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

• Published in: Tectonophysics Vol. 760; pp. 165 - 198
• Main Authors: Trønnes, R.G., Baron, M.A., Eigenmann, K.R., Guren, M.G., Heyn, B.H., Løken, A., Mohn, C.E.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 05.06.2019; Elsevier BV
• Subjects: Accretion; Basal magma ocean; Bulk density; Bulk modulus; Carbonaceous chondrites; Chondrites; Comets; Condensates; Condensation; Convection; Core segregation; Core-mantle boundary; Core-mantle exchange; Cores; Crystallization; Crystals; Deposition; Earth; Earth mantle; Earth rotation; Entrainment; Erosion; Exchanging; Fractionation; High temperature; Iron; Iron sulfides; Lava; Low pressure; Lower mantle; Magma; Mars; Mercury; Ocean circulation; Oceans; Olivine; Organic chemistry; Oxygen; Peridotite; Planetary accretion; Planets; Ratios; Segregation; Silica; Silicates; Silicon dioxide; Solar system; Solidification; Solidus; Temperature; Terrestrial environments; Terrestrial planets; Thermal conductivity; Upwelling; Velocity; Venus; Viscosity; Wave velocity; Basal magma ocean; Core segregation; Core-mantle exchange; Planetary accretion
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2018.10.021

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 FeOmantle concentration, ranging from 68 wt% core and 0.5 wt% FeOmantle for Mercury to 18 wt% core and 24 wt% FeOmantle 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) 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 FeOmantle contents, because increasing temperature shifts the equilibrium:SiO2MO+2Fecore=2FeOMO+Sicoretowards 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 SiO2MO + 2Fecore = 2FeOMO + Sicore towards the reactants. Transfer of silica crystals and a liquid SiO2 component from the core to the BMO, and liquid FeO and Fe2O3 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 <200–300 K above the present temperature of the outermost core, and the melting interval of late-stage BMO melt enriched in Al, Fe, Ca and Na would be lower than that of peridotite, the BMO might have persisted through the Hadean and possibly also the Archean. Low solid state diffusion rates, especially in bm, would have restricted the core-mantle interaction upon BMO solidification, but limited core-mantle interaction could possibly occur via partially molten ultra-low velocity zones. An outermost stagnant low-density and low-velocity core layer (E′-layer), with reduced Si and elevated O contents relative to the convecting core, appears to trace the core-BMO exchange. The E′-layer is compositionally gradational towards the convecting core at 445 km below the CMB. High thermal conductivity and minimal convective entrainment in the low-viscosity core fluid might have developed and stabilised such a gradational layer since the Hadean or early Archean. 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. [Display omitted] •Core and mantle compositional estimates for the terrestrial planets are provided•Melting and silicate-metal partitioning led to magma oceans (MO) and cores•Core-mantle separation in Earth and Venus occurred at high temperature and pressure•Mineral-melt density relations led to a separate basal MO (BMO) in Earth and Venus•Cooling of Earth and Venus cores led to core-BMO chemical exchange