Redox state of southern Tibetan upper mantle and ultrapotassic magmas

The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In contrast, oxygen fugacity ( fO2 ) data of upper mantle under orogens worldwide are rare, and the mechanism responsible for the mantle fO2 condition under orogens is not well constrained. In this study, we investigated the fO2 of mantle xenoliths derived from the southern Tibetan lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic volcanic rocks hosting the xenoliths. The fO2 of mantle xenoliths ranges from ΔFMQ = +0.5 to +1.2 (where ΔFMQ is the deviation of log fO2 from the fayalite-magnetite-quartz buffer), indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and oceanic mantle, and it falls within the typical range of mantle wedge fO2 values. Mineralogical evidence suggests that water-rich fluids and sediment melts liberated from both the subduct- ing Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized the southern Tibetan lithospheric mantle. The fO2 conditions of ultrapotassic magmas show a shift toward more oxidized conditions during ascent (from ΔFMQ = +0.8 to +3.0). Crustal evolution processes (e.g., fractionation) could influence magmatic fO2 , and thus the redox state of mantle-derived magma may not simply represent its mantle source.