[SVC45-P29] Temporal and spatial variations of helium isotope ratios in volcanic gases at Kirishima volcanic group
Keywords:Helium isotopes, Kirishima volcanic group, Volcanic gas
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 2020. 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) observed in and around the central craters of the active volcanos (i.e., Iwo-yama, Shinmoe-dake, and Ohata-ike) are higher than those (3.5 to 5.8 Ra) at the other sites. The 3He/4He ratios of non-crater sites decrease with the increase of their distances to each sampling site from a magma reservoir, location of which is estimated as pressure source of crustal deformation associated with the 2011 Shinmoe-dake eruption. This can be accounted for by an increase of contribution of radiogenic 4He in old groundwater to the magmatic helium with migration distance of the fluid from the magma to the site . On the other hand, former or present active craters show constant and high 3He/4He ratios irrespective of the distance from the magma reservoir, indicating there would be pathways of magmatic gas from the magma without or less interaction with the old groundwater.
The 3He/4He ratios of fumaroles in Iwo-yama slightly increase before Shinmoe-dake eruptions, and decrease after each 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 reservoir has high 3He/4He ratio, the increase of 3He/4He ratios of the fumaroles before the eruption would result from increase of the supply of the gas from the magma reservoir 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 reservoir. Since the last eruption at Shinmoe-dake in June 2019, the 3He/4He ratios of Iwo-yama fumaroles have been constant at high values (7.2–7.6 Ra), suggesting magmatic gas has been continuously supplied to Iwo-yama fumaroles at constant rate.
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