Connectivity of molten Fe alloy in peridotite based on in situ electrical conductivity measurements: Implications for core formation in terrestrial planets

The connectivity of molten Fe-S in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 GPa. Starting materials were powdered mixtures of peridotite KLB-1 with various amounts (0, 3, 6, 13, 19, 24 vol.%) of the 1 GPa eutectic composition in the Fe-FeS binary system. At temperatures above the eutectic point in the Fe-FeS system (∼980 °C) and below the solidus of KLB1 (∼1200 °C), molten Fe-S in a solid silicate matrix interconnects when the volume fraction is over ∼5%. Conductivity-temperature paths indicate that in the presence of partial silicate melting the connectivity of molten Fe-S in a peridotite matrix is inhibited. Based on observations of retrieved samples, the percolation threshold of Fe-S melts in the presence of low to moderate degrees of silicate melt is estimated at 13±2 vol.%. These results indicate that if the volume fraction of Fe-alloy in a planetesimal was initially greater than 5%, and if early heating by decay of radionuclides raised the temperature of the interior above the Fe-alloy melting point, initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix. These conditions were likely met by many terrestrial objects in the early solar nebula. Efficient removal of residual Fe-alloy (5 vol.%) from silicate requires high-degree melting of silicate so that metal can segregate as droplets. Giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high-degrees of melting. Alternatively, if initial metal segregation were delayed until a planetary object grew to large size (∼1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean.