ALBERT

Description: [1]  The concentration and distribution of volatiles in the Earth's mantle influences properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H 2 O, P, and F were measured in olivine, orthopyroxene and clinopyroxene by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H 2 O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and co-existing pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H 2 O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 10 19  Pa s at 1250 °C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 10 21  Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for re-equilibration.

Description: Abstract Shear localization in the upper mantle, a necessity for plate tectonics, can have a number of causes, including shear heating, the presence of melt, the development of a strong crystal‐preferred orientation, and the presence of water. The Josephine Peridotite of southwestern Oregon contains shear zones that provide an excellent opportunity to examine the initiation of shear localization. These shear zones are relatively small‐scale and low‐strain compared to many shear zones in peridotite massifs, which typically have extreme grain size reduction indicating extensive deformation. We use major, trace, and volatile element analyses of a large suite of harzburgites from the Fresno Bench shear zones to evaluate the mechanisms leading to shear localization. Lithological evidence and geochemical transects across three shear zones show a complex history of melting, melt addition, and melt‐rock interaction. The distribution of aluminum and heavy rare earth elements across the shear zones suggest that melt flow was focused in the centers of the studied shear zones. Water concentrations in orthopyroxene grains of 180–334 ppm H2O indicate a comparatively high degree of hydration for nominally anhydrous minerals. The correlation of water with aluminum and ytterbium in orthopyroxene is consistent with a melt source for this hydration, suggesting that water equilibrated between the melt and peridotite. The presence of melt and hydration of the host rock provide mechanisms for initial weakening that lead to localized deformation.