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

<jats:title>Abstract</jats:title><jats:p>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 SO<jats:sub>2</jats:sub> and H<jats:sub>2</jats:sub> 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 H<jats:sub>2</jats:sub>O 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 CO<jats:sub>2</jats:sub>/SO<jats:sub>2</jats:sub> ratio of the fumarolic gas was higher than that estimated for magmatic vapor due to SO<jats:sub>2</jats:sub> 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 CO<jats:sub>2</jats:sub>/SO<jats:sub>2</jats:sub> ratios in fumarolic gases. The high apparent equilibrium temperature defined for reactions involving SO<jats:sub>2</jats:sub>, H<jats:sub>2</jats:sub>S, H<jats:sub>2</jats:sub> and H<jats:sub>2</jats:sub>O, together with low CO<jats:sub>2</jats:sub>/SO<jats:sub>2</jats:sub> and H<jats:sub>2</jats:sub>S /SO<jats:sub>2</jats:sub> 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. </jats:p>