Sulfur-controlled iron isotope fractionation experiments of core formation in planetary bodies

A series of high pressure and temperature experiments were conducted to better constrain the Fe isotope fractionation during core–mantle differentiation in planetesimal and planetary bodies. Synthetic mixtures of oxides and metal having varying amounts of sulfur, approximating terrestrial and Martian compositions, were melted at 1–2 GPa and 1650 °C. Iron isotopic equilibrium between the resulting metal and glass run products was verified for all experiments using the three-isotope technique. Purified Fe from metal and glass was analyzed by multiple-collector ICP-MS in high resolution mode. Iron alloy and silicate glass show a well-resolved Δ57Femetal–silicate of +0.12 ± 0.04‰ in a sulfur-free system. Isotope fractionation increases with sulfur content to +0.43 ± 0.03‰ at 18 wt.% sulfur in the metal. These results cannot be easily interpreted within the context of known Fe isotope ratios in most natural samples of planetary and asteroidal mantles and therefore suggest more complex processes affected the Fe isotope fractionation therein. However, to reconcile Martian meteorite iron isotopic signatures with geophysical models using this new experimental data requires a smaller amount of sulfur in the Martian core than previous estimates, with an upper limit of ∼8 wt.%.