HR: 16:30h
AN: V44A-03    [Abstracts]
TI: Discovery and drilling of on- and off-axis hydrothermal sites in backarc spreading center of southern Mariana Trough, Western Pacific
AU: * Urabe, T
EM: urabe@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Dept. of Earth & Planetary Science, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Ishibashi, J
EM: ishi@geo.kyushu-u.ac.jp
AF: Kyushu University, Dept. of Earth & Planetary Sciences, 6 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Maruyama, A
EM: maruyama-aki@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8566 Japan
AU: Marumo, K
EM: k.marumo@aist.go.jp
AF: GSJ/AIST, Environment, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Seama, N
EM: seama@kobe-u.ac.jp
AF: Kobe University, KURCIS, 2746 Iwaya, Awaji, Hyogo, 656-2401 Japan
AU: Utsumi, M
EM: utsumi@bsys.tsukuba.ac.jp
AF: University of Tsukuba, 1 Tennodai, Tsukuba, 305-8572 Japan
AB: The Mariana Trough is an actively spreading backarc basin that is located along the eastern margin of Philippine Sea Plate. GPS monitoring indicates that the rate of spreading is about 45 mm/yr in the southern section (Kato et al., 2003). No transform fault offsets exist despite significant changes in the trend of the spreading center. Fryer et al. (1998) pointed out the close proximity of submarine arc volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank. Three hydrothermal sites were discovered along such tectonic lineament in southern Mariana Trough (12$^{o}$55-57'N, 143$^{o}$37-39'E). On-axis site (so-called Fryer site, depth: 2,850 m) consists of a hydrothermal mound about 20 m in diameter that develops on pillow lava of a segment center of the spreading axis. The segment is characterized by highly variable rock composition (up to 68% SiO2). Repeated temperature measurements revealed rapid cooling of the hydrothermal system from 240$^{o}$C in April 2003, through 112$^{o}$C in October 2003 to 69$^{o}$C in March 2004. On the other hand, two off-axis sites seem to have longevity of life: The Archaean site which locates about 2 km off-axis on the eastern (arc side) skirt is characterized by its huge sulfide spire; 50 m in height and 20 m in diameter. It is composed of pyrite, chalcopyrite and sphalerite, and is emanating hydrothermal fluids up to 220$^{o}$C. In the third site (Pika site), active black smokers (max. temp. = 330$^{o}$C), numerous dead chimneys and sulfide mounds were found on a basaltic seamount about 5 km off-axis. These lines of evidence support the idea of Fryer et al. (1998) that the backarc magma is replenished by arc/off-axis magma along the tectonic lineation. The first and third sites been drilled and cased using a tethered, submarine rock-drill system BMS (Benthic Multi-coring System) on-board the {\it R/V Hakurei-Maru # 2} as a part of {\bf Archaean Park Project}*. Rocks from two holes (7.5 m and 4.1 m) in the Fryer site are porous basaltic lava with frequent fractures. Fluid 76$^{o}$C was discharged from one of the holes. The Pika site has two holes; one hole (5.61 m) is entirely composed of massive sulfides (pyrite-sphalerite) beneath thin manganese oxide cover. The other (4.61 m) shares common features with those of Fryer site. No venting was witnessed during the drilling but the latter hole was found venting fluid (10.1$^{o}$C) month later during the {\it ROPOS/ Thompson} cruise when extensive sampling and monitoring were done on these holes. * Funded by MEXT through the Special Coordination Fund.
DE: 4832 Hydrothermal systems
DE: 5480 Volcanism (8450)
DE: 3035 Midocean ridge processes
DE: 1749 Volcanology, geochemistry, and petrology
DE: 0400 Biogeosciences
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting