Pore fluid chemistry beneath active hydrothermal fields in the mid-okinawa trough: Results of shallow drillings by BMS during taiga11 cruise

TAIGA11 cruise of R/V Hakurei-maru No.2 was conducted in June, 2011 to study subseafloor geochemical environment below active hydrothermal fields using a shallow drilling system BMS (Benthic Multi-coring System). Three active hydrothermal fields were selected as target fields; the Iheya North Knoll field (27_470 N, 126_540 E), the Jade field in the Izena Hole (27_160 N, 127_050E), and the Hakurei field in the Izena Hole (27_150 N, 127_040 E). In this chapter, we will report chemical composition and isotope ratios of pore fluids extracted from collected sediments. At the Hakurei field in the Izena Hole, BMS drilling attained to 610 cmbsf (cm below the seafloor) in the vicinity of a large massive sulfide mound. The obtained core showed evidence for sulfide and sulfate mineralization below 223 cmbsf. Pore fluid from the corresponding depth showed enrichment in Si, K and Ca, which could be attributed to influence of formation of alteration minerals rather than to involvement of the hydrothermal component. At the Jade field in the Izena Hole, BMS drilling attained to 529 cmbsf at about 300 m apart from the area where high temperature fluid venting congregate. The obtained core comprised grayish white hydrothermal altered mud below 370 cmbsf, although pore fluid showed seawater like composition. At the Iheya North Knoll field, BMS drilling attained to 453 cmbsf at about 200 m apart from the central mound area. The obtained core consisted almost entirely of grayish white hydrothermally altered mud. Pore fluid below 180 cmbsf showed substantial enrichment in major cations (Na, K, Ca and Mg) and Cl, which would be explained as a result of hydration during hydrothermal alteration.