Hydrothermal system of the Kirishima volcanic area inferred from the chemical and isotopic composition of spring waters and fumarolic gases

Abstract

The chemical and isotopic composition of hot spring waters and fumarolic gases from the Kirishima volcanic area were investigated in order to delineate the geochemical structure of the hydrothermal systems. There are active volcanoes, Iwoyama, Shinmoe and Ohachi locate in the area, and each of them has characteristic hydrothermal systems. At Iwoyama volcano, chloride-sulfate type waters are discharging from central part, and acid sulfate type waters are located on its north and west flanks. On the south west flank of Shinmoe volcano, CO 2 type waters are located. At a deep part on the west Sank of both volcanoes chloride type waters are distributed. This variety of different type of waters can be derived from differentiations of a hypothetical parent fluid. The chloride type water seems to be a liquid phase separated from the mixture of the parental fluid and ground water. The vapor phase separated from the liquid phase heats ground water and produces the sulfate type and the CO 2 type waters. In the hydrothermal systems beneath Iwoyama volcano, a parental fluid is differentiated to a chloride-sulfate type water and a H 2 S rich fumarolic gas. The parent fluid in the hydrothermal system of Shinmoe volcano seems to be depleted in sulfur species than the parental fluid for Iwoyama. Ohachi volcano has a faint fumarolic activity at the summit. The fumarolic gas losses most of water vapor prior to the discharge due to a conductive cooling and condensation of the vapor up to the summit. A model calculation evaluates the original composition of the fumarolic gas from Ohachi volcano. The original composition resembles to those of the fumarolic gases from the summit of Shinmoe volcano.