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

摘要

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 d ¹⁸O 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 d ¹⁸O 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.