Formation of Thermochemical Heterogeneities by Core-Mantle Interaction

Earth's core-mantle boundary (CMB) shows a complex structure with various seismic anomalies such as the large low shear-wave velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs). As these structures are possibly induced by chemically distinct material forming a layer above the CMB, models of mantle convection made ad hoc assumptions to simulate the dynamics of this layer. In particular, density and mass were prescribed. Both conditions are critical for the dynamics but hardly constrained. Core-mantle interaction is considered as one possible origin for this dense layer. For example, diffusion-controlled enrichment of iron has been proposed. We here apply a chemical gradient between the mantle and the denser core to analyze the penetration of dense material into the mantle. As such, we employ 2D Cartesian models where a thermochemical layer at the base of the mantle develops self-consistently by a diffusive chemical influx. Our simulations indicate that chemical diffusion is strongly affected by the convective mantle flow. This convection-assisted diffusion yields a compositional influx mainly in the areas where slabs spread over the bottom boundary and sweep dense material aside to form accumulations with rising plumes atop. Like for a prescribed dense layer this process leads to chemically distinct piles, which are typically smaller (therefore more suited to explain ULVZs) but more persistent due to the constant chemical influx. Combining the influx scenario with the primordial layer can possibly explain the simultaneous existence of LLSVPs and ULVZs along with the observation of a core-like isotopic composition in the mantle.

Automated summary

The Earth's core-mantle boundary (CMB) has complex seismic anomalies such as large low shear-wave velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs). To simulate the dynamics of these structures, ad hoc assumptions are made in models of mantle convection. Density and mass, which are critical for the dynamics, are hardly constrained. One possible origin for the dense layer above the CMB is core-mantle interaction. The study analyzes the penetration of dense material into the mantle using a chemical gradient and suggests that convection-assisted diffusion plays a crucial role in the formation of chemically distinct piles and can explain the coexistence of LLSVPs and ULVZs.