Carbonate melts in the Earth's mantle

We perform a molecular dynamics study of the properties of the carbonated silicate melts at realistic thermodynamic conditions of the Earth’s mantle. We employ the Qbox package based on a highly efficient plane wave and pseudopotentials implementation of density-functional theory. We work on three distinct compositions: Mg₂SiO₄, 16Mg₂SiO₄+CO₂ and 16Mg₂SiO₄+MgCO₃ and study the effect of the carbonization on the melt properties as well as the difference in effects between the CO₂ molecule and the CO_3^2- anionic group. We focus on the Earth-relevant isotherm at 3000K. At ambient pressure the silicon is in tetrahedral coordination as SiO₄ with no polymerization between the tetrahedra. The C atoms are the most mobile in the system followed by O. The diffusion of the CO₂ molecule takes place through intermediate short-lived CO_3^2- states. In agreement with previous studies on pure magnesium silicate melts the polymerization of the tetrahedra is enhanced by pressure; the onset of the five-fold coordination of the silicon atoms occurs after 40 GPa. The thermal dilatation of the CO₂-bearing fluid is 17kbars/1000K at ambient pressure and 3000K. The density differences due to the addition of CO₂ and of MgCO₃ to the Mg₂SiO₄ melts are small at ambient pressures and 3000K. Most significantly, we find that independent linear CO₂ molecules at low pressures change to CO₃ groups that are part of the melt structure with increasing pressure.