Abstract
We present new experimental data on Mg tracer diffusion in oriented single crystals of forsterite (Fo100) and San Carlos olivine (Fo92) between 1000–1300° C. The activation energies of diffusion are found to be 400 (±60) kJ/mol (≈96 kcal/mol) and 275 (±25) kJ/mol (≈65 kcal/ mol) in forsterite and San Carlos olivine, respectively, along [001] at a fO2 of 10−12 bars. There is no change in activation energy of Mg tracer diffusion within this temperature range. Mg tracer diffusion in a nominally pure forsterite is found to be anisotropic (D∥c > D∥a > D ∥b) and a function of fO2. This fO2 dependence is different from that in olivine containing Fe as a major element, which suggests that the diffusion mechanism of Mg in forsterite is different from that in Fe-bearing olivine at least over some range of fO2. The diffusion mechanism in nominally pure forsterites may involve impurities present below the limits of detection or alternately, Si or Fe3+ interstitial defects, Fe being present as impurity (ppm level) in forsterite. Pressure dependence of Mg tracer diffusivity in forsterite measured to 10 GPa in a multianvil apparatus yields an activation volume of approximately 1–3.5 cm3/ mol. It is found that presence of small amounts of hydrogen bearing species in the atmosphere during diffusion anneal (fH2 ≈ 0.2 bars, fH20 ≈ 0.24 bars) do not affect Mg tracer diffusion in forsterite within the resolution of our measurement at a total pressure of 1 bar. The observed diffusion process is shown to be extrinsic; hence extrapolation of the diffusion data to lower temperatures should not be plagued by uncertainties related to change of diffusion mechanism from intrinsic to extrinsic.
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Chakraborty, S., Farver, J.R., Yund, R.A. et al. Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P, T and fO2 . Phys Chem Minerals 21, 489–500 (1994). https://doi.org/10.1007/BF00203923
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DOI: https://doi.org/10.1007/BF00203923