The influence of three-dimensional grain size distributions on the rheology of polyphase rocks

In this study, the relation between the deformational behaviour of two-phase rocks and the grain size distributions of the constituent phases is investigated. Samples of hot isostatically pressed 50:50 volume percent mixtures of calcite and anhydrite were experimentally deformed in a triaxial apparatus at a temperature of 500°C, a confining pressure of 300 MPa, and a strain rate of 2×10−5 s−1. For calcite and anhydrite these are the deformational conditions for dislocation and diffusion creep, respectively, and for a grain size of 8 μm these are the conditions of the equiviscous point. It was found that samples from different batches of the synthetic material, while having an identical average two-dimensional grain size, displayed significantly different rheological behaviour. The ensuing full analysis revealed that the volumetric three-dimensional grain size distributions of the different phases and the different batches varied significantly. Since the anhydrite is in the transition between the dislocation and the grain size sensitive diffusion creep regime, the deformational behaviour of the bulk depends critically on the grain size distribution of the anhydrite phase. The usefulness of the concept of an average grain size is discussed.