Fluid-rock interaction: from simple single crystals to complex natural systems

Fluid is present in many tectonic settings in the Earth's crust and plays a fundamental role in controlling chemical, physical and kinetic properties of rocks in both static and dynamic environments. This PhD project aims to provide a deeper understanding of the interaction between fluid, rock microstructure and chemical reactions by combining experimental and field approaches. In the experimental studies, a variety of aspects of fluid-mediated mineral replacement are explored in controlled static conditions. This study demonstrates that in simple salt systems fluid-mediated replacement reactions can create deformationresembling microstructures. Similar microstructures in natural samples may be misinterpreted as resulting from crystal-plastic deformation. The experiments on polycrystalline materials reveal that the microstructures of reaction products as well as the reaction pathways can vary dramatically depending on the rate-limiting step in the replacement process (either dissolution, component transport or precipitation) and can be modified by varying fluid composition. Even slight changes in the chemistry of the reactive fluid determine if the reaction is controlled by grain boundary geometry, crystallographic orientation or reaction-generated porosity. Furthermore, this study demonstrates that in systems, where fluid undergoes continuous compositional evolution during interaction with the parent material, rapid dissolution-precipitation processes can create mineralogically and structurally complex, symplectite microstructures. The thesis is concluded with a field study, comparing samples deformed in fluid-limited versus fluid-abundant conditions. The two cases display significant differences in the activated deformation mechanisms and chemical processes, indicating dramatically different rheologies and paths of microstructural evolution. Results of this thesis show that the presence of a reactive fluid in geologic systems is more than just a kinetic factor. Fluid can control the mechanisms that govern deformation and mineral reactions on a fundamental level. In-depth understanding of these controls can lead to more accurate models for predicting the consequences of fluid-rock interaction in a variety of physio-chemical systems.