Light Elements Exsolution in the Fe–Si–C–(H) System of Terrestrial Planet Liquid Cores

Light elements are thought to be essential components of liquid cores in terrestrial planets and play a key role in core formation, chemical evolution, and the generation of planetary magnetic fields. In multicomponent iron–light element (Fe–LE) systems, when multiple light elements coexist in liquid iron, their solubilities are mutually constrained, forming an anti-correlated solubility relationship, referred to here as simultaneous solubility.

Here we investigate the simultaneous solubility and exsolution behavior of light elements in the Fe–Si–C–(H) system using a combination of high-pressure and high-temperature experiments and machine-learning force field accelerated molecular dynamics (MLFF-MD) simulations. Multi-anvil experiments conducted at pressures of 9–21 GPa and temperatures of 1400–2200 °C reveal that these light elements can dissolve simultaneously in liquid iron and exhibit simultaneous solubility limits, with exsolution of Si, C, and H observed during melting and quenching. Complementary MLFF-MD simulations of the Fe–Si–C system provide atomic-scale insights into light element interactions in metallic melts and reproduce the experimentally observed anti-correlated solubility trends under core-relevant conditions.

By combining experimental and computational results, we derive simultaneous solubility relationships in the Fe–Si–C–(H) system and show how they vary with temperature and pressure. These results suggest that in reduced planetary cores, such as those of Mercury and Earth, Si, C, and H may coexist as simultaneously dissolved light elements. As the liquid core cools, the progressive decrease in simultaneous solubility drives continuous exsolution of light elements, providing an additional potential energy source for core dynamics and offering a potential explanation for chemical heterogeneity at the core–mantle boundary (CMB).