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Abstract
Early in the history of the solar system, planetesimals were differentiated into metallic cores. In some planetesimals, this differentiation took place by percolation of the denser core forming liquid through a lighter solid silicate matrix. A key factor in core formation by percolation is the establishment of a connection threshold of the melt. In this work, we report new results from pore network modeling of 3D microtomographic images of 11 synthetic olivine aggregates containing Fe-FeS melt. Our results demonstrate that a melt volume fraction of 0.14 is required to achieve connectivity of the melt. We also show that surface-tension driven melt segregation during annealing experiments plays an important role in controlling this threshold melt fraction. We also report that, contrary to the generally accepted notion, melt pinch-off is caused by reduction in pore size, rather than melt drainage out of throats. Using the results of our study, we estimate that the peak melt segregation velocity in a planetesimal of 100 km radius can be as high as 41 m/yr and core segregation can be completed in less than 0.5 million years.
Highlights
Planetesimals—parent bodies of meteorites and building blocks of terrestrial planets—were differentiated into metallic cores overlain by a silicate mantle and crust early in the history of the solar system (Rubie, 2007; Weiss and Elkins-Tanton, 2013)
The second mechanism of core formation, percolation of Fe-FeS melts through a solid silicate matrix, operated when the internal temperature of the planetesimal was above the melting point of metallic melts in the Fe-FeS composition space, but below the melting point of the silicates (Stevenson, 1990; Rubie, 2007)
To address the discrepancy between these two sets of observations, we present the first results of direct pore network modeling of melt connectivity from microtomographic images of samples annealed by Solferino et al (2015) and Solferino and Golabek (2018)
Summary
Planetesimals—parent bodies of meteorites and building blocks of terrestrial planets—were differentiated into metallic cores overlain by a silicate mantle and crust early in the history of the solar system (Rubie, 2007; Weiss and Elkins-Tanton, 2013). The heat required to produce such large scale melting could be provided by decay of short-lived radioactive isotopes such as 26Al and 60Fe, energy released due to impact with other planetesimals, or release of gravitational energy arising from the process of core segregation (Stevenson, 1990; Tonks and Melosh, 1990; Rubie, 2007; Elkins-Tanton, 2012). The second mechanism of core formation, percolation of Fe-FeS melts through a solid silicate matrix, operated when the internal temperature of the planetesimal was above the melting point of metallic melts in the Fe-FeS composition space, but below the melting point of the silicates (Stevenson, 1990; Rubie, 2007). The eutectic melting temperature of FeS melts—at which the first melting takes place, independent of relative abundances of Fe and S in the bulk composition—is 988oC at atmospheric pressure
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