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 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 He/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 He/CO ratios and carbon isotopic ratios of CO. The He/Ar* ratios (Ar*: magmatic Ar) show significant increases in the period of high He/He ratios. The temporal variation in He/Ar* ratios may reflect changes in magma vesicularity. Therefore, the He/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.