Carbon-bearing silicate melt at deep mantle conditions

Knowledge about the incorporation and role of carbon in silicate magmas is crucial for our understanding of the deep mantle processes. CO₂ bearing silicate melting and its relevance in the upper mantle regime have been extensively explored. Here we report first-principles molecular dynamics simulations of MgSiO₃ melt containing carbon in three distinct oxidation states - CO₂, CO, and C at conditions relevant for the whole mantle. Our results show that at low pressures up to 15 GPa, the carbon dioxide speciation is dominated by molecular form and carbonate ions. At higher pressures, the dominant species are silicon-polyhedral bound carbonates, tetrahedral coordination, and polymerized di-carbonates. Our results also indicate that CO₂ component remains soluble in the melt at high pressures and the solution is nearly ideal. However, the elemental carbon and CO components show clustering of carbon atoms in the melt at high pressures, hinting towards possible exsolution of carbon from silicate melt at reduced oxygen contents. Although carbon lowers the melt density, the effect is modest at high pressures. Hence, it is likely that silicate melt above and below the mantle transition zone, and atop the core-mantle boundary could efficiently sequester significant amounts of carbon without being gravitationally unstable.