Hysteresis in melt network topology allows core formation by porous flow

Abstract:

The formation of the core via percolation is an attractive pocess to form planetary cores early in the planets evolution. Even magma ocean scenarios require percolation of the iron-sulfide melt through underlying silicate cumulates. There is currently a debate whether the dihedral angle of iron-sulphide melts in an olivine matrix is low enough to allow the formation of a percolating network. We present first computations of equilibrium melt distributions in realistic irregular grains and show that the percolation threshold at dihedral angles above 60 degrees is significantly larger than those previously reported for simple geometries. This explains why the percolation cut-off at 60 degress is such an effective barrier to porous flow at low porosities. However, given typical compositions of the terrestrial planets initial porosites after the onset of melting of iron are large, 20-40%, and likely to overcome the precolation threshold to form a connected melt network. As the porosity decreases due to melt segregation the network remains connected and allows core formation by porous flow. Only as the porosites approach 1% does the network snapp-off and the iron become isolated in pockets along triple junctions. This residual iron may provide an explanation for the formation of dense layers near the core mantle boundary such as D”.