Pore-scale simulation of two-phase melt percolation during core formation in planetesimals

Abstract:

Geochemical and paleomagnetic evidence indicates that core formation in primordial planetesimals is very rapid and likely driven by decay heating from short-lived radionuclides. While some planetesimals clearly experienced wholesale melting and differentiation in a magma ocean, others differentiated in a partially molten state. The latter is supported by the various degrees of partial melting and partial differentiation recorded in primitive achondrites. Even differentiation during partial melting will inevitably lead to the onset of silicate melting and the presence of two melt phases, a dense sulphide melt and a buoyant silicate melt. As such, core formation involves the percolation of two interacting melt phases.

Here we present the first 3D pore-scale simulations of the interaction of sulphide and silicate melts in a texturally equilibrated melt network provided by our previous work using a level set method. We assume, for now, that the sulphide melt is entirely non-wetting so that the geometry of the melt network is entirely determined by the silicate melt. We use two-phase flow lattice Boltzmann simulations to determine the relative permeabilities and the capillary pressure curves for sulphide and silicate melts. Unlike previous work, which generally assumes no interaction between the two melt phases, in our simulations there is a strong interaction when both phases are flowing. Our simulations further show the formation of immobile residual saturations that indicate it is difficult to drain all sulphide melt – at least in the absence of compaction. The constitutive functions for two-phase melt percolation reported here are also essential inputs for large-scale simulations of planetesimal differentiation.