Abstract
Oxygen partial pressure is supposed, by analogy with olivines, to influence the kinetics of the Fe-Mg exchange reaction in orthopyroxene. It has been demonstrated for olivines that the Fe−Mg interdiffusion coefficient is dependent on P(O2), according to D Fe−Mg ∝ P(O2)∼1/6 [1−3].
By means of thermogravimetric analyses performed at different P(O2) on orthopyroxene grains from a volcanic rock it was possible to detect a certain degree of non-stoichiometry which is function of P(O2).Oxygen moves into or out of the orthopyroxene lattice in response to a compositional gradient. Therefore, in orthopyroxene too, the Fe/Mg interdiffusion and hence the kinetics of the Fe−Mg intracrystalline exchange should be affected by P(O2). The oxygen chemical diffusion coefficients at P(O2)∼5⋅10−19atm were calculated at ∼400, 500 and 600°C.
It was also verified on the orthopyroxene from the TPK-30F granulite that, at the operating conditions normally used for single-crystal annealing experiments, oxygen quickly responds to a chemical potential gradient in order to maintain the system in equilibrium conditions.
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Zema, M., Ghigna, P., Domeneghetti, M.C. et al. Oxygen Diffusion in Orthopyroxene. TG study. Journal of Thermal Analysis and Calorimetry 67, 713–720 (2002). https://doi.org/10.1023/A:1014325425976
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DOI: https://doi.org/10.1023/A:1014325425976