Growth Kinetics of a Reaction Rim Between Iron and Graphite/Diamond and the Carbon Diffusion Mechanism at High Pressure and Temperature

Thermodynamic calculations of the fo₂ on modeled bulk silicate Earth mantle composition predict the formation of Fe-Ni metal alloy at about 250-300 km in depth. At such conditions the speciation of subducted carbon will be mainly affected by the local Fe(Ni)/C ratio, with diamond, Fe₃C and C-bearing Fe-Ni alloys being the most likely stable phases. To date however, no data are available to determine the effect of pressure and temperature on 1) the transport of carbon by diffusion in iron metal and 2) the kinetics of formation of carbide phases.

We performed multianvil experiments between 3 and 10 GPa and temperatures of 700-1200 ºC with the aim of measuring C diffusion in γ-Fe. Glassy carbon and synthetic diamond were used as diffusants, placed directly in contact with pure iron rod rods with a thickness of 800-1400 μm. FE-SEM was used for accurate analyses of the Fe-C interface and concentration profiles of carbon in iron were measured by electron microprobe.

Results show that the diffusion coefficient for carbon in iron metal (~3x10^-11 m²s^-1) and the activation energy (~62 kJ/mol) are similar to previous data from 1 atm and suggest a small pressure effect. The activation volume (~1.5x10^-6 m³/mol) determined from isothermal runs is in agreement with that determined for other elements for which an interstitial diffusion mechanism in iron has been established. At the interface between carbon and Fe the growth of a reaction rim was often observed. Time series experiments were therefore performed, to investigate the growth kinetics of iron carbide (Fe₃C). Results will be used to 1) determine a model for the storage of C in metallic phases in the Earth's interior and 2) provide an experimental constraint on the formation of carbide phases during subduction, with implications for the deep carbon cycle and isotopic fractionation.