Stability of the large low shear velocity provinces : numerical modeling of thermochemical mantle convection We study segregation of the subducted oceanic crust (OC) at the core mantle boundary and its ability to accumulate and form large thermochemical piles (such as the seismically observed Large Low Shear Velocity Provinces LLSVPs). Our high resolution numerical simulations suggest that the longevity of LLSVPs for up to three billion years and possibly longer can be ensured by a balance in the rate of segregation of high density OC material to the CMB and the rate of its entrainment away from the CMB by mantle upwellings. For a range of parameters tested in this study a large scale compositional anomaly forms at the CMB similar in shape and size to the LLSVPs. Neutrally buoyant thermochemical piles formed by mechanical stirring where thermally induced negative density anomaly is balanced by the presence of a fraction of dense anomalous material best resemble the geometry of LLSVPs. Such neutrally buoyant piles tend to emerge and survive for at least 3Gyr in simulations with quite different parameters. We conclude that for a plausible range of values of density anomaly of OC material in the lower mantle it is likely that it segregates to the CMB gets mechanically mixed with the ambient material and forms neutrally buoyant large scale compositional anomalies similar in shape to the LLSVPs. We have developed an efficient FEM code with dynamically adaptive time and space resolution and marker in cell methodology. This enabled us to model thermochemical mantle convection at realistically high convective vigor strong thermally induced viscosity variations and long term evolution of compositional fields.