Changgui Gao

China University of Geosciences

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Yongsheng Liu

China University of Geosciences

Zhaochu Hu

China University of Geosciences

Keqing Zong

China University of Geosciences

Shan Gao

Zhongda Hospital Southeast University

Detao He

China University of Geosciences

Haihong Chen

China University of Geosciences

Juan Xu

Tongji University

Hujun Gong

Northwest University

Stephen F. Foley

Macquarie University

Wenli Ling

China University of Geosciences

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China University of Geosciences

Chinese Academy of Sciences

State Key Laboratory of Continental Dynamics

Northwest University

ETH Zurich

State Key Laboratory of Geological Processes and Mineral Resources

Tongji University

Institute of Geochemistry

Board of the Swiss Federal Institutes of Technology

Tsinghua University

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SiC-dominated ultra-reduced mineral assemblage in carbonatitic xenoliths from the Dalihu basalt, Inner Mongolia, China

American Mineralogist (2017)

Detao He Yongsheng Liu Changgui Gao Chunfei Chen Zhaochu Hu

Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20–50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These Sic are featured by 13C-depleted isotopic compositions (δ13C = –13.2 to –22.8‰, average = –17.7‰) with obvious spatial variation.

Xenolith

Assemblage (archaeology)

Inner mongolia

10.2138/am-2017-5721

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Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH-fluids

Progress in Earth and Planetary Science (2014)

Max W. Schmidt Changgui Gao Anastasia Golubkova Arno Rohrbach J. A. D. Connolly

Natural moissanite (SiC) is reported from dozens of localities, most commonly from ultramafic rocks where it may be associated with diamond and iron silicides. Yet, formation conditions of moissanite remain in the realm of speculation. The key property of SiC is its extremely reduced nature. We have experimentally equilibrated SiC with olivine and orthopyroxene at 1300-1700°C, 2 and 10 GPa, to determine the oxygen fugacity of the C + orthopyroxene = SiC + olivine + O2 buffer (MOOC) and the equilibrium X Mg of coexisting mantle silicates. The experiments resulted in olivine and orthopyroxene with X Mg of 0.993-0.998 in equilibrium with SiC and iron silicides. Calculated oxygen fugacities are 5-6.5 log units below the iron-wustite (IW) buffer at 2-10 GPa. The experimental results concur with calculated phase relations for harzburgitic mantle under reducing conditions that include metal alloys, carbides and silicides. The extremely reducing character of SiC precludes coexistence with silicates with appreciable Fe2+, and hence excludes equilibrium with mantle phases with typical X Mg 's of ~0.9. Calculated Fe-Mg diffusion lengths reveal that SiC grains of 1 mm would react with the Fe-component of olivine to iron carbide or metal and orthopyroxene within <1 Ma at temperatures above 800°C. We thus conclude that SiC forms through a low-temperature process (<700-800°C) where equilibrium is only reached at the grain scale. The most plausible formation mechanism is a strong fractionation of a C-O-H fluid from metamorphosed sediments originally rich in organic material. Such a fluid is initially saturated with graphite or diamond and is slightly more reduced than the H2O-C join. Fluid percolation in the mantle leads to H2O-sequestration by crystallizing hydrous phases (most likely serpentine, brucite or phase A), and hence O2-removal from the fluid causing its reduction and continuous graphite or diamond precipitation. A small, highly fractionated fluid fraction may then reach CH4 and H2 concentrations that allow SiC formation on grain boundaries without equilibration with the bulk rock on a larger scale. Such a mechanism is corroborated by the strongly negative δ13C of moissanites (-20 to -37), consistent with reduced fluids originating from metamorphosed organic carbon.

Mineral redox buffer

Wüstite

Chromite

10.1186/s40645-014-0027-0

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Recycling of lower continental crust in the Trans-North China Orogen: Evidence from zircon dating of mantle composite xenoliths

Geochimica et Cosmochimica Acta Supplement (2008)

Yongsheng Liu Shan Gao Changgui Gao Keqing Zong

Xenolith

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