Kinetic effects of microscale plasticity at grain boundaries during pressure solution

Abstract

It is generally assumed in kinetic models for pressure solution in materials such as quartz that the effective dissolution rate coefficient in grain boundaries is equal to the conventional geochemical dissolution rate coefficient on a free surface. However, predictions based on this assumption usually overestimate both natural and experimental pressure solution rates even when evidence for dissolution rate control is strong. A possible explanation for this discrepancy is that grain boundary structure and dissipative processes such as microscale plasticity in the grain boundary can decrease this effective rate coefficient. On the basis of a simple grain boundary model assuming an island‐channel structure, we have derived a preliminary model for the effect that dissipation by plastic deformation (work‐hardening flow and creep) of grain boundary islands has on dissolution‐controlled pressure solution rates. Comparing the predictions of this model with the experimental data on quartz pressure solution rates, we see that microscale plasticity at grain boundary islands does slow down pressure solution and can help explain the discrepancies between observed and theoretical pressure solution rates. When applied to pressure solution creep of sandstones or fault rocks in nature, our model predicts that grain boundary plastic deformation in quartz might have a significant effect at depths beyond ∼9–10 km.