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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 d13Cd13C 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 d13C 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 d13C values in the range -4 to -6‰ from mantle-derived fluids with an average d13C 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 d13Cd13C, d15Nd15N 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 µW/m3 (n = 3) whereas mafic granulites have an average heat production of 0.13 ± 0.03 µW/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 µW/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.
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.
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 µm) 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: 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) Ĺ, b = 90.3556(7)°, V = 1076.50(5) Ĺ3, Z = 2. The ten strongest lines in the powder X-ray diffraction pattern are [dobs in Ĺ (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)S4.15Fe0.97(Ti5.68Nb0.22Si0.04)S5.94O18(OH)2. The mineral is monoclinic, space group C2/m, a = 5.290(2), b = 14.575(6), c = 5.234(2) Ĺ, ß = 97.233(7)°, V = 400.4(5) Ĺ3, Z = 1. The ten strongest lines in the X-ray micro-diffraction pattern are [dobs in Ĺ (I) hkl]: 2.596 (100) 002; 1.935 (18) 170; 1.506 (14) 133; 1.286 (13) 1.11.0; 2.046 (12) 2I41; 1.730 (12) 003; 1.272 (12) 0.10.2; 3.814 (11) 1I11; 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 ŕ 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 eNd(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 d26MgDSM3 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) Ĺ, ? = 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 eNd(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 d26MgDSM3 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 eNd(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 d13CPDB 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 d13CPDB (-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 d13CPDB 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.
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: 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 (d44/40Ca and d44/42Ca 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 d44/40Ca 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 d44/40Ca values. The lack of correlations between d44/40Ca and Mg# as well as La/Yb(N) values suggest that the variability in d44/40Ca is not controlled by the degree of partial melting. The d44/40Ca values of the carbonatites (0.58-1.1‰) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the d44/40Ca 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 d44/40Ca of the carbonatites. In plots of d44/40Ca versus eNd(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 eNd(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 d44/40Ca and highly radiogenic 87Sr/86Sr(t) is explained by hydrothermal alteration which is also invoked for the formation of massive fluorspar deposits with high d44/40Ca (1.44‰) at Ambadongar. In a plot of d44/40Ca versus K/Rb, the carbonatites plot towards the phlogopite end-member (d44/40Ca?=?1‰, K/Rb?=?40-450) while the igneous silicate rocks plot towards the amphibole end-member (d44/40Ca?=?0.44‰, K/Rb >1000). Phlogopite, especially if F-rich, is stable at greater depths in the mantle compared to amphibole. Hence, the correlated d44/40Ca 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 eNdinitial (- 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 (2s ), 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 (eNd(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 eNd(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.
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° - 784şC and aSiO2 in the range of 0.41 - 0.44. With decreasing temperature (~ 575şC) 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 547ş - 455şC 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: 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 d 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. d44/40Ca 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). d44/40Ca 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 d44/40Ca 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.
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 eNd(T) values of 2.1 ± 0.6 to 6.7 ± 0.3 whereas the ~ 90 Ma perovskites have enriched eNd(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’ eNdinitial (-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 (2s ), 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 (eNd(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 eNd(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.
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 eNd 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 .
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 Boli´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 sourceQµ-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%; SREE, 1025-12 317 ppm) and ferrocarbonatite (CaO, 15.6-31 wt%; BaO, 0.3-7 wt%; SREE, 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 eNd(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: 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 ˛°6Pb/˛ł8U 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 × 10ą8 g?ą (GZ7) and 2.53 × 10ą8 g?ą (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 µg g?ą (GZ7) and 1305 µg g?ą (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 µg g?ą ± 0.18 µg g?ą; 95% confidence uncertainty) may prove useful as reference material for Ti-in-zircon temperature estimates.
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. 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 µm 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 (<- 1%) at comparable depths. The P velocity increases from - 0.5% above 70 km depth to 1.5% at 330 km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160 km) and Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.
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 µm 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.
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 d13C values between -28 and -8 ‰, d15N 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 d15N decoupled from d13C 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.
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’ eNdinitial (-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?
