Browsing by Subject "grain boundary sliding"
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Item Influence of Grain Boundaries and their Composition on the Deformation Strength of High-purity, Synthetic Forsterite(2016-06) Dillman, AmandaGrain boundaries are an important feature of the mantle. With recent studies suggesting the majority of the upper mantle deforms by grain boundary sliding (Hirth and Kohlstedt, 2003; Hansen et al., 2013), understanding the role grain boundaries play is key. As grain boundary sliding always requires an accommodation mechanism, directly determining the contribution of grain boundary sliding to total strain on a sample is important for modeling deformation in the mantle. Altering grain boundary composition can change the structure and viscosity of the boundary. Understanding the effects of grain boundary composition is necessary for comparing data sets of different olivine as well as for accurately extrapolating experimental data to represent the mantle. In Chapter 2, uniaxial deformation experiments on high-purity synthetic forsterite at high temperature and ambient pressure are used to characterize the contribution of grain boundary sliding to strain in diffusion creep. Experiments were conducted in a one-atmosphere deformation rig, which allowed the polished surfaces of the samples to be analyzed with atomic force microscopy. The high temperature necessary for deformation enabled a great deal of thermal grooving, which can dramatically alter the topography of an initially polished surface. A methodology was developed to correct for the effect of thermal grooving and determine the amount of grain boundary sliding as a function of grain size and stress. A comparison is also made between two popular methods for determining grain size: the line intercept method and the equivalent area circle method. The line intercept method consistently produces larger grain sizes than the equivalent area circle method. In Chapter 3, triaxial compression experiments on forsterite are used to determine the effect of grain boundary chemistry on deformation strength. High-purity synthetic forsterite was doped with either Ca or Pr and then deformed at high temperature and a confining pressure of 300 MPa. Both impurities made the sample stronger, and the presence of Ca induced abnormal grain growth. This supports the theory that grain boundary composition can have a large effect on deformation strength. The hypothesis that the difference in strength between natural and high-purity synthetic olivines is due to the difference in grain boundary composition is not supported by these results. In Chapter 4, the results of experiments on forsterite with a small amount of melt are detailed. Two methods of adding melt were used. The first involved adding Pr to forsterite in concentrations greater than can dissolve in the grain boundary, which induced melting as well as enhanced grain growth. Even with a grain size over an order of magnitude greater than the melt-free sample, the melt bearing samples were weaker than the melt free samples. The second method involved synthesizing forsterite with a composition in equilibrium with a synthesized anorthitic melt. Samples were created with melt fractions < 0.01 and then deformed at a temperature of 1300°C and a confining pressure of 300 MPa. The drop in viscosity at very small melt fractions predicted by Takei and Holtzman (2009) was observed, although the drop occurred over a shorter change in melt fraction than predicted. This result suggests that, at the onset of melting, the mantle will become significantly weaker. In addition, the presence of as little as 0.1% melt in a high purity, synthetic olivine sample brings its deformation strength into agreement with natural samples. This suggests that deformation experiments on natural samples are never entirely melt free. The results of this study establish the role of grain boundary chemistry on polycrystalline deformation. The presence of large cations in olivine grain boundaries makes diffusion creep slower, which limits the regions of the mantle predicted to deform in diffusion creep and expands the regions predicted to deform in a dislocation accommodated grain boundary sliding or dislocation creep. At the onset of melting, this changes, as the melt would remove the impurities from the grain boundaries. Future studies on different types of impurities will allow the grain boundaries of natural olivines to be more accurately modeled.Item The influence of hydrogen, deformation geometry, and grain size on the rheological properties of olivine at upper mantle conditions(2015-09) Tielke, JacobMany important geophysical processes, including mantle convection and the associated movement of Earth's tectonic plates, are strongly dependent upon the rheological properties of Earth's upper mantle. Olivine is the most abundant mineral in the upper mantle and therefore largely controls the mechanical behavior of this region of Earth's interior. Many experimental investigations have been carried out to study the rheological properties of olivine single crystals, synthetically produced aggregates, and naturally occurring mantle rocks at asthenospheric temperatures. In contrast, relatively few studies have focused on measuring the rheological properties of olivine deforming at lithospheric temperatures. Furthermore, there are several unanswered questions about the microphysical processes that control deformation of olivine at upper mantle conditions. One outstanding question in the field of rock and mineral physics is "Do different microphysical processes control the rate of deformation of olivine at asthenospheric compared to lithospheric mantle conditions?" To address this question we carried out direct shear experiments on olivine single crystals at temperatures that span the transition from lithospheric to asthenospheric mantle conditions. The results of these experiments, which are presented in Chapter 2, demonstrate that the dependence of strain rate upon stress transitions from a power-law relationship at high temperatures to an exponential dependence at lower temperatures. This transition in rheological behavior is consistent with deformation that is controlled by the climb of dislocations at high-temperature conditions and deformation that is controlled by the glide of dislocations at low-temperature conditions. Furthermore, the direct shear geometry allows for isolation of the (001)[100] and (100)[001] dislocation slip systems, which cannot be individually activated in triaxial compression. At high-temperature conditions, crystals oriented for shear on the (001)[100] slip system are observed to be weaker than crystals oriented for shear on the (100)[001] slip system. At low-temperature conditions the opposite relationship is observed: crystals oriented for shear on the (100)[001] slip system are weakest. Another important outstanding question is "Do the mechanisms of hydrolytic weakening in olivine differ at asthenospheric compared to lithospheric mantle conditions?" In Chapter 3 we report the results of experiments carried out on olivine single crystals under hydrous conditions at both asthenospheric and lithospheric temperatures. For crystals deformed at high-temperatures and under hydrous conditions, the dependence of strain rate on stress follows a power-law relationship with a stress exponent (n) of ~2.5, consistent with deformation that is rate limited by diffusion of silicon through the olivine lattice. In contrast, crystals deformed at high-temperatures and under anhydrous conditions yield n values of ~3.5, consistent with deformation that is rate limited by diffusion of silicon through the cores of dislocations. At low temperature conditions, the strain rate of both hydrous and anhydrous crystals are equally well described by the same exponential dependence of stress. These observations demonstrate significant hydrolytic weakening occurs at asthenospheric temperatures, but hydrolytic weakening cannot be resolved at lithospheric temperatures for our experimental conditions. Lastly, we address a question about polycrystalline deformation: "What deformation mechanism is responsible for grain-size sensitive (GSS) power-law creep of olivine aggregates?" In Chapter 4 we compare strain rates measured during deformation experiments on olivine aggregates to strain rates calculated from a micromechanical model of intragranular slip. The micromechanical model uses the measured stress from deformation experiments and grain orientations determined from post-deformation electron backscatter diffraction measurements to approximate the contribution of dislocation creep to the strain rate. Olivine aggregates deform up to a factor of 4.6 times faster than the maximum possible rates determined from the micromechanical model of intragranular slip. The ratio of experimentally determined strain rates to those from the micromechanical model is strongly dependent upon grain size, but is independent of stress and strength of lattice-preferred orientation. These observations indicate that GSS power-law creep occurs in both weakly and strongly textured olivine aggregates at the studied conditions. We consider three explanations for the observed rheological behavior, (1) a combination of diffusion and dislocation creep, (2) the operation of dynamic recrystallization creep, and (3) the operation of dislocation-accommodated grain-boundary sliding. Our analyses indicate that the microstructural and mechanical behavior of olivine aggregates deforming in the grain-size sensitive power-law regime are most consistent with the operation of dislocation-accommodated grain-boundary sliding at the studied experimental conditions.