We have measured Fe-Mg interdiffusion coefficients, DFe-Mg, parallel to the three main crystallographic axes in two natural orthopyroxene single crystals [approximately En98Fs1 (XFs = XFe = 0.01) and En91Fs9] using diffusion couples consisting of a 20–90 nm thick silicate thin film deposited under vacuum on polished and oriented pyroxene single crystals. The thin films were prepared using pulsed laser ablation of polycrystalline olivine pellets (composition: Fo30Fa70). Samples were annealed for 4–337 h at 870–1100 °C under atmospheric pressure in a continuous flow of CO + CO2 to control the oxygen fugacity, fO2, between 10–11 and 10–7 Pa within the stability field of pyroxene. Film thickness and compositional profiles were measured using Rutherford backscattering spectroscopy (RBS) on reference and annealed samples, and Fe concentration depth profiles were extracted from the RBS spectra and fitted numerically considering a compositional dependence of DFe-Mg in orthopyroxene. We obtain an Arrhenius relationship for both types of crystals, but only for the more Fe-rich composition a dependence on fO2 could be clearly identified. For diffusion along [001] in the composition Fs9, least-squares regression of the log DFe-Mg vs. reciprocal temperature yields the following Arrhenius equation:
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