Chromite Compositions Research Articles

Podiform chromitites in lherzolitic mantle rocks (Andaman ophiolite, India): the role of magma/rock interaction and parental melt composition

Abstract In this study, Alpine-type (ophiolite hosted) podiform chromitites from the lherzolite dominant mantle sequence of the Andaman ophiolite, India are described. The mantle host rocks for the chromitite pods show features of melt-mantle interaction. The chromites of the podiform chromitites have high-Cr# (= Cr/[Cr + Al] atomic ratio) ranging between 0.69 and 0.75 and high-Mg# (= Mg/[Mg + Fe2+] atomic ratio) ranging between 0.58 and 0.72. Geochemically, the chromites of these high-Cr chromitites have an arc-spinel trend and suggest a supra-subduction zone setting. Contrarily, the low-Cr chromitites (reported elsewhere) show MORB-spinel trend. Parent melt composition is assessed on the basis of liquidus chromite compositions using various methods. The deduced composition shows that the melt parental to the high-Cr chromitite of the Andaman ophiolite had 12–13 wt.% Al2O3 and 0.2–0.5 wt.% TiO2 whereas the same for the low-Cr chromitites had ~ 16 wt.% Al2O3 and 0.5–0.9 wt.% TiO2. A correlation of the deduced parent melt compositions with the volcanic rocks of the Andaman ophiolite suggests a close affinity with the East Coast volcanics (south-Andaman) for the high-Cr chromitites. Contrarily, a similar correlation for the low-Cr chromitites is not so certain.

Genesis and tectonic implications of podiform chromitites in the metamorphosed ultramafic massif of Dobromirtsi (Bulgaria)

Podiform chromitites occur in the meta-dunite horizon of the Dobromirtsi ultramafic massif, in the Central Rhodope metamorphic core complex (southern Bulgaria). Although these were deformed and metamorphosed together with their host rocks several chromite cores from massive chromitite samples still preserve compositions unaffected by metamorphism – at least in terms of major elements. In situ laser ablation ICP-MS analyses of these chromite cores show a bimodality in the composition of chromite between high-Cr and high-Al {Cr#=[Cr/(Cr+Al)], atomic ratio=0.74–0.55} with Mg# between 0.69 and 0.60 [Mg#=Mg/(Mg+Fe2+), atomic ratio]. These compositions of chromite and the whole rock enrichment of Os–Ir–Ru relative to Pt–Pd in the chromitites resemble that reported for podiform chromitites in ophiolites elsewhere.Magmatic platinum-group minerals (PGM), including laurite (RuS2), Os–Ir alloys and sulfarsenides (irarsite, IrAsS), exhibit a wide range of 187Os/188Os (from 0.1097 to 0.1272) whereas 187Re/188Os is nearly zero. The distribution of the Os model ages calculated for these PGM ranges between 0.13 and 2.6Ga, with age peaks at 0.25, 0.4, 0.7, 2.1 and 2.6Ga. Two xenocrystic zircons were identified in the chromitites and yield concordant U–Pb age of 2257±80Ma (2σ) and 1952±82Ma (2σ). The oldest zircon exhibits a depleted-mantle Hf model age (TDM) of 2.77Ga and a “crustal” Hf model age (TCDM) of 3.1Ga, whereas the youngest has a TDM of 2.21Ga and a TCDM of 2.35Ga.We interpret the coexistence of both types of chromitites in a horizon of meta-dunites as the result of their precipitation from fractionating island arc tholeiite melts. The first reactions produced dunite; later the batches of magma were isolated from reaction with the peridotite wall-rocks and progressively fractionated in a network of dunite channels developed in the upper mantle beneath the back-arc spreading centre. The Os and Hf model ages suggest that the ultramafic protolith of the Dobromirtsi ophiolite was derived from an original Archean mantle at least 3Ga old, which underlay a continental crust and was reworked in the Paleo-proterozoic and possibly even younger times.

