Compressional and shear wave velocities of serpentinized peridotites up to 200 MPa

Compressional and shear wave velocities of serpentinized peridotites were measured at room temperature and high confining pressures of up to 200 MPa. Rock samples were collected from the Hida outer belt, Central Japan, and classified into High-T (containing antigorite) and Low-T (containing lizardite and/or chrysotile) types. Antigorite is stable up to 600-700°C, while lizardite and chrysotile are stable below 300°C. High-T type samples have distinctly higher velocities than their Low-T type counterparts with the same density. The High-T type with strong foliation shows significant velocity anisotropy, and the azimuthal anisotropy of the compressional wave velocity reaches 30%. These properties can be explained by the crystallographic structure of antigorite. Poisson's ratio increases with serpentinization in both types. The High-T type shows a lower Poisson's ratio than the Low-T type with the same density. The High-T type requires a higher degree of serpentinization than the Low-T type to give a certain value of Poisson's ratio. Observations of high Poisson's ratio have been interpreted using Low-T type properties. However, High-T type serpentinized peridotite is expected in warm subduction zones. The use of Low-T type properties will lead to a significant underestimation of serpentinization. For good interpretations, it is essential to use the properties of the appropriate type of serpentinized peridotite.