16:00 〜 16:15
[SVC33-09] 霧島硫黄山噴気中のヘリウム・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、霧島火山
In Kirishima volcanic group in Kyushu, Southwest Japan, Mt. Shinmoedake recently erupted in 2011, 2017, and 2018, and Mt. Ioyama also erupted in 2018. Some fumaroles are continuously discharging volcanic gases at Mt. Ioyama. Volcanic gases are also emitted from the Shin-yu fumaroles at ~2 km west of Mt. Shinmoedake. Chemical and isotopic compositions of volcanic gases may reflect volcanic activity related to the eruption such as increase in magmatic gas flux. Helium (3He/4He) and carbon (δ13C = [(13C/12C)gas/(13C/12C)PDB – 1)] × 1000 (‰)) isotope 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 is equal to the atmospheric 3He/4He) and <0.02 Ra in the crust. Similarly, δ13C values are −6.5±2.5‰ in the mantle, ~0‰ for marine carbonate, 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 temporal variations in 3He/4He, δ13C-CO2, and 3He/CO2 of volcanic gases from five fumaroles (a, b, c, h, V2) at the Ioyama volcano and those from a Shin-yu fumarole, which is located 4 km south of the volcano, between August 2016 and June 2021.
The 3He/4He ratios (corrected for atmospheric He contamination by 4He/20Ne) of the five Ioyama fumaroles (6.8–7.7 Ra) roughly synchronized each other, indicating that volcanic gases are supplied from a single gas reservoir. The average 3He/4He value increased and decreased before and after the Mt. Shinmoedake eruptions, respectively. This pattern probably reflects the variation in magmatic gas supply to the Ioyama fumaroles due to the pressure variation of the magma reservoir [2]. The average 3He/4He value increased again since July 2018 and has been stable until June 2021 (7.4–7.6 Ra). The 3He/CO2 variations at the Ioyama fumaroles ((0.6–2.1) × 10−10) were less synchronized and did not correlate with 3He/4He ratios. This indicates that 3He/CO2 cannot be explained by a simple mixing of magmatic gas and crustal CO2. Alternatively, the results suggest that a significant amount of CO2 is chemically removed from the volcanic gases before reaching the surface, or presence of more than two magmatic gas components with different 3He/CO2 ratios. On the other hand, the variations of δ13C-CO2 (−5.1 to −2.5‰) roughly synchronized each other. The average δ13C-CO2 increased ~1‰ after the 2018 eruption and decreased ~1‰ since July 2020. The former may reflect CO2 contribution from a newly supplied magma, and the latter may indicate that δ13C-CO2 of the magma became lower due to continuous degassing. Occurrences of magmatic δ13C-CO2 shifts to the lighter values over time have been reported in some field studies. This is probably induced by progressive loss of CO2 relatively enriched in 13C compared that in the magma, resulting in the CO2 in the residual melt being isotopically lighter with time [3].
The Shin-yu fumarole shows low 3He/4He ratios (4.3–5.8 Ra) relative to the Ioyama fumaroles, reflecting larger contribution of crustal He probably incorporated during transportation of hydrothermal fluid from the Shinmoedake volcano. The 3He/4He gradually decreased from one month after the 2017 eruption and started to increase in August 2018. The decrease in 3He/4He after volcanic eruption was also observed at some fumaroles around the Kusatsu-Shirane volcano [4]. Both may reflect decrease in hydrothermal fluid supply after the eruptions, resulting in larger contribution of crustal He.
References
[1] Padrón et al. (2013) Geology 41(5), 539–542. [2] Sumino et al. (2020) JpGU, SVC45-P29, abstract. [3] Bergfeld et al. (2017) J. Volcanol. Geotherm. Res. 343, 109–121. [4] Obase et al. (2021) JpGU, SVC29-P03, abstract.
The 3He/4He ratios (corrected for atmospheric He contamination by 4He/20Ne) of the five Ioyama fumaroles (6.8–7.7 Ra) roughly synchronized each other, indicating that volcanic gases are supplied from a single gas reservoir. The average 3He/4He value increased and decreased before and after the Mt. Shinmoedake eruptions, respectively. This pattern probably reflects the variation in magmatic gas supply to the Ioyama fumaroles due to the pressure variation of the magma reservoir [2]. The average 3He/4He value increased again since July 2018 and has been stable until June 2021 (7.4–7.6 Ra). The 3He/CO2 variations at the Ioyama fumaroles ((0.6–2.1) × 10−10) were less synchronized and did not correlate with 3He/4He ratios. This indicates that 3He/CO2 cannot be explained by a simple mixing of magmatic gas and crustal CO2. Alternatively, the results suggest that a significant amount of CO2 is chemically removed from the volcanic gases before reaching the surface, or presence of more than two magmatic gas components with different 3He/CO2 ratios. On the other hand, the variations of δ13C-CO2 (−5.1 to −2.5‰) roughly synchronized each other. The average δ13C-CO2 increased ~1‰ after the 2018 eruption and decreased ~1‰ since July 2020. The former may reflect CO2 contribution from a newly supplied magma, and the latter may indicate that δ13C-CO2 of the magma became lower due to continuous degassing. Occurrences of magmatic δ13C-CO2 shifts to the lighter values over time have been reported in some field studies. This is probably induced by progressive loss of CO2 relatively enriched in 13C compared that in the magma, resulting in the CO2 in the residual melt being isotopically lighter with time [3].
The Shin-yu fumarole shows low 3He/4He ratios (4.3–5.8 Ra) relative to the Ioyama fumaroles, reflecting larger contribution of crustal He probably incorporated during transportation of hydrothermal fluid from the Shinmoedake volcano. The 3He/4He gradually decreased from one month after the 2017 eruption and started to increase in August 2018. The decrease in 3He/4He after volcanic eruption was also observed at some fumaroles around the Kusatsu-Shirane volcano [4]. Both may reflect decrease in hydrothermal fluid supply after the eruptions, resulting in larger contribution of crustal He.
References
[1] Padrón et al. (2013) Geology 41(5), 539–542. [2] Sumino et al. (2020) JpGU, SVC45-P29, abstract. [3] Bergfeld et al. (2017) J. Volcanol. Geotherm. Res. 343, 109–121. [4] Obase et al. (2021) JpGU, SVC29-P03, abstract.