Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone

Abstract

The influence of pyroxene on upper mantle viscosity remains unclear but may occur either through its difference in strength relative to olivine or through its effect on olivine microstructure. To determine the role of pyroxene in moderating peridotite viscosity, we analyzed microstructures in paired harzburgites and dunites from a natural shear zone exposed in the Josephine Peridotite. Olivine crystallographic textures evolve similarly in harzburgites and dunites with increasing strain, indicating the operation of similar deformation mechanisms in olivine. The mean olivine grain sizes are~1.5 times larger in dunites than in harzburgites, whereas the mean olivine subgrain sizes are a factor of~1.2 smaller in dunites than in harzburgites. The average stresses in olivine, estimated with a subgrain-size piezometer, are inversely correlated with pyroxene volume fraction and directly correlated with the mean olivine grain size. The calculated ratio of pyroxene viscosity to olivine viscosity for each harzburgite/dunite pair varies from 1.2 to 3.3. Notably, our data indicate that olivine viscosity is non-Newtonian with a finite grain-size sensitivity. We suggest that at the conditions of the shear zone, pyroxene is more viscous than olivine but that the increase in aggregate strength with larger pyroxene fractions is offset by correspondingly smaller olivine grain sizes. Thus, major rheological weakening associated with pyroxene content may only be possible if (1) temperatures are higher than those characterizing this shear zone, increasing the viscosity contrast between olivine and pyroxene or (2) stresses are high enough that grain-size reduction promotes a transition to a deformation mechanism with extreme grain-size sensitivity, such as diffusion creep.

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due To Phase Mixing: Part 2. Microstructural Development

“…It is often argued that the grain size reduction associated with phase mixing and Zener pinning causes a transition from a dislocation‐dominated creep mechanism to diffusion creep (Bercovici & Skemer, ; Hansen & Warren, ; Herwegh et al, ; Linckens et al, , ; Warren & Hirth, ). Two results from the present study—one based on deformation results and the other on microstructural observations—indicate that this transition does not occur in our experiments.…”

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior

“…Grain‐size reduction is considered critical to the formation of mantle shear zones that are often composed of very fine grained mylonites and ultramylonites (e.g., Michibayashi & Mainprice, ; Warren & Hirth, ). In naturally deformed peridotites, fine‐grained regions composed of thoroughly mixed olivine and pyroxene grains provide zones in which strain localization occurs (Hansen & Warren, ; Linckens et al, , ; Tasaka et al, ; Warren & Hirth, ). If significant grain size reduction occurs, deformation by a grain‐size sensitive creep process, such as diffusion creep (diffusion‐accommodated grain‐boundary sliding) and dislocation‐accommodated grain‐boundary sliding, will enable strain to localize in the finest grain‐size regions.…”

Evolution of the Josephine Peridotite Shear Zones: 1. Compositional Variation and Shear Initiation

“…These three subparallel shear zones are among the best exposed in the Fresno Bench outcrop. The microstructures of all three shear zones are closely examined in a companion study (Kumamoto et al, in revision), as well as previous work (Hansen & Warren, ; Recanati et al, ; Skemer et al, ; Warren et al, ). Our data set indicates that the relationship between water and shear localization is more complex than previously estimated.…”