HR: 17:15h
AN: V24B-06 INVITED     [Abstracts]
TI: Real Images of Magmatic Conduit: Progress of the Conduit Drilling in Unzen
AU: * Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Earth. Res. Ins., Univ. Tokyo, Yayoi, Bunkyo, Tokyo, 113-0032 Japan
AU: Sakuma, S
AF: Japan Metals Chemicals, co. Ltd., Takizawa, Iwate, 020-0172 Japan
AU: Uto, K
AF: Geol. Surv. Japan, AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Shimizu, H
AF: Inst. Seism. Volcanol. Kushu Univ., Shin-yama, Shimabara, 855-0843 Japan
AU: Eichelberger, J C
AF: Geophys. Inst., Univ. Alaska Fairbanks, University Rd, Fairbanks, AK 99775 United States
AB: The conduit of the 1990-1995 lava dome at Unzen was penetrated in the scientific drilling project (USDP). The drilling target was set in the hyprocenter zone of isolated tremors that occurred prior to magma extrusion in 1991. The drilling operation was sponsored by MEXT-Japan and ICDP and completed in the summer of 2004. Drilling was started vertically from the northern slope of Mt. Unzen at 840 m asl and directional to below the summit. The conduit zone was reached near the sea level (ca. 1.3 km below the crater). Rock sampling and logging in and around the conduit were succeeded. The conduit zone consists of multiple EW-oriented vertical dykes, up to about 4 m thick and of different formation ages, within volcanic breccias. Both the dykes and breccias are altered hydrothermally. Pyroclastic veins of the similar orientations are abundant in the conduit zone. The volcanic breccias of the conduit zone are physically homogeneous. Their porosity is less than 0.2 and density is close to 2.5. Mud loss seldom occurred during drilling in the volcanic breccias. These facts imply little cracks developed and very low permeability of the conduit zone. Relatively fresh dyke with the highest temperature was located in the 1975 to 1999 m drilling depth, beneath a little south of the previous Unzen peak. The temperature was as low as 200 deg C, much cooler than expected. It is likely that hydrothermal fluid circulation within the conduit zone accelerated cooling and alteration of the newest conduit. Identification of the conduit as the latest eruption was based on its chemical inconsistency with that of the new dome lava. It is important that the conduit is neither single nor bundled into older ones throughout the growth history of the volcano. Magma prefers intrusion isolatedly from older conduits in every eruption event as far as in this depth. It is likely that forming cracks through one of which magma ascended was recorded as pyroclastic veins and is responsible for isolated tremor events.
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