Chromite Grains Research Articles

Ultra-high pressure inclusion in Archean ophiolitic podiform chromitite in mélange block suggests deep subduction on early Earth

The discovery of ultrahigh pressure (UHP) minerals in orogenic belts has revolutionized our understanding of subduction and the return flow of previously deeply subducted material to Earth's surface as part of the cycling and interaction of crustal and mantle systems. One class of UHP minerals is found as inclusions in orogenic peridotite-hosted podiform-chromite systems, within Phanerozoic ophiolites and ophiolitic mélanges. Such inclusions have opened a window into processes of recycling of crustal materials to the deep mantle and back through subduction and mantle convection in Phanerozoic orogens. Here, we document the first occurrence of an UHP mineral hosted in an ophiolitic podiform chromitite mélange from the Neoarchean paired metamorphic belt of the Central (Taihang) Orogenic Belt, Northern China. Numerous inclusions of rutile, apatite, dolomite, and amphibole are interpreted to be crustal-derived; they occur in podiform chromite grains hosted in a 2.6–2.5 Ga ophiolitic mélange now part of the North China Craton and formed by subduction of oceanic and continental material. Microstructures and phase relationships in a multi-phase inclusion of TiO2(II), rutile, apatite, and tremolite yield minimum P-T conditions of 7.5 GPa at 1000 °C, indicating that the crustal host, including carbonates, were subducted to depths > 270 km, transferred to the mantle of the overriding plate, and returned to the surface by 2.5 Ga. We suggest that slab rollback forced upward mantle flow, incorporating entities from the lower plate, perhaps in serpentinite diapirs, resulting in adiabatic melting that allowed crustal material to be trapped in chromite grains crystallizing in high-Mg melts. Contrasting bulk moduli and thermal contraction of the inclusions and host chromite protected the inclusions from P-induced back-reaction during exhumation. Together, these features show that the 1600 km long Central (Taihang) Orogenic Belt is emerging as the world’s first well-documented Phanerozoic style orogen, with classic tectonic zonation, ophiolitic mélanges, paired metamorphism, local evidence for UHP conditions, foreland basins, and late to post orogenic magmatism. Applying the null hypothesis, we explain this high degree of similarity by invoking the operation of Phanerozoic style plate tectonics, at least throughout the 1600 km length of the COB, and by geological comparison, in other similar aged geological terrains globally. From this we infer modern-style plate tectonics was operating in the Neoarchean.

Textural insights into the significance of ophiolitic chromitites, with special reference to Oman

This paper analyses the process and depth of formation of chromite crystals occasionally including ultrahigh pressure (UHP) minerals, recovered from some investigated ophiolites. We present direct microstructural evidence for crystallization of chromite ores from the Semail ophiolite, in place they are exposed. Similar crystal preferred orientation in olivine-rich layers and in septa from chromite layers indicate deformation of the olivine solid frame by dislocation creep at high temperature, synchronous with the crystallization of chromite. The preservation of the olivine framework throughout the development of joined chromite grains suggests that chromite replaced olivine. Chromite grains crystallized from a melt circulating in an open-system, stress-bearing network of dunitic aggregate, controlled by the olivine high-T slip system. The misorientation of joined crystals at grain boundaries define the ratio of twin vs. random boundaries that were closed to vs. open to fluid circulation, respectively, suggesting that chromite layers may represent planes of channelized melt circulation. The relationship of chromite deposits to ridge tectonics structures suggests that Cr-bearing melt was produced at the transition from an active spreading ridge to deep mantle shear zones acting as possible source of hydration. The occurrence of potential UHP inclusions in chromite crystals is assigned to continental contamination of melts ascending through the Arabian margin.

