Stability and Reactions of CaCO3 Polymorphs in the Earth's Deep Mantle

As an important component of carbonates in the mantle, CaCO3 is a major carrier of carbon from the surface to the deep interiors. In recent years, new varieties of CaCO3 polymorphs have been continuously predicted by first principles simulations and verified by experiments. The findings of these polymorphs open the possibility of stabilizing CaCO3 component in the lowermost mantle. Here, through extensive first principles simulations, we inspect the stability and reactions of high-pressure CaCO3 polymorphs at high temperatures. Systematic errors from approximations to the exchange-correlation functional in density functional theory have been essentially eliminated with a generalized rescaling method to increase the predictability of the simulations. We find temperature has important effects on the stabilities and the reactions of CaCO3 polymorphs with mantle minerals. In particular, the tetrahedrally structured CaCO3-polymorph (space group P2(1)/c) is found to be sensitive to temperature with a positive Clapeyron slope of 15.81(6) MPa/K. Reacting with MgSiO3, CaCO3 is shown to be less stable than MgCO3 over the whole mantle pressures (to similar to 136 GPa) above similar to 1500 K. And CaCO3 is demonstrated to readily react with SiO2 even in the cold subduction slabs. Thus, high temperature greatly increases the tendency of partitioning calcium into the silicates, and CaCO3 is not likely to be the major host of carbon in the Earth's deep mantle.