The effects of pressure on immiscibility in metallic, core‐forming liquids: Implications for protoplanetary differentiation

Mechanisms for metal core formation in rocky planetesimals and planetary embryos remain poorly constrained, in part due to complexities arising from immiscibility in core-forming liquids at low pressures. To assess the pressure dependence of immiscibility and its role in protoplanetary differentiation, we performed experiments at 3 and 5 GPa in the system Fe_0.9Ni_0.1 + S, P, C. Immiscibility arises due to the highly non-ideal nature of light element mixing in Fe liquids, and results in separation of Fe-rich (S-depleted, C-rich, P-rich) and FeS-rich (S-rich, C-rich, P-depleted) liquids. For a broad range of core-forming liquid compositions, a miscibility gap is only present at pressures <5 GPa. With increasing pressure, the behavior of complex systems converges on that of the Fe-S-C ternary, although S-P-C interactions continue to influence metal liquid compositions and stability. Comparison with planetary core compositions derived from meteorite data suggests that immiscibility can play a significant role during melting in planetesimals, and up to medium-sized planetary embryos. In turn, the importance of immiscibility during differentiation depends on the extent of melting, mechanisms for core formation, and corollary degree of light element loss from planetary bodies.