Abstract
Two of the three processes making up the deformation mechanism of intergranular pressure solution, being dissolution and diffusion, take place in the grain boundary fluid phase. Hence, the structure and physical properties of wet grain boundaries under stress can be expected to influence the kinetics of both dissolution and diffusion,
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as a result of effects such as molecular structuring of the fluid film, the grain boundary surface area available for dissolution, the dissipation of energy otherwise available for driving dissolution/diffusion by other processes operating in the grain boundary, and expulsion of fluids from the grain boundary by surface energy driven grain boundary healing. Despite recent interest and experimental studies, the properties of fluid-filled grain boundaries, such as the fluid film thickness and contact roughness, are not well-constrained, especially in quartz. As a result of this poor understanding of grain boundary structures and processes, dissolution and diffusion kinetics in grain boundaries undergoing pressure solution are not well quantified. Quantification of these kinetics is crucial for the prediction of pressure solution rates in for instance fault gouges and sedimentary basins using theoretical rate models. The work presented in this thesis concentrates on investigating the structure of fluid-filled grain boundaries in geological materials undergoing pressure solution and the influence of this structure on pressure solution kinetics. In situ electrical resistivity measurements made on halite-glass and halite-halite contacts undergoing pressure solution showed that crystallographic orientation influences grain boundary structure and that contact type and relative misorientation have a significant effect on contact evolution. Models are presented that describe the effects of grain boundary structure and of plastic deformation of contact points in a rough (island-channel type) grain boundary on dissolution controlled pressure solution rates. When applied to quartz, these models show that, whereas grain boundary structure has negligible influence on dissolution rates, microscale plastic deformation at grain boundary islands could slow down pressure solution considerably. Another model is presented that predicts the onset of surface energy driven healing of a grain boundary with an island-channel structure when contact normal stresses fall below a critical stress, or “yield stress”, offering an explanation for the cessation of pressure solution in nature and experiment at low effective stresses. A second suite of experiments investigated the structure and diffusive properties of grain boundaries in silicate materials undergoing pressure solution, using electrical impedance spectroscopy on single glass-glass and glass-quartz contacts under hydrothermal conditions. Cracking and irregular dissolution on and around the contacts likely influenced the resulting grain boundary diffusivity and fluid film thickness estimates, but post-mortem study of the contacts formed did show that these contacts were rough. Finally, a set of isostatic compaction experiments performed on quartz aggregates, with special focus on investigating grain boundary structures and processes, showed that during quartz pressure solution at temperatures in the range 300-600C, dissolution in rough grain boundaries is the most likely rate controlling mechanism, and that cracking is an important mechanism for creating and maintaining contact roughness. Furthermore, evidence was found for limited plasticity at high stress grain contacts.
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