HR: 0800h
AN: V21A-0596    [Abstracts]
TI: The Effects of Strain and Strain Rate on the Spatial Separation/Segregation of Olivine and Orthopyroxene in a Synthetic Harzburgite
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 rheology of polyphase aggregates incorporates the grain-matrix deformation of the component phases plus grain- and solid-state phase-boundary sliding. The grain- and phase-boundary sliding acts in kinetic series with the grain-matrix mechanisms; in rheologies involving dislocation deformation, the boundary sliding component is almost never rate-limiting [e.g., Crossman and Ashby, 1975]; on the other hand, for diffusional rheologies, the boundary sliding can be both mechanically dominant and rate-limiting. For a given set of thermodynamic (e.g., temperature, pressure, deviatoric stress) and microstructural (e.g., grain size, lattice-preferred orientation) conditions, the energy-dissipation process for the deforming polyphase aggregate must involve the strain-effected separation of the phases, based upon the relative sliding (effective) viscosities of the grain boundaries and phase boundaries. The periodicity of the phase separation should be a function of the strain rate (or, conversely, for a given set of potentials, the aggregate strain rate will be one manifestation of the phase periodicity). We are engaged in an experimental study of the phase-separation scaling physics involved in the solid-state deformation of harzburgite. Specifically, we have prepared synthetic aggregates consisting of a 50:50 (by weight) mixture of ferromagnesian olivine and orthopyroxene, employing pulverized natural material; the hot-pressed aggregates have a grain size of approximately 5 micrometers. For the conditions employed in our experiments (Griggs molten salt confining-medium apparatus in simple shear ; 17 kb; $1200\,^{\circ}$C; 10$^{-5}$-10$^{-4}$ s$^{-1}$; strains of 3-4), the aggregates deform by boundary diffusional creep, conditions that specifically interrogate the relative viscosity of ol-ol and opx-opx grain boundaries and ol-opx phase boundaries. Backscattered electron imaging is employed to characterize the morphology of phases before and after deformation.
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 3939 Physical thermodynamics
DE: 3947 Surfaces and interfaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting