Diversity of fluid geochemistry affected by processes during fluid upwelling in active hydrothermal fields in the Izena Hole, the middle Okinawa Trough back-arc basin

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  • ISHIBASHI JUN-ICHIRO
    Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University Submarine Resources Research Project (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • NOGUCHI TAKUROH
    Faculty of Science, University of the Ryukyus
  • TOKI TOMOHIRO
    Faculty of Science, University of the Ryukyus
  • MIYABE SHUNSUKE
    Graduate School of Natural Science and Technology, Okayama University
  • YAMAGAMI SHOSEI
    Graduate School of Natural Science and Technology, Okayama University
  • ONISHI YUJI
    Graduate School of Natural Science and Technology, Okayama University
  • YAMANAKA TOSHIRO
    Graduate School of Natural Science and Technology, Okayama University
  • YOKOYAMA YUKA
    Graduate School of Science, Hiroshima University
  • OMORI ERIKO
    Graduate School of Science, Hiroshima University
  • TAKAHASHI YOSHIO
    Graduate School of Science, Hiroshima University
  • HATADA KENTA
    School of Science and Engineering, Kinki University
  • NAKAGUCHI YUZURU
    School of Science and Engineering, Kinki University
  • YOSHIZAKI MOTOKO
    Graduate School of Science and Engineering, Tokyo Institute for Technology Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • KONNO UTA
    Institute of Biogeosciences (Biogeos), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Submarine Resources Research Project (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • SHIBUYA TAKAZO
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Submarine Resources Research Project (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • TAKAI KEN
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Institute of Biogeosciences (Biogeos), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Submarine Resources Research Project (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • INAGAKI FUMIO
    Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • KAWAGUCCI SHINSUKE
    Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Institute of Biogeosciences (Biogeos), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Submarine Resources Research Project (SRRP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

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Abstract

Two active hydrothermal fields, Jade and Hakurei fields have been discovered within the Izena Hole, a rectangular 6 km × 3 km shape depression located in the middle Okinawa Trough back arc basin. In both fields, intense hydrothermal activity is represented by venting of high-temperature fluid (>300°C) and occurrence of sulfide/sulfate ore deposits. We collected hydrothermal fluids during dive expeditions of ROV Hyper Dolphin conducted in 2003, 2010 and 2011, in order to analyze both elemental and gas species. The geochemistry of high temperature hydrothermal fluids collected from the Jade and Hakurei fields is very similar to each other with exceptions in minor gas composition. Little temporal variation was observed in geochemistry of the high-temperature hydrothermal fluid of the Jade field over two decades, since a previous study carried out in 1989. These results suggest that these fluids are derived from a common fluid reservoir where fluid chemistry is basically controlled by fluid-mineral equilibria and gas species are dominantly contributed from the same magma. Venting of low temperature fluid (about 104°C) was discovered in the distal part of the Jade field, which was named as the Biwako vent. Chemical composition of the Biwako vent fluid was distinctive from that of the high temperature fluid in the proximal part of the Jade field, and could not be explained by simple dilution or cooling. This intra-field chemical diversity could be caused by phase separation and segregation during fluid upwelling, based on relationships in concentrations of Cl and major cations. On the other hand, the chemical diversity recognized in minor gas composition between the Jade and Hakurei fields is in accordance with results from previous plume survey. Difference in concentrations of minor gases such as H2 is attributed to contribution from thermal degradation of organic matter in the sediment, during fluid upwelling.

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