Different, but reasonable and well-accepted assumptions made about grain-boundary structure during pressure-solution (PS) creep may easily have an effect of more than 10 orders of magnitude on the calculated PS deformation rate. Understanding of grain-boundary structure during PS creep is therefore extremely important. Experimental evidence is presented in support of a grain-boundary model previously proposed by A. J. Gratz on the basis of observations on naturally deformed rocks. In this model, boundaries are assumed to have a static island-channel network structure. Channels are located where microcracks intersect the boundary. The rate of material transport is governed by thin-film diffusion at the islands. The model predicts that PS deformation rates are sensitive to the microcracking process. PS rate is therefore essentially dependent on whether microcracking can occur and how intense the microcracking is. These factors, in turn, depend on, among others, stress, temperature, effective pressure, and crystallographic orientation. The model qualitatively explains the observation made in experimentally and naturally deformed quartz rocks that the PS rate in single grains depends on crystallographic orientation. The model predicts that sudden increases in stress or fluid pressure may lead to microcracking and subsequently, many-orders-of-magnitude increase in PS deformation rate.

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