Seismic wave dispersion and attenuation within the asthenosphere: the role of sub-solidus viscoelastic relaxation revisited

It is well known that increasing pressure and temperature along upper-mantle geotherms combine to produce a zone of low seismic wave speeds. Beyond such behaviour arising from the anharmonicity of the crystal lattices of the constituent minerals, viscoelastic relaxation may result in further reduction of the wave speeds, along with appreciable attenuation of seismic waves. In order to better constrain such sub-solidus relaxation in olivine-dominated lithologies, we have recently prepared and tested in torsional forced oscillation several new specimens of synthetic polycrystalline olivine (Fo₉₀ olivine buffered by ~5 wt% En₉₀). These specimens were prepared by hot pressing sol-gel precursor powder encapsulated within metal foils (of Ni₇₀Fe₃₀ or Pt) at high temperature (1200-1350ºC) and pressure (300 MPa). Enclosure within Ni-Fe foil yields relatively reducing anhydrous conditions and average grains sizes d ≤ 5 μm. The more oxidising and hydrous conditions associated with Pt encapsulation are conducive to grain growth to at least 20 μm. Our forced-oscillation methods have been refined by replacement of the polycrystalline alumina control specimen with single-crystal sapphire, discontinuation of the use of Ni-Fe foils at the ends of the specimens in favour of direct contact with alumina torsion rods, and selective use of austenitic stainless steel as an alternative to the usual mild-steel material for the enclosing jacket. Such testing of fine-grained olivine polycrystals at periods of 1-1000 s and shear strain amplitudes < 10^-5 has consistently revealed an essentially monotonically period- and temperature-dependent high-temperature background. The onset of high-temperature anelastic relaxation involves a superimposed dissipation peak of only modest amplitude plausibly attributed to elastically accommodated grain-boundary sliding. Grain-size sensitivity is incorporated into a Burgers type creep-function model fitted to the (G,Q^-1) data for multiple specimens through power-law grain size dependencies of the key characteristic times. The Maxwell time τ_M, varying as d^-mV, defines the transition from anelastic to viscous background behaviour, and τ_P ~ d^-mA, the centre of the distribution of relaxation times for the dissipation peak. The data for the newly prepared pure synthetic specimens of 4-22 mm grain size, tested with the refined experimental methodology, require m_V ~ 3 and m_A < 1.5. These inferences are consistent with micromechanical models for grain-boundary sliding, but yield markedly stronger grain-size sensitivity than previously reported. However, mapping of the tested samples by electron back-scattered diffraction indicates that the density of geometrically necessary dislocations, responsible for lattice curvature, decreases systematically with increasing grain size, raising the possibility that any contribution from dislocation damping might enhance the apparent grain-size sensitivity. A preliminary extrapolation of the new model for grain-size sensitive viscoelastic relaxation in dry, melt-free dunite to upper-mantle conditions of grain size and pressure suggests shear modulus relaxation < 2% and dissipation Q^-1 < 0.01 – thus unable to account for seismological observations of the mantle beneath young oceanic lithosphere and in subduction zones. Uncertainties in such extrapolation will be discussed, along with other factors that might enhance sub-solidus viscoelastic relaxation including the segregation of trace-element impurities to olivine grain boundaries, and the influence of oxygen and water fugacities.