Dynamic recrystallization and phase mixing in experimentally deformed peridotite

- Recrystallization, GBS and phase mixing explain observations from shear zones.
- Dynamic recrystallization and phase mixing preserve weak mantle shear zones.
- Phase mixing does not initiate shear localization.

Ductile shear zones evolve through complex feedbacks between microstructure and rheology. In the mantle, shear zones often display mylonitic or ultramylonitic microstructures, characterized by extensive grain-size reduction and well-mixed polymineralic domains. The feedback between the formation of these microstructures and grain-size sensitive deformation is often considered an important weakening mechanism in high-temperature mantle shear zones. To understand better the processes that transform coarse-grained tectonites into fine-grained and well-mixed mylonites, we have performed triaxial deformation experiments on synthetic peridotite samples comprised of mm-scale olivine and orthopyroxene clasts. Experiments were conducted in a Griggs apparatus at a confining pressure of ∼1 GPa, temperatures of 1400 to 1550 K and strain rates of 10−5-10−6 s−1 under nominally dry conditions. Experiments yield deformed samples with macroscopic natural strains ranging from 0.31 to 0.74. Samples are partially recrystallized, with neoblasts ranging in grain size from 2-25 μm. At these deformation conditions, unrecrystallized mm-sized clasts deform by grain-size insensitive dislocation creep, while recrystallized grains deform by grain-size sensitive dislocation accommodated grain boundary sliding (disGBS). Using electron-backscatter diffraction (EBSD) we investigate the interfaces between recrystallized olivine and orthopyroxene domains. At the majority of these interfaces no mixing is observed. However, many boundaries are serrated, which is interpreted to be the result of surface tension driven phase boundary migration. In a few cases we observe small degrees of mixing, with enclaves of one phase completely surrounded by the other phase. This mixing occurs only within a few recrystallized grain length scales of the interface. These results demonstrate that the serial processes of dynamic recrystallization, phase boundary migration and disGBS deformation are a plausible mechanism for solid-state phase mixing. However, more extensive deformation may be needed to reproduce the near steady-state microstructure observed in highly deformed mantle shear zones.