Oxygen diffusion in orthopyroxene TG study

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 DFe-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) ∼5·10-19 atm 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.