Equation of state of molten iron-10wt% sulfur: Implications for the composition of metallic cores of planetary bodies

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The Equation of State (EOS) of molten Fe-10wt%S was investigated by means of static high pressure density measurements using a newly developed variation of the sink/float technique. The method involves the use of tailored density composite spheres made out of a Pt or WC core and an Al2O 3 (ruby/sapphire) mantle, as bracketing markers for comparative density measurements of liquid Fe-10wt%S, at pressures up to 20 GPa and at temperatures up to 2123 K. The composite spheres permit tailored densities for use in a variety of liquid compositions of different densities. The inert mantle also prevents chemical reaction with molten metallic samples. Isothermal compression data were obtained at 1773 K, 1923 K and 2123 K. Density of the composite spheres at high pressures and temperatures was derived from post-experiment determination of the ratio between the metal core and ruby/sapphire mantle, and the known compressibilities and thermal expansion coefficients of Al 2O3, Pt and WC. The results of isothermal data confirm, within the experimental error, the validity of extrapolating to 20 GPa an EOS obtained earlier using an X-ray absorption technique by Sanloup et al. at pressures up to 6 GPa. The analysis of new data obtained in the 1773 K 2123 K interval up to 20 GPa, revealed the need for a well- determined melt density value at 1 atm, as an anchoring point for constructing a reliable EOS. For a 1 atm density value of 5.48 g/cm3, different EOS formulations yielded bulk modulus values K0T between 53.9 GPa and 56.2 GPa, and K0T' values between 5.8 and 7.1. The results of density measurements also showed that the effect of temperature on the density is negligible in the 1773 K 2123 K interval. Application of these results to the identification of composition of liquid metallic cores in planetary bodies, such as Ganymede, Io and Mars, yielded Fe-10wt%S to be a possible candidate in all these planetary bodies. A different EOS was used to extrapolate the results to the pressure and temperature conditions of the Earth's core. At 4273 K, the extrapolation made using the Poirier and Tarantola EOS gave a density value closest to the density at the core-mantle boundary pressure of 136 GPa. At 5773 K and 329 GPa, the extrapolation produced with the Vinet EOS resulted in a density value bracketed by the densities offered by Preliminary Reference Earth Model for the bottom of the liquid outer core and the solid inner core. Such results can be interpreted in the support of a core of the Earth containing S as a dominant light element.

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