Carbonate–Iron Interaction: Kinetic Approach for Carbonate Subduction Down to the Mantle Transition Zone

Abstract:

One of the principal redox exchange reactions at the slab-mantle interface during subduction is CaCO₃–Fe⁰ interaction. Using uniaxial press multianvil apparatus, we investigate this reaction at temperatures from 650 to 1400 °C, and pressures from 4 to 16 GPa. These conditions are representative for the PT path of the slab from the surface to the mantle transition zone. Based on our results, redox reaction can be written as: 3 CaCO₃ (aragonite) + 13 Fe⁰ (metal) = Fe₇C₃ (carbide) + 3 CaFe₂O₃ (Ca-wüstite). It is a diffusion-controlled process with the reaction rate constant (k) being log-linear in 1/T.

The experimental results allow to calculate the length scale over which the reaction kinetics between aragonite and metallic iron is likely effective. Using relatively simple relationship between the characteristic distance of diffusion as a function of k and duration of the process, we estimate the length scale of the reaction for time scales 4-16 Myr, which corresponds to subduction rates of 2-8 cm/year from 250 km (metal-saturation boundary) to 470 km, for different slab PT-profiles. Assuming that carbonates are in a direct contact with iron, the maximum degree of carbonate reduction can be evaluated using the results of our study and the data on CO₂ distribution in altered oceanic basalts. The estimates suggest that up to 0.5, 12, and 20 vol. % of carbonates can be reduced in such case during subduction down to the mantle transition zone at the conditions of cold, medium and hot geotherms, respectively. The sluggish kinetics of established CaCO₃–Fe⁰ interaction suggests that even over the entire history of the Earth, carbonates could survive during subduction from metal saturation boundary near 250 km depth, down to the transition zone, and presumably to the lower mantle.