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Geochemical Journal, Vol. 50 (No. 6), pp. 493-525, 2016
doi:10.2343/geochemj.2.0449

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

Tomohiro Toki,1* Michihiro Itoh,1 Daigo Iwata,1 Shogo Ohshima,1 Ryuichi Shinjo,2 Jun-ichiro Ishibashi,3 Urumu Tsunogai,4 Naoto Takahata,5 Yuji Sano,5 Toshiro Yamanaka,6 Akira Ijiri,7 Nobuaki Okabe,8 Toshitaka Gamo,8 Yasuyuki Muramatsu,9 Yuichiro Ueno,10,11 Shinsuke Kawagucci10,12 and Ken Takai10,12

1Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
2Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
3Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
4Earth and Planetary System Science, Faculty of Science, Hokkaido University, N10 W8 Kita-ku, Sapporo, Hokkaido 060-0810, Japan
5Marine Analytical Chemistry, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan
6Graduate School of Natural Science and Technology, Okayama University, 1-1 Naka 3-chome, Tsushima, Kita-ku, Okayama 700-8530, Japan
7Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), B200 Monobe, Nankoku 783-8502, Japan
8Marine Inorganic Chemistry Group, Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan
9Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
10Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
11Global Edge Institute, Tokyo Institute of Technology, G1-25 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
12Subsurface Geobiology Advanced Research (SUGAR) Project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

(Received March 1, 2015; Accepted September 16, 2016)

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.
Key words: hydrothermal fluid, Hatoma Knoll, Okinawa Trough, phase separation, sediment, subducting material


*Corresponding author E-mail: toki@sci.u-ryukyu.ac.jp


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