11:30 AM - 11:45 AM
[SVC32-19] Volcanic activity assessment using temporal and spatial variations of helium isotope ratios around Ebinokogen Iwo-yama, Kirishima volcanic group, Japan
Keywords:Helium, Volcanic gas, precise leveling survey, noble gas, Kirishima
The Kirishima volcanic group has experienced multiple eruptions. Small-scale eruptions occurred at the Shinmoe-dake Volcano from 2008 to 2010, followed by a sub-Plinian eruption with lava effusion in January 2011. The Shinmoe-dake Volcano resumed activity in October 2017, experiencing lava effusion and intermittent eruptions in March 2018. Subsequently, a phreatic eruption occurred at the Ebinokogen Iwo-yama Volcano (hereafter, Iwo-yama) in April 2018. Geophysical observations after the 2011 Shinmoe-dake eruption suggest the magma reservoir is located 5 km northwest of the Shinmoe-dake at a depth of 8 km [1], which is approximately 3 km west-southwest of the Iwo-yama, indicating that these two volcanoes may share the common magma reservoir.
We study the spatial distribution of helium isotope ratios in volcanic gases throughout the Kirishima volcanic group and investigate their temporal variations mainly in the areas surrounding the Iwo-yama Volcano within the group. Noble gases are chemically inert, and their variations can fundamentally be explained by the mixing of end-member components. Helium isotope ratios of volcanic gases serve as a useful indicator for the quantitative evaluation of magmatic fluid contributions with characteristic values of < 0.02 Ra for crustal components [2] and 8±1 Ra [3] for mantle-derived magmas, relative to the atmospheric ratio (1 Ra = 1.4×10-6). To assess the volcanic activity of the Iwo-yama Volcano, we investigated the correlation between temporal variations in the 3He/4He ratio and the volume changes of pressure source beneath the volcano, as estimated through precision leveling surveys. At depths of 200-700 m beneath Iwo-yama, geophysical surveys have identified a low-resistivity layer interpreted as a clay layer [4]. This layer is thought to act as a cap rock, leading to the formation of a hydrothermal water reservoir.
To monitor temporal variations, we collected volcanic gas samples every few months from 2016 to 2024 from the fumaroles of the Iwo-yama Volcano and the Shinyu fumarole, located at the southwest fiank of the Shinmoe-dake Volcano. The air-corrected 3He/4He ratios of fumaroles in the Iwo-yama (monitored throughout the study period) and in the Shinmoe-dake Volcanoes (sampled in January 2017) ranged from 6.8 to 8.0 Ra, roughly consistent with the mantle value. These values suggest that volcanic gases at both sites originated from sources with minimal interaction with low-3He/4He crustal fluids. In contrast, the Shinyu fumarole exhibited relatively lower values of 2.1-5.9 Ra, indicating the contribution of crustal fluids during the migration from the hydrothermal system beneath Shinmoe-dake Volcano.
Significant temporal variations in the 3He/4He ratios were observed at the Iwo-yama Volcano before and after the eruptions in 2017 and 2018. The 3He/4He ratio increased prior to the October 2017 eruption, decreased afterward, and then increased again during the 2018 eruption. During this period, the volume of the pressure source beneath the Iwo-yama Volcano showed rapid expansion. The pressure source then gradually expanded from December 2018 to March 2021 and settled into a steady state from 2021 to March 2023. However, it resumed expanding from March 2023 to March 2024. Mainly in the Iwo-yama summit, sulfur and mud effusions were observed from late 2022 to late 2023 [5]. Among the fumaroles in the Iwo-yama summit, the H fumarole, which exhibits high SO2/H2S ratios [6] suggesting a high contribution from magmatic components, showed a significant increase in the 3He/4He ratio during the expansion period of the pressure source (from March 2023 to March 2024). The series of results suggests that the supply of volatile components from deep-seated magma reservoirs significantly influences volcanic activity.
The spatial distribution survey revealed high values exceeding 7 Ra around the recently active volcanic centers, including the Iwo-yama and Shinmoe-dake Volcanoes, with values gradually decreasing toward peripheral areas. This distribution pattern is consistent with the 3-D resistivity structure [7] and may reflect the mixing ratios of magmatic and crustal fluids decreasing with distance from the magma reservoir.
