HR: 0800h
AN: T11B-1257    [Abstracts]
TI: Effect of Water and Iron Content on the Rheological Behavior of Olivine
AU: * Zimmerman, M E
EM: zimme030@umn.edu
AF: University of Minnesota, Dept of Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Zhao, Y
EM: zhaoy@pku.edu.cn
AF: Peking University, Department of Geophysics, Beijing, 100871 China
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, Dept of Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: We have undertaken an experimental investigation of the effect of water and iron content on the viscosity of aggregates of Fe-Mg olivine in order to provide a basis for conparing convection models for the mantle of Earth with those for the more iron-rich mantle of Mars. Our study builds on three experimental observations: (i) At a given temperature, the viscosity of single crystals of San Carlos olivine [Fo$_{90}$ = (Fe$_{0.1}$Mg$_{0.9}$)$_{2}$SiO$_{4}$] is signficantly higher than that of crystals of fayalite, Fo$_{0}$, (ii) the viscosity of San Carlos olivine decreases with increasing water concentration, and (iii) the solubility of water in olivine increases with increasing iron concentration. To extend deformation experiments to polycrystalline samples of olivine of higher Fe content, powders of Fo$_{50}$ and Fo$_{70}$ were fabricated from mixtures of natural olivine, Fo$_{90}$, and synthetic fayalite, Fo$_{0}$. The resulting materials were ground into fine ($<$10 $\mu$m) powders, cold-pressed into Ni capsules, and then hot-pressed at 300 MPa and 1533 K for 2 to 12 h. For experiments under hydrous conditions, two drops of water, each $\sim$0.03 ml, were added before sealing a sample within telescoping Ni cans for deformation. The average grain size of the resultant hot-pressed samples were between 20 and 57 $\mu$m . In the samples deformed under hydrous conditions, water bubbles were present both within olivine grains and along grain boundaries, demonstrating that the samples were water-saturated. High-temperature, high-pressure compressive creep experiments in both the diffusion and the dislocation creep regimes were carried out using a gas-medium apparatus at temperatures of 1223 to 1473 K and a confining pressure of 300 MPa. Under both anhydrous and hydrous conditions, the viscosity of samples of Fo$_{50}$ is a factor of $>$10 lower than the viscosity of samples of Fo$_{70}$, which is a factor of $>$10 lower than the viscosity of samples of Fo$_{90}$. The viscosity of a sample of a specific Fe:Mg ratio deformed under hydrous conditions is a factor of $\sim$10 lower than its counterpart deformed under anhydrous conditions. Therefore, at the same thermodynamic conditions (e.g., P, T, water fugacity), the viscosity of the more Fe-rich mantle ($\sim$18 wt % FeO) of Mars will be a factor of $\sim$3 lower than the mantle ($\sim$8 wt % FeO) of Earth.
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