Application of the cBΩ model to the calculation of diffusion parameters of He in olivine

The validity of the thermodynamic cB Omega model is tested in terms of the experimentally determined diffusion coefficients of He in a natural Fe-bearing olivine (Fo(90)) and a synthetic end-member forsterite (Mg2SiO4) over a broad temperature range (250-950 A degrees C), as reported recently by Cherniak and Watson (Geochem Cosmochim Acta 84:269-279, 2012). The calculated activation enthalpies for each of the three crystallographic axes were found to be (134 +/- A 5), (137 +/- A 13) and (158 +/- A 4) kJ mol(-1) for the [100], [010] and [001] directions in forsterite, and (141 +/- A 9) kJ mol(-1) for the [010] direction in olivine, exhibiting a deviation of < 1 % with the corresponding reported experimental values. Additional point defect parameters such as activation volume, activation entropy and activation Gibbs free energy were calculated as a function of temperature. The estimated activation volumes (3.2-3.9 +/- A 0.3 cm(3) mol(-1)) of He diffusion in olivine are comparable with other reported results for hydrogen and tracer diffusion of Mg cations in olivine. The pressure dependence of He diffusion coefficients was also determined, based on single experimental diffusion measurements at 2.6 and 2.7 GPa along the [001] direction in forsterite at 400 and 650 A degrees C.