Seismic velocities, anisotropy, and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones

Antigorite, the high-temperature (HT) form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P-and S- wave velocities (V-p and V-s), anisotropy and shear-wave splitting of 17 serpentinite samples containing > 90% antigorite at pressures up to 650 MPa. The new results, combined with data for low-temperature (LT) lizardite and/or chrysolite, reveal distinct effects of LT and HT serpentinization on the seismic properties of mantle rocks. At 600 MPa, V-p = 5.10 and 6.68 km/s, V-s = 2.32 and 3.67 km/s, and V-p/V-s = 2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack-closure pressure (similar to 150 MPa), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is > 300 degrees C should be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high-pressure fluids, or partial melt is also needed to interpret HT serpentinized mantle with V-p< 6.68 km/s, V-s< 3.67 km/s, and V-p/V-s> 1.81. The intrinsic anisotropy of the serpentinites (3.8-16.9% with an average value of 10.5% for V-p, and 3.6-18.3% with an average value of 10.4% for V-s) is caused by dislocation creep-induced lattice-preferred orientation of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S-, L-, and LS-tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.