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Title :Hydrothermal fluid geochemistry at the Iheya North field in the mid-Okinawa Trough: Implication for origin of methane in subseafloor fluid circulation systems
Authors :Kawaguchi, Shinsuke
Chiba, Hitoshi
Ishibashi, Jun-ichiro
Yamanaka, Toshihiro
Toki, Tomohiro
Muramatsu, Yasuyuki
Ueno, Yuichiro
Makabe, Akiko
Inoue, Kazuhiro
Yoshida, Naohiro
Nakagawa, Satoshi
Nonoura, Takuro
Takai, Ken
Takahata, Naoto
Sano, Yuji
Narita, Taku
Teranishi, Genta
Obata, Hajime
Gamo, Toshitaka
Issue Date :Jun-2011
Abstract :Geochemical characteristics of hydrothermal fluids in the Iheya North hydrothermal field, mid-Okinawa Trough, was investigated. Twelve-years observation reveals temporal variation of vent fluid chemistry potentially controlled by temporally varying pattern of the phase-separation and -segregation, while the constant Element/Cl ratios among the periods and chimneys indicate the stable chemical composition of the source hydrothermal fluid prior to undergoing phase-separation. The high K contents in the estimated source fluid are typical in the arc-backarc hydrothermal systems due to the hydrothermal reaction with the K-enriched felsic rocks. The high I, B and NH4 contents and alkalinity are derived from decomposition of the sedimentary organic matters. Compositional and isotopic properties of gas species, CH4, H2, CO2, and C2H6, strongly suggest a dominance of biogenic CH4 associated with the sedimentary organic matter. Based on the carbon mass balance calculation and the multidisciplinary investigations of the Iheya North hydrothermal system since the discovery, we hypothesized that the microbial methanogenesis occurs not only within the Central Valley where hydrothermal vents exist, but also in the spatially abundant and widespread basin-filling sediments surrounding the Iheya North Knoll, and that the microbially produced CH4 is recharged together with the source fluid into the deep hydrothermal reaction zone. This “Microbial Methanogenesis at Recharge area in hydrothermal circulation” (MMR) model would be an implication for the generation and incorporation of hydrothermal fluid CH4 in the deep-sea hydrothermal systems but also for those of cold seep CH4 and for the presently uncertain hydrothermal fluid paths in the subseafloor environments. In the near future, the IODP drilling will be conducted in the Iheya North hydrothermal system, and give an excellent opportunity to testify our MMR model.
URL :https://doi.org/10.2343/geochemj.1.0105
Type Local :雑誌掲載論文
ISSN :1880-5973
Publisher :The Geochemical Society of Japan
URI :http://hdl.handle.net/20.500.12000/37848
Citation :Geochemical Journal Vol.45 no.2 p.109 -124
Appears in Collections:Peer-reviewed Journal Articles (Faculty of Science)

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