Fe-Mg interdiffusion in orthopyroxene: Complex interdependencies of temperature, composition and oxygen fugacity

Knowledge of Fe-Mg interdiffusion coefficients ( D Fe-Mg ) in orthopyroxene (opx) is relevant for diffusion chronometry as well as for thermometers based on Fe-Mg exchange between opx and other common mafic minerals. We extended the existing data set for D Fe-Mg to quantify the effect of the molar fraction of Fe, X Fe = Fe/(Fe + Mg) = 0.1 to 0.4, and oxygen fugacity, f O 2 = 10 -7 to 10 -11 Pa, in the temperature range, T = 950–––1100 °C, where we build up on our recently developed experimental and analytical approach. Thin film diffusion couples using different natural opx with different trace element contents were annealed in vertical gas mixing furnaces. The diffusion profiles were extracted by acquiring backscattered electron images on foils cut from the samples using a focused ion beam-scanning electron microscope. We found complex interplays between the effect of T , X Fe and f O 2 on D Fe-Mg . The effect of X Fe increases with T for f O 2 = 10 -7 to 10 -9 Pa but decreases with T for more reducing conditions. For T > 1000 °C and f O 2 > 10 -10 Pa, D Fe-Mg depends on f O 2 and X Fe . For these conditions D Fe-Mg is described by an activation energy of 284 ± 19 kJ/mol. For T = 950 °C to 1000 °C and for f O 2 < 10 -10 Pa, D Fe-Mg seemingly ceases to depend on f O 2 , indicating a change of the diffusion mechanism, and is described by an activation energy of 246 ± 78 kJ/mol. Based on these diffusion data, we propose a qualitative point defect model for opx where the majority point defects changes over the explored Fe content, f O 2 and T conditions from vacancies on the metal site/electron holes to vacancies on the metal site/Fe 3+ to Mg interstitials/electrons, each one responsible for a different effect of f O 2 on D Fe-Mg . Based on the re-evaluation of kinetic data of order–disorder in orthopyroxene we propose that for T < 950 °C D Fe-Mg becomes insensitive to f O 2 and the effect of Fe is relatively constant and smaller than at higher T . We derived two parameterizations valid for temperatures between 950 and 1100 °C and f O 2 > 10 -10 Pa and for the same temperature range but for log f O 2 ≤ 10 -10 Pa. In addition, we explore how the new diffusion data would affect the re-equilibration of Fe-Mg during cooling between opx and spinel as well as opx and clinopyroxene. Based on our results, it is likely that the peak compositions at the core of opx are easily modified for more oxidizing conditions, higher Fe-contents and slower cooling rates, which has implications for the evaluation of geothermometric data using the distribution of Fe-Mg between opx and the exchange partner.