Time variations in the chemical and isotopic composition of fumarolic gases at Hakone volcano, Honshu Island, Japan, over the earthquake swarm and eruption in 2015, interpreted by magma sealing model

Definite increases in the components ratios of CO 2 /H 2 O, CO 2 /H 2 S, CO 2 /CH 4 and He/CH 4 were observed at the fumarolic gases from Owakudani geothermal area located at the center of Hakone volcanic caldera (Honshu Island, Japan), synchronized with the earthquake swarm in 2015. Such variations were due to the dominance of a magmatic component over a hydrothermal component, suggesting the earthquake swarm was produced by the injection of magmatic gases into the hydrothermal system. The CO 2 /H 2 O ratio of magmatic gas was estimated to be 0.0045 before the earthquake swarm, which increased up to 0.013 during the earthquake swarm, likely produced by the pressurization of magma as a result of magma sealing where the pressure increment in magma was estimated to be 3% to the lithostatic pressure. The H 2 O and CO 2 concentration in magma were estimated to be 6.3 wt% and 20 wt ppm, respectively, assuming a temperature 900 °C and a rhyolitic composition. In May 2015, a few months prior to the earthquake swarm in May 2015, a sharp increase in the Ar/CO 2 and N 2 /He ratios and a decrease in the isotopic ratio of H 2 O were observed at the fumarolic gas. The invasion of air into the hydrothermal system increased the Ar/CO 2 and N 2 /He ratios. The decrease in the isotopic ratio of H 2 O was induced by partial condensation of H 2 O vapor.