The effect of porosity on the strength of quartz aggregates experimentally deformed in the dislocation creep regime

Abstract

The presence of porosity results in significant although transient strengthening of quartz aggregates experimentally deformed in the dislocation creep regime. Experiments were conducted on four quartz aggregates with porosities of < 1 to~ 8%, at 1073 K, 10 −6/s, and 1.5 GPa confining pressure. Optical and transmission electron microscopy show that non-porous quartzite deforms at these conditions by climb-accommodated dislocation creep with recrystallization by progressive subgrain rotation; steady state flow is achieved at ~ 10% strain. In the porous aggregates, very high densities of tangled dislocations develop around the pores when the samples are taken to run pressure and temperature. When sample shortening begins, strain is accommodated dominantly in the non-work hardened material away from the pores; the increased effective strain rate produces an increased flow stress. After ~ 10% strain the work hardened regions soften by grain boundary migration recrystallization, resulting in a lower effective strain rate and flow stress in the sample. After ~ 30% strain the initially porous samples have the same flow stress and microstructures as the non-porous samples. The results of this study have important implications for the experimental determination of flow law parameters, and possibly for the strength of rocks naturally deformed at metamorphic conditions where pores form in the presence of non-wetting fluids.