Oyxgen diffusion in olivine: effect of oxygen fugacity and implications for creep

Oxygen self-diffusion experiments on single crystals of San Carlos olivine (~FO₉₂) at 1200° ≤ T ≤ 1400°C, oxygen fugacities (f_O2) along the Ni-NiO and Fe-FeO buffers, and silica activity at the olivine-orthopyroxene buffer yielded results that follow the relationship D = 2.6 × 10^-10 f_O2 ^0.21±0.03 exp [-266 ± 11 (kJ mol^-1/RT), where D is the diffusion coefficient in m² s^-1 and f_O2 is given in pascals. The activation energy compares reasonably well with results for pure forsterite. The positive dependence of f_O2 implies that the oxygen defect responsible for diffusion is an interstitial rather than a more sterically reasonable oxygen vacancy. The rate of creep of single-crystal olivine at fixed orthopyroxene activity also shows a positive f_O2 dependence. Compared with data for silicon diffusion in forsterite, our data indicate that oxygen is not the slowest diffusing species in olivine. The activation energy for oxygen diffusion is also low compared to that obtained in a majority of measurements of creep in single-crystal olivine. Hence if oxygen diffusion contributes to the control of creep in olivine, it must be coupled to another process having a nonzero activation energy. -from Authors