Computational Insights on Carbonate-Silicate-Metal Melt Behavior in the Lower Mantle

Abstract:

Carbonate melt is an important carbon-bearing phase in the mantle. While its role in the upper mantle has been well studied, its importance in the lower mantle is less understood. The behavior of carbonate melt in the lower mantle is complicated by pressure, temperature, and fO₂ conditions, as well as reactions with common lower mantle phases such as MgSiO₃ and metallic iron. We performed ab initio molecular dynamics simulations on an Mg₂₄Si₁₂C₁₂O₇₂Fe₁₃ melt composition (i.e. 12 MgCO₃ + 12 MgSiO₃ + 13 Fe) at conditions up to 148 GPa and 4000 K to understand speciation and coordination of carbonate melts in the lower mantle and at the core-mantle boundary. We find a rich diversity of species in our carbonate-silicate-metal melt, with our system displaying various carbon bonding environments. We determine that as the overall coordination of carbon increases with pressure, the abundance of C-C and C-Fe bonds increases with pressure and is compensated by a decrease in C-O and C-Si bonding. We evaluate the implications of these chemical and structural changes for the storage and cycling of carbon in Earth’s mantle.