Behavior of magmatic components in fumarolic gases related to the 2018 phreatic eruption at Ebinokogen Ioyama volcano, Kirishima Volcanic Group, Kyushu, Japan

Takeshi Ohba,Urumu Tsunogai,Ryo Shingubara,Muga Yaguchi,Masanori Ito

doi:10.1186/s40623-021-01405-4

Takeshi Ohba, Urumu Tsunogai + Show 3 more

Open Access

https://doi.org/10.1186/s40623-021-01405-4

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Abstract

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.

Highlights

Phreatic eruptions are generally small in scale, they can cause injuries and fatalities
Notable changes in the chemical composition related to volcanic activity included increased concentrations of SO2 and H2 in May 2017
Based on the isotope ratio of H 2O in fumarolic gas, the observation period was divided into three periods: Period-1 (December 2015 to December 2016), Period-II (January 2017 to May 2018), and Period-III (June 2018 to July 2020)

Summary

Introduction

Phreatic eruptions are generally small in scale, they can cause injuries and fatalities. HR components are composed of reducing chemical species such as H 2S and C H4 Those H 2S and CH4 are thought to be reduced from the magmatic S O2 and CO2, respectively, by the F e2+ contained in the crustal rocks contacting with the fluid in hydrothermal reservoir (Giggenbach 1997). Before the phreatic eruption at Hakone Volcano in 2015, volcanic earthquakes occurred, and the ratio of MV to HR components increased according to the frequency of earthquakes (Ohba et al 2019a). Such changes in fumarolic gas can be attributed to the injection of MV into the HR. This injection induces increases in fluid pressure in the HR and may initiate volcanic earthquakes (Ohba et al 2019a)

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