Microstructural evolution of polycrystalline ice during non-steady state creep
The mechanics (rheology) of polycrystalline ice Ih are important to understand the dynamics of glaciers, ice sheets and icy moons in the outer solar system. Moreover, the microstructure and rheology of deforming polycrystalline ice are analogous to the creep of quartz in the Earth's crust. The exact relationship between naturally evolving microstructures in rocks (including polycrystalline ice) and aggregate rheology is still unconstrained. Creep experiments on polycrystalline ice offer a way to study this link but have been limited by the assumption that flow under mechanically constant conditions corresponds to a microstructural steady-state, meaning that deformation proceeds with some average microstructural property that does not change with time. To experimentally investigate non-steady state creep, a reliable sample-scale (10x mm) and grain-scale (µm) imaging method providing full crystallographic orientations is required. In this thesis, the creep of ice under constant stress conditions is explored using experimentally deformed, initially fine-grained (~250 µm) laboratory-made polycrystalline ice with a homogeneous microstructure, random crystallographic preferred orientation and no impurities. High-temperature (-2°C),low-stress (0.2-0.6 MPa) uniaxial compression experiments target deformation conditions relevant for glaciological purposes. Particle image velocimetry on the surfaces of deforming samples was used in this context to reconstruct velocity fields and strain rate distributions. A triaxial creep apparatus is employed to reach the high confining stresses (50 MPa) and low temperatures (-33° C) needed to use polycrystalline ice as a rock analogue. Microstructural information was extracted post-deformation from statistically significant datasets and grain-scale maps collected with cryogenic electron backscatter diffraction. Under glaciological deformation conditions, lobate grain boundaries and an increase in grain size from the starting material is observed at stresses of 0.6 MPa indicating that dislocation creep accompanied by grain boundary migration was prevalent. Samples deformed at 0.2 MPa show straight grain boundaries and an increase in grain size from the starting material and are interpreted to undergo normal grain growth. Grain boundary migration produces grain size distributions that are skewed towards large grain sizes, whereas normal grain growth yields bell-shaped distributions. Arithmetic mean and median grain size results suggest that the driving force for strain-induced grain boundary migration is larger than for normal grain growth. Constraints on the driving forces and the finding that grain size statistics can be employed to distinguish between normal grain growth and grain boundary migration are relevant for glacier field studies, in which these recystallisation mechanisms can be superimposed. Particle image velocimetry results show bands of high strain rate one order of magnitude above their surroundings, which change their location with progressive strain (transient shear zones). A spatial comparison of the particle image velocimetry results with post-deformation microstructures reveals that transient shear zones do not cause permanent microstructural modifications. The particle image velocimetry findings offer a new explanation for previously unexplained deviations in ow velocities in glaciers and ice sheets: the inert deformation heterogeneity of polycrystalline ice. Rheological weakening during deformation, strain localisations with local thickening of the deformed samples and microstructural shear zones with a reduced grain size from the starting material are observed under deformation conditions allowing extrapolations to the creep of quartz. The local appearance of a few recrystallised grains with a reduced grain size from the starting material correlates with strain weakening. Constant strain rates are concurrent with interconnected bands of recrystallised grains in the strain-localised areas. The mechanical evolution can be explained with a two-phase model if the small recrystallised grains deform by grain-size sensitive creep. A loadbearing framework model with a few highly viscous recrystallised grains in a less viscous remnant grain matrix is associated with strain weakening. A constant strain rate is established when the highly viscous bands of recrystallised grains form a grid, representing an interconnected weak phase model. The coherence between the two-phase model and the experimental observations demonstrates the impact of a single changing microstructural property, as the grain size, on aggregate rheology. Recrystallised grains are found to have straight grain boundaries, mostly ~90°angles and the same crystallographic orientation to the shortening direction in adjacent grains. Straight grain boundaries and subgrains with 90°angles are also detected in kinked remnant grains. These similarities between kinked grains and recrystallised grains indicate that grain boundary formation by kinking could cause the grain size reduction necessary for shear zone development. Kinking in single grains is characterised by slip on two crystallographic a-axes. The same slip direction is statistically preferred for the aggregate, confirming that kinking is the dominant strain accommodation and grain size reduction mechanism. The extension of kink boundaries into neighbouring grains suggests the ability of kinking to influence the local stress and strain field and to thus trigger shear zone propagation. This kink investigation illustrates the influence of grain-scale processes on aggregate rheology. In summary, the findings of this thesis demonstrate that dynamically evolving rock microstructures are subject to localisation. These localisations are the determining factor of the rheology in cases where no change in deformation conditions or composition occurs and are the key to understand and predict the rock rheology. Knowledge of rock (including polycrystalline ice) rheology is crucial for related research disciplines as ice sheet modelling for climate purposes, rock mechanics in the crust associated with earthquake nucleation and our general understanding of the solar system.
Advisor: Prior, David; Langhorne, Pat
Degree Name: Doctor of Philosophy
Degree Discipline: Geology
Publisher: University of Otago
Keywords: microstructure, rheology, ice, creep, deformation, ductile, localization, cryo-EBSD, polycrystalline, recrystallization, slip-systems
Research Type: Thesis