Detrital chromites reveal Slave craton's missing komatiite

Abstract Komatiitic magmatism is a characteristic feature of Archean cratons, diagnostic of the addition of juvenile crust, and a clue to the thermal evolution of early Earth lithosphere. The Slave craton in northwest Canada contains >20 greenstone belts but no identified komatiite. The reason for this dearth of komatiite, when compared to other Archean cratons, remains enigmatic. The Central Slave Cover Group (ca. 2.85 Ga) includes fuchsitic quartzite with relict detrital chromite grains in heavy-mineral laminations. Major and platinum group element systematics indicate that the chromites were derived from Al-undepleted komatiitic dunites. The chromites have low 187Os/188Os ratios relative to chondrite with a narrow range of rhenium depletion ages at 3.19 ± 0.12 Ga. While these ages overlap a documented crust formation event, they identify an unrecognized addition of juvenile crust that is not preserved in the bedrock exposures or the zircon isotopic data. The documentation of komatiitic magmatism via detrital chromites indicates a region of thin lithospheric mantle at ca. 3.2 Ga, either within or at the edge of the protocratonic nucleus. This study demonstrates the applicability of detrital chromites in provenance studies, augmenting the record supplied by detrital zircons.

Characteristics and implications of podiform-chromite hosted silicate inclusions in the Zedang ophiolite, Southern Tibet

The Zedang ophiolite, containing pods of massive chromitite, crops out in the eastern segment of the Yarlung-Zangbo suture zone, Southern Tibet. Chromite grains in the Zedang chromitite bodies have high Cr# [Cr/(Cr + Al)] (0.73–0.82) and Mg# [Mg/(Mg + Fe)] (0.73–0.79), and low TiO2 content (0.13–0.25 wt%); they contain diopside (Di) exsolution lamellae and abundant inclusions of diopside, enstatite, hydrous silicates (e.q., Cr-bearing amphibole), and serpentine. The inclusions occur as isolated single-phase or multiphase (clinopyroxene + amphibole ± platinum group mineral) grains. Diopside lamellae-bearing chromite was probably a precursor CaFe2O4-phase (CF phase) forming at >12.5 GPa. Individual octahedral serpentine inclusions are likely pseudomorphs after isometric ringwoodite. Both the CF phase and ringwoodite crystallized within the mantle transition zone (MTZ). Equilibrium pressure-temperature (P-T) conditions of coexisting clinopyroxene-orthopyroxene (Cpx-Opx) inclusions are 8.5–21.5 ± 1.5 kbar, and 996–1097 ± 30 °C. These P-T conditions suggest that the inferred parental melts of the Zedang chromitites were boninitic. The boninitic liquid in turn was generated from depleted, metasomatized mantle in a supra-subduction zone (SSZ) environment. Linear alignment of hydrous silicate inclusions in chromitites imply fluid infiltration along fracture zones at shallow depths during consolidation of the ophiolite. Based on our new data, we propose a new three-stage model to explain the formation and evolution of the Zedang chromitites. It experienced a long journey from (i) the MTZ forming high-P chromite (containing ultrahigh-pressure minerals), (ii) to the upper mantle attending to the formation of the major part of the chromite, whereas it's host chromitites formed at melt-rock reaction process, and (iii) where high-P chromite was incorporated in low-P neoblastic chromite during subducting slab rollback-induced channel return flow to shallow depths (involving fluid input and alteration).

Chromian spinel neomineralisations and the microstructure of plastically deformed ophiolitic peridotites (Kraka massifs, Southern Urals, Russia)

Results of a microstructural study of spinel peridotite samples obtained from the Kraka massif in the Southern Urals, involving findings of Cr-spinel neomineralisations within intensive ductile deformed silicates (olivine and orthopyroxene), are presented. The new-formed Cr-spinel grains show different stages of syn-deformation growth as evidenced by investigations combining petrography, decorated dislocation structure analysis, scanning electron microscopy and electron backscatter diffraction (EBSD). Initial precipitations appearing as rods or lamellae are observed to form around structural defects of host silicate grains (olivine and orthopyroxene) by means of impurity segregation or heterogeneous nucleation in the most distorted lattice regions (i.e., in the predominant recrystallisation zones). Syn-deformational crystal growth leads to a complication and coarsening of the grain morphology by coalescence due to a reduction in grain boundary (interfacial) energy. While in the process of growing, the Cr-spinel grains capture fragments of silicate matrix in the solid-state process. The final stage of Cr-spinel growth involves a change in morphology resulting in their characteristic crystallographic forms (spheroidisation). The presence of euhedral Cr-spinel grains, typical for ophiolitic dunite bodies, is a result of interfacial energy reduction in areas of grain boundaries of the hardest phase. The general trend of the observed stages relates closely with the localisation of deformation zones in the upwelling upper mantle (diapir), which are composed by the weakest phase of olivine (dunites). The concentration of the weakest olivine phase in the mobile zones, which is energetically beneficial, explains why dunite bodies having euhedral chromite grains comprise the dynamic equilibrium rocks in the plastic flow localisation zones in upper mantle diapirs. Conversely, assemblages having pyroxene phases, which are stronger and larger in size compared to olivine, are not stable in these zones.

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

AbstractA number of bulk‐rock and mineral Fe and Mg isotope data have been increasingly reported from ophiolitic rocks, but their anomalous fractionations are still not well addressed. This study presents new Fe, Mg, and O isotopes and published Li isotope data of olivine and/or chromite in chromitite, dunite, and harzburgite from the Pozantı‐Karsantı ophiolite (southern Turkey) to reveal the mechanisms of inter‐ and intra‐mineral isotope fractionation during chromitite formation. In the studied samples, both olivine and chromite have highly variable δ56Fe ranging from −0.125‰ to 0.101‰ and −0.261‰ to 0.191‰, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with δ26Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ26Mg values ranging from −0.02‰ to 0.45‰. The δ18O values of olivine range from 4.65‰ to 5.86‰ and co‐vary with δ7Li (−5.43‰ to 16.65‰), δ56Fe, and δ26Mg, particularly at lithological scale. These isotopic features cannot be explained solely by subsolidus diffusion between olivine and chromite, or by partial melting and fractional crystallization. Instead, they require fluids as a diffusive medium for high efficient element exchange between chromite and olivine. The fluids were likely the hydrous fluids that were collected on chromite grain surfaces during crystallization and released later from chromite piles during precipitation. The fluid‐olivine reaction preferentially transferred light Fe from olivine grains resulting in elevated δ56Fe, but caused small O and Mg isotopic variations in the olivine. The Li diffusion depends on Li concentration gradient between olivine and fluid and olivine/fluid ratio, which can account for large δ7Li variations in olivine between different rock types. As these reactions proceeded, they induced elemental and isotopic variations in the fluids. These fluids could continuously penetrate the harzburgites through the dunite envelopes around the chromitites, producing some of the observed anomalous geochemical features of the harzburgites. Thus, we conclude that the hydrous fluids acted as critical media for ion exchange between chromite and olivine and for significant modification of the host harzburgites.

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.

Laboratory investigation into the dielectric properties of a L-chondrite (NWA 12857)

Ground Penetrating Radar (GPR) is a terrestrial geophysical exploration method that has recently become one of the most promising technique for planetary, asteroidal and cometary subsurface exploration. The capability of GPR to sound Solar System's bodies relies on the electromagnetic properties of the constitutive materials. Enstatite and ordinary chondrites represent class of asteroids occurring in the inner asteroid belt whereas carbonaceous chondrites and their icy mixtures are reasonable analogues for cometary material as well as constituent of shallow part of some Jovian satellite crusts. Therefore, the knowledge of electromagnetic properties of meteorites is very important because it allows to estimate the radar response in terms of signal velocity and attenuation. In this work we measured the real and imaginary parts of the permittivity of a L5 chondrite meteorite as a function of frequency (20 Hz-1 MHz) by using a capacitive cell connected to a self-balancing bridge. We studied the spatial variability of dielectric properties of the sample that exhibits areas with different textures characterized by a darker appearance. In general, the meteorite sample shows a stronger dispersive behavior compared to terrestrial rocks with higher values for both real and imaginary part of permittivity. In particular, the occurrence of very small grains (<10 μm) of FeNi metal, troilite and chromite scattered in some areas of the meteorite seems to be the cause of such behavior.

