High-Pressure Stability and Elasticity of Multi-Cation Carbonates: Implications for the Deep Carbon Cycle

The deep carbon cycle is crucial for understanding Earth's global carbon budget and the exchange of volatiles between the surface and the interior. Carbonates are the primary carriers of carbon into the deep mantle via subducting oceanic lithosphere, yet their stability and distribution remain subject to significant uncertainty. While current understanding of carbonate stability and elasticity is largely based on end-member minerals, the effects of complex cation mixing, which is prevalent in natural systems, are poorly constrained. In this study, we synthesized a series of multi-cation carbonates with varying cation compositions (Ca, Mg, Fe) and investigated their crystal structures and elastic properties under in situ high-pressure and high-temperature conditions using synchrotron X-ray diffraction and Brillouin scattering. Our results demonstrate that the random substitution of cations significantly modulates the phase stability and sound velocities of these carbonates. Specifically, the combined effects of Mg and Fe substitution for Ca induce distinct elastic anomalies and anisotropy variations that deviate from the behavior of end-member phases. These findings provide critical constraints for interpreting seismic observations in subduction zones and the deep mantle. By clarifying the influence of cation chemistry on carbonate elasticity, this work enhances our ability to quantify deep carbon reservoirs and understand the dynamic processes governing the global carbon cycle.