[S-VC31] Active Volcanism

[SVC31-15] Determination on the stable hydrogen and oxygen isotopic composition of water vapor in the volcanic plume emitted from Nakadake crater, Aso volcano, just after the eruption in October 2021

*Urumu Tsunogai¹, Masanori Ito¹, Yuhei Morishita¹, Fumiko Nakagawa¹, Shin Yoshikawa², Mitsuru Utsugi², Akihiko Yokoo² (1.Graduate School of Environmental Studies, Nagoya University, 2.Graduate School of Science, Kyoto University)

Keywords:fumarolic gases, volcanic plume, Water vapor, Stable isotopic composition, Unoccupied Aerial Systems, automatic volcanic plume sampler

The water vapor (H₂O) is the most major component in volcanic gas, followed by carbon dioxide, sulfur compounds (H₂S and SO₂), and variety of minor and trace gas species. Not only magmatic H₂O but also meteoric H₂O is assumed for the origin of volcanic H₂O. Based on H and O isotope ratios of H₂O in volcanic gas (volcanic H₂O), we can decide the origin of volcanic H₂O. Because the volcanic eruptions are mainly classified to either magmatic or phreatic, it might be useful to decide the origin of volcanic H₂O. In the case of magmatic H₂O in convergent-plate volcanoes, we can expect δ²H values of -24.5 ± 7.3 ‰ and δ¹⁸O values of +6.0 ± 3.0‰ (the average and the 1σ dispersion range of the volcanic H₂O samples taken in high temperature fumaroles). On the other hand, we can expect the isotopic compositions on the meteoric line for meteoric H₂O. Because phreatic eruptions are caused by meteoric water (surface water, ground water, and sea water, etc.) heated by magma, H₂O emitted during phreatic eruptions should be meteoric H₂O. On the other hand, we can expect that magmatic H₂O will occupy most portion of H₂O emitted during magmatic eruption. Thus, the isotopic compositions (δ²H and δ¹⁸O) of volcanic H₂O emitted during eruptions must be highly useful to differential the type of volcanic eruptions (magmatic or phreatic). We will be able to consider the changes in hydrothermal system through the changes in the isotopic compositions of volcanic H₂O. Direct sampling on fumarolic H₂O in eruptive volcanoes, however, is often neither practical and nor safe. Instead, we can sample of volcanic plumes at a distance from volcanic crater more safely. Since the volcanic plume is formed by a mixing of fumarolic gas and atmospheric air, the isotopic compositions of fumarolic H₂O can be estimated from the concentrations and the isotopic compositions of the H₂O in the volcanic plumes, by subtracting atmospheric contributions to the isotopic compositions of the H₂O in the volcanic plume.
This study was supported by MEXT Integrated Program for Next Generation Volcano Research and Human Resource Development (Theme B).