ABSTRACT

On June 29, 2015, a small phreatic eruption occurred in the Owakudani geothermal area on Hakone volcano, central Japan. Ashfall was observed on June 29 and 30. In this study, constituent minerals, whole-rock composition, and water-soluble components of the ash sampled at the Owakudani geothermal area on June 30 were analyzed to determine the source depth and hydrothermal condition of this ash. The ash sample included smectite, pyrite, tridymite cristobalite, gypsum, and anhydrite with plagioclase and quartz, indicating that the ash was discharged from a relatively low-temperature (lower than 100–150°C) and a near-neutral part of a hydrothermal alteration zone of the Owakudani geothermal area. The whole-rock chemical composition was characterized by an elevated Al₂O₃ content, supporting the inference that the source of the ash was in a near-neutral hydrothermal condition. Comparison of the mineral assemblage of the ash with that of the drilling core of the steam well evidently showed that the source depth of this ash was shallower than 350 m, as smectite is a mineral predominantly found at depths shallower than 350 m. In the ash leachate, 12,200 mg/kg of Cl and 6,600 mg/kg of SO₄ were detected and their origin was deduced to be thermal water adhesion rather than volcanic gas because of their large adhesion amount and high Cl/S molar ratio. This Cl/S ratio corresponds to that of thermal waters approximately 23–351 m deep. This depth range is consistent with the lower limit depth estimated from the mineral assemblage.