Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Soheil Ghanbarzadeh, Marc A. Hesse, and Maša Prodanović

PNAS 2017 ; published ahead of print December 4, 2017
doi: 10.1073/pnas.1707580114



It has long been thought that percolative core formation is prevented by high dihedral angles that trap the majority of the metallic phases in the mantle, even if the percolation threshold is overcome. Here we use pore-scale simulations to show that hysteresis in melt network topology allows the melt to remain connected during drainage, suggesting that percolative core formation is possible on bodies that contain enough metallic phases to overcome the percolation threshold.”

“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. “