Author's personal copy Experimentally determined Si isotope fractionation between silicate and Fe metal and implications for Earth's core formation Anat Shahar a,b, ⁎, Karen Ziegler c , Edward D. Young a,c , Angele Ricolleau b , Edwin A. Schauble a , Yingwei Fei b a Department of Earth and Space Sciences, University of California Los Angeles, 595 Charles Young Drive East, Los Angeles, CA 90095, USA b Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC 20015, USA c Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 3845 Slichter Hall, Los Angeles, CA 90095, USA abstract article info Article history: Received 20 April 2009 Received in revised form 14 August 2009 Accepted 12 September 2009 Available online 12 October 2009 Editor: L. Stixrude Keywords: core formation silicon isotope metal–silicate partitioning high pressure high-temperature Stable isotope fractionation amongst phases comprising terrestrial planets and asteroids can be used to elucidate planet-forming processes. To date, the composition of the Earth's core remains largely unknown though cosmochemical and geophysical evidence indicates that elements lighter than iron and nickel must reside there. Silicon is often cited as a light element that could explain the seismic properties of the core. The amount of silicon in the core, if any, can be deduced from the difference in 30 Si/ 28 Si between meteorites and terrestrial rocks if the Si isotope fractionation between silicate and Fe-rich metal is known. Recent studies (e.g., [Georg R.B., Halliday A.N., Schauble E.A., Reynolds B.C., 2007. Silicon in the Earth's core. Nature 447 (31), 1102–1106.]; [Fitoussi, C., Bourdon, B., Kleine, T., Oberli, F., Reynolds, B. C., 2009. Si isotope systematics of meteorites and terrestrial peridotites: implications for Mg/Si fractionation in the solar nebula and for Si in the Earth's core. Earth Planet. Sci. Lett. 287, 77–85.]) showing (sometimes subtle) differences between 30 Si/ 28 Si in meteorites and terrestrial rocks suggest that Si missing from terrestrial rocks might be in the core. However, any conclusion based on Earth–meteorite comparisons depends on the veracity of the 30 Si/ 28 Si fractionation factor between silicates and metals at appropriate conditions. Here we present the first direct experimental evidence that silicon isotopes are not distributed uniformly between iron metal and rock when equilibrated at high temperatures. High-precision measurements of the silicon isotope ratios in iron–silicon alloy and silicate equilibrated at 1 GPa and 1800 °C show that Si in silicate has higher 30 Si/ 28 Si than Si in metal, by at least 2.0‰. These findings provide an experimental foundation for using isotope ratios of silicon as indicators of terrestrial planet formation processes. They imply that if Si isotope equilibrium existed during segregation of Earth's core-forming metal and silicate mantle, there should be an isotopic signature of Si in the core. Our experiments, combined with previous measurements of Si isotope ratios in meteorites and rocks representing the bulk silicate Earth, suggest that the formation of the Earth's core imparted a high 30 Si/ 28 Si signature to the bulk silicate Earth due to dissolution of ~6 wt% Si into the early core. © 2009 Elsevier B.V. All rights reserved. 1. Introduction As early as 1952, Francis Birch noted that seismic velocities were inconsistent with an outer core of pure iron and nickel; the core exhibits a density deficit of about 5 to 10% (Birch, 1952). In that paper, and the one succeeding it (Birch, 1964), Birch suggested that the low density was due to the presence of one or more light elements dissolved in Fe–Ni alloy. Candidate elements include carbon, hydrogen, sulfur, silicon, and oxygen. Since then many studies have sought to determine which of these elements are responsible for lowering the density of the core (see Li and Fei, 2004, for a review). Chondritic meteorites (the most primitive rocks of the solar system) are thought to be a first-order proxy for the bulk composition of Earth because the relative abundances of refractory major elements comprising these rocks are indistinguishable from those of the Sun. Chondritic ratios of rock-forming elements are also found for dust surrounding other stars (Zuckerman et al., 2007). On close examina- tion, however, one finds that the bulk silicate Earth (BSE) has a greater Mg/Si than chondrites by ~ 15% (Ringwood, 1989). Since volatility was a primary mechanism for fractionating elements in the early solar system, and silicon and magnesium have similar volatilities, this discrepancy is unexpected. There are several possible explanations for the super-chondritic Mg/Si, including non-chondritic starting materi- als, or sequestration of Si into the core. A deficit in Si in bulk silicate Earth (as opposed to Mg-excess) is supported by super-chondritic Al/ Si of Earth relative to chondrites (Palme and O'Neil, 2003). Geochemical arguments for the light element composition of the core based on calculations of mass balance assuming a chondritic Earth and Planetary Science Letters 288 (2009) 228–234 ⁎ Corresponding author. Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC 20015, USA. Tel.: +1 202 478 8910. E-mail address: ashahar@ciw.edu (A. Shahar). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.09.025 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl