Experimental determination of melting relations in Fe-S-O system and its subsystems under Mars’ core conditions

Space missions, along with ground-based observations, are providing unprecedented geophysical data regarding the interiors of the telluric planets in the solar system. Results from the Insight lander mission indicate that Mars has a large core, mostly, if not entirely, molten, composed of an iron alloy rich in light elements. Chemical analysis of Martian meteorites and planetary differentiation models point to sulfur and oxygen as the most abundant light elements in the core. Yet, the phase diagram and the thermo-elastic properties of solid and liquid alloys in the ternary Fe-S-O system under the pressure and temperature conditions of the Martian core remain largely unconstrained.We thus investigated the Fe-S-O system and its subsystems by performing X-ray diffraction measurements at the PSICHÉ beamline of the SOLEIL synchrotron using laser-heated diamond-anvil cells. Data were collected on FeS and FeO end-members, as well as on alloys in the Fe-O binary and Fe-S-O ternary systems, in the 10-85 GPa range up to 4000 K. The ability to control the shape of the heating laser combined with temperature mapping enabled by the 4-color pyrometry system, ensured homogenous heating and precise temperature determination. Melting was constrained by tracking the appearance and evolution of the diffuse scattering signal typical of liquids, along with parallel assessment of discontinuities in the optical properties of the investigated samples.In this presentation, we will outline the developed experimental protocol and present the subsolidus phase diagram and melting curves obtained for FeS and FeO as well as the eutectic melting curve for the Fe-O binary system. Preliminary results for the Fe-S-O ternary system will also be shown. Our results will be compared with previous determinations, addressing ongoing controversies and providing a foundation for an improved understanding of the melting relations in the Fe-S-O ternary system under the conditions of the Martian core.