Geochemistry of hydrothermal fluids at the Hatoma Knoll in Okinawa Trough

Hatoma knoll is a caldera volcano which exists in the southern part of Okinawa Trough, and the hydrothermal field was discovered in the caldera in 1999. A lava dome exists in the center part of the caldera, and clear smokers up to 324.5°C, benthic organism colony and liquid CO2, and CO2 hydrate have been observed around the dome. Since 2000, the investigation cruise (NT00-06, YK07-04, NT07-12, NT08-13, and NT09-10) by "Shinkai2000", "Shinkai6500" and the "HYPER-DOLPHIN 3K" has been carried out. Hydrothermal fluid samples were taken from the hydrothermal system, and chemical and isotopic compositions of the hydrothermal fluid samples were investigated. The chemical composition of hydrothermal fluid has high pH compared with the hydrothermal fluid in the mid-ocean ridge, and ammonium concentration is high, suggesting that the sediments covered the Okinawa Trough contribute to the chemical composition of hydrothermal fluid. The end-members of hydrothermal fluid show a variation, but the ratios of the end-members are consistent with each other, suggesting that the hydrothermal system has a single source and subcritical phase separation occurs below the seafloor. The equilibrium temperature with the quartz based on Si concentration was 350-400°C at 1-2 km below the seafloor. CO2 concentration in hydrothermal fluid showed the high-level value in the hydrothermal system in the world. The origin of the abundant CO2 is the carbonate on the subducting plate and the sediment in the Okinawa Trough based on the carbon isotope and the helium isotope. Methane is also the high-level concentration in the hydrothermal system in the world. Most of methane is generated through methanogenesis based on the carbon isotope ratio. Sr isotopic ratio in the hydrothermal fluid suggests the influence of sediment. However, the knoll surface was covered by rhyolite, the influence of sediment would occur in the recharge zone of the hydrothermal system. The methane would be microbially produced in the sediment of the recharge zone, and entrained by the hydrothermal fluid.