Seismic Velocities of Subducted Serpentinites Through the Lizardite-Antigorite Transition

The mineralogical transition (lizardite to antigorite) occurring in serpentinites dragged down in subduction zones depends on the metamorphic pressure and temperature conditions. The seismic Vp/Vs ratio is useful for distinguishing serpentinites from other mantle rocks. To understand the effects of the metamorphic conditions and the associated microstructural evolution on the serpentinite velocity, we collected seven natural serpentinite rocks with different metamorphic imprints in the Western Alps, from blueschists to eclogite facies conditions. We measured the ultrasonic velocity of the samples at confining pressures up to 70 MPa. The measured seismic Vp and Vs increase with the increasing metamorphic conditions, while the Vp/Vs ratio decreases. The petrological results are used to explain the velocity anisotropy and the weak pressure-dependency of velocity. The new Vp/Vs transition trend provides new constraints for tracking the lizardite/antigorite transition with increasing metamorphic condition from field seismic data and thus studying geodynamic evolution in the subduction environment.