Contribution of Magmatic Fluid to the Active Hydrothermal System in the JADE Field, Okinawa Trough: Evidence from Fluid Inclusions, Oxygen and Helium Isotopes

The Okinawa trough is an active, backarc-spreading basin in which hydrothermal fluids are venting from black and white smoker chimneys along a felsic volcanic ridge. The Okinawa seafloor hydrothermal precipitates are comparable to Kuroko-type massive sulfide deposits in many respects. Both systems occur in backarc settings and are hosted by a suite of bimodal volcanic rocks. They show mound-like features and contain predominantly Zn-Pb-Cu metal associations. Aqueous and CO2-rich hydrothermal fluids from the JADE field exhibit high trapping temperatures (up to 420°C), and moderate salinities (up to 8% NaCl equiv.). We provide evidence for the contribution of volatiles from a magmatic source to the actively forming submarine hydrothermal system. The following features in the JADE field support a magmatic component within this system: (1) the presence of a shallow-level felsic magma chamber 1-2 km beneath the seafloor; (2) anomalously high heat flow within the hydrothermal field; (3) a high content of CO2 and CH4 in the hydrothermal plume; (4) effervescence of volatile species (e.g., CO2, CH4, H2, N2,) in the hydrothermal fluid that forms independent discharges as CO2-rich fluid through CO2 hydrate pipes near black smoker chimneys; (5) helium isotope data demonstrating that these gases were derived from a mantle source; (6) high-temperatures and moderate to high fluid-inclusion salinities; (7) oxygen isotopic compositions of altered rocks and quartz indicating that mineralizing fluids have a δ18O composition consistent with a mixture of magmatic fluid and seawater.