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Recent seismological studies challenge the traditional view that the interface between the core and mantle is a straightforward discontinuity. As seismology is pushed to its observational limits, a complex - potentially compositionally layered - region between the core and mantle is emerging. [ABSTRACT FROM AUTHOR]

The large low‐shear‐velocity provinces (LLSVPs) are thought to be thermo‐chemical in nature, with recycled oceanic crust (OC) being a contender for the source of the chemical heterogeneity. The melting process which forms OC concentrates heat producing elements (HPEs) within it which, over time, may cause any collected piles of OC to destabilize, limiting their suitability to explain LLSVPs. Despite this, most geodynamic studies which include recycling of OC consider only homogeneous heating rates. We perform a suite of spherical, three‐dimensional mantle convection simulations to investigate how buoyancy number, geochemical model and heating model affects the ability of recycled OC to accumulate at the core‐mantle boundary. Our results agree with others that only a narrow range of buoyancy numbers allow OC to form piles in the lower mantle which remain stable to present day. We demonstrate that heterogeneous radiogenic heating causes piles to destabilize more readily, reducing present day CMB coverage from 63% to 47%. Consequently, the choice of geochemical model can influence pile formation. Geochemical models which lead to high internal heating rates can cause more rapid replenishment of piles, increasing their longevity. Where piles do remain to present day, first order comparisons suggest that old (hot) OC material can produce seismic characteristics, such as Vs anomalies, similar to those of LLSVPs. Given the range of current density estimates for lower mantle mineral phases, subducted OC remains a contender for the chemical component of thermo‐chemical LLSVPs. Plain Language Summary: Large, pile‐like structures at the base of Earth's mantle may be partially composed of accumulations of recycled oceanic crust. When oceanic crust is formed, heat producing elements are concentrated within it. This causes oceanic crust to experience relatively high heating rates compared to surrounding material when it is recycled into the mantle. A consequence of the high heating rates is that piles of oceanic crust may become unstable and so may not survive to present day. We conduct three‐dimensional numerical mantle simulations to investigate how the excess chemical density of recycled oceanic crust affects the survival of piles. In line with previous work, we find only a narrow range of excess chemical densities allow the survival of piles to the present day. Piles are more readily destroyed when internal heating is controlled by the concentration of heat producing elements compared to when internal heating is distributed evenly through the mantle. Consequently, assumptions made in the geochemical model, which controls the distribution of heat producing elements, can affect the long‐term stability of piles. Key Points: Heterogenous heating rates cause piles to destabilize more rapidly than homogeneous heating ratesA narrow range of buoyancy numbers allows piles to persist to present day without being destroyed or forming a layerFaster mantle processing rates can replenish piles more efficiently, increasing their longevity [ABSTRACT FROM AUTHOR]

Seismic observations indicate that the lowermost mantle above the core–mantle boundary (CMB) is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the CMB and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional and dynamic evolution of the Earth. [ABSTRACT FROM AUTHOR]