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

Abstract

Knowledge of Fe-Mg interdiffusion coefficients (DFe-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 DFe-Mg to quantify the effect of the molar fraction of Fe, XFe = Fe/(Fe + Mg) = 0.1 to 0.4, and oxygen fugacity, fO2 = 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, XFe and fO2 on DFe-Mg. The effect of XFe increases with T for fO2 = 10-7 to 10-9 Pa but decreases with T for more reducing conditions. For T > 1000 °C and fO2 > 10-10 Pa, DFe-Mg depends on fO2 and XFe. For these conditions DFe-Mg is described by an activation energy of 284 ± 19 kJ/mol. For T = 950 °C to 1000 °C and for fO2 < 10-10 Pa, DFe-Mg seemingly ceases to depend on fO2, 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, fO2 and T conditions from vacancies on the metal site/electron holes to vacancies on the metal site/Fe3+ to Mg interstitials/electrons, each one responsible for a different effect of fO2 on DFe-Mg. Based on the re-evaluation of kinetic data of order–disorder in orthopyroxene we propose that for T < 950 °C DFe-Mg becomes insensitive to fO2 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 fO2 > 10-10 Pa and for the same temperature range but for log fO2 ≤ 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 peak T > 1100 °C, more oxidizing conditions, higher Fe-contents and slower cooling rates (e.g. 10 °C/Myr), which has implications for the evaluation of geothermometric data using the distribution of Fe-Mg between opx and the exchange partner.