Thermodynamics of the MgO‐FeO‐SiO₂system up to 140 GPa: Application to the crystallization of Earth's magma ocean

Abstract

At the end of Earth's accretion and after the core‐mantle segregation, the existence of a basal magma ocean at the top of the core‐mantle boundary (CMB) depends on the physical properties of mantle materials at relevant pressure and temperature. Present‐day deep mantle structures such as ultralow‐velocity zones and low‐shear velocity provinces might be directly linked to the still ongoing crystallization of a primordial magma ocean. We provide the first steps toward a self‐consistent thermodynamic model of magma ocean crystallization at high pressure. We build a solid‐liquid thermodynamic database for silicates in the MgO‐FeO‐SiO2 system from 20 GPa to 140 GPa. We use already published chemical potentials for solids, liquid MgO, and SiO2. We derive standard state chemical potential for liquid FeO and mixing relations from various indirect observations. Using this database, we compute the ternary phase diagram in the MgO‐FeO‐SiO2 system as a function of temperature and pressure. We confirm that the melt is lighter than the solid of same composition for all mantle conditions but at thermodynamic equilibrium, the iron‐rich liquid is denser than the solid in the deep mantle. We compute a whole fractional crystallization sequence of the mantle and show that an iron‐rich and fusible layer should be left above the CMB at the end of the crystallization.

Fate of Hydrous Fe‐Rich Silicate Melt in Earth's Deep Mantle

“…Figure 4 shows a contour of density difference equal to zero between a silicate melt and solid (i.e., neutral density buoyancy) at the CMB condition. For simplicity, (Mg + Fe)/Si of the melt is taken as 2 (Boukaré et al, 2015;Ohnishi et al, 2017) as densities of MgO and MgSiO 3 melt are similar at high pressure (Karki et al, 2018). The solid density of Earth's lowermost mantle is taken as 5.566 g/cm 3 (Dziewonski & Anderson, 1981).…”

Onset of solid‐state mantle convection and mixing during magma ocean solidification

“…The fractional crystallization of a magma ocean, possibly followed by a whole-mantle overturn, is a mechanism that has been proposed to explain various features of the early evolution of Mercury [Brown and Elkins-Tanton, 2009], Mars [Elkins-Tanton et al, 2005a;Scheinberg et al, 2014], the Moon [Zhong et al, 2000;de Vries et al, 2010;Zhang et al, 2013], and the Earth [Foley et al, 2014;Boukaré et al, 2015]. In the context of Mars, in particular, the fractional crystallization of a global magma ocean, originally proposed by Elkins-Tanton et al [2003], has been invoked to explain the formation of an ancient crust with composition consistent with surface and meteoritic data [Elkins-Tanton et al, 2005b]; to explain radioactive isotopes systematics of the SNC meteorites in terms of distinct mantle reservoirs that may have been sustained by the stable structure resulting from a global overturn [Debaille et al, 2007[Debaille et al, , 2009] (see section 4.2.2); and to account for an early episode of dynamo activity induced by the overturn of mantle cumulates [Elkins-Tanton et al, 2005a].…”

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…MO crystallization timescales strongly depend on the nature and thickness of the proto-atmosphere, which ultimately controls the heat flux from Earth to space (Lebrun et al, 2013;Marcq, 2012). As long as some degree of chemical fractionation occurs and the MO freezes from the bottom upwards, the MO is predicted to become progressively Fe-rich as Mg-rich bridgmanite in the lower mantle (Andrault et al, 2012;Boukare et al, 2015;Fiquet et al, 2010;Nomura et al, 2011;Tateno et al, 2014), and other Mg-rich phases (mostly olivine and pyroxenes) in the upper mantle, are successively removed. The related Fe-enrichment in the coexisting cumulates leads to unstable compositional density stratification in the early mantle (Elkins-Tanton, 2008).…”