Core formation, mantle differentiation and core-mantle interaction within Earth and the terrestrial planets

Abstract The terrestrial planets accreted from a diverse suite of solar system materials ranging from strongly O-deficient materials similar to enstatite chondrites via ordinary chondrite materials to fully oxidised carbonaceous chondrite and cometary materials. Heliocentric zoning with increasingly oxidised planetesimals outwards through the protoplanetary disc is broadly reflected in core fraction and FeO mantle concentration, ranging from 68 wt% core and 0.5 wt% FeO mantle for Mercury to 18 wt% core and 24 wt% FeO mantle for Vesta. Mercury, Venus and Earth grew mostly from materials which were isotopically similar to enstatite chondrites, although Earth and Venus also received more oxidised material. The elevated (Mg + Fe)/Si ratio, compared to chondrites, in the bulk silicate fraction of the terrestrial planets, except for Mercury, may be related to a combination of nebular fractionation associated with forsterite condensation, concentration of olivine-rich chondrules near the mid-plane of the accretion disc and multi-cycle impact erosion of protocrusts. For the extremely reduced Mercury the silicate magma ocean (MO, list of abbreviations in Table 1) and a core with 15 wt% Si might have equilibrated with a melt layer of FeS at the core-mantle boundary (CMB). Recent data from the MESSENGER mission combined with experimentally derived phase relations, support estimates of about 0.5 wt% FeO and 10 wt% S in the MO and the current mantle. Core segregation at very high temperatures for the largest planets Venus and Earth, led to cores with high Si content, even at relatively high oxygen fugacities and FeO mantle contents, because increasing temperature shifts the equilibrium: SiO 2 MO + 2Fe core = 2FeO MO + Si core towards the products (right side). The hot protocores of Venus and Earth might have started with about 5–7 wt% Si and 2–3 wt% O. Mars and Vesta segregated S-rich cores at high oxygen fugacity and low pressure. Strong partitioning of Fe and Mg to melt and solids, respectively, caused neutrally buoyant bridgmanite (bm) to crystallise from the MO at 1700–1860 km depth (72–80 GPa), resulting in a separate basal magma ocean (BMO) within Earth, and probably also in Venus. Slow cooling of a thermally insulated BMO and core, accompanied by protracted crystallisation of bm and ferropericlase (fp), would facilitate core-BMO chemical exchange by reversing the equilibrium SiO 2 MO  + 2Fe core  = 2FeO MO  + Si core towards the reactants. Transfer of silica crystals and a liquid SiO 2 component from the core to the BMO, and liquid FeO and Fe 2 O 3 components from the BMO to the core, would increase the Si/Mg, Mg/Fe and bm/fp ratio of the resulting cumulates. Because the solidus temperature of peridotite is The primordial bm-dominated cumulates with high Mg/Fe ratios and viscosities may have become convectively aggregated into large refractory domains, remaining neutrally buoyant in the middle to upper parts of the lower mantle and resistant to convective destruction. Late-stage dense BMO cumulates with elevated Fe/Mg ratios relative to the bulk mantle composition might represent a suitable material for 100–200 km thick thermochemical piles at the bottom of the large low S-wave velocity provinces (LLSVPs) under Africa and the Pacific. A degree-2 convection pattern, possibly initiated and stabilised during Earth's early rapid rotation, involving antipodally ascending columns in equatorial positions and an intermediary descending longitudinal belt, might have swept the late-stage, dense bridgmanitic cumulates with high Fe/Mg-ratios, viscosity and bulk modulus towards the root zones of the upwelling columns.