Oxidation Event and, therefore, peeling-off plate tectonics could be the geodynamic trigger for this event. A transition to the slab break-off controlled syn-orogenic recycling occurred as the Earth aged and cooled, leading to reduced recycling and enhanced preservation of the continental crust of present-day composition.
Geochimica et Cosmochimica Acta, in press, 61p. Pdf
Mantle
petrology
Abstract: Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep global sulfur cycle. Given that sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur concentration at sulfide saturation (SCSS) of mantle-derived magmas. CO2?±?H2O-induced melting beneath oceanic and continental mantle produces incipient CO2-rich melts. Although, SCSS of silicate melts of a variety of compositions is extensively studied, the SCSS of carbonatitic and carbonated silicate melts have not received much attention. Here we present experiments in graphite capsules at pressures (P) of 2.5-6.0 GPa and temperatures (T) of 1350-1650?°C investigating the SCSS of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe?±?Ni-sulfide melt blobs?+?carbonated melt matrix?±?ol?±?cpx?±?opx?±?gt, with melt composition on a CO2-free basis varying from 7 to 40 wt.% SiO2, 0.5 to 7 wt.% Al2O3, and 9 to 17 wt.% FeO* (total FeO). SCSS measured using EPMA increases with SiO2 and T but is not affected by P; the effect of composition being more pronounced than P-T. The composition of sulfide melt phase also affects SCSS. With increasing Ni in the molten sulfide phase, the SCSS changes from 2000 to 4000?ppm (Ni-free) to is 800-3000?ppm (33 wt.% Ni). Comparison of our measured SCSS with the existing SCSS models for nominally CO2-free silicate melts and with one study for carbonated melts show that these parameterizations fail to capture the sulfide saturation values in CO2-rich melts from our study. Using our new SCSS data and previous SCSS data for melt compositions that span the range from carbonatite to basalts via carbonated silicate melts, we develop a new empirical SCSS parameterization. Unlike a previous model, which suggested SCSS of carbonated melt is only affected by melt FeO* (other than P-T) and did not constrain how SCSS evolves from low-silica carbonatitic melt to low-CO2 basaltic melt, our new parameterization captured complex effects of many melt compositional parameters, including silica on SCSS. Using our new SCSS model, we constrained the efficiency of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated melts beneath ridges are expected to mobilize 5-15% of the initial sulfur before nominally-volatile-free peridotite melting begins. In continental mantle, deep kimberlitic melt can act as an agent to mildly enrich the shallow mantle in sulfide as it evolves to a carbonatitic melt upon reactive cooling. Application of our data to subduction zones suggests that low degree carbonatitic melt is not an efficient agent to extract residual sulfide from the subducting oceanic crust.
Third World Quarterly, Vol. 42, 8, pp. 1770-1787. pdf
India
economics
Abstract: Rio Tinto had been developing a diamond mining project in Madhya Pradesh for a decade when in 2017 it hastily abandoned the project. We analyse this counterintuitive exit through an ethnographic approach nested within a qualitative case study framework. We argue that the exit was caused by multi-scalar politics. Local protests over livelihood and labour issues -pre-emptively rearticulated by regional civil society groups through an ecological ‘framing’ - led to litigation. The national forest bureaucracy posed regulatory hurdles, and a change in the national political regime in 2014 brought to power a party that leveraged national capital of a certain variety, which weakened Rio Tinto’s political position. Lastly, a slump in the global diamond market created economic uncertainties, finally leading to its exit. It has not, however, deterred the government from facilitating investment by Indian mega-corporate houses in mining diamonds, once again ignoring local dissent. Under the current regime in India, the space for activism is increasingly restricted, and that restriction, we contend, can lead to the disarray in strategising alliances and goals between ecological and social justice concerns.
Abstract: Estimates of the volume of the earliest crust based on zircon ages and radiogenic isotopes remain equivocal. Stable isotope systems, such as molybdenum, have the potential to provide further constraints but remain underused due to the lack of complementarity between mantle and crustal reservoirs. Here we present molybdenum isotope data for Archaean komatiites and Phanerozoic komatiites and picrites and demonstrate that their mantle sources all possess subchondritic signatures complementary to the superchondritic continental crust. These results confirm that the present-day degree of mantle depletion was achieved by 3.5 billion years ago and that Earth has been in a steady state with respect to molybdenum recycling. Mass balance modelling shows that this early mantle depletion requires the extraction of a far greater volume of mafic-dominated protocrust than previously thought, more than twice the volume of the continental crust today, implying rapid crustal growth and destruction in the first billion years of Earth’s history.
Fahrig, W.F., Christie, K.W., Chown, E.H., Janes, D., Machado, N.
The tectonic significance of some basic dyke swarms in the Canadian Superior province with special reference to The geochemistry and paleomagnetism of th
Canadian Journal of Earth Sciences, Vol. 23, No. 2, February pp. 238-253
Abstract: Lamprophyres from the greenstone belts play a crucial role in deciphering tectonic and geodynamic processes operating during the Archean. This study presents a comprehensive mineralogical and geochemical study of three lamprophyre dykes with calc-alkaline to shoshonitic affinities from the Neoarchean Kadiri schist belt, eastern Dharwar craton, southern India. These rocks display porphyritic-panidiomorphic texture, typical of the lamprophyres with amphibole (magnesio-hornblende) as phenocrysts, biotite as microphenocrysts and feldspar, epidote, titanite and apatite confined to the groundmass. Alteration of biotite to chlorite is observed along with mild deformation in the amphibole phenocrysts. Based on mineralogy and major oxide geochemistry, these rocks are classified as the calc-alkaline lamprophyres. Higher Ba/Nb and low Nb/La points to their derivation from an enriched lithospheric mantle source and higher Th/Yb ratio along with negative TNT (Ti-Nb-Ta) and Zr-Hf anomalies on the primitive mantle (PM) normalized multi-element diagram indicates dehydrated fluids from the foundering slab could be the possible metasomatic agent. Fractionated HREE ratios (GdN/YbN >1.9) and higher SmN/YbN suggests that the source region lies in the garnet stability field. Higher than PM Rb/Sr along with positive correlation between K/La and Rb/La reveals presence of metasomatic phlogopite in the source region. Strong negative initial ?Nd along with radiogenic 87Sr/86Sr ratios further support an enriched mantle reservoir involved in their genesis. Non-modal batch melting (1-5%) of a mixed source (phlogopite-garnet peridotite) assuming 5% mixing of subducted sediment with ambient mantle wedge (depleted mantle) satisfies the multi-element concentration pattern shown by the Kadiri lamprophyres. The source enrichment can be linked to the accretion-related growth of Dharwar craton and its schist belts during Neoarchean. Our study shows that a majority of lamprophyres associated with the Archean greenstone belts display a shoshonitic character; this highlights the role of subduction-related processes in the growth and evolution of the greenstone belts .
Geochimica et Cosmochimica Acta, doi.org/10.1016/j.gca.2021.06.022 31p. Pdf
Global
meteorite
Abstract: The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 ?m in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4-5 s) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4-7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.
Contrasting lower crustal evolution across an Archean Proterozoic suture: physical, chemical and geochronologic studies of lower crustal xenoliths in southern Wyoming and northern Colorado.
American Geophysical Union, Geophysical Monograph, No. 154, pp. 139-162.
Delamination and ultra deep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultra high pressure metamorphic rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Zhao, Z., Niu, Y., Christensen, N.I., Zhou, W., Hou, Q., Zhang, Z.M., Xie, H., Zhang, Z.C., Liu, J.
Delamination and ultradeep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh pressure met. rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
Delamination and ultradeep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh pressure metamorphic rocks
Journal of Metamorphic Geology, Vol. 29, 7, pp. 781-801.
The tectonic significance of some basic dike swarms in the Canadian Superior Province with special reference to geochemistry and paleomagnetism of Mistassini swarm
Canadian Journal of Earth Sciences, Vol. 23, pp. 238-53.
Fahrig, W.F., Christie, K.W., Chown, E.H., Janes, D., Machado, N.
