Fractionation of Si Isotopes during Core Formation from First Principles Calculations

Recently, we have developed an efficient ab initio based computational approach to predict equilibrium isotope fractionation factors between chemically and structurally complex crystalline solids, fluids and melts at high T and P [1,2]. Our method has been successfully applied to “nontraditional” stable isotopes of light elements such as B and Li, which are important geochemical tracers widely used in petrology. We have shown that we are able to correctly reproduce the experimentally observed fractionation sequences: fluid-tourmaline-mica for B and staurolite-fluidmica-spodumene for Li isotopes with a computational uncertainty comparable to the experimental error. Moreover, with the calculations we explained the discrepancy between the results of in-situ experiments and measurements on natural samples [2]. With this well tested methodology we investigate here the Si isotope fractionation between silicate melt and liquid metal under the extreme P-T conditions of Earth’s core formation (P~25 GPa, T~3000 K) that are not yet accessible to experiment [3]. The equilibrium Si isotope fractionation factors between these materials are of great interest as they are needed to understand the origin of the difference in the isotopic signatures observed between the Bulk Silicate Earth (BSE) and Chondrite meteorites [3]. It has been proposed that the observed difference reflects the fractionation of Si isotopes between silicate melt and iron-rich metal during the formation of Earth's core [4]. We will present a comparison of our predictions with the available experimental data and discuss the effect of the pressure-induced changes in the melt structure on the fractionation of Si isotopes. Our results shed new light on the origin of the excess of the heavy Si isotopes in BSE and may also help to further constrain the silicon content of the Earth's core. [1] Kowalski, P. M. and Jahn, S. (2011) Geochim. Cosmochim. Acta 75, 6112-6123. [2] Kowalski, P. M. Wunder, B. and Jahn, S. (2013) Geochim. Cosmochim. Acta 101, 285-301. [3] Shahar, A. et al. (2011) Geochim. Cosmochim. Acta 75, 7688-7697. [4] Georg, R. B. et al. (2007) Nature 447, 1102-1106