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 confi rm 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.

Numerous ultra-high-pressure minerals have been recovered from podiform chromities in the Luobusa ophiolite, Tibet. Recovered minerals include diamond, moissanite, Fe-silicides, wüstite, Ni-Fe-Cr-C alloys, PGE alloys and octahedral Mg-Fe silicates. These are accompanied by a variety of native elements, including Si, Fe, Ni, Cr and graphite. All of the minerals were hand-picked from heavy-mineral separates of the chromitites and care was taken to prevent natural or anthropogenic contamination of the samples. Many of the minerals and alloys are either enclosed in, or attached to, chromite grains, leaving no doubt as to their provenance. The ophiolite formed originally at a mid-ocean ridge (MOR) spreading centre at 177±33 Ma, and was later modified by suprasubduction zone magmatism at about 126 Ma. The chromitites were formed in the suprasubduction zone environment from boninitic melts reacting with the host peridotites. The UHP minerals are believed to have been transported from the lower mantle by a plume and incorporated in the ophiolite during seafloor spreading at 176 Ma. Blocks of the mantle containing the UHP minerals were presumably picked up by the later boninitic melts, transported to shallow depth and incorporated in the chromitites during crystallization.

PGE and base-metal alloy minerals from podiform chromitites of the Luobusa ophiolite, southern Tibet include Os-Ir, Os-Ir-Ru, Pt-Fe, Ir-Ni-Fe, Fe-Ni-Cr, and Fe-Co with highly variable compositions. Small amounts of diamond, graphite, SiC, an unnamed chromium carbide, iron silicide (Fe-Si), and native Fe, Ni, Cr, Au, Cu and Si also are present. The alloys and native elements were recovered primarily from heavy-mineral separates of the chromitites, but some are included in, or attached to, magnesiochromite grains. They are mostly subhedral to anhedral and range in size from about 0.05 to 0.5 mm. Some grains have euhedral crystal forms and others have round shapes, suggestive of melt droplets. Fe-Ni-bearing, Ru-rich PGE alloys, Fe-Ni and Fe-Co alloys and native Fe and Ni in the Luobusa chromitites are interpreted as secondary minerals formed by alteration of PGE sulfides. The Pt-Fe, Os-Ir, and Ir-Ni-Fe alloys, Fe silicide and perhaps the native Si are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume and then captured by the melts from which the Luobusa chromitites crystallized.

A wide variety of unusual mantle has been reported from podiform chromitite orebodies Cr-31 and Cr-74 in the Luobusa (罗布莎) ophiolite, Tibet. A detailed investigation of chromitite orebody Cr-11, located in the Kangjinla (康金拉) district at the eastern end of the ophiolite, has revealed many of the same minerals, including diamond, moissanite, and some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. This orebody is particularly rich in diamonds, with over 1 000 grains recovered from about 1 100 kg sample of chromitite. More detailed studies and experiments are needed to understand the origin and significance of these unusual minerals because they have not been found in situ. It is a great breakthrough in mineralogical research that we have picked up more than 40 kinds of minerals from the Kangjinla chromite deposit in Luobusa. It is notable that a large amount of diamonds were firstly discovered from the Kangjinla chromite deposit as well as many other unusual minerals, such as moissanites, rutiles, native irons, and metal alloys. Especially, that diamond was found again in different chromitites in the same ophiolite belt provided new key evidence for discussing the origin of the diamond and the hosted chromitite and ophiolite. The mantle mineral group in Tibet has great significance in mineralogy and geodynamics.