Iron isotope electroplating: Diffusion-limited fractionation

Electron-transfer related isotope fractionation may produce large stable isotope geochemical signatures as a result of a variety of Earth processes, including biology, chemical weathering, fluid–rock interactions, and deep Earth redox reactions. In the laboratory, isotope fractionation at a charged electrode has been observed for electroplating of Fe and Zn; however these can arise from a variety of effects besides the electron transfer process. Here, we examine the effect of mass transport on observed isotope fractionation during potentiostatic electroplating of iron. We examine the observed isotope fractionation as a function of the ratio of the observed plating current to the mass-transport limited current (the Cottrell current). When the electroplating experiments are run at currents greater than the Cottrell current, the observed fractionation is ~− 1.15(± 0.40) ‰, and the extent of fractionation shows a tendency to decrease with increasing plating rate. When electroplating experiments are run at currents below the Cottrell current, observed fractionations are strongly dependent on the plating rate, with a maximum value of δ56Fe = − 4.8. The data set demonstrates that mass transport to the electrode tends to attenuate a large fractionation factor associated with other, non-mass transport, processes at the electrode.