HR: 1340h
AN: V33D-1491    [Abstracts]
TI: Petrology of Conduit Lava at Unzen Volcano; Result of Unzen Scientific Drilling Project
AU: * Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Yoshimoto, M
EM: myoshi@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Shimano, T
EM: shimano@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Kurokawa, M
EM: masaru@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Nakai, S
EM: snakai@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Sugimoto, T
EM: sugimoto@sevo.kyushu-u.ac.jp
AF: Inst. Seism. Volcanol., Kyushu Univ., Shinyama, Shimabara, 855-0843 Japan
AU: Hoshizumi, H
EM: h.hoshizumi@aist.go.jp
AF: Geol. Surv. Japan, AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Oguri, K
EM: k.oguri@aist.go.jp
AF: Geol. Surv. Japan, AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Noguchi, S
EM: noguson@geo.kyushu-u.ac.jp
AF: Dep. Earth Planet. Sci., Kyushu Univ., Hakozaki, Higashi, Fukuoka, 812-8581 Japan
AU: Goto, Y
EM: ygoto@mmm.muroran-it.ac.jp
AF: Dep. Civil Eng. Architech., Muroran Inst. Technol., Mizumoto, Muroran, 050-8585 Japan
AB: Lava samples were recovered from the conduit of the 1991-1995 eruption during the scientific drilling project in summer of 2004. Lava from the 1975-1997 m-depth conduit is most fresh among conduit samples and was nested in the hottest zone. The chemical composition including Sr isotopic ratio is very close to the 1991-1995 dome lava. The conduit lava is porphyritic dacite with phenocrysts of plagicolase, bioite and hornblende, though it is altered hydrothermally, such that hornblende was replaced by chlorite, carbonate and rutile, contrasting to freshness of biotite. The groundmass is composed of devitrified glass even in the chilled margins and contains small crystals of pyrite. As 3 wt % of water can be involved in the melt at the depth of 1.3 km where the conduit sample was recovered (about 40 MPa), about half of the initial water (6 wt %) should have been lost until when the magma reached this level. However, bubbles and microlites whose crystallization is induced by degassing are very scarce, compared in the groundmass of the dome lavas. Scarce amounts of microlites and bubbles indicate that degassing-induced crystallization was least in the ascending magma in this level, probably due to slow ascending (low cooling rate), and that bubbles could be escaped smoothly from the fluidal melt. As permeability of the country rocks is very low based on the logging data of the drilling, it is likely that magmatic volatiles should have escaped upward through cracks formed by ascending magma itself. Bubbles in the magma might have migrated into the tip of dyke magma where vapor-dominated tip was pressurized to form cracks. Crystallization of most of microlites commonly found in the dome lava had occurred in the shallower depth due to faster degassing (effective cooling rate).
DE: 8414 Eruption mechanisms
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting