Chromite Inclusions Research Articles

Characterization of bulk interior and fusion crust of Calama 009 L6 ordinary chondrite

AbstractFragment of Calama 009 L6 ordinary chondrite recovered in the Atacama Desert was chosen for a complex study of the bulk interior and the fusion crust by scanning electron microscopy (SEM) with energy‐dispersive spectroscopy (EDS), X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. SEM demonstrated the presence of Fe‐Ni‐Co grains, troilite and chromite inclusions in both the bulk interior and the fusion crust as well as many veins with ferric compound. EDS showed variations in the Ni concentration within the metal grains and within one metal phase in the grain. XRD revealed some differences in the contents of various phases in the bulk interior and in the fusion crust. XRD indicated the presence of magnesioferrite in the fusion crust as well as the formation of goethite nanoparticles with the mean size of 9 nm in both the bulk interior and the fusion crust. Magnetization measurements demonstrated the ferrimagnetic–paramagnetic phase transition in chromite at 44 K and low values of the saturation magnetization moments (6.46 and 3.26 emu g−1 at 100 K) for the bulk interior and the fusion crust, respectively, due to the lack of Fe‐Ni‐Co alloy as a result of weathering. The Mössbauer spectra of the bulk interior and the fusion crust showed some differences in the number and relative areas of spectral components. The revealing of the Mössbauer spectral components related to 57Fe in the M1 and M2 sites in olivine and orthopyroxene as well as determining the Fe2+ occupations of these sites from XRD permitted us to estimate the temperature of equilibrium cation distribution for these silicates which are (i) 662 K (XRD) and 706 K (Mössbauer spectroscopy) for olivine and (ii) 893 K (XRD) and 910 K (Mössbauer spectroscopy) for orthopyroxene.

To the transition zone and back: Records from diopside exsolution lamellae in chromite from Archean ophiolitic podiform chromitites

Plate tectonics and deep subduction provide the conveyor belt for elemental cycling between Earth's surface and deep interior. Here, we report Archean diopside exsolution lamellae in chromite grains from 2.5 billion years old (Ga) subducted then exhumed Archean oceanic crustal fragments in ophiolitic mélange. Diopside exsolution lamellae are a key indicator of a precursor ultra-high pressure (UHP) phase of chromite, known previously only from Phanerozoic subduction zones, indicating deep subduction and elemental cycling between the surface and deep Earth. These lamellae, along with trapped dolomite, and UHP TiO2(II)-bearing multi-phase solid crustal mineral inclusions in the host chromite, suggest that a precursor chromite polymorph (chenmingite) with a crystal structure stable only at conditions in the mantle transition zone (> 413 to 660 km) preserve the history of subduction, entrapment of UHP crustal mineral inclusions, and return flow to the surface in an Archean subduction/forearc system. These nano-scale diopside lamellae and UHP TiO2(II)-bearing multi-phase solid inclusions are strong evidence for deep and steep subduction in the Neoarchean, and early operation of the deep crustal material recycling, linking surface and deep environments, forging a habitable world.

FTIR study of H2O in silicate minerals and mineral inclusions in chromite from the peridotite zone of the Stillwater complex (Montana, USA): Evidence for chromitite formation in an H2O-rich environment

Although the involvement of hydrous fluids has been frequently invoked in the formation of stratiform chromitites in layered intrusions, there is a lack of natural evidence to signify their presence and mechanism. Here, Fourier-transform infrared spectroscopy (FTIR) of H2O in silicate minerals in the lowermost layer and G chromitite layer of the Stillwater complex, Montana, USA, shows that olivine grains have 20−55 ppm H2O, orthopyroxene has 30−45 ppm H2O, and clinopyroxene has 144−489 ppm H2O. The jointly increasing H2O contents of olivine and orthopyroxene in silicate cumulates along with magma differentiation record a negative correlation in chromitites. On the basis of poikilitic clinopyroxene, we calculated that the interstitial melts had averages of 1.3 wt% and 2.3 wt% H2O in dunite and chromitite, respectively, showing significant differences between chromitites and silicate cumulates. More than 10% of the chromite grains contained polymineralic inclusions up to 100 μm in size that were composed mainly of orthopyroxene, hornblende, plagioclase, and phlogopite. Most of these minerals were characterized by higher MgO and fluid-mobile element contents, such as Na and K, than minerals in associated silicates. Based on the mineral modes of the hydrous phases and their compositions, the trapped fluids contained ∼2.6 wt% H2O, consistent with the FTIR estimates, indicating the inclusion compositions represent interstitial melts instead of parental magmas. These observations indicate that the chromite microlites collected fluids during early crystallization, leading to a heterogeneous fluid redistribution in the melt. The fluids were collected on the surface of chromite grains during crystallization and then dissolved into poikilitic pyroxene. Chromite grains could also be efficiently floated by these fluids, causing them to migrate away from the silicate minerals in the magma channel and leading to the formation of nearly monomineralic chromitite seams. This process serves as a kinetic model indicating that chromite could be completely separated from silicates during mechanical sorting in layered intrusions.

Behavior of critical metals in cumulates of alkaline ultramafic magmas in the Siberian large igneous province: Insights from melt inclusions in minerals

Ultramafic-alkaline-carbonatite complexes (UACC), which are formed from mantle-derived carbonated alkali-ultramafic melts in large igneous provinces (LIPs), are important resources of Fe, Ti, U, Th, Nb, rare earth elements (REEs), Cu, Ni and platinum group elements (PGEs). Concentration of these metals and ore formation is assumed to be largely controlled by magmatic differentiation and post-magmatic hydrothermal processes. Although basic patterns of the metals’ partitioning during formation of the UACCs are constrained by geochemical and mineralogical features of the rocks, including in experimental studies, our understanding of their pathway “from primitive melt to ore deposit” is far from complete. In order to further constrain the metals’ behavior during differentiation of a carbonated alkali-ultramafic melt, we studied multiphase inclusions in olivine, chromite, perovskite, pyroxene and magnetite in the ultramafic rocks from three UACCs (Guli, Bor-Uryakh and Odikhincha), located in the Siberian LIP. Examination of both unheated and experimentally heated and quenched inclusions reveals a variety of compositions from melanephelinitic through to highly differentiated nephelinitic to alkali-rich carbonatitic. In addition, sulfide minerals, which turn into immiscible sulfide liquids during heating experiments, are widely distributed in the inclusions’ assemblages. We consider these inclusions to be snapshots of intercumulus melts, which were entrained into olivine-rich cumulate mush, and use their compositions to delineate plutonic differentiation of a carbonated alkali-ultramafic melt. Highly differentiated silicate melts, entrapped in Fe-rich chromite (Guli dunites), were rich in U, Th and Nb and crystallized Os-Ir-Ru phases in proximity to the host chromite. Concentrations of U, Th, Nb and REEs in alkali-rich carbonatite liquids, which were present in the intercumulus of Odikhincha and Bor-Uryakh peridotites, approached levels similar to mineralized carbonatites and support the concentration of these metals in an immiscible alkali-carbonatite fraction, which was enriched in S, P and Cl. Minor sulfide liquids, which are closely associated with these carbonatite fractions, were strongly enriched in Cu and Ni, thus explaining the origin of the Phalaborwa-like sulfide ores in carbonatites as a result of magmatic differentiation. Finally, our study provides insights into the formation of peridotite-hosted types of mineralization (perovskite-magnetite ores, mineralized carbonatite veins and PGE-bearing chromitites) and shows that these ore-bearing assemblages can be formed due to the infiltration of the metal-bearing intercumulus melts through the ultramafic matrix.

Mantle-hosted ophiolitic chromitites from Colombia: implications for petrogenesis and geodynamic evolution

