HR: 1330h
AN: P12B-1069    [PDF]
TI: The Effect of Large Melt Fraction on the Deformation Behavior of Peridotite: Implications for the Rheology of Io's Mantle
AU: * Scott, T
EM: scot0039@umn.edu
AF: University of Minnesota, 310 Pillsbury Dr, Minneapolis, MN 55455 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, 310 Pillsbury Dr, Minneapolis, MN 55455 United States
AB: To date, laboratory studies of partially molten mantle rocks have reached melt fractions $\phi$ $\sim$ 0.15, a value much smaller than thought to be appropriate for the asthenosphere of Io where the degree of partial melting may be 40% or higher. Therefore, we have performed a series of high temperature, triaxial compressive creep experiments on dry synthetic peridotites in a gas medium apparatus at a confining pressure of 300 MPa and temperatures from 1450 to 1523 K in order to understand the influence of large amounts of melt (0.15 $< \phi <$ 0.40) on the rheological behavior of partially molten rocks. Mechanical mixtures of crushed and dried San Carlos olivine (1-10 $\mu$m) plus MORB ($\sim$8 $\mu$m) were isostatically hot pressed at 1450 K and 300 MPa for 4 h. Microstructural analysis indicates that after hot pressing the melt is homogeneously distributed between grain-size melt pockets at triple junctions and smaller pockets at two, three and four grain junctions. Analysis of stress vs. strain rate data from a sample containing $\phi$ = 0.23 $\pm$ 0.04 MORB with a grain size ${d}$ = 9.3 $\pm$ 2.3 $\mu$m deformed at 1450 K and differential stresses of 10 to 75 MPa in the diffusional creep regime (stress exponent ${n}$ = 1) demonstrates that the viscosity may be as much as three orders of magnitude higher than that predicted by a flow law reported for peridotites, where viscosity $\eta \propto$ exp(-25 $\phi$). For example, diffusional creep data from the sample with $\phi$ = 0.23 scaled to a grain size of 1 mm and extrapolated to a temperature of 1900 K (a possible value in Io's interior) yields a viscosity of 3x$10^{18}$ Pa s, compared to a viscosity of 2x$10^{15}$ Pa s calculated from the flow law and published results from numerical models with viscosity values of $10^{17}$ Pa s for a homogeneous convecting mantle and 10$^{8}$ to 10$^{12}$ Pa s for a thin convecting asthenosphere. Therefore, additional experiments on partially molten mantle rocks with relatively high $\phi$ are required to constrain the dynamic properties of Io's mantle.
DE: 3902 Creep and deformation
DE: 5120 Plasticity, diffusion, and creep
DE: 5430 Interiors (8147)
DE: 6218 Jovian satellites
DE: 8162 Rheology--mantle
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting