HR: 0800h
AN: V51A-1463    [Abstracts]
TI: Geological Observation of the Young Petit-Spot Volcanoes on the Early Cretaceous Pacific Plate using SHINKAI 6500
AU: * Machida, S
EM: machida@ori.u-tokyo.ac.jp
AF: Department of Ocean Floor GeoScience, Ocean Research Institute, Univ. of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo, 164-8639 Japan
AU: Hirano, N
EM: nhirano@geo.titech.ac.jp
AF: Department of Earth and Planetary Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Ingle, S
EM: ingle@hawaii.edu
AF: SOEST, Univ. of Hawaii, 1680 East-West Road, POST 615A, Honolulu, HI 9682 United States
AU: Ishii, T
EM: ishii@ori.u-tokyo.ac.jp
AF: Department of Ocean Floor GeoScience, Ocean Research Institute, Univ. of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo, 164-8639 Japan
AU: Abe, N
EM: abenatsu@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima, Yokosuka, Kanagawa, 237-0061 Japan
AB: Young volcanoes are newly discovered on the cool, thick, and old Pacific Plate approximately 600 km ESE off the northern Japan Trench, during KR04-08 cruise using R/V Kairei (JAMSTEC) in 2004. The site was predicted lava eruption based on previous study for young alkali-basalt lavas (5.95 Ma) on the Japan Trench oceanward slope (the Kaiko knolls lava field). We named the Petit-Spot for such small and young volcanoes. Four SHINKAI 6500 submersible dives (dive #877 to #880) were performed in the Petit-Spot knolls and the Kaiko Knolls lava field during May 2005. Dives were initially targeted in the hopes of finding outcrops of lava flow, sampling rocks, observation of eruptive styles, and discussion of the stratigraphy of the monogenic Petit-Spot volcanoes. Three dives were conducted at the Petit-Spot lava field. One young knoll, named the Yukawa Knoll, discovered by the dredges (D-07, 08) during KR04-08 cruise was selected for dive #877. Two other knolls for dives #878 and #879 show the similar features of bathymetry and acoustic reflectivity to the Yukawa Knoll. In contrast, one site of knoll and fault wall for dive #880 situate in the Kaiko Knolls lava field, the Japan Trench oceanward slope. Dives were planned to ascending the slope from the foot of knoll with continuously observation of lavas and volcaniclastic sequences, and sampling along the slope. The basin or flat floor at the beginning of the dive was fully covered in soft pelagic sediment. We found oblong shaped pillow lavas and the pillow robes outcrops along the slope. However, outcrops are exhibited in limited area around the summit of knoll. Lavas commonly include peperites. Rock fragments sampled appear to be volcanic breccias containing clasts of lava enclosed in sedimentary rock; manganese coating on these breccias is thin, suggesting they were deposited recently. Based on above observation, we assumed that knolls in the Petit-Spot lava field show feeder dike or cryptdome like stratigraphy with some lava flows, in-situ and redeposited hyaloclastites, and water-chilled bumps. That is, basaltic lavas mainly inject into pelagic sediment layer. Furthermore, lavas show very high vesicularity in spite of high water pressure about 6000m deep, suggesting high volatile composition. Brecciated lava clasts containing peridotitic xenolith. These features may indicate that the Petit-Spot volcanoes also have aspect as a kind of abyssal diatreme induced in the oceanic lithosphere.
DE: 1065 Major and trace element geochemistry
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005