High-pressure melting relations in Fe–C–S systems: Implications for formation, evolution, and structure of metallic cores in planetary bodies

Abstract

We present new high-pressure temperature experiments on melting phase relations of Fe–C–S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe–5 wt% C–5 wt% S and Fe–5 wt% C–15 wt% S at 2–6 GPa and 1050–2000 °C in MgO capsules and on Fe–13 wt% S, Fe–5 wt% S, and Fe–1.4 wt% S at 2 GPa and 1600 °C in graphite capsules. Our experiments show that: (a) At a given P– T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5–6 GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe–C–S ternary is significantly different compared to the binary liquidus in the Fe–C and Fe–S systems. At 6 GPa, the liquidus of Fe–5 wt% C–5 wt% S is 150–200 °C lower than the Fe–5 wt% S. (d) For Fe–C–S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe 3C at 2 GPa and Fe 7C 3 at 6 GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6 GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core–mantle boundary (CMB) pressures of ∼8, ∼5, and 7 GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ⩾136 GPa, ⩾100 GPa, and ⩾23 GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures <6 GPa and have a molten core or outer core, settling of denser carbide-rich liquid or flotation of lighter, sulfide-rich melt may contribute to an early, short-lived geodynamo.