09:30 〜 09:45
[SVC41-39] Helium isotope ratios of fumaroles and hot-spring gases at Kusatsu-Shirane volcano
キーワード：ヘリウム、同位体、草津白根火山、火山ガス、温泉
Mount Kusatsu-Shirane, an active composite stratovolcano consisting of three pyroclastic cones (from north to south, Mt. Shirane, Mt. Ainomine, and Mt. Motoshirane) erupted on 23 January 2018 at Mt. Motoshirane although active discharges of fumaroles have been observed and the historic eruptions have occurred around the summit of Mt. Shirane. The 2018 eruption was a phreatic eruption, which often occur without magma movement, hence there is no or very small precursor geophysical signals. In contrast, geochemical signatures of volcanic gases could have changed reflecting pressure rise of a gas reservoir which caused the phreatic eruption. Among noble gases, helium is regarded as an useful tracer for the volcanic activity because isotope ratio of helium (3He/4He) exhibits unique values corresponding to the origin (e.g., 7–8 RA in the mantle where 1 RA denotes atmospheric 3He/4He ratio of 1.4×10−6 [1]). Few studies have reported pre-eruptive 3He/4He anomalies [2,3], suggesting the increase of supply of the magmatic helium into the hydrothermal system preceding eruption.
We have conducted biannual monitoring of 3He/4He ratios of fumaroles and hot-spring gases in Kusatsu-Shirane volcano since 2014. 3He/4He ratios until November 2017 of Kitagawa fumaroles, which are active fumaroles north of Yugama crater lake, the largest of the three crater lakes within Mt. Shirane, have been high as 7.3-7.8 RA after the correction of atmospheric contamination based on 4He/20Ne ratios. On the contrary, hot-spring gases located relatively far from the summit of Mt. Shirane were also constant but lower, around 6.5 RA at Yubatake and 4.8 RA at Shiriyaki which are 6 km and 9 km away from the summit, respectively. This tendency has been observed by a previous study [4] and explained as that high 3He/4He magmatic gas is mixed with and/or diluted by low 3He/4He crustal gas with increasing distance from the summit. At the Sessyogawara fumarole, located on the southeastern flank of Mt. Shirane and 2 km east of the 2018 phreatic vent, 3He/4He ratio had been almost constant at 7.0-7.5 RA. However, 3He/4He ratio of fumarole collected at 13 February 2018 was 6.5 RA, which is significantly lower than the previous values. There was no change in 3He/4He ratio of Yubatake hot-spring gas after the eruption, and unfortunately we have no sample from the Kitagawa fumaroles after the eruption until now. If the decrease in 3He/4He ratio of Sessyogawara fumarole is related to the phreatic eruption on January 23, it could imply the following scenarios. (1) contribution of magmatic helium with high 3He/4He ratio similar to the Kitagawa fumaroles decreased due to sealing of gas pathway from magma to its source gas reservoir resulting in dilution by low 3He/4He crustal gas. The sealing could cause increase of gas supply to another gas source, which possibly caused the phreatic eruption. (2) The Sessyogawara fumarole is a mixture of two gases from reservoirs having different 3He/4He ratios, the one is 7.3-7.8 RA represented by the Kitagawa fumaroles and the other is 6.5 RA represented by the Yubatake hot-spring. The low 3He/4He ratio of the Sessyogawara fumarole after the eruption suggests drastic change in mixing ratio of the two gases, implying hydrothermal system beneath the Kisatsu-Shirane volcano complex could change and resulted in the phreatic eruption at unexpected location. Further monitoring is necessary to see what happened with reservoir(s) supplying gas to the Sessyogawara fumarole.
[1] Ozima & Pososek (2002), Noble Gas Geochemistry. [2] Padrón et al. (2013), Geol. [3] Sano et al. (2015), Sci. Rep. [4] Sano et al. (1994), Appl. Geochem.
We have conducted biannual monitoring of 3He/4He ratios of fumaroles and hot-spring gases in Kusatsu-Shirane volcano since 2014. 3He/4He ratios until November 2017 of Kitagawa fumaroles, which are active fumaroles north of Yugama crater lake, the largest of the three crater lakes within Mt. Shirane, have been high as 7.3-7.8 RA after the correction of atmospheric contamination based on 4He/20Ne ratios. On the contrary, hot-spring gases located relatively far from the summit of Mt. Shirane were also constant but lower, around 6.5 RA at Yubatake and 4.8 RA at Shiriyaki which are 6 km and 9 km away from the summit, respectively. This tendency has been observed by a previous study [4] and explained as that high 3He/4He magmatic gas is mixed with and/or diluted by low 3He/4He crustal gas with increasing distance from the summit. At the Sessyogawara fumarole, located on the southeastern flank of Mt. Shirane and 2 km east of the 2018 phreatic vent, 3He/4He ratio had been almost constant at 7.0-7.5 RA. However, 3He/4He ratio of fumarole collected at 13 February 2018 was 6.5 RA, which is significantly lower than the previous values. There was no change in 3He/4He ratio of Yubatake hot-spring gas after the eruption, and unfortunately we have no sample from the Kitagawa fumaroles after the eruption until now. If the decrease in 3He/4He ratio of Sessyogawara fumarole is related to the phreatic eruption on January 23, it could imply the following scenarios. (1) contribution of magmatic helium with high 3He/4He ratio similar to the Kitagawa fumaroles decreased due to sealing of gas pathway from magma to its source gas reservoir resulting in dilution by low 3He/4He crustal gas. The sealing could cause increase of gas supply to another gas source, which possibly caused the phreatic eruption. (2) The Sessyogawara fumarole is a mixture of two gases from reservoirs having different 3He/4He ratios, the one is 7.3-7.8 RA represented by the Kitagawa fumaroles and the other is 6.5 RA represented by the Yubatake hot-spring. The low 3He/4He ratio of the Sessyogawara fumarole after the eruption suggests drastic change in mixing ratio of the two gases, implying hydrothermal system beneath the Kisatsu-Shirane volcano complex could change and resulted in the phreatic eruption at unexpected location. Further monitoring is necessary to see what happened with reservoir(s) supplying gas to the Sessyogawara fumarole.
[1] Ozima & Pososek (2002), Noble Gas Geochemistry. [2] Padrón et al. (2013), Geol. [3] Sano et al. (2015), Sci. Rep. [4] Sano et al. (1994), Appl. Geochem.