Fluid geochemistry of high-temperature hydrothermal fields in the okinawa trough

Abstract

This review compiles fluid chemistries of the six known high-temperature hydrothermal fields in the Okinawa Trough (OT) and compares them to global representative fields with various tectonic/geologic backgrounds. The comparisons indicate that the chemical characteristics of the OT hydrothermal fluids are explained by linkages between (1) shallow water depth that constrains the maximum fluid temperature, (2) back-arc tectonic setting that introduces magmatic volatiles into the fluid, (3) probable silicic rock-based fluidmineral interaction at the hydrothermal reaction zone, and (4) seafloor sediment around the vents that provides both compounds derived from sedimentary organic matter and biogenic compounds, such as methane, produced by microbial ecosystems in the sedimentary environment. To explain the highly diverse gas compositions and stable isotope ratios of methane among the OT hydrothermal fields, “fluid-sediment interaction” has been further classified into several types with respect to processes (microbial or chemical) and stages of subseafloor fluid circulation (recharge or discharge). This concept, called the Microbial Methanogenesis at Recharge stage (MMR) model, enables us not only to deduce the geochemical origins of the hydrothermal fluid CH4 in each OT field but also to estimate the geographical distribution of hydrothermal fluid circulation via a two-dimension schematic illustration. The model, which links the fluid geochemistry with the subseafloor fluid migration path, will serve as a base for future studies also for any subseafloor geofluid systems that include hydrothermal systems, subseafloor methane hydrate, and seismogenic fault zone.