HR: 0800h
AN: T11B-1252    [Abstracts]
TI: Dependence of Dunite Viscosity on Oxygen Fugacity
AU: * Keefner, J W
EM: John@solarvision.org
AF: University of Minnesota, Department of Geology and Geophysics Pillsbury Hall, Minneapolis, MN 55455 United States
AU: Mackwell, S
EM: mackwell@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058 United States
AB: Although the viscosity of olivine single crystals exhibits a clear dependence on oxygen fugacity, published results for polycrystalline, olivine-rich aggregates are more ambiguous. Based on deformation experiments on single crystals of olivine (Bai, Mackwell, and Kohlstedt, 1991), strain rate increases as oxygen fugacity to the -0.03 to 0.4 power, the exact value depending on factors such as crystal orientation and oxide buffer condition. In experiments on \AA heim and Anita Bay dunite (Chopra and Paterson, 1981), samples jacketed in Ni tended to flow at lower differential stress than samples jacketed in Fe. An understanding of the dependence of oxygen fugacity on dunite will allow extrapolation of flow properties for a variety of mantle conditions. In the present study, samples were cored from blocks of \AA heim dunite. The samples were annealed for twelve hours prior to deformation in a one-atmosphere horizontal furnace at an oxygen partial pressure either near the Ni/NiO or the Fe/FeO buffer to drive off water bound in hydrous minerals and water adsorbed at free surfaces. The silica activity is buffered by the presence of orthopyroxene. Samples were then deformed in a gas-medium apparatus at a confining pressure of 300 MPa and a temperature range of 1400 to 1550 K at a series of constant stress conditions. The resulting stress exponent was in the range of 3 to 4 indicating that deformation was dominated by dislocation creep. Samples buffered by Ni/NiO deformed a factor of 5 to 10 faster than samples buffered by Fe/FeO. Analyses of the results yield an oxygen fugacity exponent of approximately 0.2. Therefore, since oxygen fugacity in the mantle is believed to have increased through time, the viscosity of the mantle would likely have decreased.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8162 Rheology--mantle
DE: 5120 Plasticity, diffusion, and creep
DE: 5455 Origin and evolution
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting