Modeling phase separation and phase change for magma ocean solidification dynamics

Abstract

Just after accretion, the Earth's mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core. The mineralogical observations and thermodynamics models of solid‐liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle. Solids were probably formed first at the bottom of the lower mantle or at midmantle leaving two possible magma oceans, a shallow one and an abyssal one. Near the bottom of the mantle, the liquid phase might become denser than solids due to iron enrichment. In the shallow magma ocean, the crystallizing solid phase was denser and sank through the magma to settle and compact at depth. To understand these complex dynamics, we develop a two‐phase numerical code that can handle simultaneously convection in each phase and in the slurry, and the compaction or decompaction of the two phases. Although our code can only run in a parameter range (Rayleigh number, viscosity contrast between phases, Prandlt number) far from what would be realistic, we think it already provides a rich dynamics that illustrates what could have happened. We show situations in which the crystallization front is gravitationally stable and situations where the newly formed solids are gravitationally unstable and can snow across the magma. Our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification.

The Thermo‐Chemical Evolution of Mars With a Strongly Stratified Mantle

“…However, our models do not account for possible melt transport within the basal layer. Indeed, if the layer is partially molten, melt‐solid density differences may lead to episodes of melt segregation (Boukaré & Ricard, 2017) and/or Rayleigh‐Taylor overturns. Because both the enriched melt and solids are denser than the overlying mantle these processes would remain confined within the layer.…”

Thermal and magnetic evolution of a crystallizing basal magma ocean in Earth's mantle

“…Exsolution mechanisms explicitly occur across a metal-silicate interface that is liquid on both sides (Badro et al, 2016(Badro et al, , 2018, thus invoking a longlived basal magma ocean (BMO) atop the core (Labrosse et al, 2007;Laneuville et al, 2018) which would initiate at mid-mantle depths and crystallize downwards to the core. A giant impact as large as one suggested to lead to the formation of the Moon may have been energetic enough that Earth's initial condition was completely molten (Canup and Asphaug, 2001;33Ć uk and Stewart, 2012;Lock et al, 2018), however the initial depth of an emergent BMO is subject to uncertainty in the equation of state of lower mantle composition, its melting curve, the adiabatic gradient as determined by its material properties, and the dynamics of phase separation (Stixrude et al, 2009;De Koker and Stixrude, 2009;Boukaré et al, 2015;Boukaré and Ricard, 2017;Wolf and Bower, 2018;Caracas et al, 2019). The scenario of whether the BMO, if electrically conductive enough, could be capable of generating a dynamo was explored as a potential mechanism for providing a magnetic field during the early Earth (Ziegler and Stegman, 2013).…”

The Four‐Stage Evolution of Martian Mantle Inferred From Numerical Simulation of the Magmatism‐Mantle Upwelling Feedback

“…where u* is the velocity of magma. I neglected the volume change from adiabatic compression and melting in this equation (Boukare & Ricard, 2017). The relative velocity u * −U * is proportional to the density difference between magma and matrix:…”