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Despite over 400 occurrences of kimberlites and related rocks in India, mantle-derived xenoliths are known only from a few occurrences. This paucity of mantle-derived xenoliths in Indian kimberlites has hampered investigations of the subcontinental lithospheric mantle (SCLM). Using a valuable selection of the rare xenolith inventory, we here report Fe3+/ΣFe measurements for garnets using the electron microprobe (EPMA) flank method, targeting six mantle eclogite xenoliths (KL2 pipe) and fourteen peridotitic garnet xenocrysts (P9 and P10 hypabyssal intrusions) from the Wajrakarur kimberlite field (WKF) on the Eastern Dharwar craton (EDC). These data provide some of the first direct constraints on the oxygen fugacity (fO2) of the lithospheric mantle beneath the Indian subcontinent. The measured Fe3+/ΣFe ratios vary between 0.02 and 0.05 (± 0.01) for the eclogite xenoliths and between 0.02 and 0.10 (± 0.01) for the peridotitic garnets. Calculated ΔlogfO2 values for the KL2 eclogites show a wide range from FMQ-3.9 to FMQ-0.9 (± 0.6), straddling the boundary between the diamond and carbonate stability fields. In terms of redox compositions, it appears that the KL2 eclogites are able to host diamond, which is consistent with the diamondiferous nature of this particular WKF locality and the presence of eclogitic garnet inclusions in diamonds from the nearby TK4 kimberlite body. The peridotitic garnet xenocrysts from the P9 and P10 kimberlite bodies, which were entrained between ~ 125 and 170 km depth, reveal ΔlogfO2 values between FMQ-4.5 and FMQ-2.6 (± 0.9). Garnet xenocrysts with 'normal' REE patterns exhibit higher Fe3+/ΣFe ratios compared to garnets with 'sinusoidal' REE patterns. Importantly, the Fe3+/ΣFe ratios of garnet xenocrysts with 'normal' REE patterns (~ 125–160 km depth) correlate with metasomatic Ti–Y–Zr–V enrichment, which suggests metasomatism-driven oxidation of the cratonic mantle at mid-lithospheric depths. Such melt-related mantle metasomatism was probably diamond-destructive within the otherwise diamond-fertile lithospheric keel. The observed wide range of ΔlogfO2 values for the Dharwar cratonic mantle lithosphere allows for stabilization of various metasomatic phases (e.g., amphiboles, micas, carbonates) that may have formed (or concentrated in) distinctly different metasome assemblages within the continental root that underpins Peninsular India. Changing the relative contributions from such highly diverse volatile-rich metasomes may explain the spatiotemporal association of kimberlites and various diamond-bearing potassic magma types such as orangeites, ultramafic lamprophyres and lamproites, a scenario that is influenced by the redox composition of the Dharwar craton root. [ABSTRACT FROM AUTHOR]

Polycomponent condensate glasses found in nature provide an insight into condensation mechanisms, which are still understood inadequately poorly. Condensate glasses found in the impactites of the Lonar crater contain nanosized inclusions of metallic Fe, Cr, Cu, Zn, Ag, In, Te, Au, Pt, and Bi, along with Fe, Cu, and Zn sulfides. This combination may be indicative either of a brief condensation window for the almost simultaneous condensation of components with so different fugacity or of a possible mechanism of cluster condensation, provided that the condensation temperatures of such clusters are close. [ABSTRACT FROM AUTHOR]

