^1,2Juliana G. Peckenpaugh, ²Meryem Berrada, ¹Peng Jiang, ²Bin Chen
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.06.033]
¹Department of Earth Sciences, University of Hawaiʻi at Mānoa, Honolulu, HI 96822, USA
²Hawaiʻi Institute of Geophysics and Planetology, University of Hawaiʻi at Mānoa, Honolulu, HI 96822, USA
Copyright Elsevier

Mercury’s highly reduced formation conditions likely promoted the incorporation of silicon into its metallic core during planetary differentiation. The resulting Si-rich core composition would reduce the capacity of the metallic liquids to dissolve carbon, making carbon saturation more likely as the core cooled and crystallized. Determining the solubility of carbon in Fe-Si liquids under Mercury’s core conditions is therefore essential for evaluating whether graphite or diamond could precipitate from the core and influence Mercury’s thermal and magnetic evolution. In this study, high-pressure and high-temperature experiments were conducted on carbon-saturated Fe-Si alloys with varying silicon content from 4 to 27 wt% using a multi-anvil press at 5–20 GPa and 1673–1873  K. The analyses of the recovered samples by Scanning Electron Microscope (SEM) and Raman spectroscopy show the Fe-Si-C liquids with precipitated carbon phases in the form of graphite or diamond. Quantitative electron probe microanalysis (EPMA) results demonstrate a trend of decreasing carbon content in the Fe-Si-C alloys with increasing silicon content, described by the following equation: CFe-Si = 8.45–––0.663[Si] + 0.0134[Si]2. Higher initial silicon content within Mercury’s core due to the highly reduced conditions may result in lower solubility of carbon, suggesting a potential mechanism for the preferential exsolution of carbon-rich materials, such as graphite or diamonds. This finding suggests that carbon could precipitate within Mercury’s cooling, solidifying core under Mercurian core compositions and conditions. The segregation and accumulation of carbon could modify heat flux across the core-mantle boundary, affecting both the strength of Mercury’s early magnetic field and the longevity of dynamo action. Such compositional convection could sustain a dynamo, contributing to Mercury’s present-day magnetic field.