[SVC45-15] Helium isotope ratios at Kusatsu-Shirane volcano
Keywords:helium isotope ratio, Kusatsu-Shirane volcano, volcanic gas, hydrothermal fluid
We report 3He/4He of fumaroles and hot/cold spring gases collected from 11 sites at Kusatsu-Shirane volcano during 2014 and 2019. The measured 3He/4He were corrected for atmospheric contamination based on 4He/20Ne. 3He/4He drops down to 6.0-6.5 Ra after the 2018 eruption and subsequent recovery to higher values were observed at Sesshogawara fumarole and Kusatsu-Yubatake hot spring. Although there are no data before the 2018 eruption, 3He/4He increases were observed at Hoshimata fumarole and Anajugoku cold spring after the eruption. Since then, 3He/4He of the four sites have been constant at high levels (7.0-7.6 Ra). The temporal 3He/4He variations seem to be associated with the 2018 eruption with different time lags, which may reflect the migration distance of fluids from the volcanic center.
Based on magnetotelluric investigation, Matsunaga et al.  suggested the presence of hydrothermal reservoir at depths of 1-3 km broadly extending from beneath Mt. Shirane to Mt. Motoshirane, which supplies fluids to the shallow hydrothermal systems. The constant and high 3He/4He (7.2-8.1 Ra) during the study period of Kitagawa fumaroles on the northern flank of Mt. Shirane near the Yugama crater may reflect the value of the hydrothermal reservoir.
On the other hand, the 3He/4He drops at the four sites after the eruption requires another reservoir having low 3He/4He value. The 3He/4He of fumaroles, hot and cold springs decrease with distance from the volcano. Some of the springs relatively far from the volcano indicate involvement of fossil salt water in groundwater flow system , which would exhibit low 3He/4He value due to accumulation of crustal helium. Therefore, the 3He/4He decrease with distance from the volcano can be accounted for by an increase of contribution of crustal helium derived from old groundwater to the magmatic helium with migration distance of the fluid from the hydrothermal reservoir to the sampling site.
Ohba et al.  showed that the Kitagawa, Sesshogawara, and Hoshimata fumaroles are derived from distinct, small hydrothermal reservoirs which share the same magmatic fluid. It has been proposed that the 2018 eruption was caused by sealing of magmatic gas pathway to the shallow hydrothermal reservoir which supplies gas to the Kitagawa fumarole . If such sealing would have similarly occurred for Sesshogawara, Kusatsu-Yubatake, Hoshimata, and Anajigoku reservoirs, it would also result in decreases of supply of magmatic gas from the deep hydrothermal reservoir, which were observed as the 3He/4He drops.
Although further study is necessary to constrain the high- and low-3He/4He reservoirs, the remarkable temporal 3He/4He variations associated with the 2018 eruption implies 3He/4He tells us the state of hydrothermal reservoir potentially causing a phreatic eruption.
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