HR: 1340h
AN: V33D-1490    [Abstracts]
TI: Magma History of Unzen Volcano, Investigated using Drilling Slime of Conduit Drilling
AU: * Kurokawa, M
EM: masaru@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Nakada, S
AF: Earthquake Research Institute, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yoshimoto, M
AF: Earthquake Research Institute, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shimano, T
AF: Earthquake Research Institute, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sugimoto, T
AF: Institute of Seismology and Volcanology, Kyushu Univ., 2-5643-29 Shin-yama, Shimabara, Nagasaki, 855-0843 Japan
AU: Oguri, K
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: Hoshizumi, H
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AB: To clarify the geologic and tectonic growth history, magma evolution and eruption mechanism at Unzen volcano, the scientific drilling project (USDP) started in 1999. Conduit drilling (USDP-4) aiming at penetration into a hot magmatic path was carried out during May-July of this year. Drilling slimes from USDP-4 were investigated for understanding geologic and magmatic historyy during the growth of Unzen volcano. Drilling slime was collected in every 2m depth interval down to 1994m. The sample was washed and sieved into three grain size groups. Based on the shape and color of particles, the original rock was estimated as lava flow, and pyroclastic- and debris-flow. The slime is porphyritic dacite to andesite, and contains phenocrysts of plagioclase, hornblende, biotite, orthopyroxene, clinopyroxene, quartz, and opaque minerals. Plagioclase always includes dusty and clear types. Color, chemistry and magnetic susceptibility were measured. The color of whole sample was expressed in the CIE L*a*b* color space. As a result, color of sample change systematically with depth. L* value express degree of porosity, and a* value and b* value express degree of oxidization or alteration. Chemical composition, for example, SiO2 and K2O/Rb are nearly constant in samples deeper than the 500m depth and variable in the shallower samples. Magnetic susceptibility of the slime is also variable with depth. The value of magnetic susceptibility depends on the quantity of Fe-Ti oxide in the sample and is concordant with total iron content and TiO2 content of the sample. As the rock texture does not largely changed and the magnetic susceptibility is almost identical between different grain size groups, the magnetic susceptibility probably represents the difference of chemical composition and state of oxidation after deposition. Since the Unzen volcanic rocks contain commonly lines of evidence showing magma mixing, it is considered that similar mixing process was operated in the early stage of volcanic history. On the other hand, the mixing process, including chemistries of end members and mixing ratios, was changeable in the late stage.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting