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

Abstract As an important component of carbonates in the mantle, CaCO 3 is a major carrier of carbon from the surface to the deep interiors. In recent years, new varieties of CaCO 3 polymorphs have been continuously predicted by first principles simulations and verified by experiments. The findings of these polymorphs open the possibility of stabilizing CaCO 3 component in the lowermost mantle. Here, through extensive first principles simulations, we inspect the stability and reactions of high‐pressure CaCO 3 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 CaCO 3 polymorphs with mantle minerals. In particular, the tetrahedrally structured CaCO 3 ‐polymorph (space group P 2 1 /c) is found to be sensitive to temperature with a positive Clapeyron slope of 15.81(6) MPa/K. Reacting with MgSiO 3 , CaCO 3 is shown to be less stable than MgCO 3 over the whole mantle pressures (to ~136 GPa) above ~1500 K. And CaCO 3 is demonstrated to readily react with SiO 2 even in the cold subduction slabs. Thus, high temperature greatly increases the tendency of partitioning calcium into the silicates, and CaCO 3 is not likely to be the major host of carbon in the Earth's deep mantle.