Evidence for enhanced deformation in two-phase rocks: Experiments on the rheology of calcite-anhydrite aggregates

We investigated the deformational behavior of fine-grained calcite- anhydri te aggregates at conditions where the end-member phases are of similar stre ngth and deform by dislocation creep and/or diffusion creep. Samples of var ious volume proportions were made by hot isostatically pressing CaCO3 and C aSO4 powders into dense aggregates. Grain sizes are 5 and 8 mu m for two di fferent anhydrites (An) and 8 mu m for calcite (Cc). In all mixtures, grain sizes are 2-4 mu m for both phases. The stress difference sigma, strain ra te (epsilon)over dot and grain size d data are fit to a power law with (eps ilon)over dot proportional to sigma(n). At 500 degrees C, grain flattening accounts for most of the sample strain for all mixtures, suggesting that di slocation creep is the dominant deformation mechanism. However, the mixture s, especially the 50:50, are often weaker than the end-member phases. At 60 0 degrees C the mixtures deformed dominantly by diffusion creep, as evidenc ed by n = 1, the sensitivity of aggregate strength to the grain size of at least one component, little evidence is found for grain flattening and chan ge in microstructures compared to the undeformed materials, and the mixture s are generally weaker than both end-members. However, the grain size diffe rence is not enough to account for the strength difference. For a small num ber of experiments, the variability in the strengths appears to correlate w ith the fraction of interphase boundaries; weaker samples have higher fract ions of boundaries between unlike phases (Cc-An) than stronger samples. We suggest that the relative weakness of the mixtures is at least in part due to enhanced boundary diffusion rates between unlike phases relative to like phases.