Centrifuge assisted percolation of Fe-S melts in partially molten peridotite: Time constraints for planetary core formation

The mechanism which segregates molten Fe-S into metallic cores of planetary bodies is still not fully understood. Due to the high interfacial energy and wetting angle between Fe-S melts and silicate mantle minerals, the continuous percolative flow of such melts cannot be efficient for the core segregation in planetary bodies. A series of percolation experiments has been realized on a partially molten fertile garnet peridotite, employing a centrifuging piston cylinder. A high temperature garnet peridotite with Mg# similar to 0.90 composed of 60 vol.% olivine, 15 vol.% orthopyroxene, 6 vol.% clinopyroxene and 19 vol.% garnet has been used as the silicate matrix. Peridotite powders with the 100-200 or 20-30 mu m grain size were mixed with 5-30 vol.% Fe-S of eutectic composition Fe70S30. The aggregates were centrifuged at 500-700 g at temperatures below and above the melting point of the peridotite. The centrifuge experiments revealed a negligible percolation of Fe-S melts through the unmolten peridotite matrix. Only at T> 1260 degrees C, i.e. above the solidus of the peridotite, and starting with 5 vol.% of Fe70S30 the vertical melt gradient achieved 1-2 vol.%/mm. In samples with 15 vol.% Fe70S30 the vertical separation achieved 2-2.5 vol.%/mm after 10 h of centrifuging at 500 g. An increase in the degree of partial silicate melting in the peridotite leads to an increase of the Fe-S separation rate from the peridotite matrix. Fe-S contents > 10 vol.% cause an increase of the Fe-S melt droplet size and of the effective percolation velocity of Fe-S melt. A threshold dividing fast (> 10 cm per year) and slow percolations (<1 mm per year) of Fe_Smeltis found around 14-15 vol.% of Fe70S30. The experimentally determined permeabilities of Fe-S melt in the unmolten peridotite with 7-10 vol.% of Fe70S30 melt are 10(-18)-10(-19)m(2), which is 2-3 orders of magnitude lower than the values calculated previously from static experiments. The presence of the silicate melt increases the segregation velocity of Fe-S melt in a partially molten peridotite by more than one order of magnitude with respect to the unmolten peridotite matrix. This could provide an effective segregation of Fe-S melt in a planetary mantle down to 2.5 vol.% of residual Fe-S melt. The extremely slow percolation of Fe-S melt in the absence of the partial silicate melting precludes a scenario of metallic core formation via percolation before temperatures allow a substantial partial melting of mantle silicates in planetary bodies. (C) 2009 Elsevier B.V. All rights reserved.