The kinetics of microstructural evolution during deformation of calcite

Abstract

There is a strong coupling between the microstructure and the strength of rocks that is thought to play a key role in the evolution of shear zones and in our ability to interpret the mechanics of natural deformation processes. To investigate the microstructural evolution of calcite‐rich rocks, we have performed a series of hydrostatic and deformation experiments on synthetic calcite aggregates at 1023 K and 300 MPa. The mechanical data from our experiments were broadly consistent with a composite flow law for concurrent dislocation and diffusion creep. Recrystallization rates responded to the deformation conditions. When the bulk strain rate was dominated by diffusion creep, calcite grains grew at the same rate as occurs by normal grain growth under isostatic conditions. When the dominant deformation mechanism was dislocation creep, the matrix grain size was reduced at a rate that varied directly with product of stress, strain rate, and the square of grain size. Thus, reduction rate was proportional to mechanical work rate. If the stable grain size achieved during deformation depends on the product of stress and strain rate, rather than stress alone, then that grain size is an indication of the work rate and can be used as a paleowattmeter. Following this line of thought suggests that the gradient of recrystallized grain sizes that is often observed in the highly deformed portions of shear zones would not require a gradient in stress but could also be explained by material softening, resulting in locally elevated strain rates under constant stresses.