Origin of saline waters distributed along the Median Tectonic Line in southwest Japan: Hydrogeochemical investigation on possibility of derivation of metamorphic dehydrated fluid from subducting oceanic plate

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  • AMITA Kazuhiro
    Department of Earth Science and Technology, Graduate School of Engineering and Resource Science, Akita University
  • OHSAWA Shinji
    Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University
  • NISHIMURA Koshi
    Natural Science Laboratory, Faculty of Economics, Toyo University
  • YAMADA Makoto
    Research Institute for Humanity and Nature
  • MISHIMA Taketoshi
    Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University
  • KAZAHAYA Kohei
    The Institute of Geology and Geoinformation (IGG)，Advanced Industrial Science and Technology
  • MORIKAWA Noritoshi
    The Institute of Geology and Geoinformation (IGG)，Advanced Industrial Science and Technology
  • HIRAJIMA Takao
    Department of Geology and Mineralogy, Graduate School of Science, Kyoto University

Bibliographic Information

Other Title
  • 中央構造線沿いに湧出する高塩分泉の起源
  • 中央構造線沿いに湧出する高塩分泉の起源 : プレート脱水流体起源の可能性についての水文化学的検討
  • チュウオウ コウゾウセン ゾイ ニ ユウシュツ スル コウエンブンセン ノ キゲン : プレート ダッスイ リュウタイ キゲン ノ カノウセイ ニ ツイテ ノ スイモン カガクテキ ケントウ
  • —プレート脱水流体起源の可能性についての水文化学的検討—

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Abstract

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 the hot spring waters having anomalous δD and δ18O 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.

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