Variations in thermal state revealed by the geochemistry of fumarolic gases and hot-spring waters of the Tateyama volcanic hydrothermal system, Japan

Abstract

This study reports the chemical and isotopic compositions of hot-spring waters and fumarolic gases sampled in the Jigokudani Valley of Tateyama Volcano (central Japan) in 2015 and 2016 to reveal the state of the underlying hydrothermal system. We discuss the cause of temporal variations in geochemical data in terms of temperature change in the hydrothermal system and clarify the relationship between hot-spring waters and fumarolic gases. The volcanic gas supplied from deep-seated magma was separated into liquid and vapor phases when it reached a shallow depth. Each phase formed the following three types of hot-spring water: (1) high anion concentrations and isotopic compositions similar to magmatic water, (2) lower isotopic compositions compared to type-1 waters and large variations in Cl − /SO 42− , and (3) low Cl − and total anion concentrations. The formation of type-1 and type-2 hot springs was influenced by magmatic components such as HCl and SO 2 . We consider that type-1 hot springs are derived from the liquid phase while type-2 hot springs are derived from the vapor phase of the two-phase zone. The temporal variations in Cl − /SO 42− are considered to result from temperature changes in the reservoir where liquid and vapor separated, as the HCl partitioning coefficient between the vapor and liquid phases is strongly dependent on temperature. Type-3 hot springs are derived from the vapor phase, which is depleted in HCl and SO 2 . We propose that the Cl − concentration of type-2 hot springs could be a measure of volcanic activity because it reflects the thermal state of the shallow two-phase zone where phreatic eruptions occur.