Temporal variation in the ratio of magmatic helium and argon in fumarolic gases at Kusatsu-Shirane volcano

Abstract

Since noble gases are chemically inert and their abundance ratios vary only by physical processes, compositional variations in volcanic gases are relatively simple. Variations in 3He/40Ar* ratios (40Ar* = 40Ar excess from atmospheric Ar) of fumarolic gases reflect variations in magmatic He/Ar ratios because both 3He and 40Ar* of fumarolic gases are mostly magmatic in origin. We have measured the concentrations and isotopic compositions of noble gases and CO2 of fumarolic gases collected on the north side of Mt. Shirane since 2015. As a result, the 3He/4He and 3He/CO2 ratios increased after May 2018, suggesting an increase in the supply of magmatic gas with high 3He/4He and 3He/CO2 ratios. The 3He/40Ar* ratios of the fumarolic gases significantly increased when the 3He/4He ratios were high. It has been suggested that He/Ar ratios in gas phases of magma increase with increasing magma vesicularity. Thus, the increase in the 3He/40Ar* ratios suggests an increase in magma vesicularity at the Kusatsu-Shirane volcano. In summary, 3He/40Ar* ratios of fumarolic gases might be an important indicator to evaluate a state of magma.