Connectivity of molten Fe alloy in peridotite based on in situ electrical conductivity measurements: implications for core formation in terrestrial planets Takashi Yoshino * , Michael J. Walter 1 , Tomoo Katsura Institute for Study of the Earth’s Interior, Okayama University, Yamada 827, Misasa, Tottori 682-0193, Japan Received 2 September 2003; received in revised form 2 December 2003; accepted 5 March 2004 Abstract 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 ( f 980 jC) and below the solidus of KLB1 ( f 1200 jC), molten Fe – S in a solid silicate matrix interconnects when the volume fraction is over f 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 F 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 ( f 1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean. D 2004 Elsevier B.V. All rights reserved. Keywords: connectivity; core formation; electrical conductivity; FeS; wetting property 1. Introduction Terrestrial planets have metallic cores (see review by [1]). Isotopic evidence from iron and rocky mete- orites indicate that core formation occurred early in some parent bodies, within a few million years of t o (e.g. [2–4]). The mechanism of metal segregation in planetary objects has been poorly understood. Theo- retical models and experimental data both indicate that 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.010 * Corresponding author. Department of Earth and Environmen- tal Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. Tel.: +1-518-276-8827; fax: +1-518-276-2012. E-mail address: yoshta@rpi.edu (T. Yoshino). 1 Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK. www.elsevier.com/locate/epsl Earth and Planetary Science Letters 222 (2004) 625 – 643