High-Pressure Nitrides and Microdiamonds from Tibetan Ophiolite

Abstract

The samples of microdiamond as an inclusion of Os Ir alloy, coesite, stishovite, high pressure form of Tio 2 , high pressure nitrides associated with SiC along with boron carbide, have been reported in the past from the placer deposits of Luobasa ophiolite of Yarlung Zangbo region of Tibet. For the formation of these ultra high findings, pressure in the range of 4 to 9 Gpa and temperature in the range of 700 ° C - 1300 ° C is required. Thus these minerals can either be incorporated into the chromitite in the deep upper mantle or they have an impact origin. In absence of impact crater, Prof. Fang and others have favored the mantle origin but for the formation of nitrides in the deep mantle, the source of N is not clear. However, the event of comet impact provides better explanation for the formation process. It also simplifies the explanation of coexistence of most of these minerals in a single crystal with the shallow origin and the perfect preservation of the coesite as consequence of rapid cooling after the shock metamorphism.