Japan Geoscience Union Meeting 2014

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Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT40_1PM1] Geofluids: their distribution and role in the Earth's dynamics

Thu. May 1, 2014 2:15 PM - 4:00 PM 416 (4F)

Convener:*Michihiko Nakamura(Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University), Hiroshi Sakuma(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Masahiro Ichiki(Graduate School of Science, Tohoku University), Tsutomu Takahashi(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Chair:Tetsuya Yokoyama(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Keika Horiguchi(Crustal Fluid Research Group, The Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

2:35 PM - 3:00 PM

★ [SIT40-13] Origin of saline waters distributed along the Median Tectonic Line in southwest Japan

*Kazuhiro AMITA1, Shinji OHSAWA2, Koshi NISHIMURA3, Makoto YAMADA4, Taketoshi MISHIMA2, Kohei KAZAHAYA5, Noritoshi MORIKAWA5, Takao HIRAJIMA6 (1.Department of Earth Science & Technology Faculty of Engineering and Resource Science Akita Universit, 2.Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 3.Faculty of Economics, Toyo University, 4.Research Institute for Humanity and Nature, 5.Geological Survey of Japan, AIST, 6.Department of Geology and Mineralogy, Graduate School of Science, Kyoto University)

Keywords:hot spring water, dehydrated fluid from subducting plate, Median Tectonic Line

To identify of metamorphic dehydrated fluid as source fluid of hot spring water, we conducted chemical and isotopic analyses of water and accompanied gas samples collected from hot-spring wells along the Median Tectonic Line (MTL) in the forearc region of the southwestern part of Japan. As a result, we found hot spring waters having anomalous dD and d18O compositions as compared with modern seawater and shallow groundwater in Wakayama and Shikoku regions. Judging from data in relative B-Li-Cl composition and He isotopic systematics, the source fluid of the hot springs in Shikoku could be identified to be one of diagenetic fluids. On the other hand, the source fluid of the hot springs of Wakayama had different B-Li-Cl composition and higher 3He/4He ratio in comparison with diagenetic dehydrated fluids and then the fluid was thought to be originated from metamorphic dehydrated fluid as well as Oita plain. There was another striking contrast between the source fluid of Wakayama and Oita and that of Shikoku and Miyazaki; accompanied gases by the former were rich in CO2, whereas those with the latter were rich in CH4, and CO2 in the accompanied gases of Wakayama and Oita is mostly derived from marine carbonate like volcanic gases in subduction zones. Moreover, the Li-B-Cl compositions of them showed transitive values between the relative composition of diagenetic fluids and those of volcanic thermal waters. Consequently, the source fluid of hot springs in Wakayama and Oita was likely to be dehydrated metamorphic fluids released from the subducting Philippine-Sea plate.