Lateral variations in lower mantle seismic velocity

An important finding of recent tomographic studies is that the lower mantle, at a constant radius, has RMS variations of P- and S-wave velocity of ~0.1-0.2% and ~0.2-0.4% from average Earth models and these lateral variations correlate spatially. Both tomographic and free oscillation studies suggest that the magnitude of relative shear velocity variations is at least twice as large as the magnitude of relative compressional velocity variations. This result can be expressed as: v= (∂nV_s/∂lnV_p)_p > 2. In contrast, laboratory studies near ambient pressure have consistently found that the ratio of relative shear velocity variations to relative compressional velocity variations is near unity for metals and minerals generally, and mantle minerals, in particular. Laboratory studies further suggest, although not yet conclusively, that v is not strongly pressure or temperature sensitive. In this paper, we seek to determine whether high observed values of vcan be explained by the presence of 0.1-2.0% volatile-rich partial melt heterogeneously distributed in the lower mantle. The H₂O budget of the lower mantle is estimated to be 0.1-0.3 wt.% based on the present He flux, equation of state data for hydrous minerals, and shock devolatilization experiments which define a maximum radius of the Earth's primitive accretion core. The effect of hydrous melts on v is calculated using theories for the elastic properties of a two phase aggregate. Results indicate that, depending on aspect ratio and geometry, 0.1-2% partial melting in conjunction with ~100 K thermal anomalies can explain the seismic result so long as the compressibility of the melt differs by less than about 20% from the surrounding solid. On this basis, we conclude that small amounts of water-rich partial melt are a possible explanation for the large values of (∂nV_s/∂lnV_P)_P observed for the lower mantle.