Partial melting in a thermo-chemical boundary layer at the base of the mantle

Seismological detections of complex structures in the lowermost mantle boundary layer (the D″ region) motivate a conceptual model of a compositionally stratified thermo-chemical boundary layer (TCBL) within which lateral temperature variations (sustained by large-scale mid-mantle flow) cause variations of partial melt fraction. Partial melt fractions of from 0 to 30% in the TCBL occur due to the eutectic of the boundary layer material lying below the high temperature at the core–mantle boundary (CMB), coupled with the presence of steep thermal gradients across the boundary layer. Regions of the TCBL with the highest temperatures have extensive partial melting, producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics. This TCBL concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as: predominance of large-scale volumetric elastic wave velocity heterogeneity in the boundary layer, laterally extensive, but intermittent abrupt shear and compressional velocity increases and decreases at the top of the D″ region, small-scale topography of D″ velocity discontinuities, thin ultra-low velocity zones at the CMB, widespread shear wave anisotropy in D″, bulk sound velocity anomalies detected in low shear velocity regions of D″, and large-scale upwellings from the most extensively melted regions of the boundary layer. Neutral or negative buoyancy of the partial melt in the TCBL is required, along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle. More complex models, in which the partially melted chemical boundary layer is laterally displaced by mid-mantle downwellings are also viable, but these appear to require additional special circumstances, such as a phase change, efficient segregation of slab crustal material, or abrupt onset of anisotropy, in order to account for rapid velocity increases at the top of D″. The precise nature of the compositional anomaly, or anomalies, in D″ and the eutectic composition of this zone are yet to be determined, but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity. Increased resolution geodynamic calculations of dense boundary layer structures with partial melting (and attendant viscosity reductions and chemical heterogeneity) are needed to quantify this TCBL concept.