HR: 0800h
AN: S51B-0152    [Abstracts]
TI: Vp/Vs Ratio of Hornblende Gabbro Xenolith from Ichinomegata Volcano, NE Japan
AU: * Nishimoto, S
EM: d03ta012@ynu.ac.jp
AF: Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AU: Ishikawa, M
EM: ishikawa@ynu.ac.jp
AF: Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AU: Arima, M
EM: arima@ynu.ac.jp
AF: Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AU: Yoshida, T
EM: tyoshida@mail.tains.tohoku.ac.jp
AF: Research Laboratory of Arc Magmatism, Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AB: In order to interpret petrologically the inhomogeneous Vp, Vs and Vp/Vs structure beneath the NE Honshu arc (Nakajima et al., 2001), we measured P-wave velocity (Vp) and S-wave velocity (Vs) of Ichinomegata hornblende gabbro xenolith simultaneously within the solidus P-T conditions (up to $700\deg$C and 0.8 GPa) in this study. The rock sample was loaded in the talc-pyrophyllite high-pressure cell. Travel times of elastic-wave through the rock sample were determined with the pulse reflection technique. A sample length was cut into about 6mm diameter core and doubly polished to a length of about 6 mm. Before velocity measurements, the rock sample was oven-dried at $120\deg$C for 24 hours and the H$_{2}$O was not added to the rock sample. The measured rock includes the hornblende gabbro of Ichinomegata xenolith, which has ultrabasic composition (41.3 wt% SiO$_{2}$O) and is mainly composed of plagioclase (>An$_{86}$, 80.6 vol% in mode ratio), aluminous hornblende (13.9 wt.% Al$_{2}$0$_{3}$, 12.5 vol%) and magnetite (6.9 vol%). At constant pressure of 0.6 GPa, the Vp of the hornblende gabbro gradually decreased from 6.83 km/s at $25\deg$C to 6.59 km/s at $600\deg$C. Similarly the Vs values reduce from 3.87 km/s at $25\deg$C to 3.74 km/s at $600\deg$C. The Vp/Vs ratio of the hornblende gabbro is 1.76 at $600\deg$C. The Vp/Vs ratio of the hornblende gabbro is comparable to that of the lower crustal layer beneath the western side of Ou back-born range to back-arc side (Vp/Vs = 1.75-1.79), except near active volcanic areas (Vp/Vs > 1.83). Nakajima et al. (2001) indicated that the low Vp, low Vs and high Vp/Vs anomalies in the lower crust to uppermost mantle beneath the active volcanic areas were perhaps caused by inclusions containing melt. These high Vp/Vs anomalies do not consist with the Vp/Vs ratio of the hornblende gabbro in this study. It is suggested that the high Vp/Vs anomalies in the lower crust to uppermost mantle is comparable to the high temperature conditions beyond the solidus with partial melting and/or other materials.
DE: 8015 Local crustal structure
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 3640 Igneous petrology
DE: 1020 Composition of the crust
SC: Seismology [S]
MN: 2004 AGU Fall Meeting