The tectonic significance of some basic dyke swarms in the Canadian Superior province with special reference to The geochemistry and paleomagnetism of th
Canadian Journal of Earth Sciences, Vol. 23, No. 2, February pp. 238-253
Abstract: The mineralogy of carbonatites reflects both the diversity of the sources of their parent magmas and their unusual chemistry. Carbonatites contain diverse suites of both primary magmatic minerals and later hydrothermal products. We present a summary of the variety of minerals found in carbon-atites, and note the economic importance of some of them, particularly those that are major sources of "critical elements", such as Nb and rare earth elements (REEs), which are essential for modern technological applications. Selected mineral groups are then discussed in detail: the REE carbonates, the alkali-rich ephemeral minerals that are rarely preserved but that may be important in the petrogenesis of carbonatites and their metasomatic haloes in adjacent rocks, and the Nb-rich oxides of the pyrochlore supergroup.-
Journal of Materials Research and Technology, Vol. 12, pp. 1473-1485.
China
nanodiamonds
Abstract: In this work, Ni/diamond composite coatings have been synthesized by electrodeposition in direct current mode. The effects of mechanical and ultrasonic agitations on the microstructural, surface characteristics and electrochemical properties have been comparatively investigated by various methods. Results show that diamond nanoparticles have been evenly dispersed in Ni metallic matrix, which could reinforce their performances. The coatings prepared under ultrasonic and mechanical agitation both exhibit compact, dense and hill-valley like morphology with pyramid-like nickel crystallite grains. The relative texture coefficient (RTC) values show that the preferred orientation of the Ni/diamond coating was (200) texture. From 3 to 5 A dm?2, the crystallite sizes of ultrasonic conditions were 59.2-81.7 nm, which were smaller than 76.3-83.2 nm of magnetic agitations. The average roughness (Ra = 78.9-133 nm) of ultrasonic-assisted coatings were lower than 103-139 nm of magnetic conditions. The mechanism of the co-electrodeposition process was proposed. Electrochemical impedance spectroscopy (EIS) results illustrate that the ultrasonic-assisted electrodeposited Ni/diamond coating has better corrosion resistance than that prepared under mechanical stirring conditions. The Ni/diamond composite coatings could be applied as protective materials in harsh mediums.
Abstract: The healed internal conjugated cleavages at the core of Aykhal octahedral diamond sample AH2 were decorated with {111}dia-facetted ultrapotassic fluid/melt inclusion pockets containing nanosized graphite, phlogopite and olivine (Fo92) inclusions. These olivines are either rounded in pockets with ample fluid, or facetted by the {111}dia mold in the pockets with a fluid film. Transmission electron microscopy revealed two distinct crystallographic characteristics of olivine inclusions: (1) pronounced crystallographic texture of olivines grouped in specific diamond domain, and (2) frequent parallelism or sub-parallelism of specific low-energy faces of the two phases, mainly (010)ol, {120}ol, (001)ol and {111}dia, {110}dia, {100}dia in the order of decreasing preference, forming prominent (010)ol/{111}dia, (010)ol/{110}dia, (001)ol/{110}dia, {120}ol/{111}dia, and {120}ol/{110}dia low-energy phase boundaries with thin liquid film of 1-2?nm in between. These findings not only testify to the extremely low adhesion energies of olivine-diamond boundary pairs, but also imply that, in the presence of a fluid phase, the interfacial energetics and the energetically favored crystallographic orientations of olivine inclusions in diamond can be controlled simply by the settlement/attachment of low-energy facets of olivine crystals precipitating from the parental fluid upon the low-energy {111}dia or {110}dia surfaces of diamond. Such interfacial energetics control and the resultant low-energy boundary pairs are characteristically distinct from the common topotaxy or epitaxy between oxide/silicate mineral pairs, but are in a sense like the Van der Waals heteroepitaxy in artificial systems.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and
Contributions to Mineralogy and Petrology, in press available, 14p.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole garnet
Contributions to Mineralogy and Petrology, Vol. 161, 6, pp. 901-920.
Abstract: Microscopic minerals excavated from an ancient outcrop of Jack Hills, in Western Australia, have been the subject of intense geological study, as they seem to bear traces of the Earth’s magnetic field reaching as far back as 4.2 billion years ago. That’s almost 1 billion years earlier than when the magnetic field was previously thought to originate, and nearly back to the time when the planet itself was formed. But as intriguing as this origin story may be, an MIT-led team has now found evidence to the contrary. In a paper published today in Science Advances, the team examined the same type of crystals, called zircons, excavated from the same outcrop, and have concluded that zircons they collected are unreliable as recorders of ancient magnetic fields. In other words, the jury is still out on whether the Earth’s magnetic field existed earlier than 3.5 billion years ago.
Earth and Planetary Science Letters, Vol. 479, pp. 1-17.
Mantle
eclogite
Abstract: The conventional wisdom holds that metamorphic reactions take place at pressures near-lithostatic so that the thermodynamic pressure, reflected by the mineral assemblage, is directly correlated with depth. On the other hand, recent field-based observations and geodynamic simulations suggest that heterogeneous stress and significant pressure deviations above lithostatic (overpressure) can occur in Earth's crust. Here we show that eclogite, normally interpreted to form at great depths in subduction zones and Earth's mantle, may form at much shallower depths via local overpressure generated in crustal shear zones. The eclogites studied crop out as lenses hosted by felsic paragneiss in a sheared thrust slice and represent a local pressure and temperature anomaly in the Taconic orogenic belt, southern New England. Sharply-defined chemical zones in garnet, which record ?5 kbar pressure rise and fall accompanied by a temperature increase of 150-200?°C, demonstrate extremely short timescales of diffusion. This requires anomalously fast compression (?500 yrs) and decompression. We use coupled phase equilibria and garnet diffusion forward modeling to fit the observed garnet profiles and test the likely paths using a Monte Carlo-type approach, accounting for off-center sectioning of garnet. The simulation shows that a ?5 kbar pressure increase after the temperature peak is necessary to reproduce the garnet zoning. Remarkably, this post-peak-T compression (from 9 kbar to 14 kbar) lasted only ?500 yrs. If the compression was due to burial along a lithostatic pressure gradient, the descent speed would exceed 30 m?yr?1, defying any observed or modeled subduction rates. Local overpressure in response to partial melting in a confined volume (Vrijmoed et al., 2009) caused by transient shear heating can explain the ultra-fast compression without necessitating burial to great depth.
Abstract: A new data compilation shows that in intermediate to felsic rocks, zircon Eu/Eu* [chondrite normalized Eu/ graphic] correlates with whole rock La/Yb, which has been be used to infer crustal thickness. The resultant positive correlation between zircon Eu/Eu* and crustal thickness can be explained by two processes favored during high-pressure differentiation: (1) supression of plagioclase and (2) endogenic oxidation of Eu2+ due to garnet fractionation. Here we calibrate a crustal thickness proxy based on Eu anomalies in zircons. The Eu/Eu*-in-zircon proxy makes it possible to reconstruct crustal thickness evolution in magmatic arcs and orogens using detrital zircons. To evaluate this new proxy, we analyzed detrital zircons separated from modern river sands in the Gangdese belt, southern Tibet. Our results reveal two episodes of crustal thickening (to 60-70 km) since the Cretaceous. The first thickening event occurred at 90-70 Ma, and the second at 50-30 Ma following Eurasia-India collision. These findings are temporally consistent with contractional deformation of sedimentary strata in southern Tibet.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Tian, W., Chen, B., Ireland, T.R., Green, D.H., Suzuki, K., Chu, Z.
Petrology and geochemistry of dunites, chromitites and mineral inclusions from the Gaositai Alaskan type complex, North Chin a craton: mantle source characters
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: The accretionary mobile belts surrounding ancient cratonic cores are an important facet of the growth and preservation of continental landmasses. Peridotites from Nuominhe in the Xing'an Mongolia Orogenic Belt (XMOB) provide an additional opportunity to examine the age, structure and evolution of mantle lithosphere separating two of the largest existing ancient continental nuclei: the North China Craton and the Siberian Craton. This suite of mantle rocks comprises fertile to refractory garnet- and spinel-facies harzburgites and lherzolites. Their lithophile element systematics show that the peridotites were metasomatized to variable extent by silicate?carbonate melts. Despite this, the highly siderophile element and Os isotope systematics appear to have been largely undisturbed. The Nuominhe peridotites have Re-depletion Os model ages (TRD) that range from 0.5 Ga to 2.4 Ga, with three peaks/major ranges at ~2.0-2.4 Ga, ~1.4-1.5 Ga and ~ 0.8 Ga, of which the latter two are closely similar to those data from other XMOB localities reported in the literature. The only section of the mantle that appears to have ages which correlate with crust formation is the suite with Neoproterozoic (~0.8 Ga) depletion ages, while the older mantle domains document older episodes of mantle depletion. Given the lack of correlation between equilibrium temperatures and bulk composition or TRD ages, the Nuominhe peridotites were inter-mixed in the mantle column, most likely as a result of incorporation of recycled older continental mantle fragments into juvenile Neoproterozoic mantle during the orogenic processes responsible for new lithosphere formation. Geothermobarometry of the Nuominhe peridotites indicates a conductive geotherm of ~60 mWm?2 and therefore a lithosphere thickness of ~125 km, which is thicker than most Phanerozoic continental terranes, and even thicker than Proterozoic regions that comprise the larger cratonic unit of the Siberian craton. This thick Proterozoic lithosphere sandwiched between the converging North China and Siberian cratons was evidently partly constructed from recycled refractory continental mantle fragments, perhaps extant in the convecting mantle, or in-part derived from the surrounding cratons, leading to a composite nature of the mantle in this re-healed continental suture. Re-accretion of recycled refractory old continental mantle fragments plays a significant role in affecting mantle composition and controlling the thickness of circum-cratonic landmasses between cratonic blocks.
Geochimica et Cosmochimica Acta, Vol. 295, pp. 207-223.
China
nephelinites, basanites
Abstract: Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this “big mantle wedge”. We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., PdN/IrN normalized to PUM?=?1.1?±?0.8, 1?), relatively high total PGE contents (e.g., Ir?=?0.25?±?0.14?ppb) and modern mantle-like 187Os/188Os (0.142?±?0.020). These characteristics are coupled with lighter Mg isotope (?26Mg?=??0.48?±?0.07‰) and heavier Zn isotope (?66Zn = +0.46?±?0.06‰) compositions compared to the mantle values (?26Mg: ?0.25?±?0.07‰; ?66Zn: +0.18?±?0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., PdN/IrN?=?4.4?±?3.3; Ir?=?0.037?±?0.027?ppb) and radiogenic 187Os/188Os (0.279?±?0.115) coupled with less fractionated Mg-Zn isotope compositions (?26Mg: ?0.39?±?0.05‰; ?66Zn: +0.35?±?0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO2, Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust.
Highly heterogeneous lithospheric mantle beneath the Central Zone of the North Chin a Craton evolved from Archean mantle through diverse melt refertilization.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mass transfer from the subducted crust to the mantle wedge through orogenic peridotites.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mas
Minerals MDPI, Vol. 10, 1117, doi:10.3390/ min10121117 20p. Pdf
China
deposit - Bayan Obo
Abstract: The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407-4184 ppm and 63-152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35-4.20 wt.% and SrO at 1.75-2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10-8.21 wt.%) and SrO (2.83-3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17-2.35 wt.%) and SrO (1.51-1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.
Russian Journal of Pacific Geology, Vol. 12, 5, pp. 408-418.
Russia, Kamchatka
picrites
Abstract: The mineral and chemical compositions of the layered subvolcanic ultrabasic rocks formed through fluid-silicate (liquid) separation of the ultrabasic magma into high-potassium and low-potassium fractions are characterized by the example of the layered picritic sill from the Late Cretaceous ultrabasic volcanic complex of Eastern Kamchatka. It is determined that the main potassium concentrator in the picrites from the high-potassium layers is a residual volcanic glass containing up to 8-9 wt % K2O, which is unique for ultrabasic melts.
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.
Physicsa Status Solidi , doi:10.1002/pssa.201900888
Global
HPHT
Abstract: Various samples of multisectoral high?pressure high?temperature (HPHT) single?crystal diamond plate (IIa type) (4?×?4?×?0.53?mm) are tested for particle detection applications. The samples are investigated by X?ray diffractometry, photoluminescence spectroscopy, Raman spectroscopy, Fourier?transform infrared, and visible/ultraviolet (UV) absorption spectroscopy. High crystalline perfection and low impurity concentration (in the {100} growth sector) are observed. To investigate detector parameters, circular 1.0 and 1.5?mm diameter Pt Schottky barrier contacts are created on {111} and {100} growth sectors. On the backside, a Pt contact (3.5?×?3.5?mm) is produced. The {100} growth sector is proved to be a high?quality detector: the full width at half maximum energy resolution is 0.94% for the 5.489?MeV 226Ra ??line at an operational bias of +500?V. Therefore, it is concluded that the HPHT material {100} growth sector is used for radiation detector production, whose quality is not worse than the chemical vapor deposition method or specially selected natural diamond detectors.
Abstract: Apatites from the Verity, Fir, Gum, Howard Creek and Felix carbonatites of the Blue River (British Columbia, Canada) area have been investigated with respect to their paragenesis, cathodoluminescence, trace element and Sr–Nd isotopic composition. Although all of the Blue River carbonatites were emplaced as sills prior to amphibolite grade metamorphism and have undergone deformation, in many instances magmatic textures and mineralogy are retained. Attempts to constrain the U–Pb age of the carbonatites by SIMS, TIMS and LA–ICP-MS studies of zircon and titanite were inconclusive as all samples investigated have experienced significant Pb loss during metamorphism. The carbonatites are associated with undersaturated calcite–titanite amphibole nepheline syenite only at Howard Creek although most contain clasts of disaggregated phoscorite-like rocks. Apatite from each intrusion is characterized by distinct, but wide ranges, in trace element composition. The Sr and Nd isotopic compositions define an array on a 87Sr/86Sr vs²Nd diagram at 350 Ma indicating derivation from depleted sub-lithospheric mantle. This array could reflect mixing of Sr and Nd derived from HIMU and EM1 mantle sources, and implies that depleted mantle underlies the Canadian Cordillera. Although individual occurrences of carbonatites in the Blue River region are mineralogically and geochemically similar they are not identical and thus cannot be considered as rocks formed from a single batch of parental magma at the same stage of magmatic evolution. However, a common origin is highly probable. The variations in the trace element content and isotopic composition of apatite from each occurrence suggest that each carbonatite represents a combination of derivation of the parental magma(s) from mineralogically and isotopically heterogeneous depleted mantle sources coupled with different stages of limited differentiation and mixing of these magmas. We do not consider these carbonatites as primary direct partial melts of the sub-lithospheric mantle which have ascended from the asthenosphere without modification of their composition.
A cathodluminescence study of calcite dolomite microstructures and Cal-Dol geothermometry in highly metamorphosed carbonatites: an example from the Fir carbonatites, east central British Columbia, Canada.
GAC-MAC 2013: GS2: Igneous and Metamorphic Petrology and Volcanology, abstract only
Abstract: The results of geochronological, mineralogical, petrographical, and geochemical study of the Ilbokich ultramafic lamprophyre are reported. The specific features in the mineral and chemical compositions of the studied ultramafic lamprophyre indicate that it can be regarded as a variety similar to aillikite, while other differences dominated by K-feldspar can be referred to damtjernite. According to Rb-Sr analysis, ultramafic lamprophyre dikes intruded at the turn of the Early and Middle Devonian, about 392 Ma ago. This directly proves the existence of Early Paleozoic alkali-ultramafic magmatism in the northern part of the southwest Siberian Platform. A finding of Devonian alkali-ultramafic lamprophyre is of dual predictive importance. On the one hand, it is indicative of the low probability of finding large diamond-bearing deposits in close association with aillikite. On the other hand, it can be indicative of a possible large Devonian diamond province in the studied territory, where diamondiferous kimberlite is structurally separated from aillikite.
Abstract: To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr-Nd-Pb-C-O isotope data was obtained. Our new 206Pb/238U perovskite age (399 ± 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 ± 3 Ma and 241 ± 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to ×200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33-84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow 87Sr/86Sr0 range (0.7032-0.7042) and ?Nd(T) (4.03-3.97). Chadobets UMLs have a similar Sr isotopic signature (87Sr/86Sr0: 0.7031-0.7043) and a more depleted Nd isotopic signature (?Nd(T) 4.09-5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between 206Pb/204Pb = 18.4-19.0, 208Pb/204Pb = 38.3-38.8, and are characterized by a narrow 207Pb/204Pb ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between 206Pb/204Pb = 18.0-18.4, 208Pb/204Pb = 37.8-38.4 and 207Pb/204Pb ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable ?18O values from +12.1 and up to +20.5‰ (SMOW). The isotopic composition of ?13C values range from ?1.3‰ to ?7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps.
Abstract: To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr-Nd-Pb-C-O isotope data was obtained. Our new 206Pb/238U perovskite age (399 ± 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 ± 3 Ma and 241 ± 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to ×200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33-84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow 87Sr/86Sr0 range (0.7032-0.7042) and ?Nd(T) (4.03-3.97). Chadobets UMLs have a similar Sr isotopic signature (87Sr/86Sr0: 0.7031-0.7043) and a more depleted Nd isotopic signature (?Nd(T) 4.09-5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between 206Pb/204Pb = 18.4-19.0, 208Pb/204Pb = 38.3-38.8, and are characterized by a narrow 207Pb/204Pb ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between 206Pb/204Pb = 18.0-18.4, 208Pb/204Pb = 37.8-38.4 and 207Pb/204Pb ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable ?18O values from +12.1 and up to +20.5‰ (SMOW). The isotopic composition of ?13C values range from ?1.3‰ to ?7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps.
Mineralogy and Petrology, Vol. 109, 2, pp. 143-152.
Russia, Urals
Mineralogy
Abstract: A new picromerite-group mineral, nickelpicromerite, K2Ni(SO4)2 - 6H2O (IMA 2012-053), was found at the Vein #169 of the Ufaley quartz deposit, near the town of Slyudorudnik, Kyshtym District, Chelyabinsk area, South Urals, Russia. It is a supergene mineral that occurs, with gypsum and goethite, in the fractures of slightly weathered actinolite-talc schist containing partially vermiculitized biotite and partially altered sulfides: pyrrhotite, pentlandite, millerite, pyrite and marcasite. Nickelpicromerite forms equant to short prismatic or tabular crystals up to 0.07 mm in size and anhedral grains up to 0.5 mm across, their clusters or crusts up to 1 mm. Nickelpicromerite is light greenish blue. Lustre is vitreous. Mohs hardness is 2-2½. Cleavage is distinct, parallel to {10-2}. Dmeas is 2.20(2), Dcalc is 2.22 g cm?3. Nickelpicromerite is optically biaxial (+), ? = 1.486(2), ? = 1.489(2), ? = 1.494(2), 2Vmeas =75(10)°, 2Vcalc =76°. The chemical composition (wt.%, electron-microprobe data) is: K2O 20.93, MgO 0.38, FeO 0.07, NiO 16.76, SO3 37.20, H2O (calc.) 24.66, total 100.00. The empirical formula, calculated based on 14 O, is: K1.93Mg0.04Ni0.98S2.02O8.05(H2O)5.95. Nickelpicromerite is monoclinic, P21/c, a = 6.1310(7), b = 12.1863(14), c = 9.0076(10) Å, ? = 105.045(2)°, V = 649.9(1) Å3, Z = 2. Eight strongest reflections of the powder XRD pattern are [d,Å-I(hkl)]: 5.386--34(110); 4.312-46(002); 4.240-33(120); 4.085--100(012, 10-2); 3.685-85(031), 3.041-45(040, 112), 2.808-31(013, 20-2, 122), 2.368-34(13-3, 21-3, 033). Nickelpicromerite (single-crystal X-ray data, R = 0.028) is isostructural to other picromerite-group minerals and synthetic Tutton’s salts. Its crystal structure consists of [Ni(H2O)6]2+ octahedra linked to (SO4)2? tetrahedra via hydrogen bonds. K+ cations are coordinated by eight anions. Nickelpicromerite is the product of alteration of primary sulfide minerals and the reaction of the acid Ni-sulfate solutions with biotite.
European Journal of Mineralogy, Vol. 28, pp. 915-930.
Technology
Mineral chemistry
Abstract: The crystal structures of the lamprophyllite-related minerals are based upon HOH modules consisting of a central octahedral O sheet sandwiched between two heteropolyhedral H sheets. The general crystal-chemical formula for these minerals can be written as [10-11]A2 [[6]M1[6]M22[6]2M3X2] [[5]L2(Si2O7)2O2], where the contents of the O and H sheets are given in square brackets in this order and A = Ba, Sr, K, Na,; M1 = Na, Mn2+; M2 = Na, Mn2+, Fe2+, Ca; M3 = Ti, Mn2+, Mg, Fe3+, Fe2+; L = Ti, Fe3+; X = OH, O, F. According to the unit-cell parameters and symmetry, lamprophyllite-related minerals can be subdivided into five structure types: I (monoclinic polytypes, C2/m); II (orthorhombic polytypes, Pnmn), III (nabalamprophyllite, BaNa[Na3Ti (OH)2][Ti2(Si2O7)2O2], monoclinic, P2/m, with an ordered arrangement of the interlayer Ba2+ and Na+ cations), IV (triclinic, P1) and V (triclinic, Embedded Image ). The triclinic members (types IV and V) include schüllerite and its analogues, which differ from the lamprophyllite-group minerals sensu stricto in their symmetry and topology of the HOH modules. The end-member formulae of lamprophyllite-related minerals and the position of schüllerite in the ranks of heterophyllosilicates are discussed.
Physics and Chemistry of Minerals, Vol. 45, pp. 745-758.
Russia, Kola Peninsula
alkaline
Abstract: The new eudialyte-group mineral siudaite, ideally Na8(Mn2+2Na)Ca6Fe3+3Zr3NbSi25O74(OH)2Cl•5H2O, was discovered in a peralkaline pegmatite situated at the Eveslogchorr Mt., Khibiny alkaline massif, Kola Peninsula, Russia. The associated minerals are aegirine, albite, microcline, nepheline, astrophyllite, and loparite-(Ce). Siudaite forms yellow to brownish-yellow equant anhedral grains up to 1.5 cm across. Its lustre is vitreous, and the streak is white. Cleavage is none observed. The Mohs’ hardness is 4½. Density measured by hydrostatic weighing is 2.96(1) g/cm3. Density calculated using the empirical formula is equal to 2.973 g/cm3. Siudaite is nonpleochroic, optically uniaxial, negative, with ??=?1.635(1) and ??=?1.626(1) (??=?589 nm). The IR spectrum is given. The chemical composition of siudaite is (wt%; electron microprobe, H2O determined by HCN analysis): Na2O 8.40, K2O 0.62, CaO 9.81, La2O3 1.03, Ce2O3 1.62, Pr2O3 0.21, Nd2O3 0.29, MnO 6.45, Fe2O3 4.51. TiO2 0.54, ZrO2 11.67, HfO2 0.29, Nb2O5 2.76, SiO2 47.20, Cl 0.54, H2O 3.5, -O?=?Cl ??0.12, total 99.32. According to Mössbauer spectroscopy data, all iron is trivalent. The empirical formula (based on 24.5 Si atoms pfu, in accordance with structural data) is [Na7.57(H2O)1.43]?9(Mn1.11Na0.88Ce0.31La0.20Nd0.05Pr0.04K0.41)?3(H2O)1.8(C a5.46Mn0.54)?6(Fe3+1.76Mn2+1.19)?2.95Nb0.65(T i0.20Si0.50)?0.71(Zr2.95Hf0.04Ti0.01)?3Si24.00Cl0.47O70(OH)2Cl0.47•1.2H2O. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3m, with a?=?14.1885(26) Å, c?=?29.831(7) Å, V?=?5200.8(23) Å3 and Z?=?3. Siudaite is chemically related to georgbarsanovite and is its analogue with Fe3+-dominant M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.38 (60) (-114), 4.29 (55) (-225), 3.389 (47) (131), 3.191 (63) (-228). 2.963 (100) (4-15), 2.843 (99) (-444), 2.577 (49) (3-39). Siudaite is named after the Polish mineralogist and geochemist Rafa? Siuda (b. 1975).
European Journal of Mineralogy, Vol. 31, pp. 13-143.
Russia, Kola Peninsula
deposit - Khibiny
Abstract: The new mineral alexkhomyakovite K6(Ca2Na)(CO3)5Cl?6H2O (IMA2015-013) occurs in a peralkaline pegmatite at Mt. Koashva, Khibiny alkaline complex, Kola peninsula, Russia. It is a hydrothermal mineral associated with villiaumite, natrite, potassic feldspar, pectolite, sodalite, biotite, lamprophyllite, titanite, fluorapatite, wadeite, burbankite, rasvumite, djerfisherite, molybdenite and an incompletely characterized Na-Ca silicate. Alexkhomyakovite occurs as equant grains up to 0.2 mm, veinlets up to 3 cm long and up to 1 mm thick and fine-grained aggregates replacing delhayelite. Alexkhomyakovite is transparent to translucent, colourless, white or grey, with vitreous to greasy lustre. It is brittle, the Mohs hardness is ca. 3. No cleavage was observed, the fracture is uneven. D meas = 2.25(1), D calc = 2.196 g cm?3. Alexkhomyakovite is optically uniaxial (-), ? = 1.543(2), ? = 1.476(2). The infrared spectrum is reported. The chemical composition [wt%, electron microprobe data, CO2 and H2O contents calculated for 5 (CO3) and 6 (H2O) per formula unit (pfu), respectively] is: Na2O 4.09, K2O 35.72, CaO 14.92, MnO 0.01, FeO 0.02, SO3 0.11, Cl 4.32, CO2 28.28, H2O 13.90, -O=Cl -0.98, total 100.39. The empirical formula calculated on the basis of 9 metal cations pfu is K5.90Ca2.07Na1.03(CO3)5(SO4)0.01O0.05Cl0.95?6H2O. The numbers of CO3 groups and H2O molecules are based on structure data. Alexkhomyakovite is hexagonal, P63/mcm, a = 9.2691(2), c = 15.8419(4) Å, V = 1178.72(5) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d Å(I)(hkl)] are: 7.96(27)(002), 3.486(35)(113), 3.011(100)(114), 2.977(32)(211), 2.676(36)(300), 2.626(42)(213, 115), 2.206(26)(311) and 1.982(17)(008). The crystal structure (solved from single-crystal X-ray diffraction data, R = 0.0578) is unique. It is based on (001) heteropolyhedral layers of pentagonal bipyramids (Ca,Na)O5(H2O)2 interconnected via carbonate groups of two types, edge-sharing ones and vertex-sharing ones. Ca and Na are disordered. Ten-fold coordinated K cations centre KO6Cl(H2O)3 polyhedra on either side of the heteropolyhedral layer. A third type of carbonate group and Cl occupy the interlayer. The mineral is named in honour of the outstanding Russian mineralogist Alexander Petrovich Khomyakov (1933-2012).
Neues Jahbuch fur Mineralogie, Vol. 196, 3, pp. 193-196.
Europe, Spain
lamproite
Abstract: Al analogue of chayesite (with Al > Fe3+) was found in a lamproite from Cancarix, SE Spain. The mineral forms green thick-tabular crystals up to 0.4 mm across in cavities. The empirical formula derived from EMP measurements and calculated on the basis of 17 Mg + Fe + Al + Si apfu is (K0.75 Na0.20 Ca0.11)Mg3.04 Fe0.99 Al1.18 Si11.80 O30. The crystal structure was determined from single crystal X-ray diffraction data ( R = 2.38%). The mineral is hexagonal, space group P 6/ mcc, a = 10.09199(12), c = 14.35079(19) Å, V = 1265.78(3) Å3, Z = 2. Fe is predominantly divalent. Al is mainly distributed between the octahedral A site and the tetrahedral T 2 site. The crystal chemical formula derived from the structure refinement is C (K0.73 Na0.16 Ca0.11) B (Na0.02)4 A (Mg0.42 Al0.29 Fe0.29)2 T 2(Mg0.71 Fe0.16 Al0.13)3 T 1(Si0.985 Al0.015)12 O30.
Abstract: The new eudialyte-group mineral sergevanite, ideally Na15(Ca3Mn3)(Na2Fe)Zr3Si26O72(OH)3•H2O, was discovered in highly agpaitic foyaite from the Karnasurt Mountain, Lovozero alkaline massif, Kola Peninsula, Russia. The associated minerals are microcline, albite, nepheline, arfvedsonite, aegirine, lamprophyllite, fluorapatite, steenstrupine-(Ce), ilmenite, and sphalerite. Sergevanite forms yellow to orange-yellow anhedral grains up to 1.5 mm across and the outer zones of some grains of associated eudialyte. Its luster is vitreous, and the streak is white. No cleavage is observed. The Mohs' hardness is 5. Density measured by equilibration in heavy liquids is 2.90(1) g/cm3. Calculated density is equal to 2.906 g/cm3. Sergevanite is nonpleochroic, optically uniaxial, positive, with ? = 1.604(2) and ? = 1.607(2) (? = 589 nm). The infrared spectrum is given. The chemical composition of sergevanite is (wt.%; electron microprobe, H2O determined by HCN analysis): Na2O 13.69, K2O 1.40, CaO 7.66, La2O3 0.90, Ce2O3 1.41, Pr2O3 0.33, Nd2O3 0.64, Sm2O3 0.14, MnO 4.15, FeO 1.34, TiO2 1.19, ZrO2 10.67, HfO2 0.29, Nb2O5 1.63, SiO2 49.61, SO3 0.77, Cl 0.23, H2O 4.22, -O=Cl -0.05, total 100.22. The empirical formula (based on 25.5 Si atoms pfu, in accordance with structural data) is H14.46Na13.64K0.92Ca4.22Ce0.27La0.17Nd0.12Pr0.06Sm0.02Mn1.81Fe2+0.58Ti0.46Zr2.67Hf0.04Nb0.38Si25.5S0.30Cl0.20O81.35. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3, with a = 14.2179(1) Å, c = 30.3492(3) Å, V = 5313.11(7) Å3, and Z = 3. In the structure of sergevanite, Ca and Mn are ordered in the six-membered ring of octahedra (at the sites M11 and M12), and Na dominates over Fe2+ at the M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.12 (70) (110), 5.711 (43) (202), 4.321 (72) (205), 3.806 (39) (033), 3.551 (39) (220, 027), 3.398 (39) (313), 2.978 (95) (?forumla?), 2.855 (100) (404). Sergevanite is named after the Sergevan' River, which is near the discovery locality.
Sanatmaria-Perez, D., Ruiz-Fuertes, J., Pena-Alvarez, M., Chulia-Jordan, R., Marquerno, T., Zimmer, D., Guterrez-Cano, V., Macleod, S., Gregoryanz, E., Popescue, C., Rodriguez-Herandez, P., Munoz, A.
Abstract: Calcium carbonate is a relevant constituent of the Earth’s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca5(Si2O7)(CO3)2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6-9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.
IEEE Photonics Journal, Researchgate 35102286, April, 28p. Pdf
Global
luminescence
Abstract: Modern-day diamond sorting is achieved through the application of x-ray luminescence (XRL) and x-ray transmission (XRT) techniques. Sorting with XRL is limited to the class range of 1.25mm to 32mm because of self-absorption associated with larger diamonds, greater than 32mm. The effect of self-absorption is also a high-energy phenomenon in XRL. XRT is limited to sorting large size diamonds as the technique suffers poor contrast for diamonds smaller than 10mm. XRT measurements are immune to self-absorption for all sample sizes, while XRL measurements have good contrast for particles smaller than 32mm. The applications of these techniques have hitherto been used independently of each other and have subsequently progressed mutually exclusively. Here we analytically show a new paradox of a dual-modality X-ray diamond sorting combining XRL and XRT techniques' strengths. Key features of our new paradoxical model performance are contrast mitigation for small particles and self-absorption rejection for a large particle at high energy as well as improved particle detectability and classification.
Abstract: We describe a new methodology to collect energy domain Mössbauer spectra of inclusions in natural diamonds using a Synchrotron Mössbauer Source (SMS). Measurements were carried out at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (Grenoble, France). We applied this non-destructive approach to collect SMS spectra of a ferropericlase inclusion still contained within its diamond host from Juina (Brazil). The high spatial resolution of the measurement (~ 15 ?m) enabled multiple regions of the 190 × 105 ?m2 inclusion to be sampled and showed that while Fe3 +/Fetot values in ferropericlase were below the detection limit (0.02) overall, there was a magnetic component whose abundance varied systematically across the inclusion. Hyperfine parameters of the magnetic component are consistent with magnesioferrite, and the absence of superparamagnetism allows the minimum particle size to be estimated as ~ 30 nm. Bulk Fe3 +/Fetot values are similar to those reported for other ferropericlase inclusions from Juina, and their variation across the inclusion can provide constraints on its history.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 16340 Abstract
South America, Brazil
Deposit - Juina
Abstract: We describe a new methodology to collect energy domain Mössbauer spectra of inclusions in natural diamonds using a Synchrotron Mössbauer Source (SMS). Measurements were carried out at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (Grenoble, France). We applied this non-destructive approach to collect SMS spectra of a ferropericlase inclusion still contained within its diamond host from Juina (Brazil). The high spatial resolution of the measurement (~ 15 ?m) enabled multiple regions of the 190 × 105 ?m2 inclusion to be sampled and showed that while Fe3 +/Fetot values in ferropericlase were below the detection limit (0.02) overall, there was a magnetic component whose abundance varied systematically across the inclusion. Hyperfine parameters of the magnetic component are consistent with magnesioferrite, and the absence of superparamagnetism allows the minimum particle size to be estimated as ~ 30 nm. Bulk Fe3 +/Fetot values are similar to those reported for other ferropericlase inclusions from Juina, and their variation across the inclusion can provide constraints on its history.
Abstract: Diamond inclusions are the only samples from the mantle transition zone (410-660 km) and the lower mantle. Majoritic garnet is a rare inclusion, limited to pressures of ~8-20 Gpa with Si content being indicative of depth of re-equilibration. These garnet inclusions can therefore provide information on properties of the transition zone such as oxidation state. In this study, we used Synchrotron Mössbauer Source (SMS) to determine the ferric-ferrous ratios of 13 small (30 to 100 micrometers diameter) majoritic inclusions in diamonds from Jagersfontein. The studied inclusions have pyroxenitic affinities [1], with compositions intermediate between typical peridotite and eclogite. They contain 4.62-11.2 wt% CaO, 0.03-0.34 wt% Cr2O3 and Mg# of 0.65-0.81. Minimum pressures for their equilibration using Beyer and Frost [2] barometer are between 8 and 18 GPa with at least 4 of these inclusions being formed in the transition zone. The Fe3+/Fetotal ratios in the garnets increase from 0.08±0.01 to 0.30±0.03 with increasing pressure. These values define a clear extension of the trend apparent in the data from peridotite xenoliths crystallised at lower pressures. Thermodynamic calculations suggest that these high ferric contents correspond to oxygen fugacities above the FeFeO (IW) buffer, which means that the high Fe3+ contents were not generated by disproportionation of Fe2+ to Fe3+ and Fe0 . It is more likely that carbonate was the oxidising agent responsible for generating the high Fe3+ of these garnets.
Abstract: The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200?km, but has not been characterized in samples from the lowermost upper mantle (200-410?km depth) or the transition zone (410-660?km depth). Natural samples from the deep (>200?km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe3+/(Fe3++ Fe2+) increasing from 0.08 at approximately 240?km depth to 0.30 at approximately 500?km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe2+ to Fe3+ plus Fe0. Instead, the Fe3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe3+ contents of the inclusions.
Cerantola, V., Bykova, E., Kupenko, I., Merlini, M., Ismailova, L., McCammon, C., Bykov, M., Chumakov, A.I., Petitgirard, S., Kantor, I., Svityk, V., Jacobs, J., Hanfland, M., Mezouar, M., Prescher, C., Ruffer, R., Prakapenka, V.B., Duvbovinsky, L.
Dorfman, S.M., Potapkin, V., Lv, M., Greenberg, E., Kupenko, I., Chumakov, A.I., Bi, W., Alp, E.E., Liu, J., Magrez, A., Dutton, S.E., Cava, R.J., McCammon, C.A., Gillet, P.
American Mineralogist, Vol. 105, pp. 1030-1039. pdf
Mantle
redox
Abstract:
Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10-50% Fe/total cations, 0-25% Al/total cations, 12-100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4-3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.
Abstract: The Earth’s crust-mantle boundary, the Mohorovi?i? discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (?-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure-temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth’s mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth’s virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth’s geomagnetic data9, and especially in studies of planetary bodies that no longer have a dynamo10, such as Mars.
Kovach, V.,Salnikova, E., Wang, K-L., Jahn, B-M., Chiu, H-Y., Reznitskiy, L., Kotov, A., Lizuka, Y., Chung, S-L.
Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high grade complex, southeastern Siberia: implication for continental growth and evolution of the Central Asian orogenic belt.
Journal of Asian Earth Sciences, Vol. 62, pp. 18-36.
Abstract: Synthetic diamonds have inspired much interest for their unique photophysical properties and versatile potential applications, but their phosphorescent phenomenon and mechanism have been paid much less attention. Here, phosphorescent diamonds with a lifetime of 5.4?s were synthesized by high-pressure and high-temperature method, and the diamonds exhibit an emission band at around 468?nm under the excitation wavelength of 230?nm. The quantum yield of the phosphorescent diamonds is about 4.7% at ambient temperature and atmosphere, which is the first report on the quantum yield of diamonds. The unique phosphorescence emission can be attributed to the radiative recombination from iron related donors and boron related acceptors.
Geochemistry, Geophysics, Geosystems, Vol. 20, 9, pp. 4426-4456.
Russia
xenoliths
Abstract: This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500-700 °C. These fluids are derived from seawater that was transported to 30? to 50?km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka?Pacific Plate boundary that led to an increase in temperature and the formation of small quantities of melt that crystallized to a distinctive group of secondary minerals that are present in the samples and that postdate (overprint) the initial effects of fluid addition. The final step in the evolution of the samples was infiltration by an Fe? and Mg?rich magma that crystallized principally amphibole?group minerals.
Abstract: Nickel is a strongly compatible element in olivine, and thus fractional crystallization of olivine typically results in a concave-up trend on a Fo-Ni diagram. "Ni-enriched" olivine compositions are considered those that fall above such a crystallization trend. To explain Ni-enriched olivine crystals, we develop a set of theoretical and computational models to describe how primitive olivine phenocrysts from a parent (high-Mg, high-Ni) basalt re-equilibrate with an evolved (low-Mg, low-Ni) melt through diffusion. These models describe the progressive loss of Fo and Ni in olivine cores during protracted diffusion for various crystal shapes and different relative diffusivities for Ni and Fe-Mg. In the case when the diffusivity of Ni is lower than that for Fe-Mg interdiffusion, then olivine phenocrysts affected by protracted diffusion form a concave-down trend that contrasts with the concave-up crystallization trend. Models for different simple geometries show that the concavity of the diffusion trend does not depend on the size of the crystals and only weakly depends on their shape. We also find that the effect of diffusion anisotropy on trend concavity is in the same magnitude as the effect of crystal shape. Thus, both diffusion anisotropy and crystal shape do not significantly change the concave-down diffusion trend. Three-dimensional numerical diffusion models using a range of more complex, realistic olivine morphologies with anisotropy corroborate this conclusion. Thus, the curvature of the concave-down diffusion trend is mainly determined by the ratio of Ni and Fe-Mg diffusion coefficients. The initial and final points of the diffusion trend are in turn determined by the compositional contrast between mafic and more evolved melts that have mixed to cause disequilibrium between olivine cores and surrounding melt. We present several examples of measurements on olivine from arc basalts from Kamchatka, and several published olivine datasets from mafic magmas from non-subduction settings (lamproites and kimberlites) that are consistent with diffusion-controlled Fo-Ni behaviour. In each case the ratio of Ni and Fe-Mg diffusion coefficients is indicated to be?
Abstract: In this study, we present a number of experiments on the transformation of graphite, diamond, and multiwalled carbon nanotubes under high pressure conditions. The analysis of our results testifies to the instability of diamond in the 55-115 GPa pressure range, at which onion-like structures are formed. The formation of interlayer sp3-bonds in carbon nanostructures with a decrease in their volume has been studied theoretically. It has been found that depending on the structure, the bonds between the layers can be preserved or broken during unloading.
Journal of Geophysical Research, Vol. 122, 10.1002/2017JB014501
Mantle
thermodynamics
Abstract: We present a newly developed software framework, MMA-EoS, that evaluates phase equilibria and thermodynamic properties of multicomponent systems by Gibbs energy minimization, with application to mantle petrology. The code is versatile in terms of the equation-of-state and mixing properties and allows for the computation of properties of single phases, solution phases, and multiphase aggregates. Currently, the open program distribution contains equation-of-state formulations widely used, that is, Caloric-Murnaghan, Caloric-Modified-Tait, and Birch-Murnaghan-Mie-Grüneisen-Debye models, with published databases included. Through its modular design and easily scripted database, MMA-EoS can readily be extended with new formulations of equations-of-state and changes or extensions to thermodynamic data sets. We demonstrate the application of the program by reproducing and comparing physical properties of mantle phases and assemblages with previously published work and experimental data, successively increasing complexity, up to computing phase equilibria of six-component compositions. Chemically complex systems allow us to trace the budget of minor chemical components in order to explore whether they lead to the formation of new phases or extend stability fields of existing ones. Self-consistently computed thermophysical properties for a homogeneous mantle and a mechanical mixture of slab lithologies show no discernible differences that require a heterogeneous mantle structure as has been suggested previously. Such examples illustrate how thermodynamics of mantle mineralogy can advance the study of Earth's interior.
Abstract: High-Mg lavas are characteristic of the mid-Miocene volcanism in Inner Asia. In the Vitim Plateau, small volume high-Mg volcanics erupted at 16-14 Ma, and were followed with voluminous moderate-Mg lavas at 14-13 Ma. In the former unit, we have recorded a sequence of (1) initial basaltic melts, contaminated by crustal material, (2) uncontaminated high-Mg basanites and basalts of transitional (K-Na-K) compositions, and (3) picrobasalts and basalts of K series; in the latter unit a sequence of (1) initial basalts and basaltic andesites of transitional (Na-K-Na) compositions and (2) basalts and trachybasalts of K-Na series. From pressure estimation, we infer that the high-Mg melts were derived from the sub-lithospheric mantle as deep as 150 km, unlike the moderate-Mg melts that were produced at the shallow mantle. The 14-13 Ma rock sequence shows that initial melts equilibrated in a garnet-free mantle source with subsequently reduced degree of melting garnet-bearing material. No melting of relatively depleted lithospheric material, evidenced by mantle xenoliths, was involved in melting, however. We suggest that the studied transition from high- to moderate-Mg magmatism was due to the mid-Miocene thermal impact on the lithosphere by hot sub-lithospheric mantle material from the Transbaikalian low-velocity (melting) domain that had a potential temperature as high as 1510 °?. This thermal impact triggered rifting in the lithosphere of the Baikal Rift Zone.
Abstract: The Mokami and Saglek formations are comprised of Middle Eocene to Plio-Pleistocene deltaic deposits in the Labrador Sea, at the mouth of the Hudson Strait. In this study we use the provenance of KIM minerals to investigate the origin of these sediments. Fifty one mineral grains were obtained from Miocene to possibly Pliocene Mokami and Saglek formation strata by sub-sampling ocean cuttings from the Petro-Canada et al. Rut H-11 well. These grains were examined by optical methods, micro X-ray diffraction (?XRD) and Electron Probe Microanalysis (EPMA) at Western University for identification purposes, and 20 grains were determined to be of peridotitic mantle origin, based on the well-established compositional and mineral-formula discrimination criteria. The compositions of these Kimberlite Indicator Minerals (KIMs) have been compared to equivalent mineral grains from known Canadian kimberlite deposits, in a preliminary attempt to determine their provenance. Out of eleven garnets in the suite, nine garnets were classified as G9, thus establishing their lherzolitic mantle origin; one garnet was wehrlitic (G12), and one garnet was crustal (G0) (Fig 1A). The presence of G9 garnets, however, does not indicate provenance, as G9 garnets are ubiquitous in the mantle. Three Cr-diopside grains were found in the suite. They all passed compositional and mineral-formula criteria established by Ziberna et al. (2016) to be recognized as peridotitic. On Al+Cr-Na-K versus Ca/(Ca+Mg+Fe) plots (e.g. Grütter 2009, Fig. 4), these grains plotted in a region occupied by both garnet peridotite and spinel-garnet peridotite, such that formation in the presence of garnet is confirmed, but the type of peridotite is not definitive. These grains were used to calculate P-T conditions of formation using the Nimis and Taylor (2000) thermobarometer, and the Cr-diopside grains revealed P-T formation conditions ranging from 1304-1417 °C and 4.5-5.2 GPa (Fig 1B). These grains plot in the P-T region representing an extension of that occupied by both Somerset and Kirkland Lake kimberlites, however, calculated temperatures significantly above 1300 °C should be treated the caution because this has not been reported for Cr-diopside from any Canadian kimberlites. It is worth noting that the Cr-diopside grains definitively do not match those from the Chidliak kimberlites, although that kimberlite field is located geographically proximal to the Saglek deposit. Seven orthopyroxene grains found in the suite had compositions matching kimberlites from the Slave craton (Fig. 1C). This provenance agrees with the paleo-drainage pattern of the Bell River basin, which extended from the Northern Interior plains to the Sea of Labrador until the late Pleistocene.
Brovarone, A.V., Butch, C.J., Ciappa, A., Cleaves, H.J., Elmaleh, A., Faccenda, M., Feineman, M., Hermann, J., Nestola, F., Cordone, A., Giovannelli., D.
American Mineralogist, Vol. 105, pp. 1152-1160. pdf
Mantle
carbon
Abstract: Water plays a key role in shaping our planet and making life possible. Given the abundance of water on Earth's surface and in its interior, chemical reactions involving water, namely hydration and dehydration reactions, feature prominently in nature and are critical to the complex set of geochemical and biochemical reactions that make our planet unique. This paper highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, biology, and man-made materials, as well as their connections to carbon cycling on our planet.
Abstract: The number of studies investigating the vesiculation of natural samples and their implications to volcanic degassing and eruption mechanisms has been growing rapidly within the last decades. To interpret the natural rock textures, the geoscience community has produced a range of experimental and theoretical data sets on bubble nucleation, growth, and coalescence in magmatic systems. A robust experimental database is required to calibrate (theoretical and empirical) modeling approaches, which allow the calculation of magma ascent rates from volcanic ejecta mainly by the determination of the bubble number density (BND). Although, the available data set is still limited, it already shows that variations in melt (and volatile/fluid) composition can have a significant effect. In this manuscript we (re-)evaluate the existing experimental data set, while focusing mainly on the review and discussion of continuous decompression experiments. One aim of this review article is to encourage scientists to fill the gaps in the existing experimental data sets and help to acknowledge, use, and further develop the most promising experimental techniques. Therefore, we highlight different methods and discuss their advantages and possible limitations. We also discuss possible ways of how to better account for the influence of melt composition in models, which link BND to decompression rate.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 252-262.
Mantle
geochemistry
Abstract: The Earth and other terrestrial planets formed through the accretion of smaller bodies, with their core and mantle compositions primarily set by metal -silicate interactions during accretion. The conditions of these interactions are poorly understood, but could provide insight into the mechanisms of planetary core formation and the composition of Earth's core. Here we present modeling of Earth's core formation, combining results of 100 N-body accretion simulations with high pressure -temperature metal -silicate partitioning experiments. We explored how various aspects of accretion and core formation influence the resulting core and mantle chemistry: depth of equilibration, amounts of metal and silicate that equilibrate, initial distribution of oxidation states in the disk, temperature distribution in the planet, and target:impactor ratio of equilibrating silicate. Virtually all sets of model parameters that are able to reproduce the Earth's mantle composition result in at least several weight percent of both silicon and oxygen in the core, with more silicon than oxygen. This implies that the core's light element budget may be dominated by these elements, and is consistent with ?1 -2 wt% of other light elements. Reproducing geochemical and geophysical constraints requires that Earth formed from reduced materials that equilibrated at temperatures near or slightly above the mantle liquidus during accretion. The results indicate a strong tradeoff between the compositional effects of the depth of equilibration and the amounts of metal and silicate that equilibrate, so these aspects should be targeted in future studies aiming to better understand core formation conditions. Over the range of allowed parameter space, core and mantle compositions are most sensitive to these factors as well as stochastic variations in what the planet accreted as a function of time, so tighter constraints on these parameters will lead to an improved understanding of Earth's core composition.
