[S-VC26] Active Volcanism

[SVC26-P09] Gas transport system inferred from chemical compositions of Iwatsubo-dani fumarole, Yakedake Volcano, Japan

*Kenshin Morita¹, Takeshi Saito¹, Ryunosuke KAZAHAYA², Kazuhiro Amita³, Taketoshi Mishima⁴, Shinji Ohsawa⁴ (1.Faculty of Science, Shinshu University, 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 3.Graduate School of Engineering Science, Akita University, 4.Institute for Geothermal Sciences, Kyoto University)

Yakedake, located in the south part of Northern Japan Alps, is an active volcano which made phreatic eruptions in 1907-39 and in 1962-63.Since the latest phreatic eruption in 1963, Yakedake volcano has remained dormant, but fumarolic activity around the summit area has continued steadily. Recently, temperature, chemical, and isotopic compositions of fumarolic gases discharged from the summit fumaroles were ascertained for 2013-2017 (Saito et al., 2019). Iwatsubo-dani fumarole, located at about 500 m southwest of the summit, remained to be investigated because of steep terrain and the location deviated greatly from the trail. In 2022, remote observation using gas detector tubes and a multicomponent gas analyzer system (Multi-GAS; Shinohara, 2005) was conducted. The results showed that concentrations of H2 and SO2 were higher than those of other fumaroles, indicating that high temperature magmatic gas is supplied to southwestern region of the volcano (Saito et al., 2022). In this study, we succeeded in accessing Iwatsubo-dani fumarole and sampling volcanic gas.
Iwatsubo-dani fumarole is located at an elevation of 2170 m at the head of Iwatsubo valley. We can reach the fumarole by traversing westward the northernmost part of Shimohorizawa lava and by descending about 100 m in elevation on the southwestern slope of the volcano. One main fumarole was observed in 2023, though two strong fumaroles were observed in 2022. Discharge temperatures of Iwatsubo-dani fumarole were about 101°C, lower than the fumaroles at the northern summit (110°C). The sampled gas from Iwatsubo-dani fumarole was higher in not only H2 and SO2 but CO2 and He than those from the fumaroles at the northern summit. He/Ar ratios and hydrogen oxygen isotopic composition of fumarolic condensate also indicated strong influence of deep magmatic fluid. CO2/He from Yakedake fumaroles are quite similar, suggesting one common gas source. The apparent equilibrium temperatures (AETs, Ohba et al. 1994) calculated from the sulfur-containing gas components were about 455°C, higher than those of the other fumaroles (about 290°C). This also indicates that the Iwatsubo-dani fumarole preserved deep magmatic conditions. On the other hand, CH4 at the Iwatsubo-dani fumarole were about 10 times higher than those at the summit fumaroles, suggesting Iwatsubo-dani fumarole may be influenced by hydrothermal systems. This result is inconsistent with other results which indicate strong influence of magmatic fluid.