HR: 0800h
AN: V41B-1375    [Abstracts]
TI: Hydrothermal circulation within modern sediment layer in a shallow submarine volcano, Wakamiko crater, south Kyushu, Japan
AU: * Yamanaka, T
EM: tyama@rc.kyushu-u.ac.jp
AF: Department of Evolution of Earth Environments, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka, 810-8560 Japan
AU: Ishibashi, J
EM: ishi@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Seguchi, M
EM: sc00163e@yahoo.co.jp
AF: Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Yamashita, T
EM: iwtc_wmpic@hotmail.com
AF: Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Nakashima, M
EM: miwa@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Shimada, N
EM: nshimada@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Kusakabe, M
EM: kusakabe@misasa.okayama-u.ac.jp
AF: Institute for Study of the Earth_fs Interior, Okayama University, Yamada 827, Misasa, Tohaku-gun, Okayama, 682-0193 Japan
AU: Miyake, H
EM: hrs_miyake@ybb.ne.jp
AF: Enoshima Aquarium, 2-19-1 Katasekaigan, Fujisawa, 251-0035 Japan
AB: A vigorous fumarolic bubble emission from the seafloor has been well known in the submarine Wakamiko crater ($39\deg$39.5$\prime$N, $130\deg$46.5$\prime$E, depth=200m) in Kagoshima Bay, which is located within adjacent to the active volcano Sakurajima. Previous studies of the Wakamiko crater confirmed evidence for high-temperature ($>$$300\deg$C) hydrothermal activity based on occurence of hydrothermal-origin authigenic minerals such as pyrite and barite and hydrothermal petroleum in the surface sediments. During NT03-09 cruise conducted at Dec. 2003 using ROV Hyper-Dolphin (JAMSTEC), however, only gentle fluid shimmering was located at a small mound about one meter in height and three meters in a diameter, where fumarolic bubbling was associated in the vicinity. We found temperature of the mound reached up to $150\deg$C by stabbing a temperature probe (20cm length) into the sediment, and collected a few core samples (up to 30 cm length) to obtain interstitial water samples. Chemical composition of the interstitial waters is well explained by mixing of hydrothermal fluid endmember (Mg=0) and seawater. The silica concentration of the endmember (approx. 5mM) is comparable to quartz solubility at the seafloor pressure and the measured mound temperature. Chemical composition of the shimmering fluid follows the same relationship as the interstitial water, which strongly suggests the hydrothermal fluid ascend within the sediment layer. Their chemical characteristics were (1) Significant lower chloride concentration ($<$300mM) than that of seawater, and enrichment in ammonium and hydrogen sulfide, and (2) negative $\delta$D values and positive $\delta$$^{18}$O values. These are attributed to contribution of meteoric water and hydrothermal interactions within sediment layer. Although not confirmed by fluid chemistry, involvement of magmatic volatiles is implied by occurrence of stibnite and realgar in the mound sediments, which is considered as precipitated during hydrothermal fluid ascend.
DE: 8424 Hydrothermal systems (8135)
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 1050 Marine geochemistry (4835, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting