Change in rare gas composition of the fumarolic gases from the Showa-shinzan volcano

抄録

Isotopic compositions of He and elemental abundance of rare gases were determined in 7 fumarolic gas samples of the Showa-shinzan volcano collected during 1958 to 1977 and in one gas sample collected in 1978 from the Usu volcano. ³He/⁴He and ⁴He/²⁰Ne ratios were significantly higher than those for the atmospheric rare gases. This suggests a mantle He emanation. These ratios decreased and approached the atmospheric ratios with time, in harmony with the decline of the volcanic activity of the Showa-shinzan. The decrease can be accounted for by the increase in the atmospheric contamination. Estimation of He permeation through a glass wall shows that the permeation is so small that a borosilicate glass flask can preserve the initial ³He/⁴He and ⁴He/²⁰Ne ratios even though the gas sample has been stored for a long time of about 20 years. The gas sample from the Usu volcano 2km west of the Showa-shinzan, collected 14 months after the eruption of the volcano, showed the highest ³He/⁴He and ⁴He/²⁰Ne ratios of the gas samples studied. However, the gas sample from the Showa-shinzan collected immediately after the eruption of the Usu volcano showed no sign of increase in ³He/⁴He and ⁴He/²⁰Ne ratios. This suggests that the eruption of the Usu volcano did not affect the degassing of He from the Showa-shinzan, and the intrusion of lava was quite restricted.