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Rheological Consequences of Redox State

Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, Texas 77058, U.S.A., mackwell{at}lpi.usra.edu

Within the upper mantle of Earth, there is a gradient from a relatively more oxidizing near-surface region, with oxygen fugacities near those of the fayalite-magnetite-quartz buffer (FMQ), to more reducing conditions at depth near the iron-wüstite buffer (IW). Oxygen fugacity appears to vary laterally, as well as vertically, by as much as a factor of 10⁴. As flow within the interior of the Earth and other terrestrial planets occurs due to the (mostly) solid-state deformation of rocks, an understanding of the effect of oxygen fugacity on creep is critical in modeling planetary interior dynamical behavior. This is especially important for the asthenosphere, that anomalously weak region in the uppermost mantle that accommodates isostasy and largely decouples mantle convection from plate motions. Experimental studies of the rheological behavior of iron-bearing minerals have demonstrated that oxygen fugacity can play an important role in deformation. We have shown that olivine rich rocks deformed near FMQ deform in the dislocation creep regime about a factor of 6 faster when buffered near FMQ than at IW. Experiments on olivine single crystals and aggregates indicate that this difference in behavior results from an increase in the concentration of silicon vacancies under more oxidizing conditions, as dislocation creep is rate-limited by the climb of dislocations, which is controlled by diffusion of silicon defects. Although fewer data are available for the effects of oxygen fugacity on pyroxene deformation, clinopyroxene appears to be stronger under more oxidizing conditions, while the data on orthopyroxene deformation show no dependence on oxygen fugacity. These results indicate that vertical and lateral variations in oxygen fugacity may result in, at most, an order of magnitude difference in viscosity, while other factors, such as water fugacity and lithology may be more significant.