Physical mechanisms of planetary core formation: Constraints from in-situ X-ray microtomography

Abstract:

Segregation of the metallic core from a silicate mantle is a crucial aspect of early planetary evolution. Although a magma ocean scenario is often used to explain differentiation of large planets such as Earth, smaller planets and planetesimals likely never achieved the high temperatures necessary for wide scale melting. In these smaller bodies, silicates may have only partially melted, or not melted at all. Furthermore, isotopic signatures in meteorites suggest that some planetesimals differentiated within just a few million years. Achieving core segregation on this time scale whereby core material drains through a solid silicate mantle via an interconnected network of melt faces two major problems: (1) in a hydrostatic situation, the percolation threshold is above 5 vol% melt, so the process would lead to inefficient core formation, and (2) the permeability of fully connected melts at microstructural equilibrium is low enough that some planetesimals may still not be able to differentiate on this short time scale. It has been suggested that shear deformation can cause isolated melt pockets to become connected even at low melt fractions. Here, we have measured the change in permeability of core forming melts in solid silicate and partially molten silicate matrix due to deformation. Mixtures of olivine or KLB-1 peridotite and FeS close to the equilibrium percolation threshold (~5 vol% FeS) were pre-synthesized to achieve an equilibrium microstructure, and then loaded into the high pressure X-ray tomography apparatus at GSECARS, sector 13-BMD, at the Advanced Photon Source (Argonne National Laboratory). The samples were then pressed to ~2GPa, and heated to ~1100°C. Alternating cycles of rotation to collect X-ray tomography images, and twisting to deform the sample were conducted. Starting materials and run products have also been analysed at high resolution in three dimensions using FIB/SEM cross-beam tools. Quantitative analyses have been performed on the resulting 3-dimensional x-ray tomographic images to evaluate the effect of shear deformation on permeability of core forming melts, and place constraints on the timing of core formation in smaller terrestrial planets and planetesimals.