2:00 PM - 2:15 PM
[SVC29-02] Deep-seated fluid supply around Kusatsu-Shirane volcano revealed by helium and carbon isotope ratios
Keywords:helium isotope ratio, Kusatsu-Shirane volcano, hydrothermal fluids, carbon isotope ratio
In January 2018, a phreatic eruption occurred at Motoshirane, one of the three pyroclastic cones of Kusatsu-Shirane volcano. From September to December of the same year, a swarm of earthquakes was observed in Takayama Village, Nagano Prefecture, west of the volcano (Terada et al., JpGU-AGU Joint Meeting 2020). Both of these phenomena were caused not by magma itself but by deep hydrothermal fluids involving volatile components separated from magma. Therefore, it is important from the viewpoint of disaster prevention to obtain knowledge of the origin and migration path of deep-seated hydrothermal fluid in and around Kusatsu-Shirane volcano. In this study, we collected fumaroles, hot spring water, well water, and hot spring gas in the area, and measured the helium and carbon isotope ratios.
The 3He/4He ratio (defined as 1 Ra in the atmosphere and corrected for atmospheric contamination using the 4He/20Ne ratio) shows a decreasing trend with distance from the crater at the top of Shirane cone, where all documented (after 1882) phreatic eruptions have occurred until the 2018 eruption at Motoshirane cone. This suggests that the contribution of magmatic helium with a high 3He/4He ratio of about 8 Ra is diluted by the crustal helium with a low 3He/4He ratio during migration of the fluids. Based on the decreasing trend, degree of the dilution is more significant in the west than in the east. However, hot springs with almost constant 3He/4He ratios (4-5 Ra) are widely distributed from 9 to 15 km away from the volcano to the west. It is unlikely that fluids from the hydrothermal system beneath the volcano (e.g. Matsunaga et al., JVGR 2020) are supplied to the groundwater system in this region with a constant mixing ratio with crustal helium. The distribution of the 3He/4He ratio almost overlaps with the source distribution of the 2018 swarm of earthquakes, suggesting that deep hydrothermal fluid may have been efficiently supplied through the fault that may have been the epicenter. A higher 3He/4He ratio (6.1 Ra) was observed in the hot spring gas at a sampling site 26 km to the west, which is located near an active fault system on the western margin of the Nagano Basin, suggesting that deep-seated fluids are also supplied through this fault.
The carbon isotope ratios (δ13CPDB) within 10 km from the crater were in the range of -1 to -8‰, but tended to become lower as the distance from the crater increased. At the farthest site in the west, a very low value of -76‰ was observed. The carbon in this hot spring gas is derived from methane, and since the Green Tuff is distributed in this area, there is a possibility that the natural gas reservoir in Green Tuff, which is reported to contain mantle-derived helium (Sakata, Sekiyu Gakkaishi 1997), is supplied from deep within the basin through the active fault system at the western margin of the Nagano Basin.
The 3He/4He ratio (defined as 1 Ra in the atmosphere and corrected for atmospheric contamination using the 4He/20Ne ratio) shows a decreasing trend with distance from the crater at the top of Shirane cone, where all documented (after 1882) phreatic eruptions have occurred until the 2018 eruption at Motoshirane cone. This suggests that the contribution of magmatic helium with a high 3He/4He ratio of about 8 Ra is diluted by the crustal helium with a low 3He/4He ratio during migration of the fluids. Based on the decreasing trend, degree of the dilution is more significant in the west than in the east. However, hot springs with almost constant 3He/4He ratios (4-5 Ra) are widely distributed from 9 to 15 km away from the volcano to the west. It is unlikely that fluids from the hydrothermal system beneath the volcano (e.g. Matsunaga et al., JVGR 2020) are supplied to the groundwater system in this region with a constant mixing ratio with crustal helium. The distribution of the 3He/4He ratio almost overlaps with the source distribution of the 2018 swarm of earthquakes, suggesting that deep hydrothermal fluid may have been efficiently supplied through the fault that may have been the epicenter. A higher 3He/4He ratio (6.1 Ra) was observed in the hot spring gas at a sampling site 26 km to the west, which is located near an active fault system on the western margin of the Nagano Basin, suggesting that deep-seated fluids are also supplied through this fault.
The carbon isotope ratios (δ13CPDB) within 10 km from the crater were in the range of -1 to -8‰, but tended to become lower as the distance from the crater increased. At the farthest site in the west, a very low value of -76‰ was observed. The carbon in this hot spring gas is derived from methane, and since the Green Tuff is distributed in this area, there is a possibility that the natural gas reservoir in Green Tuff, which is reported to contain mantle-derived helium (Sakata, Sekiyu Gakkaishi 1997), is supplied from deep within the basin through the active fault system at the western margin of the Nagano Basin.