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 olivin e at high temperatures (to 1300 degreesC, seismic frequencies and low strai n 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 tr ace amounts (much less than0.1 vol%) of quenched melt glass. For T less tha n or equal to 900 degreesC, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal fr om single-crystal elasticity data confirming the suppression of thermal mic rocracking in this study. At higher temperatures, pronounced absorption-ban d dissipation and associated dispersion of the shear modulus provide eviden ce of linear viscoelastic behaviour. Both recoverable (anelastic) and perma nent (viscous) strains are involved and the proportion of the latter increa ses with increasing temperature and decreasing frequency. Comparison of the results for the three specimens provides a clear indication that the visco elastic behaviour, attributed to diffusional processes, is grain-size-sensi tive with the dissipation and associated dispersion increasing with decreas ing grain size. Both elastically accommodated and diffusionally accommodate d grain-boundary sliding appear to be implicated.