Fluid circulation is essential for several geological processes. In the upper crust, brittle fractures and faults can provide pathways for surface fluids to reach ductilely deforming regions in the mid to lower crust where fluids can substantially weaken the rocks. Although this phenomenon is well established, the mechanism(s) that transport and redistribute fluids in the ductilely deforming regime of the lithosphere, and the resulting rheological consequences are incompletely known. This dissertation combines field observations and experiments to reach a better understanding of how fluids may penetrate into and be redistributed in low permeability, ductilely deforming rocks.
The field-based part of this project (chapter 2) focuses on observations from the northern Snake Range metamorphic core complex where meteoric fluids are known to have interacted with ductilely deformed footwall rocks below the detachment fault. Samples collected from a well-exposed, 150 m thick section of footwall quartzite mylonite contain abundant fluid inclusions (FIs) related to micro- and outcrop-scale structures. Higher salinity, miscible CO<sub>2</sub> and H<sub>2</sub>O fluids were trapped along healed fractures at conditions between 270 and 345 °C and 1 kbar confining pressure, and lower salinity CO<sub>2</sub> and H<sub>2</sub>O FIs formed later along healed fractures. Results suggest that meteoric fluids infiltrated this detachment system at pressure and temperature conditions above CO<sub>2</sub>-H<sub>2</sub>O miscibility during the latest stages of ductile deformation, but the precise pathway of infiltration, through brittle fracture or along grain boundaries, is unclear. Principal exhumation of the detachment system occurred along a high geothermal gradient (~ 70 °C/km), consistent with a previous estimations, and with fluid circulation driven by rapid exhumation of hot, mid to lower crustal rocks.
This field-based study is complemented by an experimental approach to the problem of hydration of ductilely deforming rocks. This approach necessitates the use of mineral aggregate that can deform to high finite strain at high temperature under accelerated laboratory conditions. Torsion experiments of fine-grained olivine aggregates at 1200 °C fulfill these conditions: olivine deforms in the dislocation creep regime, and the assemblage olivine + water is stable at that temperature. Therefore, olivine is used in the experimental part of this project; results obtained from olivine experiments can help understand the physical processes that operate in other mineral species, such as quartz, during plastic deformation in the presence of excess water.
A suite of anneal (static condition) and torsion experiments on olivine were designed to test various hypotheses for sample hydration and to determine the fate of FIs during high temperature deformation (i.e. dislocation creep). Talc was used in experiments to add water to the sample (via dehydration at elevated temperature), and was fitted as a sleeve around the outer cylinder of samples or inserted as a cylinder into cored samples.
Results from anneal experiments (chapter 3) indicate fluids infiltrated the sample along grain boundaries, and in some cases along fluid-filled fractures. Samples annealed with talc contain FI-depleted areas representing olivine grains that recrystallized before becoming water saturated, whereas FI-rich parts of the sample were over-saturated with water during recrystallization. FIs restrict grain growth by Zener pinning, and are more abundant in samples with higher water content, particularly along grain boundaries. High temperature torsion experiments (chapter 4) were performed on wet and dry olivine aggregates with and without talc. Samples deformed with talc (whether initially dry or wet) are substantially weaker owing to the presence of water. All samples display reduced grain size, shape preferred orientation of olivine grains, and a pervasive C'-fabric defined by the alignment of FIs. Like in the anneal experiments, samples that were hydrated by talc contain FI-rich and FI-depleted domains, but in this case domain boundaries have been distorted by deformation. Olivine pole figures obtained from EBSD-based crystallographic fabric analysis confirm deformation of olivine was accommodated by dislocation creep by activation of the (010) slip system in the recrystallization regime of grain boundary migration. Results indicate that shearing accommodated by dislocation creep rearranges and aligns FIs into bands at a low angle to the shear plane. The arrangement of FIs into low-angle bands is interpreted to have established under a pressure gradient that focuses fluids in low-pressure bands, locally enhancing permeability, and creating high diffusivity pathways in the sample.
Results of this work demonstrate that microstructures may be highly affected where fluids are introduced in excess into ductilely deforming rocks. Abundant FIs may prevent grain growth, which in turn may affect several grain size sensitive processes, and FIs may also be reorganized by deformation accommodated by dislocation creep (i.e. grain boundary migration recrystallization). These micro-scale results have important rheological implications for a broad range of tectonic settings where water may be available in excess in the Earth's lithosphere.
University of Minnesota Ph.D. dissertation. October 2013. Major: Geology. Advisor: Christian Teyssier. 1 computer file (PDF); xii, 148 pages.
Carter, Matthew James.
The role and fate of fluid inclusions in natural and experimental deformation.
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