Variations in fumarolic gas compositions during the 2018 eruption of Kirishima Iwoyama volcano, Japan, observed by a continuous Multi-GAS station

Abstract

At Kirishima Iwoyama volcano, an eruption occurred on April 19, 2018. In this study, continuous observations of fumarolic gas composition using Multi-GAS were conducted in the vicinity of fumarole H from July 2017 to January 2020 to monitor volcanic activity at the volcano. The results of the continuous observation showed that the SO₂/H₂S ratio and the daily maximum values of SO₂ concentration had decreased since the observation started but turned to an increasing trend in January 2018 and showed a remarkable increase of about 100 times from February to March 2018. On the other hand, the CO₂/H₂S ratio showed an increase or decrease corresponding to the change in the SO₂/H₂S ratio during the same period, but the range of the change was smaller than that of the SO₂/H₂S ratio. After the eruption, the SO₂/H₂S ratio remained lower than before the eruption, but showed a sharp increase in June 2019 and then decreased again. Magmatic gases of deep origin are supposed to be supplied to the shallow hydrothermal system, and the observed variations in fumarolic gas compositions can be explained by reactions of sulfur precipitation and sulfur dioxide disproportionation.