HR: 0800h
AN: MR11A-0906    [Abstracts]
TI: Marked Decrease in Vp of Anorthosite above 400 $\deg$C
AU: * Kono, Y
EM: d03ta007@ynu.ac.jp
AF: Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AU: Ishikawa, M
EM: ishikawa@ed.ynu.ac.jp
AF: Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AU: Arima, M
EM: arima@ed.ynu.ac.jp
AF: Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
AB: We measured ultrasonic compressional wave velocity (Vp) of anorthosite (An content=49-53) with pulse reflection method. We carried out Vp measurement with piston-cylinder apparatus at 1 GPa up to 700 $\deg$C. The anorthosite exhibits reversible and nonlinear change in Vp during heating and cooling. $\delta$Vp/$\delta$T of the anorthosite changes from -1.9 to -4.8 x10$^{-4}$ km s$^{-1}$ $\deg$C$^{-1}$ at around 400 $\deg$C. In previous study, we observed abrupt change in $\delta$Vp/$\delta$T from -2.4 and -2.1 to -4.7 and -4.5 x10$^{-4}$ km s$^{-1}$ $\deg$C$^{-1}$ at around 400 $\deg$C in the gabbronorite and pyroxene granulite, which are mainly composed of clinopyroxene, orthopyroxene and plagioclase (An content=47-55), and it is difficult to explain by the previously proposed mechanisms, such as thermal cracking, dehydration reaction, and/or partial melting (Kono et al., 2003). Similarly to the gabbronorite and pyroxene granulite, some papers have reported marked decrease in Vp of plagioclase-rich rocks at high temperatures (e.g. Matsushima, 1981). Abrupt change in $\delta$Vp/$\delta$T of the anorthosite observed in this study explains these marked Vp decrease at high temperatures. Contrary to nonlinear change in Vp of plagioclase-rich rocks, we observed linear decrease in Vp of peridotite (wehrlite) up to 900 $\deg$C at 1 GPa. We therefore consider that marked decrease in Vp above 400 $\deg$C is a unique property of plagioclase-rich rock. We expect that this marked decrease in Vp of plagioclase above 400 $\deg$C cause seismic low velocity zones in plagioclase-rich crusts.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 5102 Acoustic properties
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting