Geochemical evaluation for the fumarolic gases collected at Ojigokudani, Iwate volcano, Japan in September 2024

Abstract Anomalous earthquakes and crustal movements have been observed at Iwate volcano in northern Honshu, Japan, where phreatic eruptions have been feared since February 2024, and the volcanic alert level was raised from 1 to 2 on October 2, 2024. We collected fumarolic gases at two locations in Ojigokudani at Iwate volcano on September 5, 2024, and analyzed their chemical composition and hydrogen and oxygen stable isotope ratios (δD and δ 18 O) to assess the magmatic contribution to the hydrothermal system, which is useful information for evaluating the activity of volcanoes that produce phreatic eruptions. The characteristics of δD and δ 18 O indicate that the fumarolic gases are a mixture of local meteoric water and magmatic gas, with the meteoric water component being predominant. The fumarolic gases were poor in HCl, and the SO 2 /H 2 S ratio, which is high in high-temperature gases, were low at 0.01 and 0.02. The apparent equilibrium temperatures, which is useful for estimating the temperature of the gas source, were estimated to be 212 and 227 °C. These geochemical properties of fumarolic gases do not indicate that the contribution of magmatic gas was as intense as that during the 1998 unrest when the magmatic intrusion was suspected, but a failed eruption occurred. Graphical Abstract