Multiphase Dynamics of the Very Young Earth's Mantle

Abstract:

Early in the history of terrestrial planets, heat of accretion, radioactive decay or core-mantle segregation may have significantly melted the silicate mantle. Magma ocean evolution depends both on the physical properties of solid and liquid materials at relevant P-T conditions and on the complex dynamics of a convecting crystallizing mantle. Present deep Earth mantle structures such as ultralow-velocity zones (ULVZs) or low-shear velocity provinces (LLSVPs) might be directly linked to the crystallization of a potential magma ocean. 

We propose a complete thermodynamic model of the solid-liquid equilibrium in the MgO-FeO-SiO2 system at HP/HT (Boukaré et al, 2015, in press). It synthesizes various data (observed and computed equations of state, melting curves, Mg/Fe partitioning). The present study confirms previous findings that, at similar compositions, melts are lighter than solids throughout the mantle. However, at thermodynamic equilibrium, the first solids that crystallize in the deep mantle are lighter than the liquid as they are more Mg-rich. This further enriches the melt in iron and this residual melt becomes much denser than the solid phase. Both the anti-freeze effect of iron and its high density suggest a mantle crystallization scenario similar to that described in Labrosse et al. [2007] where the ULVZ are iron rich and very fusible remnants of a primordial basal ocean. 

We also present the development of a multiphase convection code accounting for solid-liquid phase change, compaction and fractional crystallization. We discuss the effects of various temperature profiles and solid liquid density crossovers on the dynamics of a crystallizing mantle. Using this mechanical model, we also investigate the dynamics of upper mantle overturn following magma ocean crystallization. Indeed, current models of magma ocean evolution predict that fractional crystallization of the mantle leads to unstable chemical stratification of the upper mantle.