A New Method (Ball Milling and Sodium Sulfide) for Mechanochemical Treatment of Soda Ash Chromite Ore Processing Residue

Hexavalent chromium in soda ash Chromite Ore Processing Residue (COPR) is harmful to the environment, it is imperative to develop a low cost, efficient, and effective treatment. Herein, a new method (ball milling+sodium sulfide) was developed for mechanochemical treatment of water-leached COPR (W-COPR, about 900 mg/kg non-exchangeble Cr(VI) and mostly chromite bounded). Under a stoichiometric ratio of S2- to Cr(VI) of 5, milling speed of 200 rpm, milling time of 30 min, ball-to-powder weight ratios of 8.5, and, a total Cr(VI) of 76.88 mg/kg and a Toxicity Characteristic Leaching Procedure (TCLP) total Cr value of 1.15 mg/L were achieved after treatment. Results of the mineral liberation analyser (MLA) analyses showed 10% increase of chromite grains liberation and grain size reduction were beneficial to the chromite-bound Cr(VI) reduction. Similar effects were also observed on magnesioferrite encapsulated Cr(VI). Particle aggregation and formation of glass phase colloid precipitation could potentially impede Cr(VI) reduction. Results of X-ray absorption near edge structure (XANES) analyses indicated that the treatment method reduced the actual Cr(VI) concentration to 312 mg/kg with Cr(VI) reduction efficiency of 98% being achieved. Overall, the new method is simple and efficient, and provides a guidance for future industrial application.

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 (&amp;lt;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.

Chromite‐bearing quartzite in the southern fringe of Singhbhum Craton around Ghutrigaon, Eastern India: Petrogenetic implication

The chromite mineralization in the southern fringe of the Singhbhum Craton around Ghutrigaon, Dhenkanal district, Odisha, Eastern India shows a unique nature of occurrence. The structural features, mineral assemblage, and spinel composition are different from the two well‐known Sukinda and Boula‐Nuasahi chromite deposits occurring within the mafic/ultramafic complex in Odisha. The chromite‐bearing quartzite from Ghutrigaon shows typical sedimentary features, such as colour banding, cross‐bedding, broomsticks, and slump structures and is closely associated with large volumes of volcanic tuff. The chromite‐bearing quartzite contains major quartz and chromite along with minor accessories such as fuchsite, chromian muscovite, chromphyllite, chromceladonite, chromechalcedony, rutile, and zircon. Detailed mineralogical, textural, and spinel compositional analysis support to its detrital origin having been derived from a volcanic source. The Cr‐spinel composition shows very low Mg‐number (0.01–0.04) and high Cr‐number (0.69–0.79) in contrast to chrome ores from different localities of India. The EPMA results when plotted in various established discrimination diagrams show a distinctly different domain, indicating that these are affected by a pro‐grade metamorphic event. Our study shows that the Ghutrigaon chromite ore owes a mixed origin where chromite detritus from a volcanic provenance (island arc to intra‐plate terrain) got deposited along with terrestrially derived silica‐rich clastics. Paucity in magnesium content in the spinel may be the result of replacement of Mg by Fe2+ due to compositional re‐equilibration during metamorphism. Grain coarsening took place at the onset of metamorphism. The volcaniclastics added to the arenaceous sediment gave rise to different Al‐silicate minerals. The chrome‐content in most of these minerals is attributed to redistribution of Cr through solid‐state diffusion during metamorphism. Rutile and Zircon detritus are considered as continental input. The presence of Zn, Mn, and Ni in the chrome spinel is most likely contributed by the precursor rock. The formation of large idioblastic chromite grains, preferred grain orientation, band development, elevated values of some elements in the spinel, Zn in particular attest to their formation under low‐grade regional metamorphism (greenschist facies).

