Effects of depth- and composition-dependent thermal conductivity and the compositional viscosity ratio on the long-term evolution of large thermochemical piles of primordial material in the lower mantle of the Earth: Insights from 2-D numerical modeling

Thermal conductivity plays an important role in the thermochemical evolution of Earth’s mantle. Recent mineral physics studies suggest that the thermal conductivity of the mantle varies with pressure and composition, and this may play an important role in the evolution of the Earth’s mantle. Meanwhile, the rheology of the deep mantle is also supposed to be composition-dependent. However, the dynamic influences of these factors remain not well understood. In this study, we performed numerical experiments of thermochemical mantle convection in 2-D spherical annulus geometry to systematically investigate the effects of depth- and composition-dependent thermal conductivity and the compositional viscosity ratio on the long-term evolution of the large thermochemical structure of primordial material in Earth’s mantle. Our results show that increasing the depth-dependent thermal conductivity leads to a larger core-mantle boundary (CMB) heat flow and allows the formation of more stable large thermochemical piles (e.g., Large Low Shear Velocity Provinces, LLSVPs); while decreasing the composition-dependent thermal conductivity would slightly destabilize the primordial thermochemical piles, increase the altitude of these piles and the temperature differences between the piles and the ambient mantle. If the primordial mantle material is compositionally more viscous (e.g., 20 times than that of the ambient mantle), the long-term stability of the thermochemical piles of primordial material decreases, and this destabilizing effect will be enhanced by decreasing the composition-dependent thermal conductivity. As a result, the thermochemical piles would be unstable in the core-mantle boundary region. Therefore, our study indicates that the combined effects of depth- and composition-dependent thermal conductivity and compositional viscosity ratio are pronounced to the thermochemical evolution of the mantle.