Abstract
We repeatedly measured isotopic compositions of noble gases and CO(2) in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in (3)He/(4)He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in (3)He/CO(2) ratios and carbon isotopic ratios of CO(2). The (3)He/(40)Ar* ratios ((40)Ar*: magmatic (40)Ar) show significant increases in the period of high (3)He/(4)He ratios. The temporal variation in (3)He/(40)Ar* ratios may reflect changes in magma vesicularity. Therefore, the (3)He/(40)Ar* ratio of fumarolic gases is a useful parameter to monitor the current state of degassing magma, which is essential for understanding the deep process of volcanic unrest and may contribute to identifying precursors of a future eruption. These results provide additional validation for the use of noble gas and carbon isotopic compositions of fumarolic gases for monitoring magmatic-hydrothermal systems.