DISCOVERY OF IN SITU SUPER-REDUCING, UHP PHASES IN LUOBUSA OPHIOLITIC CHROMITITES, TIBET: NEW INSIGHT INTO THE DEEP UPPER MANTLE AND THE TRANSITION ZONE

The Luobusa ophiolitic podiform chromitites consist of chromite + olivine ± diopside. Previous researches on super-reducing ultrahigh-pressure (SuR UHP) phases from these Tibetan chromitites were mainly conducted on isolated grains extracted from very large samples. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitite ore bodies. Here we report and analyze unambiguously in situ SuR UHP assemblages: (1) blue moissanite as inclusions in olivine (Fo_96-98), and in olivine domains, (2) multiple inclusions of moissanite + wüstite in olivine, (3) FeNi and FeCr alloys in olivine and chromite, and (4) native Fe and Si in chromite. Crustal albertite and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases combined with the previously inferred presence of ringwoodite + stishovite all indicate that these assemblages formed under a highly reducing environment in the deep upper mantle - possibly the mantle transition zone (MTZ). Crustal albertite and h-BN and other low-P crustal minerals, as well as diamond + moissanite with distinct ¹³C-depleted isotopic compositions from chromitites have a metasedimentary carbon source. These associations demonstrate that near-surface crustal rocks were recycled into the mantle through subduction, and locally modified the composition of the mantle. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Urals and Myanmar imply the existence of a new type of ophiolitic chromitite, providing a new window to explore physical-chemical conditions of the MTZ, and the profound recycling of crustal materials.