Core-mantle interaction as one cause for dense thermochemical structures at the base of the mantle

Seismic observations have revealed a range of distinct features at the core-mantle boundary of the Earth. To simulate these structures, typically the presence of a primordial layer (a relic of the magma ocean) is assumed. During mantle convection thermochemical structures develop from this layer for which, however, the excess density and mass need to be prescribed ad hoc and are not well constrained.

An alternative origin of the thermochemical structures could be core material penetrating the mantle by various interaction mechanisms. As a potential explanation of the observed tungsten deficits in some ocean island basalts different mechanisms have been proposed by laboratory experiments. To investigate this concept further, we developed a numerical model that incorporates a chemical gradient between the mantle and core to investigate the infiltration of dense material into the chemically depleted mantle.

In our models core material penetrates the mantle by the diffusive chemical influx in regions where slabs spread across the bottom boundary. As a consequence we observe a self-consistently growing dense layer from which thermochemical structures emerge in a similar way as observed in the primordial layer scenario. In the scenario of core-mantle interaction, however, the thermochemical structures are long-lived because of the constant chemical influx. This temporal stability agrees with plate reconstruction models that suggest a stability of the structures in the last 200-500 Ma. We performed a large parameter study in which we analyzed excess density and mass of the primordial layer as well as rheological parameters for both scenarios. Here, we will present our results on the temporal and spatial stability of the structures resulting in the core-mantle scenario and compare these to results from the primordial layer scenario.