17:15 〜 18:30
[SVC29-P03] 草津白根火山噴気ガス中のヘリウム・炭素同位体組成の時間変動
キーワード：ヘリウム同位体、炭素同位体、火山ガス、草津白根火山、時間変動、熱水系
Kusatsu-Shirane is an active stratovolcano consists 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 precursor geophysical signals, but chemical and isotopic compositions of volcanic gases may reflect some precursor events such as pressure rise at a gas reservoir. Helium (3He/4He) and carbon (δ13C = (13C/12C)gas/(13C/12C)PDB - 1) isotopic ratios are potential tracers of volcanic activity as they exhibit unique values corresponding to the origins. For example, 3He/4He ratios are ~8 Ra in the mantle (1 Ra equals to the atmospheric 3He/4He) and <0.1 Ra in the crust, and δ13C values are -6.5±2.5‰ in the mantle and <-20‰ for organic carbon. Pre-eruptive 3He/4He rises have been reported in some volcanoes, suggesting increase in magmatic He supply preceding the eruptions [e.g., 1]. Here we report 3He/4He, δ13C-CO2, and 3He/CO2 of volcanic gases from five fumaroles at Kusatsu-Shirane volcano sampled from 2014 to 2020.
The 3He/4He ratios (corrected for atmospheric He contamination by 4He/20Ne) of three Kitagawa fumaroles on the northern flank of Mt. Shirane roughly synchronized each other and temporally varied in the range of 7.1 to 8.1 Ra, suggesting that magmatic He supply changed during the sampling period. The temporal variations of δ13C-CO2 values (-4.9 – -1.4‰) were inversely correlated to 3He/4He, indicating low δ13C-CO2 during high magmatic gas supply. On the other hand, the δ13C values of the Sesshogawara and Manza fumaroles (located the east and west of Kusatsu-Shirane volcano, respectively) varied in the ranges of -4.5 – -0.8‰ and -3.7 – -0.5‰, respectively, were not correlated with each 3He/4He ratio.
The 3He/CO2 ratios of Kitagawa fumaroles ((0.3 – 2.0) × 10-9) negatively correlated with δ13C-CO2, indicating high 3He/CO2 ratios in the periods of high magmatic gas supply. A trend for Manza fumarole is completely different that the 3He/CO2 ratios show less variation ((0.3 – 0.5) × 10-9) that positively correlate with δ13C-CO2. The 3He/CO2 ratios of Sesshogawara fumarole ((0.3 – 0.7) × 10-9) were slightly higher than those of Manza fumarole and do not correlate with δ13C-CO2. The 3He/CO2 ratios of Sesshogawara and Manza fumaroles with high δ13C-CO2 values were similar to the 3He/CO2 ratios of Kitagawa fumaroles in the periods of low magmatic gas supply.
These trends can be explained by a mixing of two gas components with organic CO2. One is the magmatic gas with high 3He/CO2 and low δ13C-CO2 values directly degassed from the magma. The other is probably the gas separated from hydrothermal fluids with low 3He/CO2 and high δ13C-CO2 values produced by addition of the crustal CO2 with a high δ13C value (~1.1‰ around Kusatsu-Shirane volcano [2]) to the magmatic gas. This model suggests that both magmatic and hydrothermal gases contribute to the fumarolic gases of Kitagawa. The Manza fumarole is fed by the hydrothermal fluid only, resulting in the small temporal variation of 3He/CO2 ratios. The positive correlation between the 3He/CO2 and δ13C-CO2 values is attributable to the addition of the organic CO2 to various extents. The Sesshogawara fumarole is mainly fed by the hydrothermal fluid with small contributions of the magmatic gas and organic CO2.
The presence of a hydrothermal fluid reservoir at depths of 1-3 km broadly extending from beneath Mt. Shirane to Mt. Motoshirane was suggested by magnetotelluric investigation [3]. If the reservoir supplies fluids to the Sesshogawara and Manza fumaroles, the episodic drop of 3He/4He ratios following the 2018 phreatic eruption at Mt. Motoshirane [4] might be attributable to a decrease in the supply of fluids that carry magmatic He to both fumaroles, resulting in crustal He more apparent.
References
[1] Padrón et al. (2013) Geology. [2] Sano et al. (1994) Appl. Geochem. [3] Matsunaga et al. (2020) J. Volcanol. Geotherm. Res. [4] Sumino et al. JpGU2020.
The 3He/4He ratios (corrected for atmospheric He contamination by 4He/20Ne) of three Kitagawa fumaroles on the northern flank of Mt. Shirane roughly synchronized each other and temporally varied in the range of 7.1 to 8.1 Ra, suggesting that magmatic He supply changed during the sampling period. The temporal variations of δ13C-CO2 values (-4.9 – -1.4‰) were inversely correlated to 3He/4He, indicating low δ13C-CO2 during high magmatic gas supply. On the other hand, the δ13C values of the Sesshogawara and Manza fumaroles (located the east and west of Kusatsu-Shirane volcano, respectively) varied in the ranges of -4.5 – -0.8‰ and -3.7 – -0.5‰, respectively, were not correlated with each 3He/4He ratio.
The 3He/CO2 ratios of Kitagawa fumaroles ((0.3 – 2.0) × 10-9) negatively correlated with δ13C-CO2, indicating high 3He/CO2 ratios in the periods of high magmatic gas supply. A trend for Manza fumarole is completely different that the 3He/CO2 ratios show less variation ((0.3 – 0.5) × 10-9) that positively correlate with δ13C-CO2. The 3He/CO2 ratios of Sesshogawara fumarole ((0.3 – 0.7) × 10-9) were slightly higher than those of Manza fumarole and do not correlate with δ13C-CO2. The 3He/CO2 ratios of Sesshogawara and Manza fumaroles with high δ13C-CO2 values were similar to the 3He/CO2 ratios of Kitagawa fumaroles in the periods of low magmatic gas supply.
These trends can be explained by a mixing of two gas components with organic CO2. One is the magmatic gas with high 3He/CO2 and low δ13C-CO2 values directly degassed from the magma. The other is probably the gas separated from hydrothermal fluids with low 3He/CO2 and high δ13C-CO2 values produced by addition of the crustal CO2 with a high δ13C value (~1.1‰ around Kusatsu-Shirane volcano [2]) to the magmatic gas. This model suggests that both magmatic and hydrothermal gases contribute to the fumarolic gases of Kitagawa. The Manza fumarole is fed by the hydrothermal fluid only, resulting in the small temporal variation of 3He/CO2 ratios. The positive correlation between the 3He/CO2 and δ13C-CO2 values is attributable to the addition of the organic CO2 to various extents. The Sesshogawara fumarole is mainly fed by the hydrothermal fluid with small contributions of the magmatic gas and organic CO2.
The presence of a hydrothermal fluid reservoir at depths of 1-3 km broadly extending from beneath Mt. Shirane to Mt. Motoshirane was suggested by magnetotelluric investigation [3]. If the reservoir supplies fluids to the Sesshogawara and Manza fumaroles, the episodic drop of 3He/4He ratios following the 2018 phreatic eruption at Mt. Motoshirane [4] might be attributable to a decrease in the supply of fluids that carry magmatic He to both fumaroles, resulting in crustal He more apparent.
References
[1] Padrón et al. (2013) Geology. [2] Sano et al. (1994) Appl. Geochem. [3] Matsunaga et al. (2020) J. Volcanol. Geotherm. Res. [4] Sumino et al. JpGU2020.