A speciation model linking the fate of carbon and hydrogen during coremagma ocean equilibration

International audienceThe core – mantle differentiation in the magma ocean constitutes a major planetary event that involved two elements that are essential to life: carbon (C) and hydrogen (H). These two elements are conventionally classified as volatiles (ie. atmophile), but they can also evolve into being siderophile and lithophile at the extreme conditions found in planetary magma oceans. We report here a model for H and C species dissolved in silicate melts in equilibrium with iron-rich alloys under variable pressure, temperature and redox conditions. This speciation model is able to reconcile and reproduce a large body of experimental data on metal-silicate partitioning for H and C at carbon-saturation and in C-undersaturated systems. At low pressure, we conclude that the prevailing species in a silicate magma ocean are CO2, CO, H2O, H2, whereas CH4 appears to dominate at high pressure. These speciation changes explain recent experimental observations that (i) C evolves from being strongly siderophile at low pressure to moderately siderophile at high pressure, and (ii) H is not siderophile at low pressure but becomes increasingly so as pressure rises. Moreover, it shows that H becomes increasingly siderophile as the total H content of the silicate melt and C-activity are lowered. Despite it offers a promising reconciliation of a large set of experimental and molecular dynamics observations, this model still suffers from large uncertainties when extrapolated to high pressure. In particular, endmember and mixing properties in both the silicate melt and the molten metal must be independently deciphered.The enhanced CH4 stability in the silicate melt at high pressure couples the fate of C and H in deep magma oceans. In such cases, the solubility of C in the basal ocean depends on the H-content and is higher than C-solubility at low pressure. This implies an increase in C activity as the ascending convective cells of the magma ocean, which may cause C saturation as graphite or diamond unless Fe-metal droplets, having a great C-solubility, are present in the shallow magma ocean. Under certain conditions, enhanced siderophile behaviour for H can lead via a runaway process to the desiccation of the magma ocean