Diffusion is an important high-temperature isotope fractionation process in Earth and planetary interiors, but there are few constraints on the magnitude of the diffusional isotope effect or its dependence on ionic parameters. Here, we present the first experimental investigation on the mass dependence of Cr isotope diffusion in olivine, and how it varies with Cr valence state. Diffusion experiments performed on forsterite over a wide range of oxygen fugacity conditions were analyzed using high-precision second ion mass spectrometry (SIMS) to obtain profiles of total Cr concentration and Cr isotope ratios (δ 53/52 Cr). Our results demonstrate that the dependence of Cr diffusion on isotope mass is large and that it varies with valence state. Where the ratio of isotope diffusion coefficients depends inversely on their mass ratio raised to the power β, we find that β increases linearly with the proportion of Cr 3+ . Based on this dependence, we obtain β values of 0.315 ± 0.003 for Cr 3+ and 0.114 ± 0.008 for Cr 2+ . Using the experimentally derived β values, we modeled diffusion-limited Cr isotope exchange between olivine and spinel during subsolidus cooling. In slowly cooled cumulate and ophiolite samples, olivine is often isotopically heavier than spinel, opposite to the sense of equilibrium fractionation. We show that partial diffusional re-equilibration during cooling, with Cr diffusing from olivine to spinel, can partially account for these disequilibrium signatures.
Xiong et al. (Tue,) studied this question.