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Shoji Arai - One of the best experts on this subject based on the ideXlab platform.

  • Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites
    MDPI AG, 2018
    Co-Authors: Makoto Miura, Shoji Arai, Tomoyuki Mizukami, Vladimir R. Shmelev, Satoko Ishimaru
    Abstract:

    Ultrahigh-pressure (UHP) Chromitites containing UHP minerals such as coesite and diamond have been reported from some ophiolites in Tibet and the Polar Urals. Their nature, i.e., origin, P-T path and abundance, however, are still controversial and left unclear. Here we describe Chromitites in the Higashi-akaishi (HA) ultramafic complex in the Cretaceous Sanbagawa metamorphic belt, Japan, which experienced UHP condition (up to 3.8 GPa) at the peak metamorphism via subduction, in order to understand the nature of UHP Chromitites. The HA peridotites typically contain garnets and are associated with eclogites, and their associated Chromitites are expected to have experienced the UHP metamorphism. The Higashi-akaishi (HA) Chromitites show banded to massive structures and are concordant to foliation of the surrounding peridotite. Chromian spinels in the Chromitite and surrounding peridotites were sometimes fractured by deformation, and contain various inclusions, i.e., blade- and needle-like diopside lamellae, and minute inclusions of pyroxenes, olivine, and pargasite. The peculiar UHP minerals, such as coesite and diamond, have not been found under the microscope and the Raman spectrometer. Spinels in the HA Chromitites show high Cr#s (0.7 to 0.85), and low Ti contents (<0.1 wt %), suggesting a genetic linkage to an arc magma. The HA Chromitites share the basic petrographic and chemical features (i.e., diopside lamellae and arc-related spinel chemistry) with the UHP Chromitites from Tibet and the Polar Urals. This suggests that some of the characteristics of the UHP Chromitite can be obtained by compression, possibly via deep subduction, of low-P Chromitite

  • peridotite Chromitite complexes in the eastern desert of egypt insight into neoproterozoic sub arc mantle processes
    Gondwana Research, 2017
    Co-Authors: Mohamed Zaki Khedr, Shoji Arai
    Abstract:

    Abstract The Neoproterozoic peridotite-Chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E–W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al 2 O 3 , CaO and clinopyroxene ( 2 O 3 , 0.7 wt% Cr 2 O 3 , and 0.2 wt% Na 2 O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41–0.80, with low TiO 2 (0.03 wt%), MnO (0. 3 wt%) and Y Fe [(Fe 3 + /(Cr + Al + Fe 3 + ) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25–40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn Chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah Chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied Chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of Chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the Chromitite size. The factors controlling the size of Neoproterozoic Chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.

  • chemical homogeneity of high cr Chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of bir tuluha ophiolite northern arabian shield saudi arabia
    Ore Geology Reviews, 2017
    Co-Authors: Abdel Monem Habtoor, Ahmed H Ahmed, Norikatsu Akizawa, Hesham M Harbi, Shoji Arai
    Abstract:

    Abstract The Bir Tuluha ophiolite is one of the most famous Chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where Chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied Chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the Chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and Chromitites. Platinum-group elements (PGE) in Chromitite pods generally exhibit steep negative slopes of typical ophiolitic Chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in Chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr Chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr Chromitites and PGE-PGM distribution.

  • chemical variations of mineral inclusions in neoproterozoic high cr Chromitites from egypt evidence of fluids during Chromitite genesis
    Lithos, 2016
    Co-Authors: Mohamed Zaki Khedr, Shoji Arai
    Abstract:

    Abstract This paper details the mode of occurrence, petrography, and chemistry of mineral inclusions hosted in chromian spinels of the Neoproterozoic Chromitites in the Southern Eastern Desert of Egypt. Neoproterozoic podiform Chromitites from the Arais, Balamhindit, and Abu Dahr areas, in the Southern Eastern Desert, can be texturally and chemically classified into two main types: primary high-Al (spinel Cr#   0.75) Chromitites. The former, being free of primary-mineral inclusions, was crystallized mainly from the MORB-like tholeiitic melt generated during proto-forearc spreading at the initiation of subduction, whereas the latter was formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage. The primary mineral inclusions, such as Na- and K-phlogopites, pargasite–edenite and olivine with subordinate pyroxenes, millerite, and laurite, were trapped within the chromian spinel during the magmatic precipitation of the Chromitites. The Abu Dahr Chromitites are free of primary hydrous inclusions; on the other hand, Arais and Balamhindit high-Cr Chromitites are enriched in Na- and K-phlogopites, respectively, as a result of a difference in the K/Na ratio of the magma responsible for Chromitite crystallization at different mantle depths. This difference in the K/Na ratio can possibly be attributed to fractionation of the upward-migrating hydrous fluids/melts by the crystallization of K- or Na-rich minerals. The Balamhindit complex, where the Chromitite showed K-phlogopite inclusions within the chromian spinel, was probably derived from a deeper part of the mantle than the other areas, where the Chromitite shows inclusions of Na-rich hydrous phases. Both K- and Na-phlogopites were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromian spinels during the Chromitite formation at different mantle depths, where the K/Na ratio decreases upward through the incorporation of Na from the peridotite wall-rock, combined with the precipitation of K-rich phases at deeper depths. The chemistry of both primary mineral inclusions and chromian spinels suggests an arc-related tectonic setting for our Chromitites that were crystallized at 1000 °C–1300 °C under pressures

  • arc related harzburgite dunite Chromitite complexes in the mantle section of the sabzevar ophiolite iran a model for formation of podiform Chromitites
    Gondwana Research, 2015
    Co-Authors: Hadi Shafaii Moghadam, Mohamed Zaki Khedr, Akihiro Tamura, Robert J Stern, Ghasem Ghorbani, Shoji Arai, Chris J. Ottley
    Abstract:

    Abstract Podiform Chromitites are common within the mantle section of the Late Cretaceous Sabzevar ophiolite in NE Iran. We studied Chromitite pods and related ultramafic rocks from three Sabzevar massifs: Baghjar-Kuh Siah, Gaft Chromitite Mine and Forumad peridotite–Chromitite. These represent an upper mantle sequence just below the Sabzevar Moho. The Baghjar-Kuh Siah mantle sequence contains plagioclase lherzolites, enriched in bulk REEs, with low Cr# spinels and MORB-like clinopyroxenes. These lherzolites formed due to the impregnation of MORB-like melts. The Gaft and Forumad harzburgites are depleted in trace and rare earth elements and thus are residues after high degree of partial melting (more than exhaustion of Cpx). The Gaft Chromitite Mine includes two types of podiform Chromitites, high Cr# and low Cr#. The melt precipitating high Cr# spinel was boninitic whereas the melt forming the low Cr# Chromitites was tholeiitic. Most Forumad massif Chromitites have high Cr# spinels, although those rich in silicate inclusions are aluminous. Trace and REE element patterns of Forumad harzburgite clinopyroxene are similar to those in supra-subduction zone (SSZ) peridotites while those of Baghjar-Kuh Siah lherzolites are similar to MOR peridotite clinopyroxenes. These mineral data are also consistent with bulk rock trace and rare earth elements composition of their host peridotites. Field observations indicate that early tholeiitic magmas were followed by late boninites, as revealed in Chromitite compositions as well as mantle rocks and dikes. We suggest a time-integrated model for the evolution of the Sabzevar mantle sequence during an early stage of subduction initiation associated with formation of an incipient arc. In this scenario, MORB-like melts (forearc basalts) formed first, causing low Cr# Chromitites and plagioclase–clinopyroxene impregnations. Subsequent arc-like or boninitic melts with increasing contribution of slab-derived fluids were responsible for the formation of replacive dunites and high Cr# Chromitites.

Jingsui Yang - One of the best experts on this subject based on the ideXlab platform.

