Dependence of dislocation creep of dunite on oxygen fugacity: Implications for viscosity variations in Earth's mantle

Significant variations in flow behavior are known to exist both vertically and laterally in Earth's upper mantle. The sources of such variation may be thermal, compositional, or reflect differences in the chemical activity of components, such as oxygen, silica, and water. We report on the effects of oxygen fugacity on dislocation creep of dunite, with a view to understanding potential strength heterogeneity in the mantle. Although room pressure experiments on single crystals of olivine have shown a clear dependence of creep rate on oxygen fugacity, no prior deformation study of polycrystalline olivine-rich rocks has demonstrated such a dependency under high-pressure conditions. In this study we performed a series of dry creep experiments on a natural dunite under carefully controlled thermochemical conditions, including oxygen fugacity. The samples, cored from coarse-grained Aheim dunite with a grain size of similar to 0.9 mm, were deformed under triaxial compression at oxygen fugacities fixed by either the iron/wustite or the nickel/nickel oxide solid state buffers, temperatures between 1150 degrees and 1277 degrees C, and differential stresses up to 300 MPa. The results of a global fit to all experimental data indicate a power law dependence of creep rate on oxygen fugacity, with an oxygen fugacity exponent of m = 0.20 +/- 0.01, n = 3.6 +/- 0.1, A = 10(2.6 +/- 0.3) s(-1) MPa-3.6 Pa-0.2, and an activation energy for creep of 449 +/- 7 kJ/mol. This activation energy is significantly less than the commonly used value of 535 kJ/mol because the earlier experiments made no corrections for the effects of oxygen fugacity. When applied to planetary interiors, an increase in oxygen fugacity by a factor of similar to 10(3.5), from the iron/wustite to the fayalite-magnetite-quartz buffers, will result in a factor of similar to 5 decrease in viscosity.