Abstract
The planets and larger rocky bodies of the inner solar system are differentiated, and consist of metallic, iron-rich cores surrounded by thick shells of silicate. Core formation in these bodies, i.e. the segregation of metal from silicate, was a key process in the early solar system, and one which left a lasting geochemical signature. It is commonly assumed that extensive silicate melting and formation of deep magma oceans was required to initiate core formation, due to the inability of iron-rich melts to segregate from a solid silicate matrix. Here we assess the role of deformation in aiding segregation of core-forming melts from solid silicate under conditions of planetary deep interiors. Low-strain rate, high-pressure/temperature deformation experiments and high-resolution 2-D and 3-D textural analysis demonstrate that deformation fundamentally alters iron-rich melt geometry, promoting wetting of silicate grain boundaries and formation of extensive micron to sub-micron width Fe-rich melt bands. Deformation-aided Fe-S melt networks noted here contrast those observed in higher finite strain experiments conducted at lower pressure, and may reveal either an alternative mechanism for melt segregation at higher pressures, or an early stage process of melt segregation. Results suggest, however, that core-mantle chemical equilibration cannot be assumed in models of planetary formation, and that instead, the chemistry of rocky planets may record a complex, multi-stage process of core formation.
Highlights
Terrestrial bodies of the inner solar system are typically differentiated, consisting of metallic, Fe-rich cores surrounded by thick shells of silicate rock
Micro-XCT analysis conducted at the University of Edinburgh was used to produce datasets ranging in voxel size of 2–3 lm for 2 mm field of view (FOV), whole-sample scans
We note that the observed independence of deformation-aided percolation on strain rate in the present study indicates that the mechanism cannot readily be discounted as a mechanism for core formation in terrestrial bodies, either at higher strain rates due to impact events, or at much lower strain rates due to convection processes
Summary
Terrestrial bodies (rocky planets and larger asteroids) of the inner solar system are typically differentiated, consisting of metallic, Fe-rich cores surrounded by thick shells of silicate rock. The vast majority of studies on olivine-metallic liquid systems have found that when left to anneal to textural equilibrium, dihedral angles are consistently >60 ° (Shannon and Agee, 1996; Terasaki et al, 2007), with the pinch-off melt fraction determined by Yoshino et al (2003) to be 5%, >6% by Watson and Roberts (2011), between 6 and 9.5% by Terasaki et al (2005), and possibly even higher (Bagdassarov et al, 2009; Walte et al, 2007) This implies that core formation by percolation would have been highly inefficient, and left a significant fraction of Fe-rich melt trapped within the silicate portion of rocky bodies, inconsistent with many aspects of the observed geochemistry of the bulk-silicate Earth, including the concentration of highly siderophile elements (HSE) (Minarik et al, 1996). This permits both assessment of the merits and limitations of 2-D and 3-D analysis, and of the capability of recent advances in 3-D imaging techniques to provide insight into textural development in complex, fine-grained geological systems
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