ABSTRACT Chromitite bodies can be found within ophiolitic mantle sections in three locations in Colombia: Medellín, Planeta Rica, and La Guajira. These chromitites are typically massive pods that are often surrounded by lateritic soil in Medellín and Planeta Rica, or small discoidal bodies in La Guajira. Inclusions in chromite are preserved to varying degrees in the three localities: olivine and pargasite are partially preserved in Medellín and Planeta Rica, whereas inclusions in La Guajira chromitites are completely altered to chlorite. Compositionally, chromite from the chromitites is high-Al in Medellín (Cr#[Cr/(Cr+Al) atomic ratio] = 0.41–0.51) and Planeta Rica (Cr# = 0.49–0.57), and high-Cr in La Guajira (Cr# = 0.75–0.76), whereas TiO2 contents are low in the three localities (0.12–0.43 wt%). Regarding trace elements composition, the chromitites are similar to others formed in supra-subduction zone settings; the chromite from the high-Al chromitites is similar to that in MORB/BABB, whereas chromite from the high-Cr chromitites has a strong boninitic affinity. Chromitites from Planeta Rica are richer in Ti, possibly due to the presence of nearby gabbroic bodies and impregnated peridotites. The alteration in chromite rims and silicate matrix is related to metamorphic processes. Bulk-rock platinum-group element (PGE) contents range from 30 to 487 ppb, and chondrite-normalized PGE patterns show the typical enrichment in IPGE relative to PPGE observed in most ophiolitic chromitites. The estimated melts in equilibrium with the different chromitites correspond to MORB-like melts for the Medellín and Planeta Rica chromitites and to boninites for La Guajira chromitites. However, the estimated TiO2 content in the melt is too low for a typical MORB, indicating back-arc basin basalts and a back-arc setting for the formation of the Medellín and Planeta Rica chromitites during the Late Triassic and the Late Jurassic, respectively. La Guajira chromitites are interpreted to have formed in a fore-arc basin related to subduction-initiation during Cretaceous times.

Al Huwaysah 010: The most reduced brachinite, so far

AbstractAl Huwaysah 010 is an ungrouped achondrite meteorite, recently referred to as a brachinite‐like meteorite. This meteorite, showing a fine‐grained assemblage of low‐Ca pyroxene and opaque phases, is strongly reduced in comparison to other reduced brachinites. The occurrence of some tiny plates of graphite and oldhamite in this meteorite suggests that a partial melt residue has experienced a further reduction process. Olivine, the most abundant phase, is compositionally homogeneous (Fo83.3) as well as the clinopyroxene (En45.5Fs10.8Wo43.7) and the plagioclase (Ab69.5). Orthopyroxene (En85.4Fs13.9Wo0.7) also occurs but only in a fine intergrowth. Other accessory phases are Fe metal grains (Ni‐free or Cr‐bearing Fe‐Ni alloy), troilite, chlorapatite, pentlandite (as inclusions in chromite). The sample shows two different closure temperatures: the highest (≈900°C) is determined via the olivine–chromite intercrystalline geothermometer and the lowest temperature (≈520°C) is determined via the pyroxene‐based intracrystalline geothermometer. These temperatures may represent, respectively, the closure temperature associated with the formation and a subsequent impact event excavating the sample from the parental body. The visible to near‐infrared (VNIR) reflectance spectra of Al Huwaysah 010 exhibit low reflectance, consistent with the presence of darkening components, and weak absorptions indicative of olivine and pyroxene. Comparing the spectral parameters of Al Huwaysah 010 to potential parent bodies characterized by olivine–pyroxene mineralogy, we find that it falls within the field previously attributed to the SIII type asteroids. These results lead us to classify the Al Huwaysah 010 meteorite as the most reduced brachinite, whose VNIR spectral features show strong affinities with those of SIII asteroids.

Accessory Minerals in the Chromitite Ores of Dzharlybutak Ore Group of Kempirsai Massif (Southern Urals, Kazakhstan): Clues for Ore Genesis

The paper provides results of a detailed mineralogical study of some chromitite ores from two deposits in the Southern Urals of Kazakhstan: Almaz-Zhemchuzhina and Geofizicheskoe-VII. It is revealed that the main ore minerals are Cr-spinel with high Cr# (Cr/(Cr + Al) = 0.8–0.83), as well as serpentine and chlorite, replacing primary olivine. Chromium spinel grains contain mineral inclusions, which are distributed rather unevenly. The most common mineral inclusions are olivine (serpentine) and amphibole; phlogopite, pyroxenes, and base metal sulfides are rare. Olivine from inclusions in chromite is the highest in magnesium (Fo97–98), and is anomalously high in nickel (up to 1.8 wt.% NiO). The closure of exchange reactions between olivine and chromite occurred in the temperature range of 700–850 °C and in the oxygen fugacity range of −1.04 … +2.8 ΔFMQ, which most likely corresponds to the upper mantle settings of the fore-arc basin. A few tens of monomineral grains and polymineral intergrowths of platinum group minerals (PGMs) were found in chromite aggregates. Notably, monomineral grains are mainly represented by Ru, Os, and Ir disulfides, while in polymineral inclusions, iridium prevails (with widespread native phases, sulfides, and sulfoarsenides). PGM grains included in chromite are often associated with hydrous silicates: amphibole, and less often with phlogopite or chlorite. Discussed in the paper is the possible genesis of ores and inclusions. As a preliminary conclusion, we suggest that the solid-phase processes played the most significant role in the crystallization of Cr-spinel in the investigated chromitite ores.

Silicate mineral inclusions in chromite from the Eastern Bushveld complex: Implications for the origin and evolution of hydrous melt during chromite mineralization in the Critical Zone

Silicate mineral inclusions in chromite from the Eastern Bushveld complex: Implications for the origin and evolution of hydrous melt during chromite mineralization in the Critical Zone

Platinum‐group mineral and silicate inclusions in the low‐Al chromitites of the Manipur ophiolite, northeast India: Implications on Cr‐PGE mineralization in nascent subduction zone settings

Low‐Al chromitites from the Manipur ophiolite contain a wide range of mineral inclusions, including hydrous and anhydrous silicates, sulphides, and platinum group minerals (PGM). The common inclusions are primary silicates, such as clinopyroxene, orthopyroxene and secondary silicate inclusions, such as chlorite, serpentine, and garnet. These inclusions are small (<30 μm in PGM, <100 μm in silicates) and randomly distributed throughout their host chromite grains. The shape and texture indicate some inclusions (clinopyroxene, orthopyroxene, PGMs) trapped during the magmatic stage and most inclusions (chlorite, serpentine, and uvarovite) trapped during recrystallization of chromite. Clinopyroxene was discrete inside the chromite, with Cr2O3 content ranging from 0.62 to 1.77 wt.%, and Al2O3 varying between 0.96 to 1.63 wt.%. Orthopyroxene is enstatite (En93Wo2Fs4), Cr2O3 varies from 0.58 to 1.8 wt.% and the minor amount of CaO (0.41–0.81 wt.%), and Al2O3 (0.58–0.83 wt.%). Uvarovite is present in two forms: discrete crystal inclusions (anhydrous) in chromites and fractures (hydrous) as fillings. The chlorite occurs as lath shaped, and patchy phase is Cr‐rich variety ‐ kammererite (clinochlore) and penninite with variable Cr2O3 (0.22–7.25 wt.%) and FeO (2.5–3.9 wt.%). The PGM in the studied chromite are laurite (Ru, Os, Ir)S2. The parental melts of chromitites had low TiO2 (0.16–0.48 wt.%) and Al2O3 (10.18–14.58 wt.%) contents, and primary mineral inclusions, suggesting an arc‐like geochemical affinity, and the estimated melt composition is comparable with that of boninitic magma.

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

Recycled Crustal Components of the Iceland Plume Centre: Ultra-Dehydrated Crust and Oxidised Water-Bearing Fertile Mantle

AbstractPrimitive basalts erupted mainly above the proposed centre of the Iceland plume have been analysed geochemically. The data shed light on the distribution of plume heterogeneities and constrain their nature and origin. Low-Ca (down to 1420 ppm) olivine macrocrysts and chromite inclusions in Snæfell rocks of the eastern flank zone indicate that the magmas were hydrous (c. 2 wt% H2O) and relatively oxidised (fO2 = c. ΔQMF+1). Al-in-olivine crystallisation temperatures indicate that magmas from central Iceland magmas are not hotter than from other parts of Iceland. Some Gæsafjöll rocks with high Nb/U = 110–120 from the northern rift zone (NRZ) represent magmas widespread in Iceland that are also characterised by low Ba/La and high La/U, La/Th, Nb/Th and Ce/Pb. They reflect a mantle component with Nb/U ≈ 200 present in many basalts of all rift zones. This mantle apparently consists of peridotite refertilised by ultra-dehydrated recycled oceanic crust (ROC). This component is in clear contrast to the main source of the Snæfell magmas, which is modelled to be peridotite strongly refertilised by enriched ROC more hydrous than Mid-Ocean Ridge Basalt. In contrast to more-depleted rift zone melts, which are extracted from depths just below the thin rifted lithosphere, some rift zone basalts are extracted deeper and produced by intermediate degrees of melting of the two types of refertilised mantle. Snæfell-type melts are common in the NRZ and eastern rift zones (ERZ) and most strongly present at Upptyppingar. Mixing of magmas is indicated at crustal levels. Mantle melting may be modelled for homogeneous sources of peridotite or olivine websterite. Double-spike Pb isotope analyses suggest that the depleted mantle sources of the ERZ basalts together with Snæfell and high-Nb/U sources, both with 206Pb/204Pb = 18.3–18.6, Δ207Pb ≈ 0 and Δ208Pb = 20–30, explain both ERZ and NRZ magmas, except for Theistareykir. They are distinct from magmas of the western rift zone, which is characterised by melt contributions from a differently enriched mantle with more radiogenic lead, negative Δ207Pb, Δ208Pb ≈ 0. This difference in type of enriched mantle suggests that the Iceland mantle plume is laterally zoned.

Electron Probe Microanalysis and Microscopy of Polishing-Exposed Solid-Phase Mineral Inclusions in Fuxian Kimberlite Diamonds

Solid-phase mineral inclusions in diamond (1–3 mm in diameter) from the No. 50 kimberlite diatreme of Liaoning Province, China, were exposed by polishing. A variety of silicate, carbonate and sulfide inclusions were recovered in the diamond. The common solid-phase inclusions are olivine, chromite, garnet and orthopyroxene; the rare phases include Ca carbonate, magnesite, dolomite, norsethite, pyrrhotite, pentlandite, troilite, a member of the linnaeite group, an unknown hydrous magnesium silicate and an Fe-rich phase. Abundance and composition of the solid-phase inclusions in diamond indicate that they belong to the peridotitic suite and are mainly harzburgitic. No eclogitic mineral inclusions were found in the diamond. The slightly lower Mg # of the olivine inclusions (peak at 93) than that of harzburgitic olivine inclusions worldwide (Mg # peak at 94), the higher Ni content (0.25–0.45 wt. %) of the olivine inclusions than those of olivine inclusions worldwide (0.30–0.40 wt. %), the higher Ti contents (up to 0.79 wt. %) in some chromite inclusions in diamond than those in chromite inclusions worldwide, the existence of carbonate inclusions in diamond, and the possible presence of hydrous silicate phases in diamond all indicate a metasomatic enrichment event in the source region of diamond beneath the North China craton, suggesting that the diamond probably formed by solid-state growth under metasomatic conditions with the presence of a fluid. Solid-state growth of diamond is also supported by abundant graphite inclusions in the diamond. Sulfide inclusions in diamond often coexist with chromite and olivine or are rich in Ni content, indicating that the sulfide inclusions belong to the peridotitic suite. From the chemical compositions, most sulfide inclusions in diamond from the No. 50 kimberlite were probably trapped as monosulfide crystals, although some may have been entrapped as melts.

Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China

The origin of ultramafic rocks, especially those in suture zones, has been a focus because they are not only important mantle sources of magma, but also provide substantial information on metamorphism and melt/fluid–peridotite interaction. Ultramafic rocks in Hujialin, in the central part of the Sulu orogen, include peridotite and pyroxenite. Although many papers on their origin and tectonic evolution have been published in the past few decades, these questions are still highly debated. Here, we present mineralogy, mineral composition, and bulk-rocks of these ultramafic rocks to evaluate their origin and tectonic evolution. The garnet clinopyroxenite is low in heavy rare-earth elements (HREE, 5.97–10.6 ppm) and has convex spoon-shaped chondrite-normalized REE patterns, suggesting the garnet formed later, and its precursor is clinopyroxenite. It is high in incompatible elements (i.e., Cs, Rb, Ba) and shows negative to positive U, Nb, and Ta anomalies, without pronounced positive Sr or Eu anomalies. Clinopyroxene in garnet clinopyroxenite contains high MgO (Mg# 0.90–0.97). The mineral chemistry and bulk-rock compositions are similar to those of reactive clinopyroxenite, suggesting that it originally formed via peridotite–melt interaction, and that such silicic and calcic melt might derive from the subducted Yangtze continent (YZC). Dunite contains olivine with high Fo (93.0–94.1), low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%), chromite with high Cr# (0.75–0.96), TiO2 (up to 0.88 wt.%), and Na2O (0.01–0.10 wt.%). It has negatively sloped chondrite-normalized REE patterns. Mineral chemistry and bulk rocks suggest dunite likely represent residual ancient lithosperic mantle peridotite beneath the North China Craton (NCC) that was overprinted by aqueous fluids. The lack of prograde and retrograde metamorphic minerals in dunite and irregular shaped mineral inclusions in chromite suggest dunite did not subduct to deep levels. Dunite mingled with garnet clinopyroxenite during exhumation of the latter at shallow depths. These ultramafic rocks, especially hydrated peridotite, may be important sources of Au for the Jiaodong gold province in the NCC.

Silicate Mineral Inclusions in Chromite from the Eastern Bushveld Complex: Implications for the Origin and Evolution of Hydrous Melt During Chromite Mineralization in the Critical Zone

Silicate Mineral Inclusions in Chromite from the Eastern Bushveld Complex: Implications for the Origin and Evolution of Hydrous Melt During Chromite Mineralization in the Critical Zone

Platinum and gold placer from Tugidak Island, Alaska: Platinum-group minerals and their inclusions, gold, and chromite mineralogy

ABSTRACT Black sand beach placers from Kodiak, Sitkinak, and Tugidak Islands, Alaska, have been mined intermittently for gold and minor platinum-group alloys for more than 100 years. High-grade platinum-rich magnetic separate and accompanying black sand from the southern beach placer of Tugidak Island were studied using electron microprobe WDS and scanning electron microscope EDS; mineral classification and identification were based on these techniques. The major platinum mineral is isoferroplatinum, followed by minor tetraferroplatinum and tulameenite, and rare ferronickelplatinum. Two types of alteration were identified in about 3–4% of the alloy grains: rim formation involving Pt loss and increased Fe, Ni, and/or Cu, and fracturing and vein filling by Cu-rich alloy. Ruthenium-Ir-Os-Pt alloys occur as inclusions and veins as well as form part of composite grains. Ten percent of the alloy grains contain a large variety of platinum-group minerals (PGM). Inclusions of cuprorhodsite, malanite, cuproiridsite, laurite, erlichmanite, cooperite, braggite, bowieite, kashinite, miassite, hollingworthite, irarsite, sperrylite, stillwaterite, genkinite, stibiopalladinite, keithconnite, zvyagintsevite, and probable palladodymite and vincentite were identified. Two unidentified inclusion phases also occur. Most of the PGM inclusions are primary and were trapped by a growing crystal from a melt; some inclusions exhibit textures that suggest trapping of an As,Te,S-rich immiscible melt. Secondary inclusions and evidence of deformation were observed in a few alloy grains. Associated with PGM inclusions or as separate inclusions are various base-metal sulfides. Two silicate-melt inclusions in one isoferroplatinum grain have an andesite–shoshonite composition. Minor gold and Ag-rich gold in the high-grade magnetic separate contain magnetite, pyrrhotite, and chromite inclusions. The gold composition suggests that their sources are the numerous quartz veins and apophyses related to granitoids on Kodiak Island. The composition of the placer chromite is similar to chromite from the Border Ranges mélange fault system and suggests that the Uyak Complex ultramafic and mafic rocks are part of a supra-subduction-zone ophiolite and are the source of the platinum-group minerals.

Composition of the Sub‐Cratonic Mantle of the Guiana Shield Inferred From Diamond‐Hosted Inclusions

AbstractThe composition of diamond‐hosted inclusions provides insight into the character of the sub‐cratonic lithosphere of the Guiana Shield. Guyana's Paleoproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo ∼89.3–91.8), enstatite, chromite, and Cr‐pyrope. Raman thermobarometry of entrapped olivine and pyrope inclusions indicate entrapment pressures of ∼5.3–7.0 GPa. Unpolarized Fourier transform infrared spectroscopic measurements of forsterite and enstatite inclusions produce low absorbances from OH. Using established calibrations, those absorbances indicate forsterite and enstatite contain median values of 26 ± 14 and 14 ± 18 ppm H2O, respectively, and suggest a high effective viscosity of 1023.7 ± 2.1 Pa∙s for the lithospheric mantle. When combined with inclusion thermobarometry, diamond residence temperatures suggest paleo‐geotherms ranging from 35 to 40 mW m−2. Low‐to‐moderate Fe2O3 content (1.9 ± 0.8 wt.%) and low oxygen fugacity (log ƒO2 (ΔFMQ) −1.6 ± 1.1) determined from chromite inclusions indicate crystallization in reducing conditions. Forsterite and chromite inclusions retain evidence for metasomatic alteration, including Mn‐enrichment in forsterite and chromite rich in Zn. These characteristics indicate that the sub‐cratonic lithosphere of the Guiana Shield experienced episodes of partial melting and fluid‐driven metasomatism of dry, strongly viscous, and moderately depleted garnet‐spinel harzburgite. The Guiana Shield has been relatively stable since the Paleoproterozoic, meaning diamond inclusions may also provide the best means for understanding current conditions in the region's lithospheric mantle.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Amphibole as a witness of chromitite formation and fluid metasomatism in ophiolites

Abstract Here we present new occurrences of amphibole in a suite of chromitites, dunites, and harzburgites from the mantle sequence of the Lycian ophiolite in the Tauride Belt, southwest Turkey. The amphibole occurs both as interstitial grains among the major constituent minerals and as inclusions in chromite grains. The interstitial amphibole shows generally decreasing trends in Na2O and Al2O3 contents from the chromitites (0.14–1.54 wt% and 0.04–6.67 wt%, respectively) and the dunites (0.09–2.37 wt%; 0.12–11.9 wt%) to the host harzburgites (<0.61 wt%; 0.02–5.41 wt%). Amphibole inclusions in chromite of the amphibole-bearing harzburgites are poorer in Al2O3 (1.12–8.86 wt%), CaO (8.47–13.2 wt%), and Na2O (b.d.l.–1.38 wt%) than their counterparts in the amphibole-bearing chromitites (Al2O3 = 6.13–10.0 wt%; CaO = 12.1–12.9 wt%; Na2O = 1.11–1.91 wt%). Estimated crystallization temperatures for the interstitial amphibole grains and amphibole inclusions range from 706 to 974 °C, with the higher values in the latter. A comparison of amphibole inclusions in chromite with interstitial grains provides direct evidence for the involvement of water in chromitite formation and the presence of hydrous melt/fluid metasomatism in the peridotites during initial subduction of Neo-Tethyan oceanic lithosphere. The hydrous melts/fluids were released from the chromitites after being collected on chromite surfaces during crystallization. Different fluid/wall rock ratios are thought to have controlled the crystallization and composition of the Lycian amphibole and the extent of modification of the chromite and pyroxene grains in the peridotites. Considering the wide distribution of podiform chromitites in this ophiolite, the link between chromitite formation and melt/fluid metasomatism defined in our study may be applicable to other ophiolites worldwide.

Composition gradients in silicate inclusions in chromites from the dunitic mantle-crust transition (Oman ophiolite) reveal high temperature fluid- melt-rock interaction controlled by faulting

The transition between the mantle section and the oceanic crust in the Maqsad area (Oman ophiolite) is mainly made of variably impregnated dunites locally associated with chromitite ore bodies. There, the dunitic transition zone (DTZ) developed above a mantle diapir that fed with MORB the former oceanic spreading centre. However, orthopyroxene and amphibole impregnations in dunites from the DTZ are witnesses of a hydrated magmatism that looks restricted to this interface. The main other piece of evidence is the nature of silicate minerals included in chromite grains scattered in dunite (e.g., amphibole, orthopyroxene, mica), which are mostly issued from a hydrated and silica-rich melt or fluid. Here, we report on a study of such inclusions along a section sampled in detail in the Maqsad DTZ. It brings critical information on the processes involved in the fluid-melt-peridotite reaction below oceanic spreading centres, complementary to the one provided by the interstitial silicates forming the matrix of the dunite. We first show that both the nature and the composition of the inclusions are well-correlated to those of the impregnations in the host dunites, then that the chemical evolution along the cross-section for all materials correlate to the presence of faults that developed at an early, syn-magmatic stage. This confirms that the early tectonics in the deep oceanic lithosphere primarily controls the fluid-melt-rock reactions and can condition chemical cycling, including for halogens (Cl, F), in oceanic spreading centre setting

Compositional variations, thermometry, and probable parental magmas of Archean chromite from the Sargur greenstone belt, Western Dharwar Craton (India)

The chromite and magnesite deposits from Talur, Sindhuvalli, and Doddakanya in the layered ultramafic-mafic plutonic sequence are a part of the Archean Sargur greenstone belt within the Western Dharwar Craton of the Indian shield. The ultramafic bodies in these three areas are highly sheared and comprise of serpentinized dunite with chromitite bodies along with harzburgite and pyroxenite. Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~ 50 vol% of chromite) and serpentinite (~ 2 vol% of chromite) throughout the entire ultramafic sequence. In Talur, the chromites in massive chromitites (~ 60–70 vol% of chromite) are characterized by relatively lower Mg# (0.28–0.39) compared to chromites from massive chromitites in Sindhuvalli (Mg# = 0.63–0.69). The accessory chromite in dunite from Talur shows higher Cr#, Fe2O3, TiO2, and MnO content than other localities. The range of Fo content of cumulus olivine in dunites from Talur (Fo91-92) is relatively lower than the dunite from Doddakanya (Fo92-93) and net-textured chromitite from Sindhuvalli (Fo95-96). The range of Mg# in the intercumulus clinopyroxene in Talur net-textured chromitite varies between 94 and 99, whereas, in the Sindhuvalli net-textured chromitite, the intercumulus clinopyroxene shows a lower range of Mg# (= 96–97). Chromites in massive and silicate-rich chromitites, and from dunite contain several rounded to subhedral inclusions of silicate minerals of various sizes (~ 4–20 μm). Spherical inclusions of olivine (Fo94) are present in accessory chromites of dunite (Fo87). The Mg-rich clinopyroxene inclusions have Cr2O3 content from 1.5 to 2.3 wt%. The clinopyroxene inclusions in the Talur schlieren-banded chromitite are more magnesian (Mg# = 95–98) compared to the inclusions in chromites of the Sindhuvalli net-textured chromitite (Mg# = 96–97). The euhedral phlogopite inclusions in chromites from the Sindhuvalli net-textured chromitite are Mg-rich (~ 31.09 wt% MgO) with a significant amount of K2O (~ 9.28 wt%). The chlorite inclusion in chromites from the Talur schlieren-banded chromitite is also Mg-rich (~ 39.32 wt% MgO) with Al2O3 ~ 21.01 wt% and a significant amount of Cr2O3 (~ 3.51 wt%). Clinopyroxene (along with phlogopite) inclusions always occur as independent grain testifying their primary nature and might be genetically linked with the chromite crystallization from a primary high-Mg silicate magma. Chlorite and dolomite inclusions may be related to secondary hydrothermal alteration events. The average re-equilibration temperature of the olivine-chromite pairs in the Sindhuvalli net-textured chromitite and Doddaknaya dunite of around 545 °C represents the first stage of metamorphism under reducing conditions. Subsequently, the second hydrothermal episode involved fluids evolving to an average temperature of about 327 °C calculated from the accessory chromite and olivine pairs from Talur dunite, consistent with the abundance of magnesite veins throughout the Talur mines. The computed values of Al2O3, TiO2, and FeO/MgO ratio of the melt show that the parental melt associated with the chromitites in the Sargur greenstone belt has a close resemblance with the parental melt of the chromitites in the Nuggihalli schist belt, which is low-Al komatiitic. In the tectonic discrimination diagrams, most of the primary chromite compositions from Sargur plot within the field of chromites from komatiites from the Archean greenstone belts. Parental komatiitic magma fractionated in the subvolcanic conduits within the thin lithospheric crust and formed the sill-like layered ultramafic-mafic bodies with chromite deposits. These conduits fed the overlying extrusive system that developed the ultramafic-mafic volcanic sequence of the Sargur greenstone belt in the Western Dharwar Craton.