We present field, petrographic and mineral compositions of biotite, amphibole, and feldspars from Neoarchean Nizamabad granites from the northeastern part of the Eastern Dharwar Craton, southern India. These granites are classified as hornblende biotite granite (HBG), biotite granite (BTG), monzogranite (MG), and microgranular enclaves (ME) hosted in HBG and BTG. The temperature estimates using amphibole and biotite thermometry exhibit similar results, with higher temperatures for HBG (818 to 859 ±22°C) and ME (800 to 855 ±22°C), and slightly lower temperatures for BTG and MG (829 to 830 ±12°C and 820 to 829 ±12°C). Based on barometry, HBG amphiboles crystallized at pressures between 363 and 448 ±60 MPa (avg. PHBG = 398 MPa), whereas the MEs crystallized at pressures between 313 and 438 ±60 MPa (avg. PME = 386 MPa). The estimated pressures suggest that these granites crystallized at depths of 14–15 km, corresponding to the upper to mid-continental crust. The amphibole compositions reveal that these granites crystallized from a water-rich magma, with >5 wt.% H2O and evolved under high oxidizing conditions NNO + 2 (Nickel–Nickel–Oxide), corresponding to magnetite (oxidized) series granites. The amphibole and biotite compositions suggest a crust-mantle mixed source for HBG, ME, and BTG, while the MG is purely crustal derived. The water-rich and highly oxidizing conditions of the parental magmas rule out a lower crustal granulitic source for the Nizamabad granites. The amphibole and biotite compositions suggest their crystallization from calc-alkaline parental magma in a subduction setting at high oxygen fugacity (fO2) conditions. This study infers the role of convergent margin tectonics in the emplacement of these granites, and their compositional variability is attributed to crust–mantle interactions in this domain of the Eastern Dharwar Craton. Research Highlights: The granitoids from the NE part of the Eastern Dharwar Craton are characterized as hornblende biotite granites (HBG), biotite granites (BTG), monzogranite (MG), and mafic enclave (ME). Estimated pressures suggest that these granites crystallized at depths of 14 to 15 km, corresponding to the upper to mid-continental crust. The water-rich (> 5 wt.% H2O content) and high oxidizing conditions (NNO+2) of HBG, BTG, and ME corresponds to magnetite (oxidized) series granites. The amphibole and biotite compositions from the Nizamabad granites suggest the crust-mantle mixed source for HBG, ME, and BTG, while the MG are purely crustal derived. The compositions of these granites suggest their crystallization from calc-alkaline parental magma in subduction settings at high oxygen fugacity (fO2) conditions. The study infers the role of convergent margin tectonics in the emplacement of compositionally variable granitoids in the NE part of the Eastern Dharwar Craton. [ABSTRACT FROM AUTHOR]

One atmosphere experimental studies have been conducted on two amygdule‐free representative samples from Khandwa, Madhya Pradesh, Central India, belonging to Eastern Deccan Volcanic province in order to understand mineral petrogenesis, crystallization history of the parent magma and role of liquid immiscibility. One atm experiments were carried out on these two samples at specified temperatures on designated time in order to understand crystallization history. The run products were identified both by optical means and electron microprobe analysis. The identified run products include pyroxenes, feldspar, opaques phase (oxide and sulphide), silica phases, and glass (both brown and colourless variety). The run product pyroxenes belong to quadrilateral pyroxene, and species‐wise correspond to augite, clinoenstatite, and pigeonite. Feldspars in majority are found to be anorthoclase (which are interpreted to have been formed during sudden crystallization in contact with water at shallow depth during experiments) and less commonly calcic plagioclase (in the range of labradorite to anorthite). Opaque phases have both oxide and sulphide, namely hematite, magnetite, martite, and pyrite, respectively. The zoned nature between the oxide and sulphide phases is suggestive of oscillation of oxygen fugacity during crystallization. The run product silica phase corresponds to tridymite. The silica‐rich glass (colourless) is overwhelmingly dominant, whereas silica‐poor glass (brown glass) is a manifestation of liquid immiscibility. The silica‐rich glasses give rise to direct crystallization of tridymite, whereas silica‐poor glasses act as the main driving liquid (or parent liquid) to further the crystallization towards lower temperature. The crystallization behaviour of the parent magma, and liquid immiscibility can be well documented with reference to forsterite‐silica phase diagram; in this phase diagram, the parent liquid corresponds to normative Qtz: 2.542, Fo: 0.098, and En: 0.34. The ambient parent liquid plots very close to Ab apex with reference to the Di‐Ab‐An phase diagram. The clear and distinct zonation pattern present in the opaque phases signifies that crystallization straddles between both oxidizing and reducing condition yielding crystallization of both oxide and sulphide phases. Our study indicates a unique role of liquid immiscibility of the parent magma supplemented by oscillation of oxidizing and reducing conditions giving rise to ensemble of variety of glass compositions, silicate, and opaque phases. [ABSTRACT FROM AUTHOR]