Syncollisional tholeiitic magmatism induced by asthenosphere upwelling owing to slab detachment at the southern margin of the Central Asian Orogenic Belt

The mafic–ultramafic complexes containing magmatic sulphides at the southern margins of the Central Asian Orogenic Belt have been recently proposed to result from an Early Permian mantle plume. However, in this study we show that the plume model cannot account for the observed geological characteristics of the Huangshan–Jingerquan mafic–ultramafic belt in the Northern Tianshan. Low K 2 O contents and positive correlation between TiO 2 and (Fe 2 O 3 ) T /MgO of the mafic–ultramafic complexes of this belt demonstrate a tholeiitic affinity. Enrichment of large ion lithophile elements and depletion of high field strength elements (in particular Nb and Ta) relative to mid-ocean ridge basalt indicate a subduction-modified mantle source. Lead isotope values and compositions of chromite indicate a significant contribution from the melting of asthenosphere. The absence of Late Carboniferous strata in the Huangshan–Jingerquan belt and Early Permian exhumation of blueschist and eclogite along the Aqikkuduk suture at the southern boundary of the belt indicate that an arc–continent collision occurred in the Late Carboniferous to Early Permian. We propose that the detachment of oceanic lithosphere from continental lithosphere during the collision induced asthenospheric upwelling, which resulted in melting of both the asthenosphere and the overlying metasomatized mantle wedge, and the formation of the mafic–ultramafic complexes with ages of 270–285 Ma along the Huangshan–Jingerquan belt. Supplementary materials: Zircon SHRIMP U–Pb ages of the Tudun and Huangshannan intrusions, major oxide and trace element compositions of the discussed intrusions, and clinopyroxene and chromite compositions of the selected intrusions are available at www.geolsoc.org.uk/SUP18656 .

English

  Archaean komatiites of the greenstone belt in Manica area, western Mozambique contains accessory amount of disseminated chromite grains. The komatiite rocks comprise peridotitic and basaltic komtiite. These komatiites are affected by regional metamorphism as all of the studied area. It is observed that only chromian spinel is preserved as the primary mineral and that all the other primary minerals in the komatiite samples are completely altered. The present work gives first time data on the effect of metamorphism on texture and composition of chromite enclosed in komatiite, Manica area, Mozambique. The primary chromian spinel has high Cr# (Cr/(Cr+Al) atomic ratio; approximate value = 0.9, which is comparable with Cr# value of chromian spinel in the worldwide komatiites. The chromian spinel have low Al and Mg value (up to 0.26 and 0.21 respectively, low Mg# and high Cr# and Fe# values. Most of the studied spinel grains rimmed by ferritchromite which is transformed outwards to magnetite. Textural features and mineral chemistry of chromian spinel enclosed in these komatiites indicate postmagmatic alteration and metamorphism in greenchist to amphibolite transition facies. The metamorphism also involves reequilibration of the chromian spinel cores with the surrounding silicate assemblage.   Key words: Chromite, microprobe analyses, komatiite, Mozambique.

Effect of isovalent ion substitution on electrical and dielectric properties of LaCrO3

In the present investigation, the effect of isovalent doping on the electrical and the dielectric properties of the system, La1−xGdxCrO3 was studied. A few compositions of Gd doped lanthanum chromite, La1−xGdxCrO3 (x = 0.00, 0.01, 0.05, 0.10, 0.15 and 0.20) were synthesised via auto-combustion route. The XRD patterns of all the compositions showed single phase formation having crystal structure similar to that of undoped LaCrO3. The Rietveld refinement of the XRD data of all the compositions indicated orthorhombic structure with space group Pnma at room temperature. The SEM micrographs revealed spherical, oval and irregular morphologies of all compositions having low porosity. The FTIR spectra of the investigated compositions showed various structural modes of vibrations. Real part of electrical conductivity exhibited frequency independent behaviour in the frequency range 20 Hz–1 MHz and in the temperature range 40–400 °C. The DC conductivity of the system was found to show Arrhenius behaviour up to 300 °C. The DC conductivity increased with x up to x = 0.01, the trend was reversed in the range 0.01 ⩽ x ⩽ 0.10 and thereafter it again increased with x up to x = 0.20. The temperature and composition dependent conduction mechanism was explained in terms of defects created due to change of concentration of dopant ion and the size difference between the host and dopant ions. The XPS analysis showed that the concentration of Cr6+ sites increased with the increase in Gd concentration, supporting the proposed conduction mechanism. The orientational and the space charge polarisation processes have been attributed to the total dielectric behaviour of the system in the measured frequency and temperature ranges.