  • genesis of the ray iz Chromitite polar urals inferences to mantle conditions and recycling processes
    Lithos, 2020
    Co-Authors: Paul T. Robinson, Jingsui Yang, Fahui Xiong, Basem Zoheir, Fancong Meng
    Abstract:

    Abstract Ophiolitic Chromitites in the eastern sector of the Alpine-Himalayan and Polar Ural orogenic belts preserve evidence of formation under ultra-high-pressure (UHP), deep mantle conditions. The textural and compositional characteristics of microscopic and submicroscopic inclusions in magnesiochromite from the Ray-Iz ophiolites are considered as clues of their complex evolutionary history. Bulk-rock platinum-group-element (PGE) compositions of refractory and metallurgical Chromitite ores (102–309 ppb) with generally IPGE>PPGE are consistent with supra-subduction zone (SSZ) metasomatism. The abundant unusual UHP phases (e.g., micro-diamonds, moissanite, coesite…etc.), together with clinopyroxene lamellae, and globular silicate inclusions in the Cr-spinel are seen as indications of deep mantle recycling of the mantle section of the Ray-Iz ophiolites. The calculated fO2 values for dunite and Chromitite (+1.17 to +4.16, generally above the FMQ buffer) are in line with interaction of mid-ocean ridge (MOR) or back-arc ophiolites with oxidized, Mg-rich silicic (boninitic) melts in a SSZ environment.

  • deep mantle origin and ultra reducing conditions in podiform Chromitite diamond moissanite and other unusual minerals in podiform Chromitites from the pozanti karsanti ophiolite southern turkey
    American Mineralogist, 2017
    Co-Authors: Dongyang Lian, Jingsui Yang, Yildirim Dilek, Zhongming Zhang, Fahui Xiong, Fei Liu, Wengda Zhou
    Abstract:

    The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of Chromitites are observed in the Pozanti-Karsanti ophiolite. One type of Chromitites occurs in the cumulate dunites around the Moho, and the other type of Chromitites is hosted by the mantle harzburgites below the Moho. The second type of Chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform Chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform Chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform Chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and Chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform Chromitite.

  • high al and high cr podiform Chromitites from the western yarlung zangbo suture zone tibet implications from mineralogy and geochemistry of chromian spinel and platinum group elements
    Ore Geology Reviews, 2017
    Co-Authors: Fahui Xiong, Paul T. Robinson, Jingsui Yang, Zhao Liu, Wenda Zhou, Guangying Feng, Xiaolu Niu
    Abstract:

    Abstract On the basis of their mineral chemistry, podiform Chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 ∗ Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different Chromitite compositions. We report here the occurrence of both high-Cr and high-Al Chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of Chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual Chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al Chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al Chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.

  • origin of podiform Chromitite a new model based on the luobusa ophiolite tibet
    Gondwana Research, 2015
    Co-Authors: Fahui Xiong, Paul T. Robinson, Jingsui Yang, Zhao Liu, Songyong Chen
    Abstract:

    Abstract Podiform Chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform Chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the Chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive Chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29 wt.%. Mg#s (= 100 ∗ Mg / (Mg + Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated Chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58 wt.%. In addition, magnesiochromite in massive ores has higher Fe3 +/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated Chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform Chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of Chromitite crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit Chromitite ores in melt channels.

  • compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments
    Gondwana Research, 2014
    Co-Authors: Meifu Zhou, Paul T. Robinson, Jingsui Yang, Benxun Su, Jianwei Li, John Malpas
    Abstract:

