Anisotropy as cause for polarity reversals of D″ reflections

Abstract

Recordings of seismic events that sample the deep mantle can test different hypotheses of mantle processes and composition. Seismic reflections from structures in the D″ region — the bottom 200–400 km of the Earth's mantle — can provide information on the velocity contrasts in this region. By studying the waveforms and polarities of the D″ reflections in P and S-waves, we can potentially distinguish between different explanations for the observed structures, such as phase transitions, mineral texture or thermal anomalies. Here we use source–receiver combinations that contain reflections from D″ in two different regions that are both characterised by fast seismic velocities in tomographic models. Beneath the Caribbean a positive S-velocity contrast but negative P-wave velocity contrast across the D″ reflector has been reported previously, consistent with a model of a phase change in MgSiO 3. In the second fast velocity region (Eurasia) we detect positive P- and S-wave velocity contrasts in two orthogonal paths crossing in the lowermost mantle indicating a different scenario for D″. A path that crosses this region in 45° to the other two great circle paths shows evidence for a negative P velocity contrast. One explanation to reconcile observations in both regions is a phase transition from perovskite to post-perovskite with a fraction of 12% preferred crystal alignment in the post-perovskite phase. Depending on the travel direction of the waves with respect to the flow direction in the lower mantle, positive or negative velocity jumps can be expected. Other anisotropic models are considered but cannot fully explain the range of observations we find in the data.