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Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet

Jing-Sui Yang^*,1, Larissa Dobrzhinetskaya², Wen-Ji Bai³, Qing-Song Fang³, Paul T. Robinson^*,,4, Junfeng Zhang⁵ and Harry W. Green, II⁵

¹ Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
² Department of Earth Sciences and Institute of Geophysics and Planetary Physics, University of California–Riverside, Riverside, California 92521, USA
³ Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
⁴ GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany
⁵ Department of Earth Sciences and Institute of Geophysics and Planetary Physics, University of California–Riverside, Riverside, California 92521, USA

Diamonds and other ultrahigh pressure (UHP) minerals have been reported previously from the Luobusa ophiolite of Tibet, but these minerals have thus far been found only as individual grains. Here we report the occurrence of diamond as an inclusion in OsIr alloy and coesite as part of a silicate assemblage rimming a grain of FeTi alloy, both of which were recovered from chromitite. These occurrences confirm the presence of UHP minerals in the Luobusa chromitite requiring minimum pressures of 2.8–4 GPa. Individual coesite "crystals" have external form similar to that of stishovite and are polycrystalline, suggesting pseudo morphic replacement and implying a pressure >9 GPa. We propose that the UHP minerals were incorporated into the chromitites in the deep upper mantle or that they have an impact origin; the preponderance of evidence favors the former.

Key Words: diamond • coesite • stishovite • chromitite • ophiolite

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