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

Abstract

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 P21/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 ~136 GPa) above ~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.

Polymorphism of feldspars above 10 GPa

“…Indeed, according to our results, crystalline plagioclases composing a major portion of a subducting crust may exist under the P-T conditions of (ultra)cold subducting slabs [41][42][43] , as shown in Fig. 4.…”

“…Phase relations in feldspars of the anorthite-albite series as revealed by multi-anvil experiments in the present study and in the work of Kubo et al30 . Cold slab and very cold slab geotherms are from Zhang et al41 . The dashed grey line represents the amorphization line of labradorite with a composition of Ab 45.0 An 51.8 Or 3.230 .…”

Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle

“…Previous experiments 24,25 indicate that CaCO 3 reacts with silicates to form MgCO 3 via the forward reaction up to 80 GPa and 2300 K, i.e., at least to the mid-lower mantle. Theoretical studies further predict that MgCO 3 + CaSiO 3 are enthalpically favored over CaCO 3 + MgSiO 3 throughout the lower mantle pressure and temperature regime [26][27][28][29][30] . However, although many studies have addressed the stability of individual carbonates up to higher pressures [31][32][33] , no experiments examined the carbonate-silicate cation exchange reaction up to core-mantle boundary conditions.…”

Comprehensive Density Functional Theory Studies of Vibrational Spectra of Carbonates

“…Like calcites, aragonites form solid solutions [ 49 ]. Studies of isostructural orthorhombic carbonates are important for understanding phase transition sequences under pressure and temperature and, therefore, can provide insight into a carbon behavior in the Earth mantle [ 50 , 51 , 52 , 53 ].…”