Mixing of magmatic CO2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes |
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| Authors: | Makoto Yamada Shinji Ohsawa Kohei Kazahaya Masaya Yasuhara Hiroshi Takahashi Kazuhiro Amita Hideo Mawatari Shin Yoshikawa |
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| Institution: | 1. Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, Oita, Japan;2. Research Center for Deep Geological Environments, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan;3. Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita University, Akita, Japan;4. Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Aso, Kumamoto, Japan |
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| Abstract: | To understand deep groundwater flow systems and their interaction with CO2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber. |
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| Keywords: | Groundwater flow Magmatic CO2 Dissolved inorganic carbon Stable isotope Aso volcano |
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