[S-VC28] Active Volcanism

[SVC28-25] Behavior of magmatic components in fumarolic gases related to the 2018 phreatic eruption at Ebinokogen Ioyama volcano, Kirishima Volcanic Group, Kyushu, Japan

*Takeshi Ohba¹, Muga Yaguchi², Urumu Tsunogai³, Masanori Ito³, Ryo Shingubara³ (1.Department of chemistry, School of Science, Tokia University, 2.Meteorol. Res. Inst., 3.Grad. School of Envir. Studies Nagoya University)

Keywords:Phreatic eruption, Volcanic gas, SO2

Direct sampling and analysis of volcanic gas was conducted at three fumaroles on Ebinokogen Ioyama volcano, Japan, between December 2015 and July 2020. Notable changes in the chemical composition of gases related to volcanic activity included a sharp increase in SO2 and H2 concentrations in May 2017 and March 2018. The analyses in March 2018 immediately preceded the April 2018 eruption at Ioyama volcano. The isotopic ratios of H2O in fumarolic gas suggested the formation of fumarolic gas. Up to 49% high enthalpy magmatic vapor mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100-160C. Portions of the vapor and liquid phases were discharged as fumarolic gases and hot spring water. The CO2/SO2 ratio of the fumarolic gas was higher than that previously estimated for magmatic vapor due to SO2 hydrolysis during the formation of the vapor phase. When the flux of the magmatic vapor was high, effects of hydrolysis were small resulting in low CO2/SO2 ratios in fumarolic gases. The high apparent equilibrium temperature (AETS) defined for reactions involving SO2, H2S, H2 and H2O, together with low CO2/SO2 and H2S/SO2 ratios were regarded to be precursor signals of fumarolic gas prior to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature (AETD) obtained from the hydrogen isotope ratio of H2O and H2 also increased significantly in May 2017 and March 2018, but there were cases where the changes did not coordinate between the fumaroles. For example, AETD of one fumarole increased, but AETD of another fumarole decreased at the same time. The distance between fumaroles is at most 200 m, and it is estimated that AETD is influenced by the phenomenon occurring in the shallow underground. AETS increased rapidly in May 2017 and March 2018 suggesting an increased flux of magmatic vapor. Between September 2017 and January 2018, AETS was low suggesting the suppression of magmatic vapor flux. During this period, magmatic eruptions took place at Shinmoedake volcano 5 km away from Ioyama volcano. We conclude that magma sealing and transport to Shinmoedake volcano occurred simultaneously in the magma chamber beneath Ioyama volcano.