Abstract
Temporal variations of 3He 4HE and 4He 20Ne ratios occur in high-temperature gases from a steam well located about 3 km north of the central cone of Mt. Mihara, an active volcano in Oshima Island, Japan. The 3He 4He ratio gradually increased from 1.71 R atm (1.71 times the atmospheric ratio of 1.40 × 10 −6) in October 1986 to 5.02. R atm in November 1987. Taking into account a similar 4He 20Ne change, the magmatic He component in the steam has increased since the violent eruptions of Oshima volcano from November 15 to December 18, 1986. This confirms a direct relationship between magmatic activity and elevated 3He 4He ratios in groundwater/hydrothermal systems. Based on the distance from the craters to the steam well, the time lag between the eruptions, and the increase of 3He 4He , we estimate the magmatic component velocity in the fluid flow. The calculated value, several tens of m per day, is significantly higher than typical groundwater flow in deep sedimentary basins of Japan.
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