Stability of MgCO₃ throughout the lower mantle: Phase diagram and wave velocity of a deep carbon reservoir

Carbonate minerals are key carriers of carbon into the Earth’s interior, yet their stability and melting behavior under lower mantle conditions remain incompletely constrained. Here, we develop a high-accuracy machine learning interatomic potential for MgCO₃ based on the deep potential framework, trained on an extensive ab initio molecular dynamics dataset spanning 0–140 GPa and 300–5000 K. Using large-scale two-phase coexistence molecular dynamics simulations, we determine the melting behavior of MgCO₃ across the pressure range of the entire lower mantle. At ambient pressure, the predicted melting temperature of 1857 K agrees with experiments within 0.3%, validating the potential. We show that the high-pressure C2/m phase exhibits a substantially higher melting temperature and a steeper Clapeyron slope than the low-pressure phase, leading to a pronounced increase in melting temperature across the structural transition. The resulting melting curve lies approximately 400 to 1100 K above representative lower mantle geotherms, indicating that crystalline MgCO₃ can remain thermodynamically stable under idealized anhydrous, Mg–end-member conditions. Calculated elastic wave velocities display a pressure-dependent high–low–high deviation relative to the PREM (Preliminary Reference Earth Model), reflecting changes in crystal structure and elastic softening near phase transitions. These results provide quantitative constraints on the phase stability, melting behavior, and elastic properties of MgCO₃ at deep-mantle conditions and offer a reference framework for evaluating the role of carbonates in deep Earth carbon storage when coupled with more realistic multicomponent mantle compositions.