Roots and reservoir of venting fluid at Taketomi submarine hot spring

The Ryukyu Arc is the forearc of the backarc-arc-trench system in which the Philippine Sea Plate is subducting beneath the Eurasian Plate. In the Taketomi submarine hot spring, mantle-derived helium was found, while the issuing mechanism of hot water remains unclear. In 2017, a hotel, which planned to open a hot spa, drilled a well down to a depth of 940 m in the central part of Taketomi Island. Based on the additional information of B isotopes of host rocks and well waters, we report the new perspectives of hot spring water venting from the seafloor off Taketomi Island. The top 30 m of the lithology is the Ohama Formation comprising Quaternary Ryukyu limestone, and the lower part is Jurassic accretionary complex, the Fusaki Formation. Boron (B) isotope ratios were measured for sample cutting take from 12 depths with every 100 m interval; the B isotope ratios ranged from -20 to -10. Such an extremely low isotope ratios, with the low content of boron, are consistent with the characteristics of old chert shown by Ishikawa and Nakamura (1993). Boron isotope ratios of the Taketomi submarine hot spring water is explained by a simple mixing of seawater with fresh water having about 11B = -1. Assuming the B had reached equilibrium at water-rock interaction, and the isotope fractionation from the laboratory experiments, the equilibrium temperature is estimated to be around 150C. This is consistent with the temperature estimated by a geothermometer using silica concentration. Sulfur hexafluoride (SF6) was detected in the hot spring water, and the concentration is significantly lower than that of air-saturated seawater in modern times. It suggests the contribution of a certain amount of groundwater equilibrated with the past atmospheric air. Assuming that the atmosphere was in equilibrium at the time of recharge, the groundwater was recharged in 2000-2001, and the residence time was estimated to be about 20 years. According to the water isotope ratios, the end-member of the hot spring water is different from this groundwater, and the mercury concentration up to 1.3 ppb is significantly higher than that of seawater, suggesting the input of deep-sourced fluid. These results suggest that the submarine hot spring water was venting after mixing with groundwater and deep-sourced fluid and reaching at equilibrium with the host rocks at ~150°C.