HR: 17:00h
AN: V24B-05    [Abstracts]
TI: Conduit drilling at Unzen volcano, Japan: core description and interpretation
AU: * Goto, Y
EM: ygoto@mmm.muroran-it.ac.jp
AF: Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido, 050-8585 Japan
AU: Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yoshimoto, M
EM: myoshi@eri.u-tokyo.ac.jp
AF: Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shimano, T
EM: shimano@eri.u-tokyo.ac.jp
AF: Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kurokawa, M
EM: kurokawa@eri.u-tokyo.ac.jp
AF: Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sakuma, S
EM: vistari40@nifty.ne.jp
AF: Tohoku Geothermal Engineering, Morioka, Iwate, 020-0024 Japan
AU: Sugimoto, T
EM: sugomoto@sevo.kyushu-u.ac.jp
AF: Kyushu University, Shimabara, Shinyama, Shimabara, Nagasaki, 855-0843 Japan
AU: Noguchi, S
EM: noguchi@kyushu-u.ac.jp
AF: Kyushu University, Fukuoka, Fukuoka, 920-1192 Japan
AU: Toramaru, A
EM: toratota@kyushu-u.ac.jp
AF: Kyushu University, Fukuoka, Fukuoka, 920-1192 Japan
AU: Hoshizumi, H
EM: h.hoshizumi@aist.go.jp
AF: AIST, Tukuba east 1-1-1, Tsukuba, 305-8567 Japan
AU: Oguri, K
EM: oguri@aist.ac.jp
AF: AIST, Tukuba east 1-1-1, Tsukuba, 305-8567 Japan
AU: Uto, K
EM: k.uto@aist.ac.jp
AF: AIST, Tukuba east 1-1-1, Tsukuba, 305-8567 Japan
AB: A 1996-m-long hole has been drilled on a slanted trajectory that passed beneath Unzen volcano, Japan. The hole penetrated central conduit region of the volcano at ~1600 m below the summit. Drill cores at 50 m intervals were recovered from the hole. The principal facies of the drill cores in the conduit region are polymictic breccia and coherent dacite. The polymictic breccia is non-stratified, poorly sorted and made up of heterolithologic, polyhedral dacite clasts 10-50 mm across, which are embedded in a cogenetic matrix. Larger clasts up to 50 cm across rarely occur. The clasts consist of non-vesicular, porphyritic dacite, which varies in color (greenish grey to grey), alteration, phenocryst size and phenocryst proportion. They show clast-rotated texture, and no jigsaw-fit texture. The edges of the clasts are commonly rounded. The matrix of the breccia is reddish grey, dense (low porosity) and composed of angular dacite fragments up to 5 mm across. The coherent dacite is grey, massive and porphyritic. It contains plagioclase and hornblende phenocrysts. The matrix of the dacite is mostly crystalline, but gradation from light grey, crystalline dacite to dark grey, glassy dacite occurs. These polymictic breccia and coherent dacite have been in places intruded by veins up to 10 mm wide. The veins vary in color (white, greenish grey and black) and components (fine-grain cray minerals, coarse rock fragments). Some veins show laminations along the vein wall. The morphological features of the polymictic breccia suggest that the breecia formed within the conduit region of the volcano in response to fragmentation of rocks due to explosions, but did not extrude from the vent and recycled within the conduit region. The coherent dacite facies is probably a dyke with chilled margins. The veins may have formed by injections of high-temperature gas and/or liquid into fractures around dykes during dyke intrusions. The descriptions of the drill core, together with composition of drill cuttings, electric conductivity of the borehole (FMS image) and seismological data during 1990-1995 eruption, suggest that the conduit region of the Unzen volcano is 300-350 m wide at ~1600 m below the summit, and consists of polymictic breccia and several, parallel, vertical dykes, intruding the breccia. Each dyke is tabular-shaped and 3-30 m thick. Thin dykes (3-8 m thick) have simple, uniformed interior and are inferred to have formed by single injection of magma. On the other hand, thick dykes (26-30 m) have several cooling units within, and probably formed by several magma injections.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting