HR: 0800h
AN: T41C-1318    [Abstracts]
TI: Strain Partitioning and Crystallographic Textures of Experimentally Deformed Olivine + Orthopyroxene Aggregates
AU: * Sundberg, M
EM: Marshall_Sundberg@Brown.edu
AF: Brown University, Dept. of Geological Sciences, Providence, RI 02912-1846 United States
AU: Cooper, R F
EM: Reid_Cooper@Brown.edu
AF: Brown University, Dept. of Geological Sciences, Providence, RI 02912-1846 United States
AB: The plastic deformation of polycrystalline rocks incorporates the grain matrix deformation of the individual grains (via chemical diffusion and/or dislocation propagation) acting in kinetic series with inter-grain sliding along grain- and phase-boundaries. In a polyphase aggregate where multiple types of interfaces exist, plastic deformation necessarily leads to strain partitioning and phase separation predicated on the relative effective viscosities of grain- and phase-boundaries. This segregation is sensitive both to lithology and stress, and manifests itself both in the grain-scale microstructures and in the lattice preferred orientations of the component phases. We have deformed fine-grained (~5 μm) aggregates of Balsam Gap dunite + Bamble, Norway orthopyroxene in triaxial compression and in (nominally) simple shear to investigate textural development in polyphase mantle rocks. Constant-load triaxial compression tests were conducted at P=300 MPa, T=1200°C in a Paterson-type gas medium apparatus. Simple shear tests were conducted at P=1.6 GPa and T=1200°C in a Griggs apparatus using a molten-salt cell at strain rates of 10^-5s^-1 to 10^-4s^-1. Samples were dried for 12 h (CO:CO₂ buffer at 950-1000°C) prior to sintering to ensure that these experiments were conducted in an anhydrous environment. No melt was produced during these experiments. Constant-load tests, conducted over the range of strain-rates used in the constant displacement-rate shear experiments, on a 50:50 mixture of olivine and orthopyroxene, reveal a stress exponent of 1.5±0.2. Similar experiments conducted on a 35:65 mixture of olivine and orthopyroxene reveal a stress exponent of 2.0±0.2. The fine grain size of the aggregates and the small stresses (<15 MPa) used in the creep tests would seem to preclude significant contribution of dislocation motion to the rheology. Samples composed of 35 vol% orthopyroxene deformed in shear at a constant strain-rate of 10^-5s^-1 develop a pronounced olivine lineation oriented ~35^o antithetic to the shear plane. Samples of identical composition deformed at a shear strain-rate of 10^-4s^-1 do not display this behavior. Samples with greater than 50 vol% olivine do not display an olivine or orthopyroxene lineation regardless of strain-rate. Olivine LPOs created in our two-phase aggregates are not consistent with patterns found in experimentally deformed, pure olivine aggregates. In samples with 65 vol% olivine deformed to γ=2, the [100] axes align in the shear plane perpendicular to the shear direction, while the [010] planes rotate 10-20° below the shear direction. In aggregates composed of 50 vol% or more pyroxene deformed to similar strains, the [100] axis lineation is identical, however the [010] planes back-rotate 10-20° from the shear plane.
DE: 3902 Creep and deformation
DE: 3947 Surfaces and interfaces
DE: 4485 Self-organization
DE: 5120 Plasticity, diffusion, and creep
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005