Abstract

Co-authors: Soheil Ghanbarzadeh (University of Texas at Austin), Masa Prodanovic (University of Texas at Austin)

Pore-scale melt distribution is thought to evolve towards textural equilibrium. The topology of these pore-scale melt networks is controlled by the porosity and the dihedral angle at the contact line between two solid grains and the melt. I will present three-dimensional computations of texturally equilibrated melt networks in realistic poly-crystalline materials that have been obtained from x-ray diffraction contrast tomography. Our simulations show strong hysteresis in the topology of melt networks with large dihedral angles. A percolation threshold at dihedral angles above 60 degrees is generally thought to prevent rapid core formation in planetesimals by segregation of metallic melts. However, primitive achondrites show that the incipient melt fractions are between 25% and 35% and that metallic melt is connected despite dihedral angles of approximately 90 degrees. Our simulations show that hysteresis allows such a high porosity and high dihedral angle melt network to rem ain connected during drainage of the metallic melt. This provides a mechanism for rapid core formation in planetesimals by porous flow. Only a very small melt fraction, approximately 1%, is trapped and left behind in the silicate mantle after melt segregation. This may provide an explanation for the “excess siderophile problem” in the Earth.