Compressional and shear wave velocities in serpentinized peridotites

Serpentinized peridotites in the wedge mantle play key roles in the transport of water and the slab-mantle coupling. Geophysical mapping of serpentinized regions is essential for good understanding of subduction zone processes. Tomographic studies have related low-velocity and high Poisson's ratio (high Vp/Vs) to serpentinized peridotites [e.g., Kamiya and Kobayashi (2000)]. Their interpretations are based on velocity measurements mostly on Low-T type (containing lizardite and/or chrysotile) serpentinized peridotites [e.g., Christensen (1996)]. However, it is questionable whether velocities of Low-T type are applicable to warm subduction zones like Costa Rica, where the other serpentine mineral antigorite is expected. From crystallographic studies, higher elastic stiffness is expected in antigorite. Watanabe et al. (2007) have shown that antigorite-bearing serpentinized peridotites have higher velocity and lower Poisson's ratio than Low-T type serpentinized peridotites. However, their argument was based on the arithmetic mean of velocities in three mutually orthogonal directions. Anisotropic nature of rock samples should be properly taken into account in calculating the average velocity. The temperature dependence of velocities should also be investigated. In order to solve these problems, we are now conducting (1) determination of all elastic moduli from velocity measurements in various directions and (2) velocity measurements under high pressure and temperature conditions. Compressional and shear wave velocities are measured on octahedral specimens by the pulse transmission technique in various directions of propagation and polarization. Specimens were prepared from antigorite- bearing serpentinized peridotites collected in Hida outer belt. Measurements are conducted at the room temperature and under the confining pressure up to 180 MPa. All elastic moduli with orthorhombic symmetry are determined, and isotropic velocities and Poisson's ratio are calculated assuming that similar rocks are randomly packed. The higher velocity and lower Poisson's ratio of antigorite-bearing serpentinized peridotites has been confirmed. Velocities of an antigorite rock are measured up to 600°C at 1 GPa. The rock sample shows numerous parallel lines, which are interpreted to be parallel to b-axis of antigorite grains. The compressional wave velocity was 8.4 km/s and 6.6 km/s in the direction parallel and perpendicular to the lines, respectively. No significant temperature dependence of the velocity was observed in the direction parallel to the lines, while the compressional wave velocity decreases by 4% from the room temperature to 600°C in the direction perpendicular to the lines. Observed anisotropic behaviors are consistent with crystallographic natures of antigorite.