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

• Published in: Geology (Boulder) 2007-10, Vol.35 (10), p.875-878
• Main Authors: Yang Jingsui, Yang Jingsui, Dobrzhinetskaya, Larissa F, Bai Wenji, Bai Wenji, Fang Qingsong, Fang Qingsong, Robinson, Paul T, Zhang, Junfeng, Green, II
• Format: Article
• Language: English
• Subjects: Alloys; Asia; China; chromitite; coesite; diamond; Diamonds; experimental studies; Far East; framework silicates; igneous and metamorphic rocks; igneous rocks; inclusions; intergrowths; Luobusa Ophiolite; mantle; mineral assemblages; mineral inclusions; Mineralogy; Minerals; native elements; ophiolite complexes; Petrology; plutonic rocks; polycrystalline materials; Pressure; pseudomorphism; Raman spectra; SEM data; silica minerals; silicates; spectra; stishovite; ultrahigh pressure; ultramafics; upper mantle; Xizang China
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G23766A.1

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.