Dislocation creep of polycrystalline dolomite

摘要

The field of dislocation creep and rheological parameters for coarse-grained (d=240μm) natural dolomite has been determined through experiments performed at temperatures of 700-1000°C, effective pressures of 300-900MPa and strain rates of 10~(-4)/s to 10~(-7)/s. At low strain (<7%), dolomite aggregates deform homogeneously and define a power law between strain rate and differential stress with a stress exponent of 3.0+/-0.1, but at higher strains, through-going, fine-grained (<10μm) shear zones develop in the dolomite aggregates concomitant with strain weakening. Recrystallization is limited at low strain and microstructures observed in the low strain samples include undulatory extinction, twins, grain boundary bulging, limited recrystallization along twins and fluid inclusion trails. These same microstructures are present outside of the narrow, through-going shear zones in high strain samples; however, within the shear zones the grain size is small (<10μm) with some larger porphyroclasts (20-50μm). Shear zones nucleate at fine-grained zones formed at twin boundaries, twin-twin intersections and fluid inclusion trails and is likely due to a switch in deformation mechanism due to the large strength contrast between the fine-grained zones deforming by diffusion creep and the coarse-grained protolith. The activation energy (Q) for creep of coarse-grained dolomite at low strain is 145kJ/mol. In contrast to other activation energies for dislocation and diffusion creep of minerals, Q for dislocation creep of dolomite is considerably less than that for diffusion creep (248kJ/mol). The results of this study indicate that coarse-grained dolomite will initially deform by dislocation creep at natural strain rates and temperatures between 200 and 550°C, but due to limited recovery mechanisms, fine-grained shear zones will nucleate and diffusion creep may control the rheology of these fine-grained shear zones in nature at temperatures above ~300°C.