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The large low shear velocity provinces observed in the lowermost mantle are widely accepted as chemically distinct thermochemical "piles," but their origin and long-term evolution remain poorly understood. The survival time and shape of the large low shear velocity provinces are thought to be mainly controlled by their compositional density, while their viscosity has been considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade-off between compositional viscosity and density contrasts required for long-term stability of thermochemical piles, which permits lower-density and higher-viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation-induced entrainment, for example, during initial pile formation, which suggests long-term pile survival.

Plain Language Summary Seismic images of the Earth's mantle show two anomalous continent-sized regions close to the core-mantle boundary. The inferred properties of these regions suggest that they have a different composition than the surrounding mantle. Two possible candidate materials have been proposed: accumulated oceanic crust from the Earth's surface or an iron-rich residue remaining from Earth's original magma ocean. Although both materials are denser than the surrounding mantle, it remains unclear whether piles of these chemical heterogeneities can survive at the core-mantle boundary beneath vigorous mantle convection. Numerical models show that the excess density required to preserve these structures is typically larger than indicated by seismological and gravitational observations. In this study, we show that the excess density used in numerical models can be reduced toward the observed values if the pile material is also stiffer than the surrounding mantle. Furthermore, we show that piles must be denser and/or stiffer to avoid destruction during episodes of strong deformation. Because pile formation probably includes vigorous deformation, we expect long-term survival of the piles after their formation is completed.