Grainsize-sensitive viscoelastic relaxation in olivine: Towards a robust laboratory-based model for seismological application

Torsional forced oscillation data for a newly prepared specimen of dry, melt-free polycrystalline Fo90 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 GU relative to its value GUR at reference values of temperature (TR = 900 °C) and pressure (PR = 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 GUR that is significantly (7%) less than the strictly anharmonic value for the same conditions (TR,PR). 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.