Olivine fabrics formed during deformation of two-phase rocks with different viscosity contrasts

Abstract:

Observations from field studies and experiments demonstrate that the presence of multiple mineral phases during deformation influences the evolution and preservation of microstructural and rheological properties in tectonites. Importantly, different phases in polymineralic rocks rarely have identical viscosities, and the potential effects of viscosity contrasts are poorly understood. To investigate the role of viscosity contrasts on microstructural evolution, we compare results from deformation experiments on samples of olivine + chromite, olivine + enstatite, and olivine + ferropericlase deformed at similar PT-conditions (1200–1300°C and 300 MPa) with identical strain geometry (torsion / simple shear). Each composition provides a unique viscosity contrast between olivine and a secondary phase: olivine << chromite, olivine <= enstatite, olivine > ferropericlase. Patterns of olivine crystallographic preferred orientations (CPOs) and low-angle (2–10°) intragranular misorientation axes reveal different dominant rotation axes in the mixtures, which we interpret to reflect a shift in dominant slip systems. In the olivine + chromite rocks, clusters of [100] and [001] parallel to the shear direction and lattice curvature about [010] suggest activity of (001)[100] and (100)[001] slip systems. In the olivine + enstatite rocks, shear-parallel clusters of [100]-axes and rotation about [010] and [001] are compatible with a combination of (010)[100] and (001)[100] slip systems. In the olivine + ferropericlase rocks, [100] and [010] form weak maxima within girdles inclined < 30° to the flow plane, and intragranular rotation about [001] dominates indicating prevalence of (010)[100] slip, potentially accompanied by [001] screw dislocations ± grain rotation and reorientation accommodated by grain boundary sliding. The variation in CPO patterns and inferred slip systems are not the result of differences in deformation conditions or bulk strain geometry. Our results demonstrate a need for caution when interpreting the kinematic and rheological significance of olivine CPOs in polymineralic tectonites.