DIAMOND DISCOVERED FROM SIX DIFFERENT ULTRAMAFIC MASSIFS ALONG THE YARLUNG ZANGBU SUTURE BETWEEN THE INDIAN AND EURASIAN PLATES

The more than 2000-km-long Yarlung-Zangbo suture zone in south Tibet marks the boundary between India and Asia. The ophiolites along the suture consist chiefly of mantle harzburgite and dunite with minor crustal gabbros and basalts. Only the Luobusa ophiolite hosts significant podiform chromitites, generally lenticular masses of high-chrome chromite surrounded by envelopes of dunite. This ophiolite is believed to have formed initially at an oceanic spreading center at ~176 Ma and then been modified by melt-rock reaction in a suprasubduction zone at ~126 Ma. Diamond separates have been recovered from six ultramafic massifs along a 1400-km section of the Yarlung-Zangbo suture zone, from west to east, they are the Dongbo, Purang, Dangqiong, Xigaze, Zedong and Luobusa massifs. The diamonds have been recovered from both peridotites and chromitites, and in-situ diamond has been discovered in the chromitites from the Luobusa ophiolite. The in-situ diamonds are euhedral to subhedral crystals, mostly 200-500 μm across, hosted in small, commonly spherical, patches of amorphous carbon within chromite. These diamonds usually lack lattice dislocations and contain inclusions of Ni-Mn-Co alloys, features that distinguish them from kimberlitic and metamorphic diamonds. Textural evidence and Ca-K-Cl fluid inclusions indicate that they grew from C-rich fluids. Diamonds from the ophiolitic rocks have extremely low δ13CPDB values, ranging from -18 ‰ to -29 ‰, much lower than kimberlitic diamond (-5 ‰ to -10‰) but nearly identical to all analysed moissanite, indicating derivation from a light carbon reservoir in the mantle. The presence of abundant diamonds in ophiolites indicates a major new occurrence of these minerals in the oceanic mantle, unrelated to kimberlites and UHP metamorphism.