Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Abstract

The segregation of dense core-forming melts by porous flow is a natural mechanism for core formation in early planetesimals. However, experimental observations show that texturally equilibrated metallic melt does not wet the silicate grain boundaries and tends to reside in isolated pockets that prevent percolation. Here we use pore-scale simulations to determine the minimum melt fraction required to induce porous flow, the percolation threshold. The composition of terrestrial planets suggests that typical planetesimals contain enough metal to overcome this threshold. Nevertheless, it is currently thought that melt segregation is prevented by a pinch-off at melt fractions slightly below the percolation threshold. In contrast to previous work, our simulations on irregular grain geometries reveal that a texturally equilibrated melt network remains connected down to melt fractions of only 1 to 2%. This hysteresis in melt connectivity allows percolative core formation in planetesimals that contain enough metal to exceed the percolation threshold. Evidence for the percolation of metallic melt is provided by X-ray microtomography of primitive achondrite Northwest Africa (NWA) 2993. Microstructural analysis shows that the metal-silicate interface has characteristics expected for a texturally equilibrated pore network with a dihedral angle of ∼85°. The melt network therefore remained close to textural equilibrium despite a complex history. This suggests that the hysteresis in melt connectivity is a viable process for percolative core formation in the parent bodies of primitive achondrites.

Magma ascent in planetesimals: Control by grain size

“…To date, only few studies have quantitatively investigated this effect. These were either based on melt transport models with parameterized melt ascent velocities (Moskovitz and Gaidos, 2011;Wilson and Keil, 2012;Mandler and Elkins-Tanton, 2013;Neumann et al, 2013Neumann et al, , 2014Neumann et al, , 2018, or focused on metal-silicate separation (Šrámek et al, 2012;Ghanbarzadeh et al, 2017).…”

“…However, if volatiles are exsolved before the onset of silicate melting, Fu and Elkins-Tanton (2014) argue that the segregation rate of dry melt is mostly controlled by the oxygen fugacity and the degree of melting. The oxygen fugacity determines (or is determined by) the relative abundance of FeO and Fe-FeS in the pri-mordial rock, with parts of the latter potentially lost to the core by percolation before the onset of major silicate melting (Yoshino et al, 2003;Cerantola et al, 2015;Ghanbarzadeh et al, 2017). Higher oxygen fugacity may therefore result in more Fe-rich silicate melts with reduced (or even inverted) density contrast relative to the host rock.…”

Universal Relationship Between Viscous and Inertial Permeability of Geologic Porous Media

“…Permeability thus dictates the role of subsurface fluids in many geologic phenomena such as earthquakes (Elkhoury et al, 2006), volcanic eruptions (Okumura et al, 2009), ice sheet drainage (Das et al, 2008), planetary compositional differentiation (Ghanbarzadeh et al, 2017), and weathering of the land surface (Rempe & Dietrich, 2014). Permeability thus dictates the role of subsurface fluids in many geologic phenomena such as earthquakes (Elkhoury et al, 2006), volcanic eruptions (Okumura et al, 2009), ice sheet drainage (Das et al, 2008), planetary compositional differentiation (Ghanbarzadeh et al, 2017), and weathering of the land surface (Rempe & Dietrich, 2014).…”

Acoustic and Microstructural Properties of Partially Molten Samples in the Ice–Ammonia System

“…In a study of ice doped with only trace amount (1-15 ppm) of sulfuric acid, researchers found a pronounced reduction in viscosity compared to undoped ice, suggesting that in the case of ice, the critical melt fraction may be vanishingly small [34]. Even when θ > 60 • , studies have shown that interconnectivity can occur at high enough melt fractions or in irregular media [35,36].…”