High-temperature viscoelasticity of fine-grained polycrystalline olivine

Torsional forced-oscillation and microcreep methods have been employed in a study of the viscoelastic behaviour of fine-grained polycrystalline olivine at high temperatures (to 1300 degreesC, seismic frequencies and low strain amplitudes. The Fo(90) specimens are of low porosity and low dislocation density. They vary in mean grain size from 8 to 150 mum and contain only trace amounts (much less than0.1 vol%) of quenched melt glass. For T less than or equal to 900 degreesC, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal from single-crystal elasticity data confirming the suppression of thermal microcracking in this study. At higher temperatures, pronounced absorption-band dissipation and associated dispersion of the shear modulus provide evidence of linear viscoelastic behaviour. Both recoverable (anelastic) and permanent (viscous) strains are involved and the proportion of the latter increases with increasing temperature and decreasing frequency. Comparison of the results for the three specimens provides a clear indication that the viscoelastic behaviour, attributed to diffusional processes, is grain-size-sensitive with the dissipation and associated dispersion increasing with decreasing grain size. Both elastically accommodated and diffusionally accommodated grain-boundary sliding appear to be implicated.