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[1] Laboratory and field observations note the significant role of strength recovery (healing) on faults during interseismic periods and implicate pressure solution as a plausible mechanism. Plausible rates for pressure solution to activate, and the magnitudes of ultimate strength gain, are examined through slide-hold-slide experiments using simulated quartz gouge. Experiments are conducted on fine-grained (110 mm) granular silica gouge, saturated with deionized water, confined under constant normal stress of 5 MPa and at modest temperatures of 20 and 65°C, and sheared at a maximum rate of 20 mm/s. Data at 20°C show a log linear relation between strength gain and the duration of holding periods, whereas the higher temperature observations indicate higher healing rates than the log linear dependencies; these are apparent for hold times greater than $1000 s. This behavior is attributed to the growth and welding of grain contact areas, mediated by pressure solution. The physical dependencies of this behavior are investigated through a mechanistic model incorporating the serial processes of grain contact dissolution, grain boundary diffusion, and precipitation at the rim of contacts. We use the model to predict strength gain for arbitrary conditions of mean stress, fluid pressure, and temperature. The strength gain predicted under the experimental conditions (s eff = 5 MPa and T = 65°C) underestimates experimental measurements for hold periods of less than $1000 s where other frictional mechanisms contribute to strength gain. Beyond this threshold, laboratory observations resemble the trend in the prediction by our mechanistic model, implicating that pressure solution is likely the dominant mechanism for strength gain. The model is applied to the long-term prediction of healing behavior in quartzite fault zones. Predictions show that both rates and magnitudes of gain in contact area increase with an increase in applied stresses and temperatures and that fault healing aided by pressure solution should reach completion within recurrence interval durations ranging from <1 to $10 4 years, depending on applied stresses, temperatures, and reaction rates.

[1] A mechanistic model is presented to describe closure of a fracture mediated by pressure solution; closure controls permeability reduction and incorporates the serial processes of dissolution at contacting asperities, interfacial diffusion, and precipitation at the free face of fractures. These processes progress over a representative contacting asperity and define compaction at the macroscopic level, together with evolving changes in solute concentration for arbitrarily open or closed systems for prescribed ranges of driving effective stresses, equilibrium fluid and rock temperatures, and fluid flow rates. Measured fracture surface profiles are applied to define simple relations between fracture wall contact area ratio and fracture aperture that represents the irreversible alteration of the fracture surface geometry as compaction proceeds. Comparisons with experimental measurements of aperture reduction conducted on a natural fracture in novaculite show good agreement if the unknown magnitude of microscopic asperity contact area is increased over the nominal fracture contact area. Predictions of silica concentration slightly underestimate the experimental results even for elevated microscopic contact areas and may result from the unaccounted contribution of free face dissolution. For the modest temperatures (20-150°C) and short duration (900 hours) of the test, pressure solution is demonstrated to be the dominant mechanism contributing to both compaction and permeability reduction, despite net dissolution and removal of mineral mass. Pressure solution results in an 80% reduction in fracture aperture from 12 mm, in contrast to a $10 nm contribution by precipitation, even for the case of a closed system. For the considered dissolution-dominated system, fracture closure rates are shown to scale roughly linearly with stress increase and exponentially with temperature increase, taking between days and decades for closure to reach completion.

Background and Purpose-The effect of ebselen, a seleno-organic compound with antioxidant activity through a glutathione peroxidase-like action, on the outcome of acute ischemic stroke was evaluated in a multicenter, placebo-controlled, double-blind clinical trial. Methods-Patients diagnosed as having acute ischemic stroke who could receive drug treatment within 48 hours of stroke onset were enrolled. Oral administration of ebselen granules suspended in water (150 mg BID) or placebo was started immediately after admission and was continued for 2 weeks. The major end points were the Glasgow Outcome Scale scores at 1 month and 3 months after the start of treatment. The modified Mathew Scale and modified Barthel Index scores at 1 month and 3 months were also studied as secondary outcome measures. Results-Three hundred two patients were enrolled in the trial. Intent-to-treat analysis of 300 patients (151 given ebselen and 149given placebo) revealed that ebselen treatment achieved a significantly better outcome than placebo at 1 month (Pϭ.023, Wilcoxon rank sum test) but not at 3 months (Pϭ.056, Wilcoxon rank sum test). The improvement was significant in patients who started ebselen within 24 hours of stroke onset but not in those who started treatment after 24 hours. There was a corresponding improvement in the modified Mathew Scale and modified Barthel Index scores. Conclusions-Early treatment with ebselen improved the outcome of acute ischemic stroke. Ebselen may be a promising neuroprotective agent. (Stroke. 1998;29:12-17.)

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1] A model is presented for the compaction of granular aggregates that accommodates the serial processes of grain-contact dissolution, grain-boundary diffusion, and precipitation at the pore wall. The progress of compaction and the evolution of the mass concentration of the pore fluids may be followed with time, for arbitrary mean stress, fluid pressure, and temperature conditions, for hydraulically open or closed systems, and accommodating arbitrary switching in dominant processes, from dissolution, to diffusion, to precipitation. Hindcast comparisons for compaction of quartz sands [Elias and Hajash, 1992] show excellent agreement for rates of change of porosity, the asymptotic long-term porosity, and for the development of silica concentrations in the pore fluid with time. Predictions may be extended to hydraulically open systems where flushing by meteoric fluids affects the compaction response. For basins at depths to a few kilometers, at effective stresses of 35 MPa, and temperatures in the range 75°-300°C, rates of porosity reduction and ultimate magnitudes of porosity reduction increase with increased temperature. Ultimate porosities asymptote to the order of 15% (300°C) to 25% (75°C) at the completion of dissolution-mediated compaction and durations are accelerated from a few centuries to a fraction of a year as the temperature is increased. Where the system is hydraulically open, flushing elevates the final porosity, has little effect on evolving strain in these precipitation-controlled systems, and depresses pore fluid concentrations. These effects are greatest at lower temperatures.

Flow‐through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C. Measurements of fluid and dissolved mass fluxes, and concurrent X‐ray CT imaging, are used to constrain the progress of mineral dissolution and its effect on transport properties. Under constant effective stress, fracture permeability decreases monotonically with an increase in temperature. Increases in temperature cause closure of the fracture, although each increment in temperature causes a successively smaller effect. The initial differential fluid pressure‐drop across the fracture increases by two orders of magnitude through the 900 h duration of the test, consistent with a reduction of an equivalent hydraulic aperture by a factor of five. Both the magnitude and rate of aperture reduction is consistent with the dissolution of stressed asperities in contact, as confirmed by the hydraulic and mass efflux data. These observations are confirmed by CT imaging, resolved to 35 microns, and define the potentially substantial influence that benign changes in environmental conditions of stress, temperature, and chemistry may exert on transport properties.