    Abstract Podiform Chromitites in the mantle sections of ophiolites belong to either high-Cr (metallurgical) or high-Al (refractory) varieties. Their highly variable compositions are reflected by different Cr#s [100Cr / (Cr + Al)] and Cr2O3 and Al2O3 contents of the chromite, falling in the boninitic and MORB fields, respectively. Parental magmas of high-Cr Chromitites have higher Sc, Mn, Co and Ni, and lower Ti, V, Zn and Ga concentrations than MORB melts; their trace-element patterns are similar to those of boninites, except for Ni and Zn. In contrast, high-Al Chromitites have parental magmas characterized by generally flat MORB-normalized patterns, showing slight enrichments in V, Mn and Co, and depletion in Ni and Zn. Regardless of their compositions, both types of Chromitites have chondrite-normalized platinum group element (PGE) patterns showing enrichment in IPGE and depletion in PPGE. A variety of platinum group minerals are typically present in both types, occurring either as euhedral inclusions or along fractures in chromite grains. These minerals have a wide span of Re–Os isotopic compositions, reflecting a variety of origins. There is a diversity of unusual minerals and mineral inclusions associated with podiform Chromitites. The presence of these minerals suggest that grains of amphibolite (plagioclase, amphibole and zircon) and eclogite (coesite, kyanite and garnet) were present in the magmas from which chromite crystallized. Multiphase mineral inclusions demonstrate that podiform Chromitites form from hydrous mafic magmas in suprasubduction zone environments (SSZ). We propose a new model in which Chromitite formation was involved in intra-oceanic subduction zones initiated in closing oceanic basins. Continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist, amphibolite and eclogite facies conditions. The tearing and breakoff of the subducted slab, possibly along the transitional contact between amphibolites and eclogites, create a slab window through which the underlying asthenosphere rises and melts to generate Cr-rich mafic magmas. These upward-migrating magmas pass through the subduction zone and assimilate the subducted slab. As a result of slab contamination, these magmas become more siliceous, more oxidized and more hydrous, rapidly triggering chromite crystallization. Minute grains of chromite are suspended in the upward-moving magmas as they migrate through the overlying metasomatized mantle wedge. Such chromite-bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the Moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced. Highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform Chromitites and host peridotites of ophiolites. Some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It is also possible that some diamonds may have formed in the subducted slab below about 150 km. Some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the SSZ melts. Some of these SSZ mantle wedges are emplaced on land to become podiform Chromitite-bearing ophiolites.

Ahmed H Ahmed - One of the best experts on this subject based on the ideXlab platform.

  • chemical homogeneity of high cr Chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of bir tuluha ophiolite northern arabian shield saudi arabia
    Ore Geology Reviews, 2017
    Co-Authors: Abdel Monem Habtoor, Ahmed H Ahmed, Norikatsu Akizawa, Hesham M Harbi, Shoji Arai
    Abstract:

    Abstract The Bir Tuluha ophiolite is one of the most famous Chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where Chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied Chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the Chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and Chromitites. Platinum-group elements (PGE) in Chromitite pods generally exhibit steep negative slopes of typical ophiolitic Chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in Chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr Chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr Chromitites and PGE-PGM distribution.

  • heterogeneously depleted precambrian lithosphere deduced from mantle peridotites and associated Chromitite deposits of al ays ophiolite northwestern arabian shield saudi arabia
    Ore Geology Reviews, 2015
    Co-Authors: Ahmed H Ahmed, Abdel Monem Habtoor
    Abstract:

    Abstract The mantle section of Al'Ays ophiolite consists of heterogeneously depleted harzburgites, dunites and large-sized Chromitite pods. Two Chromitite-bearing sites (Site1 and Site2), about 10 km apart horizontally from one another, were examined for their upper mantle rocks. Cr-spinels from the two sites have different chemistry; Cr-rich in Site1 and Al-rich in Site2. The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, dunites and Chromitites is remarkably high 0.78, 0.77 and 0.87, respectively, in Site1, compared with those of Site2 which have intermediate ratio averages 0.5, 0.56 and 0.6, respectively. The platinum-group elements (PGE) in Chromitites also show contrasting patterns from Site1 to Site2; having elevated IPGE (Os, Ir, Ru) and strongly depleted in PPGE (Rh, Pt, Pd) with steep negative slopes in the former, and gentle negative slopes in the latter. The oxygen fugacity (Δlog fO2) values deduced from harzburgites and dunites of Site1 show a wide variation under reducing conditions, mostly below the FMQ buffer. The Site2 harzburgites and dunites, on the other hand are mostly above the FMQ buffer. Two magmatic stages are suggested for the lithospheric evolution of Al'Ays ophiolite in response to a switch of tectonic setting. The first stage produced a peridotites–Chromitites suite with Al-rich Cr-spinels, possibly beneath a mid-ocean ridge setting, or most likely in back-arc rift of a supra-subduction zone setting. The second stage involved higher degrees of partial melting, produced a peridotites–Chromitites suite with Cr-rich Cr-spinels, possibly in a fore-arc setting. The coexistence of compositionally different mantle suites with different melting histories in a restricted area of an ophiolite complex may be attributable to a mechanically juxtaposed by mantle convection during recycling. The mantle harzburgites and dunites are apt to be compositionally modified during recycling process; being highly depleted (Site1 case) than their original composition (Site2 case).

  • highly depleted harzburgite dunite Chromitite complexes from the neoproterozoic ophiolite south eastern desert egypt a possible recycled upper mantle lithosphere
    Precambrian Research, 2013
    Co-Authors: Ahmed H Ahmed
    Abstract:

    Abstract Five Neoproterozoic ophiolitic complexes from the southern Eastern Desert of Egypt have been petrologically examined for their upper mantle harzburgite–dunite–Chromitite associations. Three of them are exceptionally fresh (Abu Dahr, Abu Siayil and Arays), while the other two localities (Belamhandeit and Umm Thagar) are severely serpentinized. Although the upper mantle harzburgite hosts in these complexes are highly depleted, they contain frequent large-sized Chromitite pods with metallurgical grade. Orthopyroxene, olivine and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo94) than those in harzburgites (Fo92). Chromian spinels in harzburgites, dunites and Chromitites are very refractory with restricted chemical compositions of high-Cr varieties. The average Cr-ratio (=Cr/(Cr + Al) atomic ratio) of chromian spinel in the Chromitites and dunite envelopes ranges from 0.76 to 0.87, while it ranges from 0.67 to 0.86 in the harzburgite hosts. Platinum-group elements (PGEs) in Chromitites exhibit steep negatively sloped distribution patterns, being highly enriched in IPGEs (Os, Ir, Ru) and strongly depleted in PPGEs (Rh, Pt, Pd). The estimated chemical composition of the primitive parental magma in equilibrium with podiform Chromitites and associated ultramafic rocks are characterized by Al2O3 contents and FeO/MgO ratio that is fairly similar to a boninitic source generated at supra-subduction zone settings. The studied complexes show constantly high oxygen fugacities (fO2), averaging Δlog fO2 + 3.33, +2.42 and +1.80 in Chromitites, dunites and harzburgites, respectively. The high oxidation state of the studied upper mantle ophiolitic complexes also suggests a boninitic source in the mantle wedge of an arc setting. The highly depleted nature of the harzburgite hosts in the studied complexes is inconsistent with the consensus that podiform Chromitite is hosted most commonly by moderately refractory harzburgites. This can be achieved if the podiform Chromitites and associated ultramafic rocks have been subsequently modified during deep recycling process within the upper mantle where the harzburgite hosts and dunite envelopes become more refractory than the original compositions. Hence, the uncommon presence of podiform Chromitites hosted by highly refractory ultramafic rocks were most probably due to a second stage melting of a depleted mantle harzburgite at a fore-arc setting, or alternatively due to deep recycling processes within the upper mantle.

