Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)

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Effect of water on the dislocation creep microstructure and flow stress of quartz and implications for the recrystallized grain size piezometer

Deformation experiments on Black Hills quartzite with three different initial water contents (as-is, water-added, and vacuum-dried) were carried out in the dislocation creep regime in order to evaluate the effect of water on the recrystallized grain size/flow stress piezometer. Samples were deformed in axial compression at temperatures of 750°–1100°C, strain rates between 2 × 10−7 s−1 and 2 × 10−4 s−1 and strains up to 46% using a molten salt assembly in a Griggs apparatus. An increase of the initial water content at otherwise constant deformation conditions caused a decrease in flow stress, an effect known as hydrolytic weakening. The total water content of the starting material was analyzed by Karl Fischer titration (KFT) and Fourier transform infrared (IR) spectroscopy, and quenched samples were analyzed microstructurally and by IR. Changes in the dynamic recrystallization microstructure correlate with changes in flow stress, but there is no independent effect of temperature, strain rate or water content. IR absorption spectra of the deformed spectra indicate that different water contents were maintained in the three sample sets throughout the experiments. However, the amounts of water measured within the vacuum-dried (∼260 ± 40 ppm H2O), the as-is (∼340 ± 50 ppm H2O), and the water-added (∼430 ± 110 ppm H2O) samples are significantly smaller than the initial content of the quartzite (∼640 ± 50 ppm H2O). Water from the inclusions in the starting material adds to the free fluid phase along the grain boundaries, which probably controls the water fugacity and the flow strength, but this water is largely lost during IR sample preparation. Vacuum-dried as well as water-added samples have the same recrystallized grain size/flow stress relationship as the piezometer determined for as-is samples. No independent effect of water on the piezometric relationship has been detected

Plagioclase preferred orientation in layered mylonites : evaluation of flow laws for the lower crust

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2008We evaluate the applicability of plagioclase and gabbro flow laws by comparing predicted and observed deformation mechanisms in gabbroic shear zones. Gabbros and layered gabbro mylonites were collected from the Southwest Indian Ridge (SWIR), ODP Hole 735B. Deformation temperatures are constrained by two-pyroxene thermometry, stress is estimated from grain size, and deformation mechanisms are analyzed by microstructure and the presence or absence of a lattice preferred orientation (LPO). Our analyses indicate that mylonite layers deformed at a strain rate in the range of 10-12 to 10- 11 s-1, while coarse-grained gabbro deformed at a strain rate of approximately 10-14 to 10- 13 s-1. Plagioclase in pure plagioclase mylonite layers exhibit strong LPOs indicating they deform by dislocation creep. Plagioclase grain size in mixed plagioclase-pyroxene mylonite layers is finer than in pure plagioclase layers, and depends on the size and proportion of pyroxenes. Progressive mixing of pyroxene and plagioclase within gabbro mylonite layers is accompanied by weakening of the LPO indicating that phase mixing promotes a transition to diffusion creep processes that involve grain boundary sliding. Our results indicate that experimental flow laws are accurate at geologic strain rates, although the strain rate for diffusion creep of fine-grained gabbro may be underestimated. At the conditions estimated for the SWIR crust, our calculations suggest that strain localization leads to a factor of two to four decrease in lower crustal viscosity. Even so, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle

The electrical conductance of fused sodium nitrate to 475 °C and 12 kbars

The electrical conductance of fused sodium nitrate has been measured in a piston-cylinder pressure apparatus between 320 and 475 °C to pressures of 12 kbars. The value of the activation volume for the specific conductance [math] equals 2.5 cm3/mole at 380 °C and is almost independent of temperature. Using PVT-data for sodium nitrate, isochoric activation energies have been calculated from measured isobaric activation energies

The structure of molten ZnCl2\mathrm{ZnCl_2}

High energy photon diffraction measurements on molten ZnCl2 have been carried out in the temperature range 623–853 K. The momentum transfer range covered is 0.15 Å−1<Q<21.6 Å−1. The sample is investigated in transmission geometry. The data are in good agreement with neutron diffraction measurements. A discrepancy at smaller Q values is to be noted, however, with earlier studies using conventional Mo–Kα X-rays in reflection geometry. By combining neutron and high energy photon results, the partial distribution functions can be separated to some extent. Almost no change of the first Zn–Cl distance is observed with increasing temperature, whereas the distribution of the first shell Cl–Cl distances increases in width and the first shell Zn–Zn distance decreases