Time Variation in the Chemical and Isotopic Composition of Volcanic Gas at Mt. Mihara of Izu-Oshima Island, Japan

Volcanic gas was sampled at three fumaroles and one borehole on Mt. Mihara, Izu-Oshima volcano. The fumarolic gas and the borehole steam possessed an excess enthalpy relative to the air saturated with water vapor. The fumarolic gas located west of the pit crater on Mt. Mihara showed a time variation in chemical and isotopic composition. The cause of the variation seems to be an enhancement of water vapor condensation. No similar variation was observed in the fumarolic gas located east of the pit crater, suggesting the above variation is a phenomena localized around the western fumarole. Hydrogen gas was detected in the sampled gases with low concentration. The change in the H2 concentration synchronized among the three fumaroles, suggesting the H2 gas originated in the hydrothermal system developed beneath Mt. Mihara.