Rheology of Two-Phase Systems

Ductile shear zones commonly contain distinctive bands of high strain rock characterized by dispersed fine-grained two-phase or polyphase material. Microstructure and weak CPO suggest that these may deform primarily by grain-boundary diffusion creep. It is unclear how such zones develop, how the phases become evenly dispersed, what controls their rheology, and what controls their grain-size evolution. We propose the following generic scenario, applicable to mixtures of materials with similar diffusion coefficients and surface energies, such as quartz-feldspar, olivine-pyroxene, or calcite-dolomite. 1) Deformation of an initially coarse-grained two-phase aggregate results in grain refinement by dynamic recrystallization, and possibly also by brittle fracture of the stronger phase. Stress concentration in the stronger phase results in a greater degree of grain-size reduction. 2) Grain-size reduction in both phases results in a transition to grain-size sensitive creep, producing alternating bands of relatively weak fine-grained material. 3) Grain-boundary sliding in the finer, strong-phase material is facilitated by grain-boundary diffusion (Coble) creep of the weaker phase. The weaker phase fills the spaces between the strong grains; hence the grain-size of the weaker phase is controlled by that of the stronger phase. This leads to mixing and dispersion of the two phases, producing a fine-grained, evenly dispersed two-phase aggregate, with a grain-size dictated by that of the stronger phase. 3) The rheology of the aggregate will be controlled by the volume concentration of the two phases and their respective individual rheologies, but will be dominated by grain-boundary diffusion creep of the weaker phase. 4) Grain-growth in the mixture is limited by the diffusion coefficient for grain-boundary diffusion of the less abundant phase, as growth of isolated grains requires diffusion along the grain-boundaries. The growth law for the aggregate is slowed relative to that of the single phase by the ratio of grain-boundary length / grain-boundary thickness for the dispersed phase, which is likely to be of the order of 10^5. This therefore provides a mechanism for developing and maintaining strongly weakened zones of two-phase mylonite that will localize deformation.