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. Plain Language Summary Seismic observations have revealed many anomalously low-velocity regions in mantle wedges above subducting slabs. The inferred reduction in mantle velocity has been attributed to the formation of low-velocity serpentine in the mantle wedge. We report the new results of high-pressure ultrasonic measurements on a hot-pressed polycrystalline sample to determine both compressional and shear wave velocities of pure antigorite up to 8.4 GPa at room temperature. We found that antigorite exhibits significantly lower P and S wave velocities and higher V-P/V-S ratio than major upper mantle minerals (e.g., garnet, clinopyroxene, orthopyroxene, and olivine) at upper mantle pressures. The experimental results were analyzed to interpret the seismic observations in slow regions and constrain the degree of serpentinization in the mantle wedge above subduction zones. We established precise serpentinization budgets from seismic observations and provided compelling evidence for the interpretation of seismic anomalies of low-velocity and high-velocity ratios by serpentinization in different subduction zones. These results will improve our ability to map the distribution of hydrous minerals in slabs through interpretation of low-velocity anomalies in subduction zones.