Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC39] Hydrothermal systems of volcanoes

Mon. May 27, 2019 9:00 AM - 10:30 AM IC (2F)

convener:Yasuhiro Fujimitsu(Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University), Wataru Kanda(Volcanic Fluid Research Center, School of Science, Tokyo Institute of Technology), Takeshi Ohba(Department of chemistry, School of Science, Tokia University), Chairperson:Yasuhiro Fujimitsu(Faculty of Engineering, Kyushu University), Takeshi Ohba, Wataru Kanda

9:45 AM - 10:00 AM

[SVC39-04] Time variation in the chemical and isotopic composition of fumarolic gases at Kusatsu-Shirane volcano Japan

*Takeshi Ohba1, Muga Yaguchi2, Nozomi Numanami1, Kotaro Toyama3, Hirochika Sumino3, Urumu Tsunogai4, Masanori Ito4, Ryo Shingubara4 (1.Department of chemistry, School of Science, Tokia University, 2.Meteorological Research Institute, JMA, 3.The University of Tokyo Graduate School of Arts and Sciences Department of Basic Science, 4.Nagoya University Graduate School of Environmental Studies)

At Kusatsu-Shirane volcano Japan, the intense seismic activity took place in 2014 and 2018. To investigate the relationship between the activity and the chemistry of fumarolic gas, the gas samples have been repeatedly analyzed. A monotonic decrease in CO2/H2O, He/H2O and N2/H2O ratios since July 2014 to November 2017 suggests the decline of magmatic component. On contrary CH4/H2O ratio significantly increased during the seismically quiet period, suggesting the reduced condition in hydrothermal system favoring to the formation of CH4. The high N2/He ratio in the quiet period suggests the addition of component originating in crustal rock hosting hydrothermal system. The N2/He ratio in 2018 was definitely lower than the ratio in 2014, indicating the evolution of magma brought by the degassing. The δD(H2O), δ18O(H2O) and CO2/H2O ratio of fumarolic gas were modeled with the following processes; the generation of vapor phase after the mixing between magmatic gas and a cold groundwater with meteoric origin, the addition of vapor phase with meteoric origin, and the partial condensation of water vapor near surface. Only a single magmatic gas is necessary for the above modeling.