Low Water Content in the Center of an Upper Mantle Shear Zone

Abstract:

Trace amounts of water in the nominally anhydrous minerals of the upper mantle can drastically affect mantle properties. The presence of water reduces viscosity and causes melting at lower temperatures, both of which weaken the rock hosting the water. We investigate the relationship between water content, deformation, and olivine LPO in an upper mantle shear zone in the Josephine Peridotite of SW Oregon. Previous work in the Josephine focused on two shear zones, showing that water concentrations were higher in the centers of shear zones compared to the edges. This study focuses on a previously unanalyzed shear zone (Shear Zone A; SZA) that reaches a maximum strain of 64 with accompanying grain size reduction to ~200 μm from ~2 mm.

Electron backscatter diffraction was performed on a transect of samples from the edge of the shear zone to the core. As strain increases, the olivine LPO evolves from a pre-existing orientation to a Type E slip system (slip on the (001) plane in the [100] direction). Olivine [100] axes are tilted past the shear plane at high strain, consistent with previous observations, though rotated to a higher angle (30°) than previously observed.

Water concentrations in opx were measured and used to predict olivine concentrations in the same transect assuming an olivine/opx partition coefficient of 0.11. Opx in the core of SZA has ~200 ppm H₂O, which corresponds to olivine with ~22 ppm H₂O (~360 ppm H/Si). This low water is unexpected as the Type E LPO is usually associated with higher water contents. In addition, the transect shows that the center of the shear zone has a low water content compared to the rest of the shear zone. This is also unexpected as higher water was predicted to localize deformation; instead, the lowest water contents are found where the degree of deformation is highest. Hypotheses to explain this behavior include: a) melt formation in the center of the shear zone due to shear heating, followed by extraction, b) water content decreasing as a function of deformation due to recrystallization, and c) interacting shear zones leading to unusual water gradients over the small range of current samples.