CHROMITITES OF THE URALS (PART 1): OVERVIEWOF CHROMITE MINERAL CHEMISTRY AND GEO-TECTONIC SETTING

Published and unpublished compositions of chromite in 333 chromitite samples from 14 ultramafic complexes of the Urals are overviewed. The chromitites occur in the mantle unit and/or the supra-Moho cumulate sequence of ophiolite complexes, as well as in Alaskan-type intrusions. They vary in size from giant ore deposits associated with ophiolites (e.g., Kempirsai, Ray-Iz, Voykar-Syninsky) to sub-economic mineralization in the Alaskan-type complexes (e.g., Svetly Bor, Kachkanar). Mantle-hosted chromitites occur either as discordant, podiform, high-Cr ore bodies and sub-concordant elongated lenses of high-Al chromite. In the supra-Moho sequences of ophiolites, chromitite is mainly of the high-Al variety, and occurs as concordant layers alternated with peridotite and pyroxenite cumulates. In the Alaskan-type intrusions of the Urals, chromitite occurs as centimeter to meter-size pods and lenses having syngenetic or epigenetic relationship with the host dunite. Calculated melt compositions in equilibrium with chromite and comparison of chromite composition with those from various volcanic suites, and chromitites from different plutonic complexes, allow division of the Urals chromitites into four different compositional groups, corresponding to different geodynamic environments of formation: 1) The high-Al, low-Ti suite (Al 2 O 3 > 20 wt%, Cr# 20 wt%, Cr# 0.70, Al 2 O 3 0.70, Al 2 O 3 < 20 wt%, TiO 2 = 0.38-1.30 wt%, Fe 3+ # = 0.20-1.29, δ logf(O 2 ) = +0.9 ÷ +5.9) is represented by chromitites from the Urals Alaskan-type intrusions and the East-Khabarny complex. They have crystallized from Fe-rich magma (av. FeO/MgO = 1.35) under oxygen- fugacity conditions well above the FMQ buffer. The melt is characterized by high-Ti, high-K, calc-alkaline composition, having many geochemical characteristics in common with ankaramites. It was generated by partial melting of a fluid-metasomatized mantle source, in a subduction-influenced arc setting. However, the close similarity with the zoned complexes emplaced in the Russian-Far-East craton suggests that formation of Alaskan-type melts may be not restricted to SSZ, island arc settings.

PETROGENESIS OF Cr-RICH CALC-SILICATE ROCKS FROM THE BANDIHALLI SUPRACRUSTAL BELT, ARCHEAN DHARWAR CRATON, INDIA

Chromium-rich calc-silicate rocks in the Archaean Bandihalli supracrustal belt in the Dharwar Craton, India, show a characteristic mineral assemblage that includes euhedral and anhedral (resorbed) green garnet associated with chromite, in addition to diopside, quartz, clinozoisite and calcite. Two distinct types of green garnets and a brown variety were identified in the calc-silicate rocks. These garnets show complex chemical compositional variations that range between the endmembers of uvarovite, grossular, and spessartine. Chromite grains in the matrix are euhedral or subhedral, whereas inclusions in garnet are rounded or amoeboid in shape. The two textural varieties of chromite from Bandihalli are separated by a compositional gap. Furthermore, chromites from the study area have high Cr# (0.80–0.97) and low Mg# (0.01–0.04), which is similar to the chromite in serpentinites and are in contrast with chromite compositions reported from igneous rocks. Therefore, the chromite in the calcsilicate rocks may have been derived from serpentinized ultramafic rocks. Textural and compositional data presented here provide evidence for chemical changes of the primary chromite with increasing metamorphic grade, presumably associated with primary hydrothermal alteration that resulted in serpentinization and carbonation of the ultramafic protolith. Therefore, Cr-rich calcsilicate rocks can be considered as products of metamorphosed carbonated and serpentinized mafic or ultramafic igneous rocks.

Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE Bulgaria: a two-stage process

The Golyamo Kamenyane serpentinite is a portion of a metaophiolite, located in the Upper High-Grade Unit of the metamorphic basement of the Eastern Rhodope Metamorphic Complex, SE Bulgaria. It consists of metaharzburgite and metadunite hosting layers of metagabbro and some chromitite bodies. All these lithologies were affected by ultrahigh-pressure (UHP) metamorphism and subsequent retrograde evolution during exhumation. Chromite from chromitites can be classified into four textural groups: (1) partly altered chromite, (2) porous chromite, (3) homogeneous chromite and (4) zoned chromite. Partly altered chromite shows unaltered, Al-rich cores with unit cell size of 8.255 A and Cr# [Cr/(Cr + Al) atomic ratio] = 0.52–0.60, Mg# [Mg/(Mg + Fe2+) atomic ratio] = 0.65–0.70 and Fe3+/(Fe3+ + Fe2+) = 0.20–0.30, surrounded by porous chromite, with a cell size of 8.325 A, Fe3+/(Fe3+ + Fe2+) < 0.20 and values of Cr# and Mg# evolving from 0.60 to 0.91 and 0.65–0.44, respectively, from core to rim. The chemical composition of porous chromite varies within the following ranges: Cr# = 0.93–0.96, Mg# = 0.48–0.35 and Fe3+/(Fe3+ + Fe2+) = 0.22–0.53. Its unit cell size is very constant (8.350 A). Most pores in porous and partly altered chromite are filled with chlorite, which also occurs between chromite grains. Homogeneous chromite has Fe3+/(Fe3+ + Fe2+) = 0.55–0.66, Cr# = 0.96–0.99, Mg# = 0.32–0.19 and a cell size of 8.385 A. The cores of zoned chromite are similar to those of partially altered chromite, but the rims are identical to homogeneous chromite. Although chlorite predominates in the silicate matrix of homogeneous and zoned chromite, it coexists with some antigorite, talc and magnesiohornblende. Mineral data and thermodynamic modeling allow interpretation of the alteration patterns of chromite as the consequence of a two-stage process developed during retrograde metamorphic evolution coeval with fluid infiltration. During the first stage, chromite reacts in the presence of fluid with olivine to produce chlorite and Cr- and Fe2+-rich residual chromite (ferrous chromite) at ~700 to ~450 °C. This dissolution–precipitation reaction involves continuous chromite mass loss resulting in the development of a porous texture. This stage takes place progressively on cooling under water-saturated and reducing conditions. The second stage mainly consists of the formation of homogeneous chromite with ferrian chromite composition by the addition of magnetite to the porous ferrous chromite during a late oxidizing hydrothermal event.

Compositional Variations in the Mesoarchean Chromites of the Nuggihalli Schist Belt, Western Dharwar Craton (India): Potential Parental Melts and Implications for Tectonic Settings

The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60–75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.

