A chemical Earth model with whole mantle convection: The importance of a core-mantle boundary layer (D″) and its early formation

Advances in seismic tomography and mantle convection modeling have led to the convincing conclusion of whole-mantle convective flow, so that compositional differences between upper and lower mantle could not have been preserved over the age of the earth. Nevertheless, there is compelling geochemical evidence favoring the occurrence of an ancient apparently isolated reservoir. We propose this reservoir is located in the core-mantle transition (termed D″). It was formed during the late stages of Earth accretion via subduction of primitive mafic to ultramafic crust along with a terrestrial regolith composed (partially) of chondritic and solar-wind-implanted material. To investigate this scenario we develop a quantitative geochemical model envisaging accretion of the Earth from chondrite-like material, formation and evolution of the principal terrestrial reservoirs (the mantle, core, D″, continental crust) via mass fluxes that include rare gas and respective parent isotopes, also Sm, Nd, Lu, Hf, Rb, Sr, W isotopes and siderophile elements. The solution shows that such a model, with a D″ mass of ∼ 2 × 10²⁶ g and a regolith proportion of ∼ 1/4, allows the apparent conflict between geophysical and geochemical observations to be resolved.