Deformation Mechanics of an Asperity under Hydrothermal Conditions

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Pressure solution is a deformation mechanism involving fluid-laden granular rocks at relatively shallow depths in the continental crust. Such solution transport mechanisms are important during natural processes, including the compaction of sediments, subsidence of sedimentary basins, fault strengthening, and crustal orogeny, and for engineering and industrial applications, such as the characterization of aquifers, transport of pollutants, oil exploration and production, geothermal energy recovery, nuclear waste disposal, and carbon dioxide sequestration. Although there is copious evidence of the importance of this ubiquitous process, a detailed constitutive law does not exist that would enable scientists to predict the rate of deformation under the natural conditions. In this project, both numerical experiments and laboratory tests are being conducted to investigate the physics and kinetics of deformation at small asperities in granular materials. The experiments are being done by students and staff at the Massachusetts Institute of Technology, in collaboration with S. Hickman and N. Beeler, USGS, Menlo Park. The experiments are designed to measure deformation of quartz asperities pressed against either quartz or sapphire plates at temperatures up to 600 C and fluid pressures of 150 MPa or less for periods of 1-4 weeks. For the initial lab experiments, sapphire is being used as a proxy "insoluble" mineral because it is easily polished, strong, and transparent enough to be used as a window material. The microstructure produced at the contact region is being observed after deformation using optical, transmission electron, and scanning electron microscopy. In addition to the laboratory experiments, numerical calculations are being performed to investigate the coupling of transport, dissolution, and precipitation processes, the effect of variations in stress distribution, the production or decay of transients, and the relation of the kinetics of the asperity deformation to the kinetics of the component processes. Initial numerical models suggest a much more intricate coupling of the processes than thought heretofore. In addition, the calculations predict a rather long transient period that will, if confirmed, be critical to understand if laboratory experiments are to be extrapolated accurately to natural conditions.