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

Abstract

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. The 3He/ 4He ratio decreases with increasing distance from the central crater of the volcano to the sampling site whereas the δ 13C value of CO 2 increases with the distance. This tendency suggests that high 3He/ 4He-low δ 13C magmatic gas is mixed with and/or diluted by low 3He/ 4He-high δ 13C crustal gas with increasing distance from the crater. Alternatively, the variation of δ 13C values may be the result of isotope fractionation during migration since the isotope shift is relatively small. Based on the apparent mixing trend in a 3He/ 4He-δ 13C diagram, the magmatic He and CO 2 in the volcano may have a 3He/ 4He ratio of 8 R atm and a δ 13C value of −3.2%, respectively. The CO 2/ 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 CO 2 and He.