Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P,T and fO2 |
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| Authors: | Sumit Chakraborty John R Farver Richard A Yund David C Rubie |
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| Institution: | 1. Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440, Bayreuth, Germany
3. Department of Geological Sciences, Brown University, 02912, Providence, RI, USA
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| 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 > Da > 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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