Interdiffusion of Mg–Fe in olivine at 1,400–1,600 °C and 1 atm total pressure
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The interdiffusion coefficient of Mg–Fe in olivine (D Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10−3.5–10−2 Pa using a diffusion couple technique. The D Mg–Fe shows the anisotropy (largest along the  direction and smallest along the  direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe3+/Fe2+ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe3+/Fe2+ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.
KeywordsOlivine Diffusion Kinetics Point defect
We would like to thank Masana Morioka for his assistance in synthesizing single crystals of Fe-free forsterite. We would also like to thank Hideto Yoshida fro his help in electron microprobe analyses. Careful reviews by Sumit Chakraborty, an anonymous reviewer, and Editor Masanori Matsui are appreciated, which made the quality of the paper much improved. This work was partly supported by Grant-in-Aid for Scientific Research (S) (16104007) and Grant-in-Aids for Young Scientists (A) (20684025).
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