The effect of compositional fluctuations in a liquid Fe–O alloy on the nucleation of Earth’s inner core

The Earth’s solid inner core plays a fundamental role in determining the past and present properties and dynamics of the Earth’s deep interior. Inner core growth powers the geodynamo, producing the protective global magnetic field, and provides a record of core evolution spanning geological timescales. However, the origins of the inner core remain enigmatic. Traditional core evolution models assume that the inner core formed when the core first cooled to its melting temperature, but this neglects the physical requirement that liquids must be supercooled to below their melting point before freezing. Prior estimates from mineral physics calculations of the supercooling δT required to homogeneously nucleate the inner core from candidate binary alloys exceed constraints of δT ≲ 400 K inferred from geophysical observations, while a plausible scenario for heterogeneous nucleation has yet to be identified. Here we consider a different possibility, that atomic-scale compositional fluctuations can increase the local melting temperature, and hence supercooling, available for homogeneous nucleation. Using molecular dynamic simulations of Fe-O alloys we find that compositional fluctuations producing O-depleted regions are too rare to aid nucleation, while O-enriched regions can reduce the undercooling by ∼50 K (δT ∼ 700 K) for a bulk concentration of 20 mol.% O or ∼400 K (δT ∼ 300 K) for a bulk concentration of 30 mol.% O. While these results do not explain the nucleation of Earth’s inner core, they do show that compositional fluctuations can aid the process of homogeneous nucleation.