Chromitites of the Kraka ophiolite (South Urals, Russia): geological, mineralogical and structural features

The paper describes disseminated tabular, podiform massive, and transitional chromitite deposits from a mantle section of the Kraka ophiolite massif, South Urals, Russia. The chromitite is hosted by dunite with no correlation between their size and quality and the size of the dunite bodies. Thick dunite bodies mostly host disseminated fine-grained banded chromitite; massive ores are composed of coarse-grained chromitite typically with a thin dunite envelope. The chromitite and host ultramafic rocks exhibit plastic deformation of silicates and chromite, which is expressed in microstructural features, preferred orientation of rock-forming olivine, and folding of the chromitite bodies. The ultramafic rocks are also characterized by deformation-induced textures leading to the formation of the small-size chromite grains on structural defects of plastically deformed rock-forming olivine and orthopyroxene. The formation of dunite bodies and associated chromitite is related to the localization of deformation of rising mantle flows under decompression conditions. Dunite was the most rheologically weak zone exhibiting a focused solid state flow and effective separation of mineral phases (olivine and chromite). The higher amount of the latter in dunite is a result of deformation-induced breakdown of enstatite and removal of trace elements from olivine. The structural features of massive chromitite aggregates indicate that they are a product of concentration and aggregation of grains under the influence of tectonic stresses at high temperatures and pressures, similar to pressure sintering.

Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.

De Kraalen and Witrivier Greenstone Belts, Kaapvaal Craton, South Africa: Characterisation of the Palaeo-Mesoarchaean evolution by rutile and zircon U-Pb geochronology combined with Hf isotopes

Abstract The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Here we report zircon and rutile in situ U-Pb ages and zircon Hf isotopic data from a variety of supracrustal rocks. The zircon cores from a metamafic amphibole-bearing gneiss from the DKGB give a protolith age of 3 441 ± 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 ± 16 Ma. The 176Hf/177Hft values for all zircon domains give a tight cluster around 0.280596 ± 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin calc-silicate layers from the WGB give a single crystallisation age of 3 230 ± 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields 176Hf/177Hf~3.23 Ga = 0.28064 ± 0.00004, corresponding to εHf~3.23 Ga = 2.4 ± 1.9 (2SD) and Hf model ages between ca. 3.51 to 3.30 Ga. These are Hf isotope characteristics for zircons from a relatively juvenile source extracted from a depleted mantle source ca. 0.28 to 0.07 Ga prior zircon crystallisation. The second population yields 176Hf/177Hf~3.23 Ga = 0.28093 ± 0.00004 with εHf~3.23 Ga = 8.1 ± 1.3 (2SD). These Hf data combined with the 206Pb/207Pb ages lead to isotope ratios that lie above those of Depleted Mantle. The unusually high Hf isotope signature for the cores of the zircons from the WGB amphibolite most likely represent a contribution from an early highly depleted mantle source. A rutile in situ U-Pb age of 3.085 Ga from a recrystallised quartzite indicate that the rocks from the DKGB experienced slow cooling following the 3.21 Ga metamorphic event or (partial) resetting due to elevated geothermal gradient caused by the ca. 3.1 Ga intrusions of the Vrede Granitiod Suite. The latter interpretation is preferred because ~145 Ma of slow cooling from the amphibolite facies conditions of peak metamorphism to the blocking temperature for mass diffusion of Pb in rutile is unlikely. While the Zr-in-rutile temperature of ca. 710°C at 7 kbar for DKGB most likely records the peak temperature of the ~3.23 to 3.21 Ga event. The trace element concentrations of the metamorphic rutile grains within the quartzite of the DKGB indicate that the source rock was enriched in Cr. Either due to silification during hydrothermal alteration of the (ultra)mafic country rock or during deposition in an atmosphere that allowed for chromite grains to be part of the sediment.

