The effect of shear deformation on planetesimal core segregation: Results from in-situ X-ray microtomography

It is well accepted that the Earth formed by the accretion and collision of small (10-100km) planetesimals. W-Hf isotopic evidence from meteorites (e.g. Kleine et al., 2002) suggest that pre-differentiated planetesimals accreted to form the Earth within 3 My. How did these planetesimals differentiate in such a relatively short time period? While a very hot, deep magma ocean is generally thought to have been the driving mechanism for core formation in large planetary bodies, it inadequately explains differentiation and core formation in small planetesimals due to temperatures being insufficient for wide-scale melting to occur. In order for these planetesimals to differentiate within such a relatively short time without a magma ocean, a critical melt volume of the metallic (core-forming) phase, and sufficient melt connectivity and grain size must exist in order to attain the required permeability and lead to efficient core formation. Deformation has been shown to improve permeability in similar studies with samples of nearly the same composition and melt fraction (e.g. Hustoft & Kohlstedt, 2006), and could have been a contributing factor in the formation of planetesimal cores. This deformation may have been caused by large impacts and collisions experienced by the planetesimals in the early Solar System. The purpose of this work is to test the hypothesis that shear deformation enhances the connectivity and permeability of Fe-S melt within a solid silicate (olivine) matrix, such that rapid core formation is plausible. A rotational Drickamer press was used to heat and torsionally deform a sample of solid olivine + FeS liquid through 6 steps of 180° rotation, while X-ray microtomography was used to obtain 3-dimensional images of the sample in-situ at each step. The resulting digital volumes were processed and permeability simulations were performed to determine the effect of deformation on connectivity and permeability within the sample. Our results indicate that deformation at high pressure and temperature increases the permeability of Fe-S in an olivine matrix by at least an order of magnitude from 0° to 840° of torsion, approaching full connectivity and the percolation required for more efficient core formation.