INVESTIGATING THE ROLE OF INITIAL MICROSTRUCTURAL FEATURES ON PHASE MIXING AND LOCALIZATION DURING DEFORMATION OF OLIVINE-BEARING ROCKS (Invited Presentation)

Mylonitic peridotites in exhumed shear zones have undergone highly localized deformation at high temperature in the lithospheric mantle. These fine-grained rocks are composed of multiple mineral phases including olivine (Ol), orthopyroxene (Opx), and clinopyroxene that are uniformly dispersed amongst one another at the grain scale. Although the physical processes responsible for producing fine grain sizes are relatively well understood, the microphysical mechanisms that lead to phase mixing are less well constrained. This is in part because many previous experimental studies on polymineralic rocks started with samples in which the individual mineral phases were already relatively well-mixed with one another at the scale of the grain size, making it difficult to examine the microstructural evolution that occurs at the onset of phase mixing.

To study these microstructural processes, we performed deformation experiments on two-phase samples composed of either Ol plus Opx or Ol plus ferropericlase (Fp). Samples were fabricated with different initial microstructures including a well-mixed, non-clustered distribution of the two phases, clusters of large Fp grains in an Ol matrix, and large single crystal clasts of Opx or Fp in an Ol matrix. Samples were then deformed in torsion at a constant strain rate in a triaxial, gas-medium deformation apparatus at T = 1523 K and P = 300 MPa to shear strains of γ > 4.

Microstructural observations of the deformed samples demonstrate differences in mixing behavior associated with different initial microstructures. In samples with single crystal clasts of the secondary phase, very little mixing took place; no mixing is observed along the boundary between the two phases and is only found to occur at the ends of the inclusion where stress is concentrated, suggesting such mechanical mixing is inefficient. Mixing is enhanced if the secondary phase grains are smaller than the single crystals and arranged in more closely spaced clusters. In this case, localization occurs, forming S-C fabrics. In contrast to experiments carried out on already well-mixed samples, these experiments demonstrate that the initial size and distribution of the secondary phase inclusions are important for the evolution of a polymineralic rock into fine-grained, well-mixed mylonites.