HR: 16:15h
AN: MR24A-02    [Abstracts]
TI: The Rheology of Dry, Melt-Free Polycrystalline Fo90 Olivine
AU: Faul, U
EM: uli.faul@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: * Jackson, I
EM: ian.jackson@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: We have recently measured the shear modulus and attenuation of pure, fine-grained solution-gelation-derived olivine at seismic frequencies (Jackson et al., JGR, 2002). These data have been fitted to a Burgers type model that includes a term for Newtonian viscous behavior. However, at the relatively short time-scales and microstrains of the torsional forced oscillation experiments the viscosity is not very well constrained. Motivated by the need to better constrain the long-term viscous behavior we have begun a program to determine the finite strain rheology of these materials. A particular aim is to determine the activation energy for diffusion creep over a similar temperature range as for the torsional forced oscillation data. After reaction of the gel to olivine in a controlled atmosphere furnace, cold-pressed pellets are hot-pressed at a range of temperatures to produce a range of mean grain sizes. The subsequent triaxial compressive creep experiments are conducted in a Paterson-type gas medium apparatus at a confining pressure of 300 MPa and temperatures in the range from 1150 to $1350\deg$C. Each sample is deformed at a range of applied loads at a constant temperature. During each segment of a test the load is held constant to determine the strain rate for a given stress before stepping up in load for the next constant load segment. Preliminary observations indicate that grain growth is minor during the deformation tests; mean grain sizes vary with hot-pressing temperature from 3 to 6 micron. The deformation tests clearly show the transition from diffusion to dislocation creep at stresses of 100 to 150 MPa. When the diffusion creep data is normalized to a common grain size with a grain size exponent of 3, strain rates at a given stress for these genuinely melt-free samples are up to 2 orders of magnitude lower than those observed by Hirth and Kohlstedt (JGR, 1995) for nominally melt-free samples with grain sizes $>$ 10 micron prepared from natural olivine. However, our strain rates are similar to those reported by Beeman and Kohlstedt (JGR, 1993) for melt-free samples with grain sizes similar to ours. These initial results suggest that either the presence of melt has a larger effect on the strength of upper mantle rocks than has previously been reported or the grain size exponent has been overestimated.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 5120 Plasticity, diffusion, and creep
DE: 3902 Creep and deformation
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting