The hydrothermal system beneath Owakudani on Hakone Volcano, Japan; traced by stable isotopic ratios of H2O

In this study, we sampled natural fumarolic gases, the gas from a steam well drilled to 800 m depth and natural hot spring waters from the Owakudani geothermal area, Hakone Volcano, Japan. We then measured the chemical composition and the D/H and 18 O/ 16 O ratios of H 2 O within the samples. On the basis of the analytical results, we investigated the differentiation processes of the magmatic fluid during ascent to the surface. Volcanic gas discharged from the No. 52 steam well has a CO 2 /H 2 O ratio that is much higher than those from fumarolic gases. The isotopic ratio of H 2 O in the gas are also much higher than those in the fumarolic gases, and the gas also contains high concentrations of SO 2 and HCl. The above observations indicate a magmatic origin for the gas from the No. 52 steam well. The hydrothermal system is principally explained by the mixing of a magmatic vapor, represented by the gas from the No. 52 steam well, and cold local meteoric water. Following mixing of the fluids, a separation of primary vapor and liquid occurs. The primary liquid is discharged as a component of hot spring water, and has high isotopic ratios. The primary vapor is mixed with vapor derived from the boiling of local meteoric water. The mixed vapor is then discharged to the surface, interacting with shallow meteoric water and undergoing partial condensation of H 2 O vapor. Hot spring water with isotopic ratios lower than those of the primary liquid is derived from the boiling of local meteoric water.