Presentation Information
[S1-01]Temporal Variations in Helium Isotope Ratios of Volcanic Gases and Volcanic Activity in the Kirishima Volcanic Group (2016–2026)
*Nanae Fukushima1, Hirochika Sumino1, Takeshi Ohba2, Muga Yaguchi3, Jun-ichiro Ishibashi4, Yuki Yasuda5, Obase Tomoya6, Kotaro Toyama7, Rai Yoneda8, Takeshi Matsushima9 (1. RCAST, UTokyo, 2. Sch. of Sci. Tokai Univ., 3. MRI, JMA, 4. KOBEC, Kobe U, 5. ERI, UTokyo, 6. Sch. of Sci. Science Tokyo, 7. HSRI, 8. Sch. of Sci. UTokyo, 9. SEVO, Kyushu Univ.)
Keywords:
Volcanic gas,Helium isotope ratio,Phreatic eruption,Precise leveling survey,Kirishima
Helium isotope ratios (3He/4He) in volcanic gases are useful indicators of magmatic fluid contributions and have the potential to assess volcanic activity, including the detection of precursory signals of phreatic eruptions. We investigate temporal variations of 3He/4He in gases from Kirishima volcanic group, southern Kyushu, Japan, focusing on Shinmoe-dake and Ebinokogen Iwo-yama (hereafter, Iwo-yama), both of which have erupted since the 20th century. Based on the volcanic subsurface structure inferred from geophysical observations (Nakao et al., 2013; Ueda et al., 2013; Aizawa et al., 2026), the two volcanoes may share the same magma reservoir.
From 2016 to 2026, gas samples were collected every few months from Iwo-yama fumaroles and Shinyu fumarole on Shinmoe-dake’s southwest flank. To evaluate whether temporal variations in the 3He/4He ratio reflect localized hydrothermal processes or broader changes in the volcanic system, we compared them with volume changes in the hydrothermal reservoir beneath Iwo-yama estimated from precise leveling surveys. The reservoir is inferred to be capped by a clay-rich, low-resistivity layer at depths of 200–700 m (Tsukamoto et al., 2018).
Significant variations in the 3He/4He ratios were observed at Iwo-yama surrounding the 2017 and 2018 eruptions. The 3He/4He ratio increased before the 2017 Shinmoe-dake eruption, decreased afterward, and increased again during the 2018 eruptions at both Shinmoe-dake and Iwo-yama. Agreement between geochemical and geophysical data indicates that volatile ascent from the magma reservoir influences volcanic activity.
Shinyu fumarole exhibited relatively low air-corrected 3He/4He ratios (2.1–5.9 Ra), with no pre-eruptive increase. A transient decrease in the 3He/4He ratio was observed only after the 2018 Shinmoe-dake eruption, which we attribute to radiogenic 4He released by rock fracturing during the magma reservoir deflation inferred from GNSS observations by GSI. In contrast, no significant changes in the 3He/4He ratios were observed at either Iwo-yama or Shinyu during the 2025 eruption of Shinmoe-dake. We are currently investigating the factors responsible for this difference in light of the contrasting eruption styles of the 2017–2018 and 2025 eruptions.
From 2016 to 2026, gas samples were collected every few months from Iwo-yama fumaroles and Shinyu fumarole on Shinmoe-dake’s southwest flank. To evaluate whether temporal variations in the 3He/4He ratio reflect localized hydrothermal processes or broader changes in the volcanic system, we compared them with volume changes in the hydrothermal reservoir beneath Iwo-yama estimated from precise leveling surveys. The reservoir is inferred to be capped by a clay-rich, low-resistivity layer at depths of 200–700 m (Tsukamoto et al., 2018).
Significant variations in the 3He/4He ratios were observed at Iwo-yama surrounding the 2017 and 2018 eruptions. The 3He/4He ratio increased before the 2017 Shinmoe-dake eruption, decreased afterward, and increased again during the 2018 eruptions at both Shinmoe-dake and Iwo-yama. Agreement between geochemical and geophysical data indicates that volatile ascent from the magma reservoir influences volcanic activity.
Shinyu fumarole exhibited relatively low air-corrected 3He/4He ratios (2.1–5.9 Ra), with no pre-eruptive increase. A transient decrease in the 3He/4He ratio was observed only after the 2018 Shinmoe-dake eruption, which we attribute to radiogenic 4He released by rock fracturing during the magma reservoir deflation inferred from GNSS observations by GSI. In contrast, no significant changes in the 3He/4He ratios were observed at either Iwo-yama or Shinyu during the 2025 eruption of Shinmoe-dake. We are currently investigating the factors responsible for this difference in light of the contrasting eruption styles of the 2017–2018 and 2025 eruptions.