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.

在2015年12月至2020年7月之间，对日本火山岩沼气进行了富马气的直接采样和分析。与火山活动有关的气体化学成分的显着变化包括2017年5月的SO ₂和H ₂浓度急剧增加，以及2018年3月。2018年3月的分析紧接于2018年4月在饭山火山爆发之前。富马气中H ₂ O的同位素比揭示了形成过程。在100–160°C时，高达49％的高焓岩浆蒸气与51％的局部冷空气混合，从而生成蒸气和液相共存。汽相和液相的部分分别作为富马酸气体和温泉水排放。CO ₂ / SO由于在气相形成过程中SO ₂水解，富马酸酯气的₂比率高于岩浆蒸汽的估计比率。当岩浆蒸气的通量较高时，水解作用较小，从而导致富马酸气体中的CO ₂ / SO _2比较低。为涉及SO ₂，H ₂ S，H ₂和H ₂ O的反应以及较低的CO ₂ / SO ₂和H ₂ S / SO ₂定义的高表观平衡温度比率被认为是饭山火山潜水爆发的前兆信号。表观平衡温度在2017年5月和2018年3月迅速增加，表明岩浆蒸汽通量增加。在2017年9月至2018年1月之间，表观平衡温度较低，表明岩浆蒸汽通量受到抑制。在此期间，在距饭山火山5公里的新月岳火山发生了岩浆喷发。我们得出结论，岩浆的密封和向新茂岳火山的运输是在饭山火山下面的岩浆室内同时发生的。