Research Projects
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Evolution of CPOs in ice sheared to large strainsIce 1h is a strongly anisotropic mineral, such that the development of a CPO in polycrystalline ice causes anisotropy in various bulk physical properties, most notably its viscosity. The development of CPOs in natural ice thus may have a strong influence on the motion of glaciers and ice sheets. Most ice deformation experiments have been conducted in axial compression; published CPOs of ice deformed in the laboratory are therefore mostly limited to axial strains <0.3, much smaller than in natural ice systems. I carried out the first-ever direct-shear experiments on ice to investigate the evolution of CPOs with strain in ice.
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The influence of particles on the rheological behavior of ice
The influence of embedded particles on the rheological behavior of polycrystalline ice is not well known. This knowledge gap is compounded by the discovery that ice deforms not by a single deformation mechanism, as classically assumed, but rather by multiple creep mechanisms, each of which dominates the flow behavior of ice over different regimes of grain size, temperature, and stress (Goldsby and Kohlstedt, 2001). Inter-granular particles, for example, may slow the rate of GBS creep, whereas intra-granular particles may impede the motion of lattice dislocations, slowing both dislocation creep and GBS creep.
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The transition in CPO-forming mechanisms in iceMy experiments on ice found that, as stress increases, the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases. Also, the CPO strength generally decrease with increasing stress. Based on these observations, collaborators and I propose that a transition in the dominant mechanism of CPO development occurs with increasing stress, from grain-boundary migration, which consumes grains with low Schmid factors, at low stress, to the rotation of basal slip planes normal to the shortening direction at high stress.
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Mechanism of the CPO developed in sheared partially molten rocksCPOs in deformed mantle rocks provide a link between anisotropy in microstructure and kinematics of flow. Recent laboratory and field studies reveal the existence of a fabric with [100] axes sub-perpendicular to the shear direction in the shear plane and [001] axes sub-parallel to the shear direction, namely the "a-c switch", in partially molten olivine-rich rocks. Based on experimental results, collaborator and I conclude that the a-c switch is best explained by a "melt-assisted SPO-induced" mechanism: the CPO is primarily developed by the alignment of the long axes of grains in the flow field, while a minor portion is contributed by dislocation glide on the (010)[100] slip system. The presence of this CPO in partially molten regions of the upper mantle would have a significant impact on the interpretation of seismic anisotropy and extrapolation of the kinematics of flow.
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Viscous anisotropy in deformed partially molten rocksThe theory of anisotropic viscosity of partially molten rocks proposes that the presence of melt fundamentally changes the style of deformation, making it anisotropic with first-order, testable consequences, one of which is the prediction for a background, base-state melt segregation in deforming partially molten rocks. My experiments on partially molten rocks testified that, in torsional flow, base-state melt segregation occurs toward the center of a cylinder.
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Pressure shadows in sheared partially molten rocksAs a two-phase, solid–melt material flows around rigid particles, melt-depleted and melt-enriched regions (i.e., pressure shadows) develop due to the coupled fluxes of melt and solid driven by pressure gradients around the particles. To study this compaction–decompaction process, samples composed of fine-grained San Carlos olivine plus mid-ocean ridge basalt containing dispersed sub-millimeter-sized, single crystal beads of olivine were deformed in torsion. These experiments are the first to produce pressure shadows in partially molten rocks. One implication of this study is that it will be possible to constrain the ratio of bulk to shear viscosity, which is inferred from the distribution of melt using a combination of experimental observations and numerical simulations.
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