Connectivity of core forming melts: Experimental constraints from electrical conductivity and X-ray tomography

The formation of a metallic core is one of the most profound events in the early evolution of a planet. Although there is much evidence that favors a very hot and deep magma ocean for the major core formation event on Earth, many smaller planetesimals likely never became hot enough to generate the wide-scale melting required for such a scenario. Furthermore, there is evidence that planetesimal cores formed rapidly (within 3 My). An inefficient percolative flow mechanism has been suggested to be viable for systems that have a metallic melt fraction in excess of the percolation threshold (approximately 5 vol%), provided that the permeability of these connected melts is high enough to remove the majority of the core liquid from the silicate matrix in such a relatively short time span. More accurate knowledge of the permeability of core forming melts requires a detailed understanding of how the melt is connected in three dimensions, and the complex relationships between melt volume, connectedness and permeability. In this study, we calculated the permeability of core forming metallic liquids (Fe67S33) within a silicate matrix by lattice-Boltzmann simulations of flow through digital volumes generated from three-dimensional, synchrotron-based X-ray tomographic images of experimental run samples synthesized at 1300 °C and 1 GPa. Electrical conductivity measurements were also conducted on the same pre-synthesized samples to independently determine the percolation (connectivity) threshold. The percolation threshold of these samples was determined to be higher than some earlier measurements (>6 vol%), and the calculated permeability is substantially lower than previously estimated. As a consequence, although percolation still appears viable for some planetesimal sized objects, it may be a secondary mechanism, acting in conjunction with flow induced through other processes such as deformation or more wide spread melting.

► We measure connectivity using tomography, 2-D imaging and electrical conductivity.
► Our results show pinch off at 3–4 vol% and full connection at ∼10 vol%.
► Permeability/migration velocity calculations are consistent with fast core formation.
► The different analytical methods are complementary to each other.