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity (XH2O) conditions obtained with the winTWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log fO2 values ranging from −11.38 to −14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks. [ABSTRACT FROM AUTHOR]

Magmatic epidotes of granitoid pluton from North Purulia Shear Zone, eastern India, are identified by textural and chemical criteria. The accessory sphene, epidote, allanite and magnetite in the granitoid indicate high fO2 during crystallization. Hornblendes were crystallized between 1.2 and 4.8 kbar, 753°C and 783°C as well as high fO2 (>Ni-NiO buffer). Preservation of magmatic epidote in pluton emplaced at low pressure is due to rapid magma ascent (>3.1 km/year). Fast upward transportation of parental magma of the pluton took place through extensional voids along the regional shear zone. [ABSTRACT FROM AUTHOR]

Gold mineralization in Kundarkocha gold deposit occurs in the eastern Indian Craton that is hosted by sheared quartz–carbonate–sulfide veins emplaced within the graphitic schist, carbonaceous phyllite and talc–chlorite–serpentine schist belongs to Gorumahisani–Badampahar schist belt of Iron Ore Group. Gold mineralization exhibits both lithological and structural controls in the study area, albeit the stratigraphic control is more ubiquitously observed. Detailed mineralogical characterization coupled with electron probe microanalysis of the sulfide phases reveal the occurrences of gold in three distinct forms (i) as lattice-bound form within sulfides especially enriched in arsenopyrite, loellingite, pyrite, pyrrhotite and chalcopyrite in decreasing order of abundance; (ii) as micro inclusions or nano-scale gold inclusions within pyrite and arsenopyrite especially along the growth zones and micro-fractures as substrates and (iii) as free milling nugget gold grains either along the grain boundaries of sulfides or within the host rocks. Three generations of pyrite (Py-I, Py-II and Py-III) and arsenopyrite (Asp-I, Asp-II, Asp-III) have been identified based on textural, morphological characteristics and mineral chemistry. The lattice-bound gold content in pyrite and arsenopyrite varies from 600 to 2700 ppm and 900 to 3600 ppm respectively and increase in concentration of such refractory gold is seen in the order of chalcopyrite > pyrrhotite > pyrite > loellingite/arsenopyrite. The evolutionary stages of different forms of gold include remobilization of the lattice-bound grains in pyrite and arsenopyrite (Py-I and Asp-I) and re-concentration along the zoned-pyrite and arsenopyrite (Py-II and Asp-II) and ultimately as native gold/nuggets surrounding the sulfides as well as within the main mineralized zone. Lattice-bound gold distribution could have resulted due to metamorphic devolatilization reactions which are further aided by the influx of hydrothermal fluids. These reactions along with additional input of hydrothermal fluid paved the way for expulsion of lattice-bound gold from sulfides to concentrate as nuggets/main lode within shear fractures channelizing the fluid flow. Thermometry results of the arsenopyrite–pyrite–pyrrhotite assemblage yielded a temperature range from 375 to 390 °C which is the ideal condition for gold precipitation. Native silver, gersdorffite and arsenian-ullmannite are being reported for the first time from this deposit indicating the complexity and wide variety of mineral phases associated with gold implying magmatic-hydrothermal input of the source fluid. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biotite and uraninite related to IOCG mineralization are associated with redox sensitive minerals such as magnetite, ilmenite, and graphite. • Calculation of Fe3+ in biotite using methods of Dymek (1983) and Guidotti and Dyar (1991) • Mineral association, Fe3+ content of biotite, and U/Th ratio of uraninite point to redox conditions reaching below the Fayalite-magnetite-quartz (FMQ) buffer. • The presented method could be useful for magnetite-poor IOCG systems. Key constraints related to the nature of alteration and mineralization in geologically complex and economically important iron oxide copper–gold (IOCG) systems remain highly speculative, especially, the physicochemical characteristics of the hydrothermal fluids. This study was conducted on samples from the Khetri Copper Belt (KCB) in western India that hosts several IOCG-type deposits. The study aims to constrain qualitative oxygen fugacity conditions during alteration-mineralization events using biotite and uraninite chemistry. The inferred redox conditions are backed up by the presence or absence of redox-sensitive minerals including magnetite, ilmenite, and graphite. Biotite is present in association with REE ± Th ± U and Cu-Co-U-REE mineralization, which has a genetic connection with IOCG-style K-Fe-Mg alteration. The first type of mineralization is mostly sulfide-free and is characterized by the presence of REE-silicate, REE-phosphate, U-Th-silicate, and U-oxides with minor ilmenite, magnetite, and pyrite. The other type is characterized by the presence of Cu-Co-Fe sulfides, U-oxides, REE-silicates and -phosphates with significant volume of graphite. The Fe3+ in biotite was calculated from electron probe data by two different methods, and U and Th concentrations in uraninite were measured by an electron probe microanalyzer (EPMA). Stability of redox-sensitive phases (magnetite, ilmenite, and graphite), Fe3+-Fe2+-Mg content of biotite, and U/Th ratio of uraninite collectively suggest redox conditions essentially below the haematite-magnetite buffer often reaching the fayalite-quartz-magnetite buffer. We conclude that the studied alteration-mineralization assemblages represent the deeper and reduced facies of the Khetri IOCG system. We suggest that biotite and uraninite chemistry can be used as an indicator of redox conditions for alteration/mineralization assemblages in IOCG systems where redox-sensitive minerals particularly magnetite is absent. [ABSTRACT FROM AUTHOR]

Along the east coast of the Andaman Islands, abundant detrital chromian spinels frequently occur in black sands at the confluence of streams meeting the Andaman Sea. The mineral chemistry of these detrital chromian spinels has been used in reconstructing the evolutionary history of the Andaman ophiolite. The chromian spinels show wide variation in compositional parameters such as Cr# [= Cr/(Cr + A1) atomic ratio] (0.13-0.91), Mg# [= Mg/(Mg + Fe2+) atomic ratio] (0.23-0.76), and TiO2 (<0.05-3.9 wt%). The YFe3+[= 100Fe3+/(Cr + A1 + Fe3+) atomic ratio] is remarkably low (usually <10 except for south Andaman). The ranges of chemical composition of chromian spinels are different in each locality. The spinel compositions show very depleted signatures over the entire island, which suggests that all massifs in the Andaman ophiolite were affected under island-arc conditions. Although the degree of depletion varies in different parts of the island, a directional change in composition of the detrital chromian spinels from south to north is evident. Towards the north the detrital chromian spinels point to less-depleted source rocks in contrast to relatively more depleted towards the south. The possibilities to explain this directional change are critically discussed in the context of the evolution of Andaman ophiolite. [ABSTRACT FROM AUTHOR]

Abstract: We document complex intergrowth involving spinel–ilmenite–magnetite–hematite–corundum–rutile in various combinations from a suite of granulite facies rocks of Eastern Ghats Belt, India. Individual oxide phase shows considerable compositional variation among textures and samples. These textures arguably developed by oxidation reaction of early spinel and ilmenite solid solution from near-peak to subsequent retrogressive stages. Oxygen fugacity is measured at a constant pressure of 8 kbar and different temperatures estimated from geothermometric analyses involving oxide and silicate-bearing equilibria in different samples. The calculated fO2 values are 2–3 log units higher than the QFM buffer except for one sample. Uncertainties in fO2 calculation in some samples arise presumably due to extensive compositional readjustment of different oxide systematics at lower temperatures. The persistence of high fO2 in mineral assemblages could be inherited from an oxidized precursor, but field evidence and presence of H2O–CO2-rich fluid inclusions in the studied samples imply possible involvement of externally-derived oxidizing fluid in the later part of the retrograde history. Recent experimental and natural data suggest CO2 charged brine solution could be suitable for oxidation of mineral assemblages. Presence of brine inclusions are not detected, but fluorine enriched nature of biotite grains could possibly provide some indication for its presence. The mixed fluid possibly has its source in the crystallizing mafic magma emplaced at the lower crust that is exposed in the adjacent area. [Copyright &y& Elsevier]