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

Abstract

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.

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

“…1). The prismatic external form of the polycrystalline coesite domains that host these inclusions establish that the coesite formed by pseudomorphic replacement of stishovite (minimum pressure for its stability Ϸ10 GPa, implying Ͼ300-km depth) (8). With a focused ion beam (FIB) instrument, we prepared several thin foils from this material that were examined in the transmission electron microscope (TEM), leading to discovery and characterization of the additional phases.…”

“…This discovery strongly enhances the conclusion of ref. 8 that this occurrence of very high-pressure minerals is not the result of meteorite impact. Thus, a subset of ophiolites (currently of unknown abundance) contains high-pressure and reduced phases, despite the overwhelming evidence that the ophiolites themselves formed at oceanic spreading centers under oxidizing conditions.…”

“…Initial description of this material (8) documented diamonds and coesite-after-stishovite, establishing a minimum depth of origin of Ϸ300 km. This surprising and curious discovery was strengthened by observation of coesite and diopside lamellae within chromite collected from the same outcrop (9) and, very importantly, discovery of diamonds and indicators of very low oxygen fugacity from similar chromitite in a second ophiolite in the Polar Ural Mountains (10), establishing that the Tibetan occurrence is not unique.…”

“…The material that makes up the subject of both ref. 8 and this article has been extracted from massive chromite ore 93 of the Luobusa ophiolite, located on the collisional boundary between Asia and India (13,14).…”

Manganilmenite in the Magnetite Ore Body from Pokphur Area of Nagaland, North East India and the Possibility of Microdiamonds in the Ophiolites of Indo-Myanmar Ranges

“…However, ophiolites are a newly documented host of diamonds on the Earth, and abundant diamonds have been separated from peridotites and chromitites of ophiolites in China, Myanmar and Russia 29 . Diamond grains have been recently discovered in chromitite from the Cretaceous Luobusa ophiolite, Tibet 30 , in ophiolitic massifs along the Yarlung-Tsangpo suture zone 31 from the early Paleozoic Ray-Iz ophiolite, polar Urals, Russia; in podiform chromitites of the Devonian Sartohai ophiolite in western China, and in peridotites of the Jurassic Myitkyina ophiolite, Myanmar 29 . These ophiolitic diamonds occur as subhedral to euhedral crystals, ~0.2-0.5 mm in diameter.…”