Textural evolution of metallic phases in a convecting magma ocean: A 3D microtomography study

Abstract

The textures of solid and molten metal in the presence of varying fractions of silicate melt at high temperature have been investigated to shed light on differentiation processes occurring in magma oceans formed on rocky bodies of the early solar system. Analogue experiments have been performed in a three-phase system (composed of coexisting metal, forsterite and silicate melt) in both static (1 GPa, 1723 K) and dynamic (i.e. agitated, at 1 bar, 1713 K and 1743 K) conditions. Micro-textures were analyzed with SEM and EBSD techniques, while meso-textures of the metallic phase were analyzed using ex-situ 3D microtomography.Although all samples exhibit the same micro-scale organization consistent with the minimization of local interfacial energies, their meso-scale textures differ significantly. Static conditions produce metal grains that have shapes close to spherical, corresponding to the state predicted by the grain-scale minimization of interfacial energies. In contrast, under dynamic conditions and in the presence of high silicate melt fractions (≥50 vol%), molten metal coalesces to form pools with sizes that are several orders of magnitude larger than those predicted by grain growth mechanisms. Furthermore, in agreement with expectations based upon an interfacial energy budget, images show that nickel grains, whether solid or molten, do not occur surrounded entirely by silicate melt, but rather in contact with both forsterite crystals and silicate melt, leading to the formation of composite aggregates.Assuming that a magma ocean has less than 50 vol% of crystals (the upper limit that permits convective motion), thermodynamic calculations indicate that at the necessary temperatures, the metallic subsystem (Fe-Ni-S) of the planetesimal is entirely molten and the silicate residue is only composed of olivine. Convective motions in such a body will drive agitation, promoting the formation of composite aggregates of olivine and molten iron-sulfide, their initial coalescence and subsequent fragmentation. In detail, these composite aggregates have a reduced density contrast with the surrounding silicate melt that reduces their settling velocities compared to pure metal. They also entrain olivine during the downward migration of iron-sulfide pools. Olivine grains concentrate at the surface of the metallic pools, hindering coalescence between pools or with a pre-existing core. An alternative differentiation scenario for core formation is explored in which the simple compaction of partially molten mixtures in the basal non-convecting layer of the magma ocean expels the interstitial silicate melt upward, such that the local fraction of iron-sulfide increases by mass-balance, reaching its percolation threshold and allowing core formation. This process is not only limited to early accreted planetesimals but may also occur in terrestrial bodies.