Earth’s D” Layer

The D” layer, the lowermost portion of the mantle, sits just above the molten iron-rich outer core.  Seismic observations have revealed a region with an intriguingly complex signature.  This relatively thin layer, varying around 250 km in thickness, may hold the key to understanding how the core and mantle interact.  The D” layer may also be where deep mantle plumes originate and where subducting slabs terminate.  Some of the most puzzling seismic features include the splitting of shear-waves travelling through this layer and the presence of ultralow-velocity zones (ULVZ).  ULVZs are thin (5- 40 km thick) patches in which the compressional and shear wave velocities are depressed by 5-10% and 10-30%, respectively, relative to the neighboring region.

We found that silicate post-perovskite (ppv), the major constituent in the D” layer, can be highly enriched in iron (Mao et al, PNAS 2004b, Mao et al, PNAS 2005), which has a dramatic effect on its physical and chemical properties (Mao et al, GRL 2006; Mao et al, AGU Monograph 2007).  The ability of post-perovskite to absorb iron may explain the anomalously low velocities in ULVZs (Mao et al, Science 2006).  More recently we studied the elastic anisotropy of this phase and found that it can exhibit large shear-wave splitting (Mao et al, PEPI 2009), consistent with seismic observations and theoretical calculations.  We have been working on transport measurements of the thermal conductivity which is a key property for studying Earth’s thermal evolution and internal dynamics (Goncharov et al, PEPI 2010), and investigating how iron enrichment affects Fe-Mg partitioning and structural variation among a number of possible post-perovskite phases.  The goal of these studies is to compile a comprehensive set of materials properties for each individual phase as well as for mineral assemblages, in order to deepen our understanding of how this boundary layer plays such a central role in global dynamics and the evolution of Earth.