HR: 1340h
AN: MR13C-1408    [Abstracts]
TI: Shear Attenuation and Dispersion in Harzburgite
AU: * Sundberg, M
EM: marshall_sundberg@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912, United States
AU: Cooper, R F
EM: reid_cooper@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912, United States
AB: The shear attenuation, and the associated dispersion in the shear modulus, of fine-grained, texturally mature, synthetic vacuum-sintered aggregates of olivine (Fo₉₁) + orthopyroxene (En₉₂) was measured over a frequency range of 10^-2.25 to 1 Hz at temperatures between 1000 and 1300°C in a 1-atm reciprocating torsion apparatus. Experiments were conducted on samples with a range of orthopyroxene contents (0-40 wt%) and a range of average grain sizes (2-10μm). All samples contain a small fraction (φ < 0.01) of silicate melt. In experiments with little or no pyroxene (<20%), high-temperature and/or low frequency measurements show a monotonic decrease in Q_G^-1 with an increase in frequency where Q_G^-1 \propto f^-0.35, consistent with diffusion-limited relaxation of normal tractions along grain boundaries. At lower temperature and/or higher frequencies this behavior gives way to a broad dissipation plateau consistent with a contribution of grain boundary sliding to the dissipation. In experiments with larger amounts of orthopyroxene (>20%) the high-temperature, low-frequency response follows Q_G^-1 \propto f^{- 0.6}. Moreover, in all pyroxene-bearing samples, a pronounced peak in dissipation occurs at f~1 Hz at T=1300°C that shifts to lower frequencies at lower temperatures. Additionally, this peak shifts to lower frequency with higher orthopyroxene contents. These attenuation data are compared critically to similar experimental studies on dunite and our preliminary results suggest that fine-grained harzburgite displays both higher attenuation and less modulus dispersion than a dunite with similar microstructure over the range of experimental conditions tested.
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 5120 Plasticity, diffusion, and creep
DE: 5144 Wave attenuation
DE: 7208 Mantle (1212, 1213, 8124)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting