Dislocation creep of fine-grained olivine

Deformation experiments conducted in a gas medium apparatus at temperatures from 1200 to 1350 degrees C with a fine-grained, solution-gelation derived Fe-bearing olivine show a stress dependence of the strain rate at stresses above similar to 150 MPa, which is much stronger than previously reported for polycrystalline samples. The data can be fit by a power law with epsilon proportional to sigma(n) with n similar to 7-8, or equally well by a Peierls creep law with exponential stress dependence. Due to the observed strong stress dependence the samples deform at significantly higher strain rates at a given stress than single crystals or coarse-grained polycrystals with n similar to 3.5. TEM observations indicate the presence of dislocations with at least two different Burgers vectors, with free dislocations predominantly of screw character. Subgrain walls are present but are only weakly developed and have small misorientation angles. Both the rheology and dislocation structures are consistent with creep rate-limited by dislocation glide or cross slip for aggregates with grain sizes smaller than or approaching the recrystallized grain size. Deformation mechanism maps extrapolated to lithospheric temperatures using the melt-free diffusion creep rheology of Faul and Jackson (2007), the dislocation creep rheology of Hirth and Kohlstedt (2003), and the results described here indicate that deformation conditions of ultramylonitic shear zones fall near the triple point of Peierls, dislocation, and diffusion creep.