MAGMATIC AND POST-MAGMATIC SIGNIFICANCE OF CHROMITITE AND ASSOCIATED PLATINUM-GROUP MINERALS (PGM) IN THE EASTERN KHOY OPHIOLITIC COMPLEX (NW IRAN)

The Khoy ophiolites are one of the largest Iranian ophiolitic complexes and are characterized by a very complicate geology, consisting of several ophiolites with different ages and geodynamic settings of formation. Few small lenticular and irregular bodies of podiform and schlieren type chromitites have been recognized in the Khoy ophiolites, occurring in the serpentinized mantle peridotites. They have massive and nodular textures. According to their chromite composition, the Khoy chromitites can be classified as Al-rich and fall clearly in the field of podiform chromitites. The magmatic chromite composition of the podiform chromitites of the Khoy ophiolites suggests that they crystallized from a MORB type melt in an extensional regime. The Khoy chromitites were altered during ocean floor metamorphisms and no magmatic silicates were preserved. The composition of chlorite occurring in the silicate matrix or in contact with chromite has been used as a geothermometer. The estimated temperatures possibly reflect the effects of low-grade metamorphism. The Khoy chromitites have very low total platinum group elements (PGE) concentration with a predominance of Os + Ir + Ru over Rh + Pt + Pd. Consistently with the geochemical data, they contain few platinum group minerals (PGM) of Ru, Ir and Os. Laurite is the most common PGM along with a minor Ir-Cu sulfide, possibly cuproiridsite, osmium and erlichmanite. Most of the PGM are magmatic in origin. They crystallized in a narrow range of temperature, around 1000 o C, at relatively low sulfur fugacity and in the absence of fluids. This observation is consistent with formation of the chromitites in a mid-oceanic ridge (MOR) rather than in a supra-subduction zone (SSZ).

Chromite–silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism

The Neoarchean layered anorthositic complex at Sittampundi in southern India is known for its chromitite layers that are mostly associated with anorthosite (An90–100). The chromitites contain FeAl-rich chromites concentrated in layers between amphibole-rich layers with a dominant mineralogy of amphibole–spinel–plagiocase±sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X-ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2O3 and Al2O3 contents varying between 34–40wt.% and 23–28wt.%. The chromite compositions are noticeably different from those in layered igneous intrusions of the Bushveld-Stillwater type. The existence of original highly calcic plagioclase, FeAl-rich chromite, and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra-subduction zone arc setting. In terms of mineralogy and field relations, the Sittampundi chromitites are remarkably similar to anorthosite-hosted chromitites in the Neoarchean Fiskenæsset anorthositic complex, Greenland. We propose that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone. This melting process produced hydrous, aluminous basalt, which fractionated at depth to give rise to a variety of high-alumina basalt compositions from which the anorthositic complex with its cumulate chromite-rich and amphibole-rich layers formed within the magma chamber of a supra-subduction zone arc.

Composition of chromites from kimberlites of the Botuobinskaya pipe in Yakutia

A representative collection (138 analyses) of chromites from kimberlites of the Botuobinskaya pipe in Yakutia was studied. With allowance for the Cr, Ti, and Al contents, the chromites are subdivided into the low-Cr aluminous group A and the high-Cr and high-Ti group B. The chromites of group A with their compositional variations controlled by the Al3+-Cr3+ isomorphism are not related to kimberlite in composition and reveal attributes of restites. The chromites of group B with heterovalent Ti4+ + Fe2+ + Fe3+-3Cr3+ isomorphism vary in their composition in line with the compositional variations of kimberlites, thus demonstrating their primary magmatic origin. The chromites of the second group crystallized simultaneously with olivines from kimberlites, and both minerals could have formed nodules of spinel dunite.

Reduction Feature and Selection Principle of Chromium Ore for Stainless Steelmaking in a Smelting Reduction Converter

This paper starts with the mineral structure, reducing properties of chromium ore and the physical and chemical properties of chromium oxide and discusses the requirements of the chemical composition of chromite and confirms the selection principle of chromium ore and reducing agent of chromium ore smelting reduction. The chemical compositions of the available chromite request the relatively high contents of chromium and iron, the lower contents of Al2O3, MgO and P in gangue and the lower Cr/Fe ratio in keeping with the case of high chromium. Chromoium ore in South Africa is considered to be suitable for smelting stainless steel crude melts in a smelting reduction converter. From the reducing power and economy angle, carbon as a reducing agent is more suited to reducing chromium ore.

