Geochemical characteristics of hydrothermal fluids at Hatoma Knoll in the southern Okinawa Trough

Tomohiro Toki; Michihiro Itoh; Daigo Iwata; Shogo Ohshima; Ryuichi Shinjo; Jun-ichiro Ishibashi; Urumu Tsunogai; Naoto Takahata; Yuji Sano; Toshiro Yamanaka; Akira Ijiri; Nobuaki Okabe; Toshitaka Gamo; Yasuyuki Muramatsu; Yuichiro Ueno; Shinsuke Kawagucci; Ken Takai

doi:10.2343/geochemj.2.0449

Special Issue: Front Edge of Submarine Mineral Resources Research in Japan (Part 2)

Geochemical characteristics of hydrothermal fluids at Hatoma Knoll in the southern Okinawa Trough

Tomohiro Toki, Michihiro Itoh, Daigo Iwata, Shogo Ohshima, Ryuichi Shinjo, Jun-ichiro Ishibashi, Urumu Tsunogai, Naoto Takahata, Yuji Sano, Toshiro Yamanaka, Akira Ijiri, Nobuaki Okabe, Toshitaka Gamo, Yasuyuki Muramatsu, Yuichiro Ueno, Shinsuke Kawagucci, Ken Takai

Author information

Tomohiro Toki
Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
Michihiro Itoh
Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
Daigo Iwata
Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
Shogo Ohshima
Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus
Ryuichi Shinjo
Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus
Jun-ichiro Ishibashi
Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University
Urumu Tsunogai
Earth and Planetary System Science, Faculty of Science, Hokkaido University
Naoto Takahata
Marine Analytical Chemistry, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
Yuji Sano
Marine Analytical Chemistry, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
Toshiro Yamanaka
Graduate School of Natural Science and Technology, Okayama University
Akira Ijiri
Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Nobuaki Okabe
Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
Toshitaka Gamo
Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
Yasuyuki Muramatsu
Department of Chemistry, Gakushuin University
Yuichiro Ueno
Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Global Edge Institute, Tokyo Institute of Technology
Shinsuke Kawagucci
Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Ken Takai
Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

Keywords: hydrothermal fluid, Hatoma Knoll, Okinawa Trough, phase separation, sediment, subducting material

JOURNAL FREE ACCESS

2016 Volume 50 Issue 6 Pages 493-525

DOI https://doi.org/10.2343/geochemj.2.0449

Details

Abstract

Chemical and isotopic compositions of hydrothermal fluids from Hatoma Knoll in the southern Okinawa Trough were investigated. The hydrothermal fluids were derived from a single pure hydrothermal fluid source, but they underwent phase separation beneath the seafloor prior to venting. Only vapor-like fluids vent at the Hatoma system, and the most active area is around the center of the crater, based on the location of the maximum temperature and the lowest Cl^– concentrations. Compared with other hydrothermal systems in the world, at Hatoma the pH and alkalinity, as well as the B, NH₄⁺, K, Li, CO₂, and CH₄ concentrations, were higher, and the Fe and Al concentrations were lower, suggesting that the characteristics of the Hatoma hydrothermal fluids are comparable to those of the other Okinawa Trough hydrothermal fluids. Helium isotope ratios were lower than those of sediment-starved hydrothermal systems, suggesting that ⁴He derived from the sediment is supplied to the hydrothermal fluids in Hatoma Knoll. The carbon isotope ratios of CO₂ in the hydrothermal fluids indicate an influence of organic carbon decomposition. The carbon isotope ratios of CH₄ in the hydrothermal fluids imply that most of the CH₄ originated from microbial methane produced in a recharge zone of the hydrothermal system. Although sediment influences are a typical feature of Okinawan Trough hydrothermal fluids, the Hatoma hydrothermal system has the lowest carbon isotope ratios of CH₄ among them, which suggests that Hatoma is the most highly influenced by the sediments in the recharge zone. Thus, the degree of the sediment influences has a variable in each hydrothermal field in the Okinawa Trough.

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