The effects of differentiated heat production on the stability of deep dense pools at the core-mantle boundary

The formation of the continental and oceanic crust is a main source of chemical differentiation. Mantle convection both generates this heterogeneity by melting at plate boundaries and destroys it by convective mixing. Thermochemical convection models had been a useful tool to study the geochemical evolution of the earth, with tracers carrying the chemical information and combining geodynamics and geochemistry (e.g., Christensen and Hofmann, 1994; Brandenburg et al., 2008). Brandenburg et al., 2008 demonstrated that the creation and destruction of oceanic crust over the age of the Earth explains the radiogenic isotope characteristics of HIMU, EM1 and DMM reasonably well. The dense oceanic crust forms deep dense piles that seem similar to the large low velocity regions seen in seismology. In this study we have extended the Brandenburg et al. by taking into account the effects of differentiation on heat production. We use high resolution 2D finite element models that incorporate a very large number of active tracers, that carry information about the local density and heat production. Unlike the additional density carried by the tracer, which influences the flow instantaneously, the additional heat productivity influence the flow in a more gradual and integrated manner: tracer heating causes localized temperature increase, and then increase of buoyancy and decrease of viscosity as well. The piles of recycled oceanic crust carry higher concentration of radioactive elements. According to the thermochemical convection models from Christensen & Hofmann 1994 and Brandenburg et al. 2008, the piles of subducted slabs, featured by higher concentration of eclogite, always have higher temperature than the surroundings, as a result of long-term heating from the bottom(core). Radiogenic heating naturally further promotes this effect and destabilizes more rapidly the deep dense pools. Our work quantifies the relative density increase that is necessary to retain the deep pools and satisfy the geochemical, seismological and geodynamical constraints.