Anelasticity and microcreep in polycrystalline MgO at high temperature: An exploratory study

The frequency dependence of the shear modulus and dissipation in polycrystalline MgO has been determined at high temperature using both microcreep (ε = 10^-4) and seismic frequency forced-oscillation (ε = 10^-5) measurements. The frequency-dependent and time-dependent data have been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The forced-oscillation measurements show that for temperatures above 700 °C the shear modulus begins to decrease dramatically and the modulus becomes frequency-dependent with increasing temperature. This is accompanied by an increase in dissipation, which also becomes frequency-dependent. The microcreep measurements resolve this frequency-dependent behaviour into an anelastic regime from 700-1050 °C, and a viscoelastic regime from 1100-1300 °C. At 1300 °C, the seismic frequency shear wave speed is ∼60% of the extrapolated low-temperature frequency-independent value, and the dissipation has risen to Q^-1 = 10^-1 from 10^-3 at temperatures below 600 °C. The mechanism by which this frequency-dependent rheology occurs appears to be diffusional creep, which produces viscous slip on the grain boundaries. It is proposed that the anelastic behaviour is due to viscous slip occurring on segments of grain boundaries, with the viscous deformation being accommodated by elastic distortion of adjacent unslipped regions of the grain boundary. At higher temperatures, slippage occurs across the entire grain boundary and viscoelastic behaviour begins to occur.