Physics
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010pepi..183..151j&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 183, Issue 1-2, p. 151-163.
Physics
8
Scientific paper
Torsional forced oscillation data for a newly prepared specimen of dry, melt-free polycrystalline Fo 90 olivine of 3.1 μm average grainsize have been used to reassess alternative strategies for the parameterisation of grainsize-sensitive viscoelastic relaxation. Our previously employed extended Burgers model has been modified by prescribing anharmonic temperature and pressure dependence of the effective unrelaxed shear modulus G U relative to its value G UR at reference values of temperature ( T R = 900 °C) and pressure ( P R = 0.2 GPa). The modified model provides an excellent description of forced-oscillation data for the newly prepared olivine polycrystal at temperatures of 900-1200 °C and oscillation periods of 1-1000 s, with a value of G UR that is significantly (7%) less than the strictly anharmonic value for the same conditions ( T R , P R ). This modulus deficit is interpreted to reflect the impact of elastically accommodated grain-boundary sliding tentatively associated with a newly recognised 'plateau' with Q -1 ˜ 0.01 that moves across the seismic band from long to short period with increasing temperature between 750 and 950 °C. The modified Burgers model is preferred over the Andrade-pseudoperiod and power-law Q -1 alternatives for its flexibility in specifying a distribution D ( τ ) of anelastic relaxation times that can account for both the monotonic background dissipation and the superimposed dissipation peak of elastically accommodated grain-boundary sliding, along with the associated modulus dispersion. Such 'background + peak' Burgers models, seamlessly describing the transition from (anharmonic) elasticity to grainsize-sensitive viscoelastic behaviour, have been fitted to the data for individual polycrystalline olivine specimens and suites of olivine polycrystals. Extrapolation of the model for our suite of essentially melt-free olivine polycrystals to mantle grain sizes and pressures suggest a significant contribution from grain-boundary relaxation under upper-mantle conditions. However, tighter constraints are expected from ongoing work-seeking confirmation of the occurrence at moderate temperatures of elastically accommodated sliding, and a cleaner separation of the roles of grainsize and water.
Faul Ulrich H.
Jackson Ian
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