Carbon distribution in planet Mercury from magma ocean evolution to graphite crust and core composition.

MESSENGER observations revealed a primary graphite flotation crust on Mercury, implying substantial carbon retention in its magma ocean rather than sequestration into the core. To investigate the conditions enabling this retention, we conducted high-pressure, high-temperature metal-silicate partitioning experiments over a wide range of oxygen fugacities. Carbon behavior is strongly redox dependent: under relatively oxidizing conditions it is highly siderophile, whereas under the reducing conditions relevant to Mercury it becomes significantly less siderophile, promoting carbon retention in silicate melts and graphite crystallization. Modeling of carbon partitioning between the core, mantle, crust, and atmosphere indicates that oxygen fugacities of IW - 6 to IW - 6.5 best reproduce the graphite crust thickness inferred from MESSENGER data. Under these conditions, Mercury's core remains relatively carbon-poor ( < 5000 μ g/g), implying that its density deficit is primarily controlled by other light elements, most likely silicon and sulfur. These results link Mercury's extreme reduction to both its graphite crust and internal chemical structure.