Stability of calcium and magnesium carbonates at lower mantle thermodynamic conditions

TL;DR

This study uses ab initio calculations to show that magnesium and calcium carbonates are stable in Earth's lower mantle and are likely the main carbon hosts there, with minimal free CO2 presence.

Contribution

It provides the first theoretical evidence supporting the stability of MgCO₃ and CaCO₃ in the lower mantle under realistic conditions.

Findings

MgCO₃ and CaCO₃ are stable in the lower mantle

Carbon is mainly hosted as MgCO₃ in the mantle

Free CO₂ is unlikely in the lower mantle

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.