Hydrothermal fluid geochemistry at the Iheya North field in the mid-Okinawa trough: Implication for origin of methane in subseafloor fluid circulation systems

Shinsuke Kawagucci, Hitoshi Chiba, Jun Ichiro Ishibashi, Toshiro Yamanaka, Tomohiro Toki, Yasuyuki Muramatsu, Yuichiro Ueno, Akiko Makabe, Kazuhiro Inoue, Naohiro Yoshida, Satoshi Nakagawa, Takuro Nunoura, Ken Takai, Naoto Takahata, Yuji Sano, Taku Narita, Genta Teranishi, Hajime Obata, Toshitaka Gamo

Research output: Contribution to journalArticlepeer-review

110 Citations (Scopus)


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 phaseseparation. 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 C2H 6, 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.

Original languageEnglish
Pages (from-to)109-124
Number of pages16
Issue number2
Publication statusPublished - 2011

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology


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