HR: 13:55h
AN: T13D-02    [Abstracts]
TI: The Effect of Large Melt Fraction on the Deformation Behavior of Peridotite: Implications for the Viscosity of Io$'$s Mantle and the Rheologically Critical Melt Fraction
AU: * Scott, T J
EM: scot0039@umn.edu
AF: University of Minnesota Dept. of Geology & Geophysics, 310 Pillsbury Dr SE, Minneapolis, MN 55455
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota Dept. of Geology & Geophysics, 310 Pillsbury Dr SE, Minneapolis, MN 55455
AB: To date, laboratory studies of the rheological properties of partially molten mantle rocks have reached melt fractions of $\phi$ $<$ 0.15, a value much smaller than thought to be appropriate for the asthenosphere of Io where the degree of partial melting may be as large as 40%. 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 1500 to 1553 K in order to determine the influence of large amounts of melt (0.15 $<$ $\phi$ $<$ 0.30) on the rheological behavior of partially molten rocks in both the diffusion and dislocation creep regimes. After hot-pressing San Carlos olivine (10 to 50 $\mu$m) plus MORB ($\sim$8 $\mu$m), the melt is homogeneously distributed between grain-size melt pockets at triple junctions and smaller pockets at two-, three- and four-grain junctions. Stress vs strain rate data from samples in the diffusion creep regime (stress exponent $\ital{n}$ = 1) reveal a drop in rock viscosity of several order of magnitude between $\phi$ = 0.25 and $\phi$ = 0.30, indicative of a rheologically critical melt fraction (RCMF). The combined results from experiments in the diffusion and dislocation creep regimes indicate that the flow behavior in both creep regimes is well described by the published flow laws [e.g. $\ital{Hirth & Kohlstedt}$, 2003] with strain rate $\dot{\epsilon}$ $\propto$ exp($\alpha\phi$) and $\alpha$ = 25 for diffusion creep or $\alpha$ = 30 for dislocation creep. By comparing these flow laws to published values of viscosity in models of whole-mantle or thin-asthenosphere convection on Io, we can place constraints on the likely grain size, melt fraction, and differential stress. For convection limited to a thin (10 to 100 km) asthenosphere, the published range for viscosity of $10^{8-12}$ Pa s requires grain sizes on the order of 10 to 100 $\mu$m in diffusion creep, or a differential stress of 5 to 120 MPa in dislocation creep with $\phi$ $\geq$ 0.25 over the temperature range 1500 to 1800 K. In contrast, for convection extending through the whole mantle, the published value for viscosity of $10^{17}$ Pa s necessitates a grain size $\ital{d}$ $\simeq$ 1 to 10 mm in diffusion creep, or differential stresses of $10^{-2}$ $<$ $\sigma$ $<$ 1 MPa in dislocation creep over the same temperature range and melt fraction. The extremely high differential stresses or the fine grain sizes required for asthenosphere convection are unlikely in a rock with the high melt fraction expected in Io$'$s mantle. A comparison of these values for $\ital{d}$, $\dot{\epsilon}$ and $\sigma$ to those for Earth$'$s upper mantle with $\ital{d}$ = 1 mm, $\dot{\epsilon}$ = $10^{-12}$ $s^{-1}$, and $\sigma$ = 0.1 MPa suggests that whole mantle convection is most likely for Io regardless of creep regime.
DE: 8147 Planetary interiors (5430, 5724)
DE: 8162 Rheology--mantle
DE: 5120 Plasticity, diffusion, and creep
DE: 6218 Jovian satellites
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting