Takeshi Ohba, Muga Yaguchi, Masanori Ito, Urumu Tsunogai
The SO2/CO2 ratio of fumarolic gas collected from vent (c) of Ebinokogen Ioyama volcano increased in response to a failed eruption in 2017 and a phreatic eruption in April 2018. Therefore, the SO2/CO2 ratio of fumarolic gas was thought to be a useful indicator for evaluating the volcanic activity of Ebinokogen Ioyama volcano. However, a gradual increase was observed in the ratio during the period 2020-2024, with no correlation to seismic activity. The SO2/CO2 ratio has not been an effective indicator of volcanic activity since 2020. The δ18O of fumarolic gas (h) increased during the period 2020-2024, reaching +4.4‰ in June 2023, although the temperature of fumarolic gas was 116°C. A fumarolic gas generation model is employed where a high-temperature magmatic vapor mixes with cold underground meteoric water, resulting in the formation of water vapor and thermal water. The fumarolic gas represents the water vapor. Applying the model, the above high δ18O (+4.4‰) requires a high mixing fraction of magmatic vapor. If the magmatic vapor temperature is assumed to be 900°C, no thermal water is generated and the water vapor temperature exceeds the observed fumarolic temperature. A possible range of magmatic vapor temperatures is 500 to 600°C, which is lower than the general magmatic temperature. Isenthalpic adiabatic expansion and conductive cooling as magmatic vapor ascends in the crust probably occurred after the degassing of magma. During the period from 2020 to 2024, the SO2/CO2 ratio of the two fumarolic gases (c and h) continued to rise with no correspondence with the number of earthquakes. The apparent equilibrium temperature (AET) calculated from the composition of the fumarolic gas also increased during the period from 2020 to 2024, suggesting a reduction in the sulfur sequestration effect for SO2/CO2 ratio and a reduction in the conductive cooling effect for AET. Effective indicators for evaluating volcanic activity at Ebinokogen Ioyama volcano include the δ18O of fumarolic gas (c), where the condensation effect of water vapor is observed in June 2024, and the AET of fumarolic gases (c and h). If the flux of magmatic vapor increases, the condensation effect of water vapor disappears, the δ18O of the fumarolic gas (c) increases, and the AET of the fumarolic gases (c and h) is also expected to increase, reflecting an increase in the temperature of the magmatic vapor.