Rocks are composed of multiple mineral phases organized as polyphase crystal aggregates. However, previous studies assumed that the viscosity of a rock is represented by the viscosity of the primary phase in the rock, which oversimplifies the systems. The rheological properties of two-phase crystal aggregates are reviewed, focusing on two minerals representative of the upper mantle (olivine and pyroxene). Our research group performed several deformation and grain growth experiments on olivine and pyroxene aggregates with different volume fractions of pyroxene. Based on the experimental results, a constitutive law is proposed that relates grain size, viscosity, and mineral fraction in two-phase systems when a sample deforms under grain size sensitive creep. High strain torsion experiments were performed on olivine and pyroxene aggregates to understand the weakening process due to phase mixing. The microstructures observed in deformed samples are consistent with those found in naturally deformed rocks. The microstructural evolution of well-mixed fine-grained olivine and pyroxene aggregates can be explained from differences in diffusivity among Me (Fe or Mg), O, and Si, with the transport of MeO being significantly faster than that of SiO2. It is proposed that pyroxene plays a key role in plastic deformation, leading to long-term weakening associated with phase mixing.
