Oxidation state of mantle xenoliths from Jeju-do, South Korea

Abstract

The common upper mantle assemblage of olivine-orthopyroxene-spinel may be used to calculate the oxygen fugacity at which mantle-derived peridotites have equilibrated. Mantle-derived ultramafic xenoliths occur in two alkali basaltic flows of Jeju-do, South Korea: Sangumburi and Sinsanri. They are mostly spinel Iherzolites with minor spinel harzburgites. Main mineral assemblage is olivine-orthopyroxene-clinopyroxene-spinel. The oxygen fugacities of 27 peridotite xenoliths from two locations fall mostly within a narrow range from −1.5 to 0.5 log units relative to FMQ buffer. The range offo2 approaching FMQ buffer indicates that dominant fluid phases in the uppermost mantle beneath Jeju-do are H2O and CO2 instead of CH4 and H2. The overall degree of basaltic element depletion in the xenoliths is independent of thefo2 they record, which implies that partial melting and melt extraction took place under approximately the samefo2 conditions. Oxidation state of the sub-continental lithospheric mantle is somewhat related to the tectonic setting. Published data suggest that hot-spot and recent subduction-related domains are more oxidized than asthenospheric (MORB-like) ones. It is commonly considered that relatively high oxygen fugacity in the mantle above subduction zone results from upward migration of slab-derived hydrous fluid. Oxygen fugacity values for lithospheric mantle beneath Jeju-do fall within the range observed in abyssal peridotites, which is similar to the range of peridotite xenoliths from other continental regions, but more reduced than for hot-spot and recent subduction-related domains. However the meanfo2 values of Jeju xenoliths are slightly higher than those of the typical continental lithospheric mantle xenoliths, suggesting a possible interaction with the fluids emanating from mantle plumes.