Interaction between magmatic fluid and meteoric water, inferred from 18O/16O and 36Ar/H2O ratios of fumarolic gases at the Kusatsu Shirane volcano, Japan

Fumarolic gases from the Kusatsu Shirane volcano, Japan were analyzed for the content and isotopic compositions of H2O, the abundances and isotopic compositions of noble gas. The observed δ18O and 36Ar/H2O ratios could not be explained by an existing hydrothermal model in which a mixing between magmatic vapor (MAV) and meteoric water (ASW), a single-step separation to vapor and liquid phases and a subsequent partial H2O vapor removal from the vapor phase have been considered. We constructed a revised model in which two-step mixing-separation and a subsequent partial H2O vapor removal by condensation were considered. In the model, the first step was the mixing between MAV and ASW, and a subsequent separation to a primary vapor and liquid phases. In the secondary step, the primary vapor mixed with another fluid which has a δ18O lower than the primary vapor phase and a low 36Ar/H2O ratio relative to ASW. The mixture separates to secondary vapor and liquid phases. The fluid with low 36Ar/H2O ratio could be ASW partially vaporized in the geothermal area. The secondary vapor ascended and discharged as fumarolic gas. In the ascending process, the secondary vapor suffers a partial H2O vapor removal by condensation in various degrees.