[SVC45-15] 草津白根火山の火山ガスのヘリウム同位体比
キーワード：ヘリウム同位体比、草津白根火山、火山ガス、熱水
At Kusatsu-Shirane, an active stratovolcano consisting of three pyroclastic cones (Mt. Shirane, Mt. Ainomine, and Mt. Motoshirane), a phreatic eruption occurred on 23 January 2018 at Mt. Motoshirane. Phreatic eruptions often occur without magma movement, thus are accompanied by no or very small precursor geophysical signals. In contrast, chemical and isotopic compositions of volcanic gases are expected to change reflecting pressure rise of a gas reservoir which causes a phreatic eruption. Helium isotope ratio (3He/4He) has a great potential as a tracer of volcanic activity, because it exhibits unique values corresponding to the origin (e.g., ca. 8 Ra in the mantle and < 0.1 Ra in the curst, where 1 Ra denotes atmospheric 3He/4He). Some studies reported pre-eruptive 3He/4He anomalies, suggesting the increase of supply of the magmatic helium into the hydrothermal system preceding eruption [1-3].
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. [4] 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 [5], 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. [6] 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 [7]. 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.
References
[1] Padrón et al. (2013) Geology. [2] Paonita et al. (2016) Geology. [3] Sano et al. (2015) Sci. Rep. [4] Matsunaga et al. (2020) J. Volcanol. Geotherm. Res. [5] Terada et al. (2020) JpGU2020. [6] Ohba et al. (2019) Front. Earth Sci. [7] Ohba et al. (2018) JpGU2018.
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. [4] 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 [5], 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. [6] 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 [7]. 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.
References
[1] Padrón et al. (2013) Geology. [2] Paonita et al. (2016) Geology. [3] Sano et al. (2015) Sci. Rep. [4] Matsunaga et al. (2020) J. Volcanol. Geotherm. Res. [5] Terada et al. (2020) JpGU2020. [6] Ohba et al. (2019) Front. Earth Sci. [7] Ohba et al. (2018) JpGU2018.