Petrology and geochemistry of the Andaman ophiolite: melt–rock interaction in a suprasubduction-zone setting

Abstract: The Andaman ophiolite occurs as thrust slices in the outer arc of the Andaman–Java subduction zone. This ophiolite preserves the mantle sequence, layered ultramafic–mafic rocks, intrusive and extrusive rocks. The mantle sequence is represented by serpentinized lherzolite and harzburgite, hosting dunite and chromitite pods. The low Cr-number (0.2–0.4), Cr-number–TiO 2 relation of the chromites, oxygen fugacity ( f O 2 ) values ( Δ log f O 2 (FMQ) = −1.90) and trace elements of mantle peridotites indicate a mid-ocean ridge basalt–suprasubduction-zone (MORB–SSZ) setting. The MORB mantle underwent a low degree of melting ( c . 10–15%) and interacted with the subduction-zone melts. Melt–rock interaction of the peridotites in a suprasubduction zone is demonstrated by the replacement of pyroxene grains by olivine grains (Fo 90 ), composition of chromites and oxygen fugacity ( Δ log f O 2 (FMQ) = –1.90 to +2.16, where FMQ is the fayalite–magnetite–quartz buffer). The chromite composition of chromitite pods (Cr-number 0.72–0.75), f O 2 levels and trace elements for layered peridotites, and occurrence of the extrusive rocks as low-Ca boninite and island arc tholeiitic (IAT) basalt indicate interplay of both boninite and IAT melts for the Andaman ophiolite. The MORB mantle of the subducting Indian plate accreted into the mantle wedge and then melting of the accreted mantle produced boninite melt at the first stage and tholeiitic melts at the second stage. Supplementary material: Sample co-ordinates from the Andaman ophiolite, mineral analysis data for all the samples, modal compositions of the units of the Andaman ophiolite and details of the laboratory analyses including details of the standard are available at http://www.geolsoc.org.uk/SUP18459 .

Magmatic and metamorphic imprints in 2.9 Ga chromitites from the Sittampundi layered complex, Tamil Nadu, India

The ca. 2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6 m thick) within clinopyroxenite and highly calcic anorthosite (An 99–100) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at ca. 2.51 Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca. 2.50–2.45 Ga (granulite facies) and at 0.72–0.50 Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place. Chromite compositions show large variations (Cr# = 26–60, Mg# = 4–45, Fe 3+# = 2–24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr# = 3–21; Mg# 45–61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe–Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean–Proterozoic boundary.

Occurrence and mineral chemistry of chromite and xieite in the Suizhou L6 chondrite

The occurrence and mineral chemistry of chromite and its high-pressure phase xieite in the Suizhou meteorite were studied by different modern micromineralogical techniques. Three types of occurrences for chromite were observed in the Suizhou L6 chondrite: coarse chromite grains, cluster of chromite fragments in molten plagioclase, and exsolution lamellar chromite in olivine. All the chromite grains of the first two types are remarkably similar in chemical compositions, but the composition of exsolution chromite is inhomogeneous and variable in Al2O3 content. Xieite is a post-spinel CT-phase of chromite firstly found in the Suizhou meteorite. Three types of occurrences of xieite have also been revealed in this meteorite: coarse xieite grains, complex three-zone-grains consisting of the inner xieite, the intermediate lamellae-like CF-phase and the outer chromite phase, and two-phase-grains consisting of xieite and one of the high-pressure silicate minerals lingunite, ringwoodite or majorite. The curved boundary between xieite and the silicate half in two-phase grains is indicative of some partial or even full melting of the silicate phase. EPMA and EDS results show that the compositions of xieite inside/contacting the shock veins are also identical to that of chromite outside the veins. However, some element diffusion appeared in between the xieite and the silicate half in the two-phase grains, namely, some of Al3+ from lingunite, or Fe2+ from ringwoodite migrated to xieite, and some of Cr3+ migrated from xieite to lingunite or ringwoodite. Majorite in two-phase grains shows remarkable decrease of SiO2 and MgO, and notable increase of Al2O3 and CaO, indicating that its host mineral pyroxene was fully molten and mixed with the surrounding silicate melt of the vein matrix. The complexity in mineral chemistry of these two-phase grains in shock veins can be explained by the much higher shock peak temperature in shock veins (1800–2000°C) than in unmelted main body (∼1000°C), and by the much lower density of the silicate minerals (2.6–3.3 g/cm3) than that of chromite (4.43 g/cm3). Being a refractory and a relatively high-impedance material, chromite is chemically more stable and easier to reflect shock wave into the silicate half causing the partial or even full melting of silicate phases, upon which some diffusion of elements between the two phases themselves, or even mixing of molten pyroxene and the surrounding silicate melt.

Low temperature magnetic transition of chromite in ordinary chondrites

[1] We present low temperature magnetic data (thermal demagnetization, hysteresis) obtained on thirty three ordinary chondrite meteorites. A magnetic transition is observed in the 40–80 K range (average 66 K), and is interpreted as a ferrimagnetic to paramagnetic phase transition. We present quantitative analyses of changes in magnetization across the magnetic phase transition and chemical elemental analyses to argue that chromite, a common accessory mineral in meteorites, is the mineral undergoing the phase transition, and not troilite or tetrataenite as proposed in previous studies. Moreover, we observe a correlation between the measured Curie temperature and chemical composition of chromites in ordinary chondrites. Low temperature magnetic measurement are thus a sensitive indicator of chromite composition in these meteorites, and as such a possible proxy to the understanding of their thermal metamorphism.

Compositional variations in the Mesoarchean chromites of the Nuggihalli schist belt, Western Dharwar Craton (India): potential parental melts and implications for tectonic setting

The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60–75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.

Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition

The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr2O3 content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.

Neoproterozoic podiform chromitites in serpentinites of the Abu Meriewa–Hagar Dungash district, Eastern Desert, Egypt: Geotectonic implications and metamorphism

AbstractPodiform chromitites hosted in serpentinites (after harzburgite and dunite) and talc‐carbonate rocks from the Abu Meriewa–Hagar Dungash district (MHD), Eastern Desert of Egypt, together with metagabbros, pillow metavolcanics, and metasediments, form an ophiolitic mélange formed during the Neoproterozoic Pan‐African Orogeny. The chromitites show massive, disseminated, and nodular textures. Chromite cores in chromitites have high and restricted ranges of Cr# (0.65–0.75) and Mg# (0.64–0.83), implying primary compositions not affected by metamorphism. Therefore, they are used as reliable indicators of parent magma composition and tectonic affinities of these highly metamorphosed rocks. On the contrary, the altered rims are high‐Cr, low‐Fe3+spinel (rather than ferritchromit) enriched in Cr, Fe, and Mn, and depleted in Al and Mg (Cr# = 0.75–0.97, Mg# = 0.29–0.79), due to equilibration with interstitial silicates during regional metamorphism up to transitional greenschist–amphibolite facies at about 500–550°C. The primary chromite compositions suggest derivation from a high‐Mg tholeiitic, to possibly boninitic, parental magma in a supra‐subduction zone (arc–marginal basin) environment, similar to the spatially associated metavolcanic rocks. The MHD chromitites are most probably formed by melt–rock interaction mechanisms. The high Cr# of the investigated chromites suggests high degrees of partial melting of a depleted harzburgite source by interaction with primitive basaltic melt of deeper origin followed by mixing. Such Cr‐rich chromites are common in chromitites from the Eastern Desert of Egypt, implying broad thermal anomalies, possibly linked to an important geodynamic feature of the Arabian–Nubian Shield (ANS) evolution. This could revive interest in models that involve asthenospheric uprise, related to plume interaction or most probably due to oblique convergence of arc terranes during early evolution of the ANS.