Prof. Jackson brief biography: B Sc Honours in Physics, University of Queensland 1972Ph D Geophysics with Liebermann and Ringwood, RSES 1976Post Doc at Caltech working with Tom Ahrens on shock compression of mineralsReturn as Research Fellow to RSES 1978, Professor since 2002, Director since 2012Pawsey and Jaeger Medals of the Australian Academy of ScienceFellowship of the American Geophysical Union
The progressive transition from high-frequency elastic behaviour, through the recoverable strains of anelastic relaxation, to long-term viscous deformation is of central significance to an understanding of seismological models for the Earth’s interior and of its geodynamical behaviour.During the past decade, we have systematically applied torsional forced-oscillation methods to explore such viscoelastic behaviour in fine-grained polycrystalline specimens of the dominant upper-mantle mineral olivine. Polycrystalline specimens have been prepared by hot-pressing of powders derived from either natural or fully synthetic precursors. These analogues for mantle rocks have been tested in torsional oscillation at low frequencies and strain amplitudes, and in complementary torsional microcreep tests, under conditions of simultaneous high pressure and temperature.Significant strain energy dissipation and associated frequency-dependent reduction (dispersion) of the rigidity (or shear modulus) attest to viscoelastic behaviour – even in genuinely melt-free polycrystalline olivine. The grain-size sensitivity of this viscoelastic behaviour is attributed to sliding on boundaries between adjacent crystals. However, a satisfactory reconciliation of the observations with micromechanical models of grain-boundary sliding has proved elusive and is only now beginning to emerge.In parallel, we have been exploring the influence on the seismic properties of prior deformation involving line defects called dislocations. Specimens pre-deformed at high stresses in compression exhibit an enhancement of the dissipation and shear modulus dispersion that increases systematically with increasing density of dislocations. Prior torsional deformation by dislocation creep is even more effective in enhancing the viscoelastic behaviour because the resulting dislocations are, on average, more favourably oriented for reactivation during torsional oscillation.Progress in the separation of the contributions from grain-boundary sliding and dislocation-related relaxation will be described, along with beckoning opportunities to study the role of water incorporated as protonated crystal defects in nominally anhydrous silicate minerals such as olivine, and to re-evaluate the effect of partial melting.