The CaCO₃–Fe interaction: Kinetic approach for carbonate subduction to the deep Earth's mantle

The CaCO₃–Fe⁰ system, as a model for redox reactions between carbonates and reduced lithologies at the slab-mantle interface during subduction or at core-mantle boundary, was investigated systematically at temperatures from 650 to 1400 °C and pressures from 4 to 16 GPa using multianvil apparatus. CaCO₃ reduction via reaction: 3 CaCO₃ (aragonite) + 13 Fe⁰ (metal) = Fe₇C₃ (carbide) + 3 CaFe₂O₃ (Ca-wüstite) was observed. The thickness of the reaction-product layer (Δx) increases linearly with the square root of time in the time-series experiments (t), indicating diffusion-controlled process. The reaction rate constant (k = Δx²/2t) is log-linear relative to 1/T. Its temperature dependences was determined to be k [m²/s] = 2.1 × 10⁻⁷exp(−162[kJ/mol]/RT) at 4–6 GPa and k [m²/s] = 2.6 × 10⁻¹¹exp(−65[kJ/mol]/RT) at 16 GPa. The sluggish kinetics of established CaCO₃–Fe⁰ interaction suggests that significant amount of carbonates could survive during subduction from metal saturation boundary near 250 km depth down to the transition zone and presumably to the lower mantle if melting of carbonates is not involved.