Abstract: Alfred Wegener (1880-1930) was a leading explorer, geophysicist, and meteorologist from Germany, and pioneer in the exploration of Greenland (1). His seminal volume of meteorology (2) is universally considered a fundamental manual for this discipline and was long a reference text for students and specialists. Wegener's preeminent scientific legacy, however, is the continental drift hypothesis. He first presented his thesis to the scientific community on 6 January 1912 at the Geological Society of Frankfurt am Main, but his famous monograph on the subject dates to 1915 (3).
Abstract: The CIM Mineral Exploration Best Practice Guidelines (the Exploration Guidelines) have been prepared to assist professional geoscientists and engineers to conduct consistently high-quality work in order to maintain public confidence. The Exploration Guidelines are meant to assist professional geoscientists and exploration practitioners in planning, supervising, and executing exploration programs. In Canada, there are generally two types of public resource reporting: “Disclosure”, as defined by NI 43-101, is the reporting of technical information to the public and market participants for securities legislation purposes where a Qualified Person (QP) must be involved, and reporting of exploration information for governmental agencies to support obligations under laws including the Mining Acts of each of the Provinces and Territories. The Exploration Guidelines are also relevant where the results will not be publicly reported but are intended for internal company use. While this document is intended as guidance for work conducted or supervised by geoscientists in Canada, many of the practices described herein can be adapted to mineral exploration activities in other countries. The Exploration Guidelines are not intended to inhibit original thinking, or to prevent the application of new approaches that may develop into fundamental components of successful mineral exploration programs. Rather than provide prescriptive solutions to specific issues, they include general guidelines for current professional practice and to demonstrate and defend the merits of new methods. These guidelines do not preclude individuals and companies from developing more detailed guidelines specific to their own requirements. The initial version of the Exploration Guidelines was prepared by the Canadian Institute of Mining and Metallurgy and Petroleum (CIM) Exploration Best Practices Committee and adopted by CIM Council on August 20, 2000. On January 9, 2018, CIM Council formed a new committee, the Mineral Resources and Reserves Committee (CIM MRMR Committee) with a mandate to, among other things, update the Exploration Best Practice Guidelines. The mandate for the committee was accepted by CIM Council on March 2, 2018. The new Exploration Guidelines document was adopted by the CIM Council on November 23, 2018.
Abstract: A high-temperature rheometer equipped with a graphite furnace, characterized by an air-bearing-supported synchronous motor, has been enhanced by a custom-made Pt-Au concentric cylinder assembly. With this adaptation, viscosity measurements of highly fluid melts can be achieved at high temperatures, up to 1273 K. Due to the air-bearing-supported motor, this apparatus can perform measurements of extremely low torque ranging between 0.01 ?Nm and 230 mNm (resolution of 0.1 nNm), extending the typical range of viscosity measurements accessible in the present configuration to 10?3.5-103.5 Pa•s and shear rates up to 102 of s?1. We calibrated the system with distilled water, silicone oils, and the DGG-1 standard glass. We further present new data for the viscosity of Na2CO3, K2CO3, and Li2CO3 liquids. Finally, a comparison between our results and literature data is provided, to illustrate the effect of chemical composition and oxygen fugacity on the viscosity of alkali carbonate melts, which serve as analogs for both carbonatitic melts and molten carbonates of industrial relevance. This study substantially improves the database of alkali carbonate melts and dramatically increases the accuracy with respect to previous measurement attempts. The very low viscosity range data and their temperature dependence also helps to constrain very well the activation energy of these highly fluid systems and confirms the estimate of a universal pre-exponential factor for non-Arrhenian viscosity-temperature relationships.
Abstract: The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440?Ma), and was subsequently impacted by a late-stage hydrothermal event (~232?Ma) involving REE-rich fluids/melt. This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by ?11B values that vary between ?7 (typical asthenospheric mantle) and?+?4‰; ?11B values and B abundances (~0.2 to ~1?ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300?Ma old) carbonatites worldwide.
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first ?11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and ?13CPDB (?5.37 to ?4.85‰) and ?18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The ?11B values for carbonates from Felix, Gum, and Howard Creek vary between ?8.67 and ?6.36‰, and overlap the range for asthenospheric mantle (?7.1?±?0.9‰), whereas two samples from Fir yield heavier values of ?3.98 and ?2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like ?11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Chemical Geology, doi.org/10.1016/j.chemgeo.2019.07.015 59p.
Canada, British Columbia
carbonatite - Blue River
Abstract: This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first ?11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and ?13CPDB (?5.37 to ?4.85‰) and ?18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The ?11B values for carbonates from Felix, Gum, and Howard Creek vary between ?8.67 and ?6.36‰, and overlap the range for asthenospheric mantle (?7.1?±?0.9‰), whereas two samples from Fir yield heavier values of ?3.98 and ?2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300?Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like ?11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.
Cimen, O., Corcoran, L., Kuebler, C., Simonetti, S.S., Simonetti, A.
Geochemical, stable ( O, C, and B) and radiogenic ( Sr, Nd, Pb) isotopic data from the Eskisehir-Kizulxaoren ( NW-Anatolia) and the Malatya-Kuluncak ( E- central Anatolia) F-REE-Th deposits, Turkey: implications for nature of carbonate-hosted mineralizati
Turkish Journal of Earth Sciences, Vol. 29, doe:10.3906/yer-2001-7 18p. Pdf
Turkish Journal of Earth Sciences, Vol. 29, pp. 798-814. pdf
Europe, Turkey
REE
Abstract: In Turkey, the largest fluorine (F)-rare earth element (REE)-thorium (Th) deposits are located within the Eski?ehir-K?z?lcaören (north-western Anatolia) and the Malatya-Kuluncak (east-central Anatolia) regions, and these are associated with Oligocene extensional alkaline volcanic and Late Cretecaous-Early Paleocene postcollisional intrusive rocks, respectively. In the K?z?lcaören region, the basement units include the Triassic Karakaya Complex and the Late Cretaceous oceanic units (Neotethyan suture) that are cut and overlain by phonolite and carbonatite intrusions and lava flows. In the Kuluncak region, the plutonic rocks are mainly composed of syenite, quartz syenite, and rare monzonite, and these cut the late-Cretaceous Karap?nar limestone, which hosts the F-REE-Th mineralization in contact zones. A carbonatite sample from the K?z?lcaören region displays both a total rare earth element (TREE) concentration (4795 ppm) and ?11B (-6.83‰) isotope composition consistent with mantle-derived carbonatite; whereas it is characterized by heavier ?13C (+1.43‰) and ?18O (+20.23‰) isotope signatures compared to those for carbonatites worldwide. In contrast, the carbonates which host the F-REE-Th mineralization in the Kuluncak region are characterized by lower TREE concentrations (5.13 to 55.88 ppm), and heavier ?13C (-0.14 to -0.75‰), ?18O (+27.36 to +30.61‰), and ?11B (+5.38 to +6.89‰) isotope ratios compared to mantle-derived carbonatites. Moreover, the combined initial 87Sr/86Sr (0.70584 to 0.70759) and 143Nd/144Nd (0.512238 to 0.512571) isotope ratios for samples investigated here are distinct and much more radiogenic compared to those for carbonatites worldwide, and therefore indicate significant crustal input and/or hydrothermal metasomatic-related alteration. Overall, stable and radiogenic isotope data suggest that the host carbonate rocks for the F-REE-Th mineralization in both the K?z?lcaören and the Kuluncak regions consist of hydrothermally metasomatized carbonatite and limestone, respectively. The mineralization in the K?z?lcaören region may potentially be related to carbonatite magmatism, whereas the mineralization in the Kuluncak region, which most likely formed through interactions between the plutonic rocks and surrounding limestone at contact metamorphism zone, involved hydrothermal/magmatic fluids associated with extensive postcollisional magmatism.
Abstract: After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the re
