Flow of the Earth's mantle allows heat to be transferred by convection outward from the planet's interior, and the style of that convection also has a primary role in determining how plate tectonics shape the outermost portions of the Earth. Knowledge of the viscosity of the Earth's mantle is essential to understanding several of these large-scale geodynamic problems, including the Earth's thermal evolution and the interaction of lithospheric plates with the underlying asthenosphere. There exists, however, great difficulty in extrapolating the results of laboratory experiments conducted on a human timescale to geodynamic processes that operate on geological timescales. Thus accurate scaling relationships must be developed that describe the dependence of mantle viscosity on key factors such as temperature, stress, and grain size. Several microscopic mechanisms contribute to deformation of mantle rocks. Grain-boundary sliding, where individual grains move past each other to accommodate deformation, has only recently been brought attention to as a viscosity-controlling mechanism in mantle rocks. This project emphasizes a new, transformative approach to laboratory experiments designed to develop a precise scaling relationship for grain-boundary sliding in olivine, the dominant mineral in the upper mantle. This relationship can then be applied to many other studies, including numerical simulations of mantle flow, geodetic observations of earth surface deformation, and field studies of exposed mantle rocks, which all rely on accurate knowledge of mantle viscosity under a given set of conditions.
This project is motivated by recent experimental results and field observations that indicate that grain-boundary sliding may be the dominant mechanism in shear zones in the Earth's mantle. However, these predictions rely on experimentally derived relationships, and previous experiments have often neglected the effect of grain size when it was likely important. Even small changes in grain size (e.g., due to grain growth) during deformation experiments can lead to significant errors in the determination of the dependence of viscosity on stress and temperature. These errors are so great that grain-boundary sliding could control the viscosity of the entire upper mantle rather than merely low-temperature shear zones. The dependence of viscosity on stress, grain size, and temperature is not currently known well enough to allow extrapolation and application of results obtained from laboratory experiments to geodynamic processes occurring in the Earth's mantle. Our new approach relies on testing temperature dependence and stress dependence in samples where the grain size is stable. To precisely control grain size, we have introduced two important changes to our deformation experiments. First, we fabricate samples with grain sizes significantly larger than the stable grain size. Second, we deform these samples in torsion to large shear strains. Because the stable grain size is a function of the applied stress, samples deformed to high strains will recrystallize to produce a stable grain size. Using this method, we can control grain size simply by controlling the applied stress. Our initial results demonstrate that a stable grain size is attained in these experiments based on flow behavior, grain-size evolution, and grain orientations. Tests of stress and temperature dependencies will be carried out after significant strains have been accumulated. The study proposed has the potential to accurately characterize the importance of grain-boundary sliding to olivine deformation, substantially furthering our ability to predict upper mantle viscosity.
