Marked Decrease in Vp of Anorthosite above 400 ° C

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 ° C. The anorthosite exhibits reversible and nonlinear change in Vp during heating and cooling. δ Vp/δ T of the anorthosite changes from -1.9 to -4.8 x10^-4 km s^-1 ° C^-1 at around 400 ° C. In previous study, we observed abrupt change in δ Vp/δ T from -2.4 and -2.1 to -4.7 and -4.5 x10^-4 km s^-1 ° C^-1 at around 400 ° 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 δ Vp/δ 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 ° C at 1 GPa. We therefore consider that marked decrease in Vp above 400 ° C is a unique property of plagioclase-rich rock. We expect that this marked decrease in Vp of plagioclase above 400 ° C cause seismic low velocity zones in plagioclase-rich crusts.