ABSTRACT

Volcanic and hot spring gases and mineral waters containing dissolved CO₂ were collected from the Unzen volcanic area in the Shimabara peninsula, Kyushu, Japan, before and after the Mt. Fugen eruption of dacitic magma on November 17, 1990. The relationships between the ³He/⁴He and ⁴He/²⁰Ne ratios for samples collected from the eastern part of the peninsula including Mt. Fugen indicate that a mixing has occurred between air and present magmatic gas with the ³He/⁴He ratio of 10 × 10^–6, which is one of the highest values reported for volcanic and geothermal gases in Japanese island. On the other hand, the gases from two spas in the western part before and a year after the onset of the eruption show the mixings between air and gases with lower ³He/⁴He ratios of 6 × 10^–6 and 7 × 10^–6, having no effects by presently active magma. The seismic data related to the present eruption suggest that the present magma migrated from the west to the center of the Shimabara peninsula during the period of Nov. 1989 to May, 1991. These incompatible observations suggest that there are several independent channels for magmatic He to reach the earth's surface. The gases sampled from Obama and Unzen spas in the western part of Shimabara peninsula have been contaminated with ⁴He which had accumulated in cracks along the ascending route from a deep-seated magma having non-contaminated ³He/⁴He ratios of 10 × 10^–6.