Spinel elasticity and seismic structure of the transition zone of the mantle

Velocity profiles for the M2SiO4 component of the mantle to depths of about 600 km are presented. It is found to be unlikely that any seismologically observable velocity discontinuity at about 520 km depth can be attributed to this component, although the contrast in impedance might be sufficient for the beta-gamma transformation to be observed in long-period seismic reflection studies at near-normal incidence. The velocity gradients in the transition zone, particularly for shear waves, are steeper than would be expected for simple adiabatic compression of likely mantle compositions, suggesting that alternative explanations including chemical heterogeneity and anelastic relaxation need to be explored.