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.