[1] Nakao et al. (2013), EPS
[2] Ballentine and Burnard (2002), Rev. Mineral. Geochem
[3] Graham (2002), Rev. Mineral. Geochem
[4] Tsukamoto et al. (2018), GRL
[5] Report of Coordinating Committee for Prediction of Volcanic Eruption, 153, 51-63, 2024
[6] Ohba et al., (2021), EPS
[7] Aizawa et al. (2014) JGR: Solid Earth
We study the spatial distribution of helium isotope ratios in volcanic gases throughout the Kirishima volcanic group and investigate their temporal variations mainly in the areas surrounding the Iwo-yama Volcano within the group. Noble gases are chemically inert, and their variations can fundamentally be explained by the mixing of end-member components. Helium isotope ratios of volcanic gases serve as a useful indicator for the quantitative evaluation of magmatic fluid contributions with characteristic values of < 0.02 Ra for crustal components [2] and 8±1 Ra [3] for mantle-derived magmas, relative to the atmospheric ratio (1 Ra = 1.4×10-6). To assess the volcanic activity of the Iwo-yama Volcano, we investigated the correlation between temporal variations in the 3He/4He ratio and the volume changes of pressure source beneath the volcano, as estimated through precision leveling surveys. At depths of 200-700 m beneath Iwo-yama, geophysical surveys have identified a low-resistivity layer interpreted as a clay layer [4]. This layer is thought to act as a cap rock, leading to the formation of a hydrothermal water reservoir.
To monitor temporal variations, we collected volcanic gas samples every few months from 2016 to 2024 from the fumaroles of the Iwo-yama Volcano and the Shinyu fumarole, located at the southwest fiank of the Shinmoe-dake Volcano. The air-corrected 3He/4He ratios of fumaroles in the Iwo-yama (monitored throughout the study period) and in the Shinmoe-dake Volcanoes (sampled in January 2017) ranged from 6.8 to 8.0 Ra, roughly consistent with the mantle value. These values suggest that volcanic gases at both sites originated from sources with minimal interaction with low-3He/4He crustal fluids. In contrast, the Shinyu fumarole exhibited relatively lower values of 2.1-5.9 Ra, indicating the contribution of crustal fluids during the migration from the hydrothermal system beneath Shinmoe-dake Volcano.
Significant temporal variations in the 3He/4He ratios were observed at the Iwo-yama Volcano before and after the eruptions in 2017 and 2018. The 3He/4He ratio increased prior to the October 2017 eruption, decreased afterward, and then increased again during the 2018 eruption. During this period, the volume of the pressure source beneath the Iwo-yama Volcano showed rapid expansion. The pressure source then gradually expanded from December 2018 to March 2021 and settled into a steady state from 2021 to March 2023. However, it resumed expanding from March 2023 to March 2024. Mainly in the Iwo-yama summit, sulfur and mud effusions were observed from late 2022 to late 2023 [5]. Among the fumaroles in the Iwo-yama summit, the H fumarole, which exhibits high SO2/H2S ratios [6] suggesting a high contribution from magmatic components, showed a significant increase in the 3He/4He ratio during the expansion period of the pressure source (from March 2023 to March 2024). The series of results suggests that the supply of volatile components from deep-seated magma reservoirs significantly influences volcanic activity.
The spatial distribution survey revealed high values exceeding 7 Ra around the recently active volcanic centers, including the Iwo-yama and Shinmoe-dake Volcanoes, with values gradually decreasing toward peripheral areas. This distribution pattern is consistent with the 3-D resistivity structure [7] and may reflect the mixing ratios of magmatic and crustal fluids decreasing with distance from the magma reservoir.
[1] Nakao et al. (2013), EPS
[2] Ballentine and Burnard (2002), Rev. Mineral. Geochem
[3] Graham (2002), Rev. Mineral. Geochem
[4] Tsukamoto et al. (2018), GRL
[5] Report of Coordinating Committee for Prediction of Volcanic Eruption, 153, 51-63, 2024
[6] Ohba et al., (2021), EPS
[7] Aizawa et al. (2014) JGR: Solid Earth