Structures and Transport Properties of CaCO3 Melts under Earth's Mantle Conditions

Carbonatitic and carbonated silicate melts generated from melting in the mantle are the chief agents for liberating carbon from the solid Earth and exert important controls on Earth's deep carbon cycle. However, significant gaps in our knowledge about the carbonatitic melts under conditions pertinent to Earth's deep interior remain due to experimental challenges. Here, we report on a first-principles molecular dynamics (FPMD) calculation of calcium carbonate (CaCO3) melts at pressures up to 52.5 GPa. Our FPMD calculations reproduce the ultralow viscosity measured by experiments and confirm the ideal liquid behavior of calcium carbonate melts at pressures below 11.2 GPa. However, calcium carbonate melts are characterized by a pronounced nonideal liquid behavior at pressures above 11.2 GPa, arising from (1) the temporal formation of small carbonate clusters and (2) increased interactions between Ca2+ and CO32- ions. It is found that the Stokes Einstein equation relating the viscosity with the diffusion coefficients still holds at high pressure provided that a suitable effective particle size can be chosen.