A statistical boundary layer model for the mantle D″ region

摘要

The D″ region above the core-mantle boundary (CMB) plays critical roles in the dynamics of both the mantle and the core; however, the complexity of this region observed over a broad range of spatial scales defies simple interpretations as either purely thermal or purely chemical heterogeneity. Here, we formulate a 1-D, time-dependent boundary layer model for the D″ region, which provides statistical properties of the dynamics and seismic heterogeneity, by coupling thermal, chemical, and phase (TCP) variability in the layer. We assume a Gaussian-like time-dependent mantle flow, compositional stratification due to variations in iron content, heat flow variations because of changes in the local temperature gradient, and a postperovskite (PPV) phase transformation. We compute a range of TCP boundary layer model cases that are consistent with the observed seismic shear wave velocity heterogeneity in the region 50–300 km above the CMB. These results imply an average CMB heat flow near 13 TW with -3 TW variations, CMB temperature of 4000 K, a large positive PPV Clapeyron slope, and an average heat transport of about 3 TW associated with deep mantle plumes.