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

Abstract

Direct sampling and analysis of fumarolic gas was conducted at 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 revealed the process of formation. Up to 49% high-enthalpy magmatic vapor mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100–160 °C. Portions of the vapor and liquid phases were discharged as fumarolic gases and hot spring water, respectively. The CO2/SO2 ratio of the fumarolic gas was higher than that 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 defined for reactions involving SO2, H2S, H2 and H2O, together with low CO2/SO2 and H2S /SO2 ratios were regarded to be precursor signals to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature increased rapidly in May 2017 and March 2018 suggesting an increased flux of magmatic vapor. Between September 2017 and January 2018, the apparent equilibrium temperature 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.

Imaging the Hydrothermal System of Kirishima Volcanic Complex With L‐Band InSAR Time Series

“…observation of days of fluid upwelling just before the 2017 blowout and 2018 eruption (Ohba et al, 2021;Tajima et al, 2020), we interpret the expanded inflation is driven by the bottom-up pressurization from the upward migration of magmatic fluid. The magmatic fluid is likely generated by the interaction between local meteoric water and magmatic vapor rising from the depth (Ohba et al, 2021).…”

“…These shallow hydrothermal reservoirs beneath Iwo‐yama are possibly connected to the subvertical conductor (2–10 km b.s.l. ; Aizawa et al., 2014) via magma degassing and hydrothermal fluid migration (Ohba et al., 2021; Tsukamoto et al., 2018), thus, sharing the same origin of deep magmatic source with Shinmoe‐dake.…”

“…Considering (1) the mix of meteoric and magmatic water and the sharp increase of SO 2 and H 2 concentrations in fumarolic gas from the geochemical analysis in 2018 before the eruption and (2) the observation of days of fluid upwelling just before the 2017 blowout and 2018 (Ohba et al, 2021;Tajima et al, 2020), we interpret the expanded inflation is driven by the bottom-up pressurization from the upward migration of magmatic fluid. The magmatic fluid is likely generated by the interaction between local meteoric water and magmatic vapor rising from the depth (Ohba et al, 2021).…”

“…The lower source at 340 m depth hosts the southern and western vents and is responsible for the April 2018 eruption. Considering (1) the mix of meteoric and magmatic water and the sharp increase of SO 2 and H 2 concentrations in fumarolic gas from the geochemical analysis in 2018 before the eruption and (2) the observation of days of fluid upwelling just before the 2017 blowout and 2018 (Ohba et al, 2021;Tajima et al, 2020), we interpret the expanded inflation is driven by the bottom-up pressurization from the upward migration of magmatic fluid. The magmatic fluid is likely generated by the interaction between local meteoric water and magmatic vapor rising from the depth (Ohba et al, 2021).…”

“…The magmatic fluid is likely generated by the interaction between local meteoric water and magmatic vapor rising from the depth (Ohba et al, 2021).…”