Stability of calcium and magnesium carbonates at Earth's lower mantle thermodynamic conditions

Abstract

We present a theoretical investigation, based on ab initio calculations and the quasi-harmonic approximation, on the stability properties of magnesium (MgCO 3 ) and calcium (CaCO 3 ) carbonates at high temperatures and pressures. The results indicate that those carbonates should be stable in Earth's lower mantle, instead of dissociating into other minerals, in chemical environments with excesses of SiO 2 , MgO, or MgSiO 3 . Therefore, considering the lower mantle chemical composition, filled with major minerals such as MgSiO 3 and MgO, calcium and magnesium carbonates are the primary candidates as carbon hosts in that region. For the thermodynamic conditions of the mantle, the results also indicate that carbon should be primarily hosted on MgCO 3 . Finally, the results indicate that carbon, in the form of free CO 2 , is unlikely in the lower mantle.

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

“…2). We note that the experimental data allow for significant uncertainty in this boundary, but are inconsistent with theoretical predictions [28][29][30] (yellow region, Fig. 2).…”

“…The error bars indicate uncertainties of pressure and temperature measurements (see "Methods" for details). The boundaries proposed by previous theoretical predictions are illustrated by yellow-shaded region [28][29][30] . The gray dotted line indicates the reversal boundary of the carbonate-silicate exchange reaction proposed by this study, whereas previous theoretical predictions are illustrated by yellow-shaded region [28][29][30] .…”

“…The boundaries proposed by previous theoretical predictions are illustrated by yellow-shaded region [28][29][30] . The gray dotted line indicates the reversal boundary of the carbonate-silicate exchange reaction proposed by this study, whereas previous theoretical predictions are illustrated by yellow-shaded region [28][29][30] . The cyan and orange lines indicate the decarbonation reactions of CaCO 3 + SiO 2 40 and MgCO 3 + SiO 2 41 , respectively.…”

“…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.…”

Structural and Chemical Modifications of Carbon Dioxide on Transport to the Deep Earth

“…Actually, the two-step mechanism of the CO 2 liberation from carbonates and the subsequent dissociation of CO 2 was questioned as a plausible theory explaining the origin of ultra-deep diamonds (Kaminsky, 2017). What is more, recent theoretical studies indicate that direct decomposition of CaCO 3 and MgCO 3 is unfavorable over the lower mantle, putting into doubt the possibility of free CO 2 existing at these depths (Pickard & Needs, 2015;Santos et al, 2019). It should be also emphasized that possible chemical processes, involving either reactions of CO 2 or carbonates with elemental iron of the Earth's outer core close to the core-mantle boundary (Dorfman et al, 2018;Martirosyan et al, 2019), or with hydrous iron minerals like goethite (Boulard et al, 2018), can dramatically change carbon speciation in the lower mantle.…”