Laboratory Measurements of Ultrasonic Wave Velocities of Crustal Rocks at High Pressures and Temperatures: Petrological Structure of Izu-Bonin-Mariana Arc Crust

Abstract

To construct petrological model for island arc crusts having relatively higher geo-thermal gradient, we developed experimental techniques capable for simultaneous measure-ment of P-and S-wave velocities of an encapsulated rock specimen up to 1 GPa and 1000°C. In this paper we introduce our experimental methods and discuss petrological models of the Izu-Bonin-Mariana (IBM) island arc architecture. The models are constructed on the basis of our velocity measurement data at high temperatures and pressures and the seismic velocity profiles of the IBM arc crust previously reported (Suyehiro et al. 1996; Takahashi et al. 1998). These seismic velocity profiles clearly defined a stratified four-layered crustal struc-ture for the IBM arc crust. Our ultrasonic laboratory measurements reveal that the IBM crust consists of a 5 km thick basaltic upper crust underlain by a 5 km thick tonalitic middle crust (Vp = 6.2–6.3 km/s) while the lower crust consist of a 3 km thick hornblende-bearing gabbro (Vp = 6.7–6.8 km/s) succeeded below by a 8 km thick pyroxenite or gabbroic rocks (Vp = 7.1–7.3 km/s). The more recent seismic experiments of the IBM arc further highlighted the relatively low-velocity (Vp = 7.4–7.7 km/s) domains located within the upper mantle imme-diately below the lower crust (Vp = 7.1–7.3 km/s) (Kodaira et al. 2007a, 2007b). Seismic ref-lectors were observed within and near the base of these low-velocity domains. Our data sug-gest that the low-velocity domains probably represent mixtures of various garnet-pyroxene-rich ultramafic rocks of crustal origin (restites after lower crustal anatexis and/or cumulates after magmatic differentiation) and mantle peridotites components.