Monitoring of magmatic–hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases

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.