Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges

Elastic properties of antigorite are important for interpretation of seismic mapping of serpentinization in the mantle wedge above subducting slabs. The compressional (V_P) and shear (V_S) wave velocities in pure antigorite aggregates were measured simultaneously up to 8.4 GPa by ultrasonic interferometry. We found that V_P increases monotonically with pressure while V_S increases with pressure up to about 3 GPa but undergoes a negative pressure dependence above 4 GPa. Compared to other mantle minerals, antigorite exhibits significantly lower P and S wave velocities as well as a higher V_P/V_S ratio at upper mantle pressures. We modeled velocity reductions manifesting through the formation of antigorite in mantle peridotite and provide compelling evidence that seismic anomalies with low-velocity and high V_P/V_S ratios is caused by varying degrees of serpentinization in subduction zones.