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Jingsui Yang, Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing (China)
Wenji Bai, Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing (China)
Qingsong Fang, Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing (China)
Songyong Chen, Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing (China)
Zhongming Zhang, Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing (China)
A. B. Makeev, Institute of Geology, Komi Scientific Center, Ural Division, Russian Academy of Sciences,Pervomaiskaya ul. (Russian Federation)
N. I. Bryanchaninova, Institute of Geology, Komi Scientific Cente Ural Division, Russian Academy of Sciences (Russian Federation)
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A controversial theme in the Earth Sciences over the last 30 years has been the origin of chromite deposits found worldwide within depleted peridotites (harzburgites) of the uppermost mantle in "ophiolites" - rock sequences representing former oceanic lithosphere. The majority of workers support a model in which massive chromites containing Os- and Ir-rich platinum group element (PGE) alloys form by interaction of H2O-saturated boninite melt with harzburgites of the mantle wedge above a subduction zone at comparatively shallow levels in the mantle, with suggested maximum pressures of ∼1.6 GPa or ∼4.5 GPa. A second model suggests that the Os/Ir-rich PGE alloys, and their host chromitites, formed in the very deep mantle or at the core-mantle boundary (P ∼90 GPa), and that they have been transported to the surface as xenoliths in deep-rooted mantle plumes.
We report here the discovery of two ultrahigh pressure phases from Luobusa chromitite of the Luobusa ophiolite. that shed new light on the origin of these enigmatic deposits: abundant coesite rimming a grain of Fe-Ti alloy and an inclusion of diamond within Os-Ir alloy. These occurrences definitively rule out contamination and establish the minimum pressure experienced by these minerals as 4 GPa. Moreover, the prismatic morphology of the coesite "crystals" and their polycrystalline nature strongly suggest that the coesite is pseudomorphic after stishovite, its higher-pressure polymorph, implying P>10 GPa. For comparison study, we also collected about 1500 kg of chromitite from two orebodies in the Polar Urals. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. Other mineral group include: (1) native elements: Cr, W, Ni, Co, Si, Al and Ta; (2) carbides: SiC and WC; (3) alloys: Cr-Fe, Si-Al-Fe, Ni-Cu, Ag-Au, Ag-Sn, Fe-Si, Fe-P, and Ag-Zn-Sn, FeCrNI, FeCrNiSi, MnNiCrFe; (4) oxides: rutile and Si-bearing rutile, ilmenite, corundum, chromite, MgO, and SnO2; (5) silicates: kyanite, pseudomorphs of octahedral olivine, zircon, garnet, feldspar, and quartz,; (6) sulfides of Fe, Ni, Cu, Mo, Pb, Ab, AsFe, FeNi, CuZn, and CoFeNi; and (7) iron groups: native Fe, FeO, and Fe2O3.
These minerals are very similar in composition and structure to those reported from the Luobusa chromitites. The possibility that the Luobusa chromitites formed at great depth and were brought to the surface by a mantle plume is consistent with our observations and with the recent discovery of coesite lamellae in Luobusa chromite, which suggests precipitation of this coesite from a high-pressure polymorph of chromite. A deep-earth origin for the high-pressure phases would appear to require a composite origin in which the high-pressure phases are older constituents of the chromitites, brought from the deep mantle perhaps by a mantle plume, and that the plume materials were subsequently reworked at shallow depths, including partial recrystallization of the chromite.
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