Chemical and Isotopic Compositions of Fumarolic Gases from Yakedake Volcano, Japan

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  • SAITO Takeshi
    Institute of Science, Academic Assembly School of Science and Technology, Shinshu University
  • SAWAMURA Shun
    Graduate School of Science and Technology, Shinshu University
  • TAMURA Rina
    Department of Geology, Faculty of Science, Shinshu University Shiojiri City Government
  • SEKI Susumu
    Department of Geology, Faculty of Science, Shinshu University Sendai Regional Headquarters, Japan Meteorological Agency
  • AMITA Kazuhiro
    Graduate School of Engineering Science, Akita University
  • MISHIMA Taketoshi
    Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University
  • OHSAWA Shinji
    Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University

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Other Title
  • 焼岳火山の噴気の化学・同位体組成
  • ヤケダケ カザン ノ フンキ ノ カガク ・ ドウイタイ ソセイ

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<p>Since the latest phreatic eruption in 1962-1963, Yakedake volcano has remained dormant, but fumarolic activity around the summit area has continued steadily during the past hundred years. Temperature, chemical, and isotopic compositions of fumarolic gases were ascertained for 2013-2017. Discharge temperatures of the most active fumarole were 113-123°C: much lower temperatures than those during the active period. Lower concentrations of HCl and H2 and a lower SO2/H2S ratio coincide with this relative quiescence of the present Yakedake volcano. The apparent equilibrium temperatures estimated from the sulfur reactions were about 250-280°C, which decreased by about 200°Cshortly after the latest eruption and which were lower than the critical temperature of water, suggesting the formation of a volcanic hydrothermal system beneath the summit region. The isotopic compositions of water (D,18O) show that the fumarolic fluids were probably formed by mixing of the magmatic fluids and local meteoric water. The contribution of magmatic fluid to the sampled fumarolic fluids was estimated as about 80-90%, which is higher than the values obtained shortly after the latest eruption. The estimated volcanic hydrothermal system under the volcano is possibly less influenced by local meteoric water, in spite of the low discharge temperature and low concentrations of gas species derived from high-temperature volcanic gas.</p>

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