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Thermal characteristics of fossil fluids from the Philippine Sea slab: Insights from fluid inclusions and thermochronology
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  • Shigeru Sueoka,
  • Hideki Iwano,
  • Tohru Danhara,
  • Masakazu Niwa,
  • Mizuho Kanno,
  • Barry P. Kohn,
  • Makoto Kawamura,
  • Tatsunori Yokoyama,
  • Saya Kagami,
  • Yasuhiro Ogita,
  • Takafumi Hirata
Shigeru Sueoka
Japan Atomic Energy Agency

Corresponding Author:[email protected]

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Hideki Iwano
Kyoto Fission-Track Co., Ltd.
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Tohru Danhara
Kyoto Fission-Track, Co
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Masakazu Niwa
Japan Atomic Energy Agency
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Mizuho Kanno
Japan Atomic Energy Agency
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Barry P. Kohn
University of Melbourne
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Makoto Kawamura
Japan Atomic Energy Agency
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Tatsunori Yokoyama
Japan Atomic Energy Agency
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Saya Kagami
Japan Atomic Energy Agency
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Yasuhiro Ogita
Japan Atomic Energy Agency
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Takafumi Hirata
Geochemical Research Center, University of Tokyo
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Abstract

For quantitative understanding of thermal features of fossil fluid activity derived from the Philippine Sea slab, we applied fluid-inclusion and thermochronometric analyses to hydrothermal veins and their host rocks outcropping in the Hongu area in southwestern Japan. Although hydrothermal events at ~150{degree sign}C and ~200{degree sign}C were identified by fluid-inclusion analyses of quartz veins, no thermal anomaly was found associated with the veins’ host rocks. Cooling ages showed no variation as a function of distance from the veins. Using zircon, we determined U-Pb ages of 77.3-66.9 Ma in the youngest population, fission-track pooled ages of 34.1-24.0 Ma, and (U-Th)/He single-grain ages of 23.6-8.7 Ma. Apatite yielded pooled fission-track ages of 12.0-9.0 Ma. All these ages can be explained by fluid flow that occurred either (1) before ~10 Ma at a depth where ambient temperature is higher than closure temperature of the apatite fission-track system (90{degree sign}C-120{degree sign}C, equivalent to ~3-km depth) or (2) after ~10 Ma but of such duration that is too short to have annealed fission tracks in apatite, which process requires ~10 yr at ~150{degree sign}C or as short as a few months at ~200{degree sign}C. Apatite fission-track ages of ~10 Ma might reflect regional mountain uplift and exhumation related to rapid subduction of the Philippine Sea slab and associated with clockwise rotation of the Southwest Japan Arc.