HR: 0800h
AN: T41B-1178    [Abstracts]
TI: Effect of Water Fugacity on Creep Strength of Anorthite
AU: * Rybacki, E
EM: uddi@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AU: Gottschalk, M
EM: mgott@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AU: Dresen, G
EM: dre@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AB: High-temperature creep of nominally water-free crustal silicates like quartz or feldspar is significantly affected by the presence of trace amounts of water. For water-saturated rocks the creep rate is expected to depend on water fugacity raised to a power of r. In this study we present first results on the fugacity exponent r and the activation volume V for creep of polycrystalline plagioclase. Fine-grained ($\sim$3 $\mu$m) anorthite aggregates were fabricated from a glass-powder by hot-isostatic pressing at 300 MPa pressure and a temperature of 1100$^{\circ}$C. To estimate the activation volume, 8 samples were subsequently dried ($\sim$ 0.003 wt% H$_{2}$O) and co-axially deformed at temperatures of 1125-1200$^{\circ}$C, pressures between 100 to 400 MPa, and axial stresses between $\sim$30 to 400 MPa, resulting in axial strain rates of about 7$\times$10$^{-7}$ - 5$\times$10$^{-4}$ s$^{-1}$. Sample deformation at constant load was dominantly accommodated by grain boundary diffusion creep for stresses below about 200 MPa. The resulting creep activation volume is about 24 cm$^{3}$mol$^{-1}$. A total of 20 samples were encapsulated in talc setting water activity aH$_{2}$O = 1. We used Fe, Ni or Cu jackets to buffer oxygen fugacity. Water content of samples after deformation was determined using Fourier-transformed infrared spectroscopy. Absorbance spectra suggest mainly molecular water located dominantly on grain boundaries. The water content is about 0.33 wt% H$_{2}$O. Wet samples were deformed at temperatures of 1000-1150$^{\circ}$C, confining pressures of 100 - 450 MPa, and stresses of $\sim$10 - 330 MPa, yielding strain rates of about 6$\times$10$^{-7}$ - 1$\times$10$^{-3}$ s$^{-1}$. In comparison to dry samples wet specimens are about 1.5 orders of magnitude weaker. In diffusion creep the fugacity exponent r is estimated to about 0.6. Samples buffered by Cu-CuO are considerably weaker than those deformed using Fe and Ni jackets. Since the strength of (dry) anorthite is independent of oxygen fugacity, our results suggest that hydrogen fugacity also affect the creep strength of wet plagioclase. The apparent hydrogen fugacity exponent is about -0.2.
DE: 8159 Rheology--crust and lithosphere
DE: 5120 Plasticity, diffusion, and creep
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting