DRILLING INTO SUB-VENT BIOSPHERE AT SUIYO SEAMOUNT, IZU-BONIN ARC, WESTERN PACIFIC

"Archaean Park Project" focuses on the influence of magma-hydrothermal activities on biological communities of a 200 x 200m size seafloor hydrothermal system developed on atop of Suiyo seamount, Izu-Bonin island-arc. Black smokers (maximum temperature is 317C) were observed at the central area of the caldera floor (depth=1,390m), meanwhile warm water with mussel colonies occur on the marginal part of the caldera. We made 2 to 6m depths drillings near the black smokers (drill holes APSK04, APSK05 and APSK07), using the Bentic Multi-coring System (BMS) and RV Daini-Hakurei Maru. We also made drillings near warm water conduits (APSK01 and APSK02) at the south-east and south margins of hydrothermal area. The temperatures of hydrothermal fluids from APSK05 and APSK07 were 304C and 156C, respectively. The temperatures of warm water from other holes were 40 to 60C. Sulfid-rich core samples contain Au (<42ppm), Ag (<1,550ppm), Cu (0.6%),Pb (<0.6%),Zn (<40%),As (<1,440ppm),Hg (<55ppm), Sb (<96ppm), Se (<44ppm). The highest Au and Ag sulfide samples were obtained from APSK07 where the measured hydtrothermal fluid temperature was 156C. Lead isotope ratios (206Pb/204Pb=18.546 to 18.562, 207Pb/204Pb=15.535 to 15.551, 208Pb/204Pb=38.322 to 38.375) of sulfide-rich samples are very similar to those of the dacite (206Pb/204Pb=18.552, 207Pb/204Pb=15.539, 208Pb/204Pb=38.333). Sulfate-rich core samples contain Ba (<10%),Ca (<30% as CaO),Sr (<0.4%). The Ca/Sr ratios of these sulfate-rich core samples are very similar to the seawater value, suggesting most of Ca and Sr of anhydrite are seawater origin. The 87Sr/86Sr ratios (0.70732 to 0.70872) of sulfate-rich sample also suggests that Sr of anhydrite is a seawater origin. Oxygen isotope analyses were performed on montmorillonite, chlorite/montromollonite mixed-layer minerals, mica and chlorite. Oxygen isotopic ratios of these clay minerals are +7.2 to +7.6ñ for montmorillonite, +3.2 to +4.6ñ for chlorite/montromollonite mixed-layer minerals, and +3.1 to +3.8ñ for mixtures of chlorite and mica. Assuming that the oxygen isotopic ratios of hydrothermal fluid responsible for the formation of these clay minerals is seawater value, we can evaluate that the formational temperatures of montmorillonite and the mixture of chlorite and mica are 170 to 230C and 250 to 290C, respectively.