The sound velocity of garnet at high pressure: Implications for high velocity anomalies in cold subducting slabs

The origin of the high velocity anomalies in cold subduction slabs revealed by seismological tomography remains unknown. Garnet is a major component of the pyrolitic mantle and subduction slabs and crucially affects their physical properties, such as densities and velocities. Based on previous studies, the predicted velocities of garnets with various compositions are much different from the measured values. Here, we measured the velocities of four hot-pressed natural garnet samples, namely, Alm65.1Grs17.5Prp8.5Sps2.0And6.9, Alm60.9Grs25.9Prp8.2Sps2.0And3.0, Alm49.6Grs5.3Prp37.8Sps1.7And5.6, and Grs89.0Prp0.4Sps6.3And4.3, at pressures up to 11 GPa using the ultrasonic interference method in conjunction with a Walker-type multi anvil, to further constrain the composition dependence on sound velocities. Our results show that the velocities and the bulk and shear moduli monotonically increase with pressure but display a complicated dependence on composition. Accordingly, we have determined the dependences of P- and S-wave velocities of garnets on pressure and composition and reconstructed the velocity profiles of pyrolite, eclogite, harzburgite, and a garnet layer at depths of 90-300 km in a cold subducted slab. Our model proposes that the presence of a garnet layer with a thickness of 5.9-15.9 km atop the slab could cause velocity contrasts of 4.0-6.0% for VP and 4.1-7.7% for VS between the slab and the overlying pyrolite. The presence of a layer with a high proportion of garnet could contribute to observed high-velocity anomalies in cold subduction zones.