Crustal anorthosite formation by deep‐seated hydrothermal circulation beneath fast‐spreading axis: Constraints from chronological approach, Sr isotope, and fluid–chromite inclusion investigation

AbstractHydrothermal circulation beneath the spreading axis plays a significant role in the exchange of energy and mass between the solid Earth and the oceans. Deep‐seated hydrothermal circulation down to the crust/mantle boundary in the fast‐spreading axis has been introduced by a number of studies regarding geological investigations and numerical models. In order to assess a reaction between hydrothermal fluid and host rock around the crust/mantle boundary, we conducted bulk trace element and Sr isotope analyses with a series of in situ investigations for crustal anorthosite, a reaction product between hydrothermal fluid and gabbro in the lowermost crustal section along Wadi Fizh, northern Oman ophiolite. In addition, we conducted titanite U–Pb isotope analyses to evaluate timing of the crustal anorthosite formation in the framework of the evolutional process of the Oman ophiolite. We estimated the formation age of the crustal anorthosite at 97.5 Ma ± 5.0 Ma, overlapping with the timing of the crust formation in the paleo spreading axis. The crustal anorthosite shows high‐Th/U ratio (~2.5) and high‐initial 87Sr/86Sr ratio (0.7050) due to seawater‐derived hydrothermal fluid ingress into the precursor gabbro. With using analytical technique of micro‐excavation at cryo‐temperature, we detected Cl from a few micrometer‐sized inclusion of aqueous fluid and chromite grains. The solubility of Cr was enhanced by complexation reactions with Cl in the hydrothermal fluid. Regarding reconstructed three‐dimensional mass distribution of the inclusion and chromite composition, maximum Cr content of parental fluid was estimated at ~69 000 μg/g. The exceptionally high‐Cr content was achieved locally by leaking of fluid and synchronous chromite crystallization during fluid entrapment. Presence of the deep‐seated hydrothermal circulation could be assigned to the segment end, where cold seawater penetrates into the lowermost crust and extract heat along widely spaced network‐like fluid channel.

Idiomorphic oikocrysts of clinopyroxene produced by a peritectic reaction within a solidification front of the Bushveld Complex

Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.

К вопросу о вхождении редкоземельных элементов в структуру хромпироповых гранатов

Rare earth elements (REE) in garnet are of interest in various fields of modern geology. The geochemistry of REEs in magmatic minerals is widely used in determining the distribution coefficients of crystal/melt and crystal/fluid, modeling the processes of melting and crystallization of magmatic rocks, studying deep mantle processes, age estimates and other issues of petrogenesis. The aim of the present work was a synthesis of a peridotite mineral association including the garnet containing REE at high pressure and high temperature. The initial sample consisted mainly of natural serpentine collected from ophiolites of the Eastern Sayan (Russia). As is known, the extreme stage of the regressive metamorphism of peridotites is serpentinization. It is depleted in calcium, but can recrystallize at high PT conditions into a harzburgite paragenesis, and at the initial stage of the experiment the chemical composition of the sample was a model harzburgite depleted in calcium and chromium, as well as a fluid of predominantly aqueous composition. As a source of chromium, chromite grains of 1–2 mm in size from peridotite xenoliths of the Udachnaya kimberlite pipe (Yakutia) were used. REE were added to the initial charge in the form of water-soluble salts. The experiment at a pressure of 5 GPa and temperature 1300 was performed on a multi-anvil high-pressure apparatus of the “split sphere” type (BARS) designed and developed at the V.S. Sobolev Institute of Geology and Mineralogy SB RAS. A container based on refractory oxide ZrO2 was used as a highpressure medium. The pressure in the cell before sample heating was estimated using the reference substances Bi and PbSe. The temperature was measured by a platinum-platinum-rhodium thermocouple PtRh30-PtRh6. The quenched was performed by switching off the voltage in the heater circuit. The experiment products contain an association of olivine + garnet + orthopyroxene + newly formed spinel. The predominant phase was olivine of a forsterite composition. A low-Fe orthopyroxene (1.49 – 1.68 wt% FeO) was found in elongated grains uniformly distributed throughout the sample. The newly formed spinel shows the faceted grains. The chromium content in the spinel significantly exceeds that of the initially added to the initial charge, 61.63 and 54.04 wt% Cr2O3, respectively. The garnet is characterized by a purple color, and was identified in the sample volume between olivine grains in the form of individual faceted crystals or their clusters. The largest garnets reached 0.5 mm in size. The synthesized garnet was determined as a high-Cr low-Ca pyrope variety. The contents of Cr2O3 and CaO are 10.15-11.21 and 0.06-0.11 wt%, respectively. The total content of REE in the garnet identified by the microprobe analysis is relatively high reaching 5-7 wt%. As a result of the work a mineral association corresponding to the peridotite paragenesis was obtained, including the subcalcic Crrich pyrope containing rare earth elements in significant amounts. It was estimated that their content in garnet mainly depends on the size of the ionic radius and, accordingly, on the atomic weight. This is consistent with the known facts about the preferable position of heavy REEs into the garnet structure compared to the light REEs.

Formation of serpentinite-hosted, Fe-rich arsenide ores at the latest stage of mineralization of the Bou-Azzer mining district (Morocco)

The Bou Azzer district in the central Anti-Atlas of Morocco is the world’s unique mining district where Co is exploited from serpentinite-hosted hydrothermal Co-Ni-Fe arsenide ores. This paper provides the first-ever mineralogical and geochemical characterization of a suite of Fe-rich arsenide orebodies found during recent mining in the Ait Ahmane area, in central Bou Azzer district. These are lens-like orebodies of massive and disseminated Fe-rich ores fully hosted in serpentinites (hereafter named as serpentinite-hosted ores) that are typically enveloped by arsenide-poor, carbonated serpentinite, evolving irregularly outwards to talc-rich serpentinite. The central part of these orebodies mainly consists of massive ore made up of aggregates of zoned spindle-like crystals of löllingite whereas the outer zone of disseminated ore consists of rosette-like löllingite. These ores often contain accessory grains of chromite that display strong fracturation, zoning and alteration.We propose a general model for the formation of the Bou-Azzer Co-Ni-Fe arsenide ores associated to the infiltration of high-temperature (>400 °C) slightly alkaline, oxidized and CaCl2-rich fluids through opening fault and fractures. When channeled through faults separating serpentinite and quartz diorite (or other rocks) the fluids deposited Ni- and Ni-Co-rich ores in these fault-related open spaces (here re-defined contact-type ores). Over the course of mineral deposition, the generated Ni-poor fluids tended to migrate into serpentinite, first through networks of thin entwined veins and progressively by intergranular percolation, promoting dissolution of the infiltrated serpentinite and precipitation of Co-Fe ores (the presence of remnants of chromite included in coexisting arsenides and calcite sustains the role played by dissolution/precipitation, ore-forming process). Further circulation of the evolving fluids through weak zones of massive serpentinite away from the serpentinite-quartz diorite (or other rocks) contact (intra serpentinite faults), formed the serpentinite-hosted, Fe-rich ore type represented by the F55 lens. These fluids most probably evolved at low temperature (~200 °C) towards slightly higher pH values and oxidizing conditions increasing the proportions of CO2 and CO32−, promoting the dissolution of serpentine, the increase in the magnetite component of chromite, the formation of Cr and Fe hydroxides, the mobilization of silica out of the reactive zone and the precipitation of calcite + Fe-rich arsenides.

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally. It has been exploited for nearly 100 years, yet its origin remains unresolved. In the present study, we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy. Our results indicate that the Merensky Reef formed through a range of diverse processes. Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth. The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow. Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation. The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts. Olivine-hosted melt inclusions containing Cl-rich apatite, sodic plagioclase, and phlogopite suggest the presence of highly evolved, volatile-rich melts. Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase, possibly triggered by flux of heat, acidic fluids, or hydrous melt. Together, these data suggest that the reef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes.

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