ABSTRACT

Variations in chemical and isotopic compositions of gases discharged from Kuju-Iwoyama, a fumarolic area developed in the 1738 crater of the Hosshozan volcano in Kyushu, Japan, were investigated from 1960 to 1984. The isotopic data suggest that the fumarolic condensate collected from KH-1 fumarole (the hottest fumarole of Kuju-Iwoyama) in 1960 is likely to be almost pure magmatic water, and that those collected from other low-temperature fumaroles in the early 1960s are mixtures of the magmatic water and local groundwaters. The relationship between the temperature and composition of the fumarolic gases shows that the mixing of high-temperature gases with liquid groundwater took place near the surface in the early 1960s. In 1984, the steam samples from KH-1 fumarole were found to be 1 : 1 mixtures of the steam, which had been derived from the groundwater by total evaporation, and magmatic steam, and those from other fumaroles to be mixtures of KH-1 steam and liquid groundwater. The mixing of the magmatic steam with the goundwater-derived steam suggests the penetration of groundwaters into deeper parts of the geothermal system along with the decline of fumarolic activity of the system. The isotopic and chemical geothermometer study on the gases suggests that the deep reservoir of volcanic gases was maintained at temperatures above 600°C during this investigation, and that the addition of some gaseous species (e.g., CH₄, H₂S and SO₂) into volcanic gases took place during the passage of the gases through the geothermal system.