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 July 2020. Notable changes in the chemical composition gases related to volcanic activity included a sharp increase SO 2 H concentrations May 2017 March 2018. The analyses 2018 immediately preceded April eruption volcano. isotopic ratios O revealed process formation. Up 49% high-enthalpy magmatic vapor mixed with 51% cold local meteoric water generate coexisting liquid phases 100–160 °C. Portions were discharged as hot spring water, respectively. CO /SO ratio higher than that estimated for due hydrolysis during formation phase. When flux high, effects small resulting low gases. high apparent equilibrium temperature defined reactions involving , S, O, together S regarded be precursor signals phreatic increased rapidly suggesting an vapor. Between September January 2018, suppression flux. During this period, eruptions took place Shinmoedake volcano 5 km away from We conclude magma sealing transport occurred simultaneously chamber beneath