Carbonate melts have been proposed to exist in the lower mantle, but
their interaction with other lower mantle melt compositions is poorly
understood. To understand miscibility in the carbonate-silicate-metal
melt system, we simulate endmember, binary, and ternary melt mixtures
and study how their Gibbs free energies of mixing evolve with pressure.
We find that carbonate-metal and carbonate-silicate melts have
miscibility gaps that close with increasing pressure, while
silicate-metal melts are immiscible at all lower-mantle pressures.
Extending this analysis to the core-mantle boundary, we suggest three
miscible melt fields near the endmember carbonate, silicate, and iron
melt compositions. Analysis of the densities of these miscible melt
compositions indicates that some carbonate-rich and some silicate-rich
melt compositions are gravitationally stable at the core-mantle boundary
and could be candidate compositions to explain ultra-low velocity zones.
Additionally, we evaluate the speciation of an example immiscible melt
composition at various pressures throughout the mantle and identify
reduced carbon species that would be expected to form in the melt. Our
analysis reveals that a majority of Earth's carbon could have been
transported to the core during core-mantle differentiation and that much
of Earth's carbon may be stored in the deep interior today.