“Pressure solution” creep: Some causes and mechanisms

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Abstract
Despite the widespread evidence of stress‐controlled dissolution and precipitation in diagenetically altered and low‐grade metamorphic rocks, a great deal of controversy remains concerning the driving forces and transport mechanisms involved. To clarify the various driving forces, the free enthalpy equation is expanded here to allow identification of different terms contributing to the overall phenomenon. It is argued that under diagenetic conditions, stress concentrations at grain‐to‐grain contacts will be the largest source of chemical potential gradients and that upon burial and cementation these inhomogeneities decline and the orientation dependence of normal stress in a quasi‐homogeneous stress field becomes important as well. These mechanisms operate efficiently enough under these relatively cold, H2O‐rich conditions that stresses can remain below the threshold for crystal plastic deformation. Water on grain boundaries provides at the very least a high diffusivity path and in cases of large volume losses must also contribute directly through fluid flow. Most experimental work on this phenomenon has not distinguished carefully between stress‐enhanced solubility and solubility enhancement due to plastic deformation or microcracking. A new thermodynamic analysis of the results of some experiments by Sprunt and Nur suggests that in at least some of their experiments, true pressure solution creep has been activated. A related phenomenon, volume transfer creep during phase transformations which involve significant volume change, displays many of the characteristics of pressure solution.