Ideal FeFeS, FeFeO phase relations and Earth's core

Abstract

Liquid-state and solid-state model fits to melting data for Fe, FeS and FeO provide constraints for calculating ideal phase relations in FeFeS and FeFeO systems in the pressure range corresponding to the Earth's outer core. The liquid-state model fit to the Fe melting data of Williams and Jeanloz places constraints on the temperature and other properties of Fe along the liquidus beyond the range of their data. The temperature along the best-fit Fe liquidus reaches 5000 K at 136 GPa and 7250 K at 330 GPa, which is somewhat lower than that implied by the Hugoniot results (∼7800 K at 330 GPa). This discrepancy may be due to reshock in experimental targets, or some inaccuracy in the extrapolation, presuming the Hugoniot results represent the equilibrium melting behavior of Fe. Constraints on the solidi of FeS and FeO from the comparison of data and solid-state model calculations imply that FeS and FeO melt at ∼4610 and 5900 K, respectively, at 136 GPa, and ∼6150 and 8950 K, respectively, at 330 GPa. Calculations for the equilibrium thermodynamic properties of solid and liquid Fe along the coincident solidus and liquidus imply that the entropy of melting for Fe is approximately independent of pressure at a value of approximately R (where R is the gas constant), while the change in the molar heat capacity across the transition increases with pressure from ∼0.5 R to 4 R between standard pressure and 330 GPa. We use these constraints to construct ideal-mixing phase diagrams for FeFeS and FeFeO systems at outer core pressures, assuming that Fe and FeS, or Fe and FeO, respectively, are the solid phases in equilibrium with the liquid FeFeS or FeFeO mixtures, respectively. Calculated FeFeO eutectic compositions at 330 GPa (15–20 mol% O) are <25 mol% O, while calculated FeFeS eutectic compositions at 330 GPa (23–30 mol% S) are generally >25 mol% S. Combined with density considerations, these calculations imply that an O-rich outer core is more likely to lie on the FeO-rich side of the FeFeX eutectic, while an S-rich outer core is more likely to lie on the Fe-rich side of the FeFeX eutectic. In addition, eutectic temperatures in both systems are ≳5000 K at 330 GPa. Widely accepted temperature profiles for the outer core, ranging from ≲3000 K at the 136 GPa, the core-mantle boundary, to ≲4200 K at 330 GPa, the outer-inner core boundary, are ⩾800 K below this value. In the context of the outer-inner core boundary-phase boundary hypothesis, this discrepancy implies that at least one boundary layer of ⩾1000 K exists in the mantle, possibly at its base in the D″ region.