Melting diagrams of Fe-rich alloys determined from synchrotron in situ measurements in the 15–23 GPa pressure range

We report in situ observations of the melting behaviour of iron alloyed with 10–20 at.% C, O, S, or Si at pressures between 15 and 24 GPa, using X-ray diffraction in a multi-anvil press (SPring8). The degree of partial melting of the iron alloys has been quantified from analysis of the intensity of diffuse X-ray scattering of molten iron as a function for decreasing temperature with a 50° step. Coupled with microanalysis of recovered samples, the in situ observations bring direct constraints on shape and positions of liquidus and solidus curves in the melting diagrams. e Fe–S system, our results are in good agreement with previous works. We observe that the eutectic temperature increases from 1023 K at 15 GPa to 1123 K at 20.6 GPa and that the eutectic composition decreases with increases pressure. Concerning the Fe–C system the eutectic temperature of 1460 K at 20.7 GPa falls slightly below a linear extrapolation of the previous work. In the case of the Fe–Ni–Si system and the Fe–O system, we find eutectic temperatures significantly lower than previously reported. For the two systems, both eutectic temperature and composition increase with increasing pressure in the 15–20 GPa range. Compare to previous work, we observe eutectic compositions (a) richer in light elements in the Fe–O system, with 9.0 and 10.5 wt% O at 16.5 and 20.5 GPa, respectively, and (b) poorer in the Fe–Ni–Si system with 11.5 wt% Si at 16.9 GPa. firm very high solubility of Si and C with solid iron, and report a Si partitioning coefficient of 1.3(2) at 16.9 GPa. The S and O solubility in solid iron appears very small. Therefore, both S and/or O could explain density jumps between liquid outer and solid inner parts of planetary cores, at least up to ∼25 GPa.