  • podiform Chromitite classification revisited a comparison of discordant and concordant Chromitite pods from wadi hilti northern oman ophiolite
    Journal of Asian Earth Sciences, 2012
    Co-Authors: Makoto Miura, Ahmed H Ahmed, Shoji Arai, Tomoyuki Mizukami, Masayuki Okuno, Shinji Yamamoto
    Abstract:

    Abstract Two types of podiform Chromitite, concordant and discordant, were examined in the mantle section of northern Oman ophiolite along Wadi Hilti, to revisit the structural classification of podiform Chromitite. They are contrasted in mineral chemical characteristics, in addition to the difference in attitude; the Cr/(Cr + Al) atomic ratio of spinel is around 0.6 for the concordant Chromitite and surrounding peridotites, but is around 0.7 for the discordant one and surrounding peridotites. Chromian spinel grains contain pargasite-rich inclusions of primary origin from the both types, but they are far less abundant and smaller in size in the concordant Chromitite than in the discordant one. Thin lamellae of pyroxenes in chromian spinel, similar to those in ultrahigh-pressure (UHP) Chromitites from Tibet, are available only from the concordant Chromitite. The dunite enveloping the concordant Chromitite is extraordinarily high in NiO (up to >0.5 wt.%), suggesting subsolidus Ni diffusion from the Chromitite. The involved melt was quite different between the two types of Chromitite; the melt to precipitate the discordant one was more hydrous than that for the concordant one because of far more abundance of hydrous minerals in the former. The difference in duration of subsolidus cooling, and probably decompression, is prominent between the two types of Chromitite. The concordant Chromitite cannot be formed from the discordant one simply by metamorphic conversion: the former is of deep magmatic origin whereas the latter, of shallow magmatic origin.

  • osmium isotope systematics of the proterozoic and phanerozoic ophiolitic Chromitites in situ ion probe analysis of primary os rich pgm
    Earth and Planetary Science Letters, 2006
    Co-Authors: Ahmed H Ahmed, Karen Hanghoj, Peter B Kelemen, Stanley R Hart, Shoji Arai
    Abstract:

    In situ 187 Os/ 188 Os ratios are determined on Os-rich platinum-group minerals in podiform Chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite. Because they have very low Re/Os, these primary minerals reflect the initial 187 Os/ 188 Os ratios of their parental magmas. The platinum-group minerals (PGM) in the central Eastern Desert Chromitites exhibit sub-chondritic to chondritic 187 Os/ 188 Os ratios, 0.1226 on average, which is lower than the primitive upper mantle evolution trend of a comparable age. Those of the southern Eastern Desert Chromitites have more radiogenic Os, with supra-chondritic 187 Os/ 188 Os ratio of about 0.1293 on average, which could be due to crustal contamination. The three Chromitite types in the northern part of the Oman ophiolite are almost indistinguishable in terms of their 187 Os/ 188 Os ratios; they have overlapping values ranging from sub-chondritic to supra-chondritic ratios. The PGE-rich, mantle Chromitite samples have a wide range of 187 Os/ 188 Os ratio from 0.1230 up to 0.1376, with an average of 0.1299. The values of the PGE-poor mantle Chromitites overlap in their 187 Os/ 188 Os ratios with PGE-rich chromites, but are less variable and have a significantly higher average ratio. The Moho transition zone (MTZ) Chromitites are highly variable in the 187 Os/ 188 Os ratio, ranging from 0.1208 up to 0.1459. The wide range of 187 Os/ 188 Os ratios, from 0.1192 to 0.1459, in platinum-group minerals in Egyptian and Oman ophiolites can be attributed to the diversity of origin of their podiform Chromitites. The Os-isotope data combined with spinel chemistry indicate that the way involved in podiform Chromitite formation was not substantially different between the Proterozoic ophiolite of Egypt and the Phanerozoic ophiolite in northern Oman. The Os-isotope compositions of the mantle Chromitites in the Proterozoic ophiolite of Egypt clearly suggest crustal contamination. The heterogeneity of 187 Os/ 188 Os ratios combined with the spinel chemistry and high PGE contents of the PGE-rich Chromitite in the Oman ophiolite may give reliable evidence for high degree partial melting at a supra-subduction zone setting. Crustal contamination from the subducted slab, and assimilation of previously altered, lower crustal gabbro, may have contributed to the high Cr# spinel and radiogenic Os characteristics in Chromitite formed in the mantle section and along the Moho transition zone, respectively.

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  • a new model for Chromitite formation in ophiolites fluid immiscibility
    Science China-earth Sciences, 2021
    Co-Authors: Xia Liu, İbrahim Uysal, Meifu Zhou, Paul T. Robinson, Chen Chen, Yan Xiao, Yang Bai, Pengfei Zhang
    Abstract:

    Although the involvement of hydrous fluids has been widely invoked in formation of podiform Chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform Chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kizildag ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ , forming podiform Chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and Chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in Chromitite formation.

  • genesis of the ray iz Chromitite polar urals inferences to mantle conditions and recycling processes
    Lithos, 2020
    Co-Authors: Paul T. Robinson, Jingsui Yang, Fahui Xiong, Basem Zoheir, Fancong Meng
    Abstract:

    Abstract Ophiolitic Chromitites in the eastern sector of the Alpine-Himalayan and Polar Ural orogenic belts preserve evidence of formation under ultra-high-pressure (UHP), deep mantle conditions. The textural and compositional characteristics of microscopic and submicroscopic inclusions in magnesiochromite from the Ray-Iz ophiolites are considered as clues of their complex evolutionary history. Bulk-rock platinum-group-element (PGE) compositions of refractory and metallurgical Chromitite ores (102–309 ppb) with generally IPGE>PPGE are consistent with supra-subduction zone (SSZ) metasomatism. The abundant unusual UHP phases (e.g., micro-diamonds, moissanite, coesite…etc.), together with clinopyroxene lamellae, and globular silicate inclusions in the Cr-spinel are seen as indications of deep mantle recycling of the mantle section of the Ray-Iz ophiolites. The calculated fO2 values for dunite and Chromitite (+1.17 to +4.16, generally above the FMQ buffer) are in line with interaction of mid-ocean ridge (MOR) or back-arc ophiolites with oxidized, Mg-rich silicic (boninitic) melts in a SSZ environment.

  • distinctive melt activity and chromite mineralization in luobusa and purang ophiolites southern tibet constraints from trace element compositions of chromite and olivine
    Chinese Science Bulletin, 2019
    Co-Authors: Benxun Su, Meifu Zhou, Paul T. Robinson, Jiejun Jing, Chen Chen, Yan Xiao, Davide Lenaz, Yan Hu
    Abstract:

    Abstract To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and Chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and Chromitite are distinctly different between the two ophiolites. In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements (e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and Chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of Chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements (e.g., Nb, Zr). Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate Chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition, the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively, and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form Chromitites requires the presence of focused melt channels.

  • high al and high cr podiform Chromitites from the western yarlung zangbo suture zone tibet implications from mineralogy and geochemistry of chromian spinel and platinum group elements
    Ore Geology Reviews, 2017
    Co-Authors: Fahui Xiong, Paul T. Robinson, Jingsui Yang, Zhao Liu, Wenda Zhou, Guangying Feng, Xiaolu Niu
    Abstract:

    Abstract On the basis of their mineral chemistry, podiform Chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 ∗ Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different Chromitite compositions. We report here the occurrence of both high-Cr and high-Al Chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of Chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual Chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al Chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al Chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.

  • iron isotopic fractionation and origin of Chromitites in the paleo moho transition zone of the kop ophiolite ne turkey
    Lithos, 2017
    Co-Authors: Pengfei Zhang, İbrahim Uysal, Meifu Zhou, Paul T. Robinson, Erdi Avci
    Abstract:

    The paleo-Moho transition zone (MTZ) of the Kop ophiolite in NE Turkey is mainly composed of dunites, which are locally interlayered with Chromitites and contain minor relics of harzburgites. Large Fe isotopic variations were observed for magnesiochromite (− 0.14 to 0.06‰) and olivine (− 0.12 to 0.14‰) from the MTZ rocks. In individual samples, magnesiochromite has lighter Fe isotopic compositions than olivine, which was probably caused by subsolidus Mg–Fe exchange between them. Both magnesiochromite and olivine display an increasing trend of δ56Fe along a profile from Chromitite to dunite. This trend reflects continuous fractional crystallization in a magma chamber, which resulted in heavier Fe isotopes concentrated in the evolved magmas. In each cumulate cycle of Chromitite and dunite, dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite. Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite, generating Chromitite layers in the cumulate pile. Before mixing happened, the primitive melts had reacted with mantle harzburgites during their ascendance; whereas the evolved melts may lie on the olivine–chromite cotectic near the liquidus field of pyroxene. Variable degrees of magma mixing and differentiation are expected to generate melts with different δ56Fe values, accounting for the Fe isotopic variations of the Kop MTZ.

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  • genesis of the ray iz Chromitite polar urals inferences to mantle conditions and recycling processes
    Lithos, 2020
    Co-Authors: Paul T. Robinson, Jingsui Yang, Fahui Xiong, Basem Zoheir, Fancong Meng
    Abstract:

    Abstract Ophiolitic Chromitites in the eastern sector of the Alpine-Himalayan and Polar Ural orogenic belts preserve evidence of formation under ultra-high-pressure (UHP), deep mantle conditions. The textural and compositional characteristics of microscopic and submicroscopic inclusions in magnesiochromite from the Ray-Iz ophiolites are considered as clues of their complex evolutionary history. Bulk-rock platinum-group-element (PGE) compositions of refractory and metallurgical Chromitite ores (102–309 ppb) with generally IPGE>PPGE are consistent with supra-subduction zone (SSZ) metasomatism. The abundant unusual UHP phases (e.g., micro-diamonds, moissanite, coesite…etc.), together with clinopyroxene lamellae, and globular silicate inclusions in the Cr-spinel are seen as indications of deep mantle recycling of the mantle section of the Ray-Iz ophiolites. The calculated fO2 values for dunite and Chromitite (+1.17 to +4.16, generally above the FMQ buffer) are in line with interaction of mid-ocean ridge (MOR) or back-arc ophiolites with oxidized, Mg-rich silicic (boninitic) melts in a SSZ environment.

  • deep mantle origin and ultra reducing conditions in podiform Chromitite diamond moissanite and other unusual minerals in podiform Chromitites from the pozanti karsanti ophiolite southern turkey
    American Mineralogist, 2017
    Co-Authors: Dongyang Lian, Jingsui Yang, Yildirim Dilek, Zhongming Zhang, Fahui Xiong, Fei Liu, Wengda Zhou
    Abstract:

    The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of Chromitites are observed in the Pozanti-Karsanti ophiolite. One type of Chromitites occurs in the cumulate dunites around the Moho, and the other type of Chromitites is hosted by the mantle harzburgites below the Moho. The second type of Chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform Chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform Chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform Chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and Chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform Chromitite.

  • high al and high cr podiform Chromitites from the western yarlung zangbo suture zone tibet implications from mineralogy and geochemistry of chromian spinel and platinum group elements
    Ore Geology Reviews, 2017
    Co-Authors: Fahui Xiong, Paul T. Robinson, Jingsui Yang, Zhao Liu, Wenda Zhou, Guangying Feng, Xiaolu Niu
    Abstract:

    Abstract On the basis of their mineral chemistry, podiform Chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 ∗ Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different Chromitite compositions. We report here the occurrence of both high-Cr and high-Al Chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of Chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual Chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al Chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al Chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.

  • origin of podiform Chromitite a new model based on the luobusa ophiolite tibet
    Gondwana Research, 2015
    Co-Authors: Fahui Xiong, Paul T. Robinson, Jingsui Yang, Zhao Liu, Songyong Chen
    Abstract:

    Abstract Podiform Chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform Chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the Chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive Chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29 wt.%. Mg#s (= 100 ∗ Mg / (Mg + Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated Chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58 wt.%. In addition, magnesiochromite in massive ores has higher Fe3 +/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated Chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform Chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of Chromitite crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit Chromitite ores in melt channels.