14:15 〜 14:30
[SVC38-38] 霧島火山群におけるヘリウム同位体比の時空間変化
キーワード：霧島火山群、ヘリウム同位体比、火山ガス、時空間変化、火山噴火
In Kirishima volcanic group in Kyushu, Southwest Japan, Shinmoe-dake is an active volcano and erupted in 2008, 2011, 2017 and 2018, and Iwo-yama also erupted in 2018. For the volcanic disaster preservation, forecasting the timing of the volcanic eruption is required. It is known that the temporal variations in chemical and isotopic compositions of volcanic gases are useful to evaluate the present state of volcanic activity [1]. Especially, helium isotope ratio (3He/4He) exhibits unique values corresponding to the origin (e.g., 7-8 Ra in the mantle and about 0.02 Ra in the crust, where 1 Ra denotes atmospheric 3He/4He ratio of 1.4×10-6 [2]). Some studies have reported pre-eruptive 3He/4He anomalies, suggesting the increase of the magmatic helium supply into the hydrothermal system preceding eruption [1, 3, 4].
We report spatial and temporal variations of 3He/4He ratios of fumaroles and hot spring gases collected from 10 sites in Kirishima volcanic group during 2016 to 2018. The measured 3He/4He ratios were corrected for atmospheric contamination based on 4He/20Ne ratios. The air-corrected 3He/4He ratios (6.8 to 7.7 Ra) of the central craters in the active volcanos (i.e., Iwo-yama and Shinmoe-dake) are higher than those (4.3 to 5.3 Ra) of the other sites. These 3He/4He ratios decrease with the increase of the distance from the magma reservoir, location of which is estimated as pressure source of crustal deformation associated with the 2011 Shinmoe-dake eruption, to each sampling site. This trend reflects that the contribution of radiogenic 4He in old groundwater increases with migration distance of the gas from the magma to the site [5].
The 3He/4He ratios of fumaroles in Iwo-yama slightly increase before Shinmoe-dake eruptions, and decrease after the eruption. This variation cannot be accounted for by the contribution of the radiogenic 4He relative to total helium in the fumaroles before and after the eruptions, because it is estimated that the amount of radiogenic 4He which volcanic gas can acquire during its migration from magma to the surface is negligible. Alternatively, the variation results from the change in mixing ratio of gases derived from two reservoirs having high and low 3He/4He ratios. Assuming that the magma chamber has high 3He/4He ratio, the increase of 3He/4He ratios of the fumaroles before the eruption would be resulted from increase of the supply of the gas from the chamber to the Iwo-yama fumaroles. Once an eruption occurs at Shinmoe-dake, magmatic gas is effectively released through the volcanic vent, resulting in decrease of its supply to the Iwo-yama fumaroles. Thus, the temporal variation of 3He/4He ratios in volcanic gases may reflect the pressure variation of the magma chamber.
References: [1]Paonita et al. (2016) Geol.; [2] Ozima and Podosek (2002) Noble Gas Geochemistry.; [3] Padrón et al. (2013) Geol.; [4] Sano et al. (2015) Sci. Rep.; [5] Sano et al. (1994) Appl. Geochem.
We report spatial and temporal variations of 3He/4He ratios of fumaroles and hot spring gases collected from 10 sites in Kirishima volcanic group during 2016 to 2018. The measured 3He/4He ratios were corrected for atmospheric contamination based on 4He/20Ne ratios. The air-corrected 3He/4He ratios (6.8 to 7.7 Ra) of the central craters in the active volcanos (i.e., Iwo-yama and Shinmoe-dake) are higher than those (4.3 to 5.3 Ra) of the other sites. These 3He/4He ratios decrease with the increase of the distance from the magma reservoir, location of which is estimated as pressure source of crustal deformation associated with the 2011 Shinmoe-dake eruption, to each sampling site. This trend reflects that the contribution of radiogenic 4He in old groundwater increases with migration distance of the gas from the magma to the site [5].
The 3He/4He ratios of fumaroles in Iwo-yama slightly increase before Shinmoe-dake eruptions, and decrease after the eruption. This variation cannot be accounted for by the contribution of the radiogenic 4He relative to total helium in the fumaroles before and after the eruptions, because it is estimated that the amount of radiogenic 4He which volcanic gas can acquire during its migration from magma to the surface is negligible. Alternatively, the variation results from the change in mixing ratio of gases derived from two reservoirs having high and low 3He/4He ratios. Assuming that the magma chamber has high 3He/4He ratio, the increase of 3He/4He ratios of the fumaroles before the eruption would be resulted from increase of the supply of the gas from the chamber to the Iwo-yama fumaroles. Once an eruption occurs at Shinmoe-dake, magmatic gas is effectively released through the volcanic vent, resulting in decrease of its supply to the Iwo-yama fumaroles. Thus, the temporal variation of 3He/4He ratios in volcanic gases may reflect the pressure variation of the magma chamber.
References: [1]Paonita et al. (2016) Geol.; [2] Ozima and Podosek (2002) Noble Gas Geochemistry.; [3] Padrón et al. (2013) Geol.; [4] Sano et al. (2015) Sci. Rep.; [5] Sano et al. (1994) Appl. Geochem.