Helium Isotopes of Sea Waters above the Venting Sites of the Submarine Volcanoes in the Southernmost Part of Okinawa Trough

Many active submarine volcanoes have been found in southern Okinawa Trough. Water column samples from the hydrothermal plumes above venting volcanoes were collected during the OR2-1897, -1984, and -2003 cruises. Meanwhile, diving at shallower depths were conducted several times to collect the water samples near the venting sites. In total, 134 water samples from various depths in the offshore area of NE Taiwan were collected for dissolved gases and helium isotopes measurement.
The dissolved gases of water column samples show that the CO2 concentration and the alkalinity increase with depth and become higher at the bottom, while the result of O2 concentration shows a reverse pattern. The 3He/4He ratios near the vicinity of active Kueishantao volcano show highest value, 1.92 to 5.57 RA, where RA is the atmospheric ratios of 1.39 x 10-6. The plot of 3He/4He and 4He/20Ne ratios suggests that the influence of 59.7 % and 80.2 % by mantle component at KST-Y and KST-W, and the excess-3He profiles in this study suggesting that the helium-3 isotope is a nice tracer for the magmatic-component input from venting volcanoes. Furthermore, the correlation between helium and carbon isotopic ratios is low in this study. Therefore, the carbon isotopic ratio along itself may not be a sensitive proxy of geothermal.