Δs, a measure of the sensitivity of the adiabatic bulk-modulus to isobaric density changes, controls lateral variations of P-wave velocities. To estimate the effect of pressure on Δs, we have performed lattice dynamic calculations of the cubic alkaline earth oxides. We used a many-body parametric Hamiltonian that reproduces the available laboratory data on thermoelastic properties. The calculations were carried out for P-T conditions that correspond to those of the lower mantle. We find that Δs decreases from 4¿1 at standard conditions to ≤2 at lower mantle conditions. The latter value is consistent with D.L. Anderson's estimates based on geophysical data.

With Δs≤2, and a corresponding parameter for shear modulus not decreasing with compression, no partial melting is required to account for the observation that ΔlnVs/ΔlnVp≥2 in the lower mantle. Although more data and calculations on additional minerals are needed to draw any firm conclusions, our results indicate that the relatively small lateral variations of lnVp, when compared to lnVs, are due entirely to compressional effects. ¿ American Geophysical Union 1990