HR: 0800h
AN: MR41A-0898    [Abstracts]
TI: Laboratory Measurements of Seismic Wave Attenuation in Natural Dunite
AU: * Aizawa, Y
EM: yoshitaka.aizawa@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT0200 Australia
AU: Barnhoorn, A
EM: auke.barnhoorn@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT0200 Australia
AU: Fitzgerald, J D
EM: john.fitzgerald@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT0200 Australia
AU: Faul, U H
EM: Uli.Faul@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT0200 Australia
AU: Jackson, I
EM: Ian.Jackson@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT0200 Australia
AB: In order to examine the processes responsible for the attenuation of seismic shear waves in the Earth's upper mantle, torsional forced-oscillation and microcreep experiements have been conducted on a natural dunite specimen at high temperatures to 1300° C and seismic frequencies from 0.001 to 1Hz. The dunite specimen (from Anita Bay, NZ) consists mainly of olivine (olivine 94%, orthopyroxene 5%, chromite 1%) of about 100 micron average grain size, but some olivine crystals of size up to several millimeters occur randomly. It also contains trace amounts of hydrous phases (loss on ignition is 0.2 wt.%). We measured both untreated and prefired (1200° C, 15hrs) specimens to assess the possible role of water on viscoelasticity. Water weakening of olivine aggregates in creep is currently interpreted in terms of increased concentrations of point defects, resulting in enhanced rates of ionic diffusion and dislocation climb. By analogy, it has been speculated that water significantly affects low-strain viscoelastic behavior as well. Our measurements suggest that shear modulus (G) and dissipation (Q^-1) of the prefired specimen are generally larger than those of untreated one. In addition, frequency-dependence of G and Q^-1 for the prefired specimen is substantially larger than those of the untreated one. Compared to calculations based on melt-free olivine polycrystals with an average grain size of 100 microns, the measured G and Q^-1 are smaller than those calculated. On the other hand, the frequency-dependence of G and Q^-1 for the untreated specimen is consistent with those of calculations, although frequency-dependence for the prefired specimen is generally larger relative to the calculations. No distinct dissipation peaks are observed related to the presence of melt, even though the untreated specimen has on the order of 1% melt, and the prefired one also contains a trace amount of melt. Because the measurements were mostly performed at temperatures higher than solidus temperatures, physical properties and distributions of melt may play major roles on interpreting the present experimental results. In the untreated specimen, melt is localized and the dimension of melt pockets is highly heterogeneous. Microstructures and water contents for both specimens will be discussed in detail.
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005