The influence of water on the formation of mantle shear zones

The formation of localized shear zones is a prerequisite for plate tectonics, yet the mechanisms that drive shear localization are poorly constrained. In the oceanic lithosphere, ductiley-deformed peridotites have been found at detachment and transform faults, indicating the importance of mantle shear localization for plate spreading. Proposed mechanisms for initiating shear localization include water or melt addition, grain size reduction, and effects due to lattice preferred orientations (LPO). However, the sequence of physical processes that leads to ductile shear localization in the mantle lack direct geological observation for the generation of fine-grained (~1-100 μm) mylonites from coarse-grained (0.5-5 mm) protoliths. The Josephine Peridotite in Oregon provides a field setting to test hypotheses for initiation of shear localization. The massif is an obducted section of oceanic lithospheric mantle, which contains a sequence of small shear zones that vary from 1 m to 60 m wide. Measurement of the passive rotation of pyroxenite layers across the shear zones provides an estimate strain. Mylonites are observed at the center of the narrowest shear zones, which also record the highest strains (>2000 %). Field observations of the presence and abundance of syn-kinematic melt veins, consisting of gabbro or dunite, provide constraints on the role of melt in shear localization. Variations in water content appear to drive strain localization in the Josephine. Water, present as hydrogen defects in nominally anhydrous minerals (e.g., olivine, orthopyroxene) can significantly influence mantle rheology. One of the highest strain shear zones has an olivine LPO that corresponds to Type-E (001)[100] slip, whereas the LPO of the widest shear zone corresponds to Type-A (010)[100] slip. Experimental constraints on olivine LPO indicate that Type-E slip corresponds to olivine with a higher water content than Type-A slip. To confirm the role of water in shear localization, we analyzed water concentrations in olivine and orthopyroxene by secondary ion mass spectrometry. Olivine was found to have diffusively lost hydrogen, whereas orthopyroxene retained its upper mantle composition. Therefore, olivine water concentrations were estimated from orthopyroxene concentrations, combined with an olivine/orthopyroxene partition coefficient of 0.11. In the widest shear zone, olivine contains 355 ±31 ppm H/Si, whereas olivine from the narrower shear zone has 502 ±44 ppm H/Si. These concentrations approximately correspond to the experimentally-determined transition between olivine Type-A and Type-E slip. To further examine the role of water, we ran 1-D models of shear zone development with water introduced as 5 m wide zones of elevated concentrations. Strain rates and viscosity were calculated using olivine dislocation creep flow laws, while water diffusion was calculated using the olivine diffusion coefficient at 1000 C. The width of the shear zone that developed was found to decrease with increasing initial water content. However, widths <100 m could not be produced for reasonable (<800 ppm H/Si) water contents. This suggests that while water may have provided the initial perturbation for strain localization, additional mechanism(s) were required to generate the Josephine shear zones. Based on additional field considerations, the most likely candidate is geometrical weakening due to LPO development.