Carbon and helium isotopic ratios at Kusatsu-Shirane Volcano,Japan

We have measured the chemical compositions, He/Ne ratios, He and C isotopes of 14 gas samples collected from the crater lake, fumaroles and hot springs associated with Kusatsu-Shirane Volcano, Japan. The³He/⁴He ratio decreases with increasing distance from the central crater of the volcano to the sampling site whereas the δ¹³C value of CO₂ increases with the distance. This tendency suggests that high³He/⁴He-low δ¹³C magmatic gas is mixed with and/or diluted by low³He/⁴He-high δ¹³C crustal gas with increasing distance from the crater. Alternatively, the variation of δ¹³C values may be the result of isotope fractionation during migration since the isotope shift is relatively small. Based on the apparent mixing trend in a³He/⁴He-δ¹³C diagram, the magmatic He and CO₂ in the volcano may have a³He/⁴He ratio of 8 R_atm and a δ¹³C value of −3.2%, respectively. The CO₂/³He ratios show a positive correlation with temperature of fumaroles or hot springs, which may be caused by a difference of temperature dependencies of the solubilities between CO₂ and He.