An exploratory study of dislocation relaxation in polycrystalline olivine

Motivated by theoretical considerations of damped vibrations of dislocations and the need to better constrain global attenuation models, a laboratory study was undertaken of the attenuation of seismic waves and associated shear modulus dispersion in upper mantle olivine, (Mgo.9 ,Feo.i)2 Si0 4 . Polycrystalline olivine specimens were synthesised from laboratory (solution-gelation) and natural (San Carlos) precursors, hot-pressed and pre-deformed in the laboratory under high temperature (< 1300°C) and pressure (300 MPa). Compressive deformation experiments were performed for differential stresses up to 290 MPa and 22% strain in the dislocation creep regime of olivine. Torsional deformation experiments of the same materials under similar conditions were performed up to maximum shear stresses of 195 MPa and maximum shear strains of 0.8. Examination of the pre-deformed specimens revealed an average grain size of ~ 4 8 pm for sol-gel and ~ 8 17 pm for San Carlos olivine. The density of dislocations with [100] and [001] Burgers vectors was increased by as much as an order of magnitude compared to hot-pressed specimens. Crystallographic preferred orientations were weak to random in all specimens. Infrared spectroscopy analysis confirmed that most sol-gel material was essentially dry, but the San Carlos specimens contained between 200 and 550 H/106 Si. Dislocation recovery experiments on both types of olivine established that a maximum temperature of 1100°C should allow a relatively stable dislocation density to be maintained during prolonged mechanical testing (> 50 hours). In addition, the recovery experiments demonstrated that there was no discernible difference in the rate of dislocation recovery between polycrystalline sol-gel and San Carlos olivine. ( Calculations of resolved shear stress and anelastic shear strain for the easy slip system in olivine as a function of grain orientation, indicated that torsional deformation prior to torsional forced-oscillation testing may generate a population of dislocations more favourably oriented to dissipate strain energy than prior compressive deformation. In addition, models for string-vibration and kink migration of dislocation segments predict a significantly enhanced relaxation strength and dependence of dissipation on oscillation period. Torsional forced-oscillation tests were performed at high temperature and pressure on hot-pressed and pre-deformed specimens at maximum strain amplitudes approximately less than 10-5 within the linear regime, yielding measurements of shear modulus and associated dissipation. A comparison of data from the pre-deformed specimens with a model for grain-size sensitive viscoelastic relaxation (Jackson and Faul 2010) demonstrated enhanced dissipation and modulus dispersion attributable to dislocation relaxation. The strength of this dislocation-related relaxation increased monotonically with frequency and temperature and with increasing dislocation density. The torsionally pre-deformed olivine specimens demonstrated generally greater levels of dissipation and modulus dispersion over a range of temperatures. The observed relationship between dislocation-related dissipation and the orientation of the stress field during prior deformation suggests the possibility of seismological attenuation anisotropy. If dislocation damping is not grain size sensitive, then its operation in coarser-grained upper mantle olivine may contribute significantly to attenuation (Q~l ~ 10—2), especially in regions with relatively high levels of prevailing tectonic stress.