Fluid inclusion and HeAr isotope evidence for subsurface phase separation and variable fluid mixing regimes beneath the Yonaguni Knoll IV hydrothermal field, SOT

Abstract The Okinawa Trough is a back-arc, initial marginal sea basin located behind the Ryukyu trench-arc system. Pb-Zn-(Cu) mineralization associated with volcanism has been recognized more than 15 hydrothermal fields in this trough. Fluid inclusions and He Ar isotopic compositions were studied in sulfide-sulfate chimneys at Yonaguni Knoll IV hydrothermal field, South Okinawa Trough. Studies of fluid inclusion in sphalerite, anglesite and barite indicate formation temperatures between 220 °C and 357 °C and varying fluid salinities (1.9 to 8.55 equiv. wt% NaCl) that fall within the range observed at modern active mid-ocean ridge hydrothermal fields. The trapping temperatures heterogeneity and wide variations in salinities of partial fluid inclusions reveal phase separation and fluid mixing beneath the seafloor. The resulting brine favored the release and transportation of additional metal ions besides Pb and Zn and led to a complex mineralization at higher levels. The 3He/4He ratio values (1.5 to 6.6Ra) of co-existing minerals also suggest a mixing fluid source, which can be calculated that basaltic magma He and seawater He mixed in varying degrees, with the proportions of basaltic magma He varied from 36% to 80% during the crystallization of sphalerite. Even in barite, the proportion of basaltic magma He can reach ~6%, supporting the contribution of the mantle He to the hydrothermal fluid. Anglesite-hosted fluid inclusions indicate that the existence of multiple hydrothermal fluid reservoirs in the sub-seafloor where the accumulation and migration of the hydrothermal fluid caused the rocks and terrigenous sediments to be altered intensely and promoted the lateral migration and mineralization of hydrothermal fluid under the seafloor.