Physics
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999pepi..115..259w&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 115, Issue 3-4, p. 259-291.
Physics
13
Scientific paper
The frequency-dependence of the shear modulus and dissipation in polycrystalline CaTiO3 and SrTiO3 perovskites has been determined at high temperature using micro-creep (ɛ=10-4), seismic frequency forced oscillation (ɛ=10-5), and ultrasonic interferometric measurements. Both of these perovskites display viscoelastic behaviour in the seismic frequency regime; with the temperature of onset of this frequency-dependent behaviour being grainsize-dependent. 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 relatively smooth variation with temperature of G and Q-1 for CaTiO3 (sampled at relatively widely spaced (50°C) temperature steps) suggests that the high temperature phase transitions occurring in CaTiO3 have little effect on the viscoelastic behaviour. In the absence of grain-boundary impurities, twinning and dislocations, the long-timescale viscoelastic deformation of SrTiO3 is attributed to grain-boundary sliding accommodated by lattice or grain-boundary diffusion, controlled by the slowest diffusing species, which is expected to be either Sr or Ti. The low viscosities determined for the CaTiO3 samples, together with the low values (with respect to the extrapolated ultrasonic data) of shear modulus associated with Andrade model fits to the data suggest that the long-timescale viscoelastic deformation of CaTiO3 may reflect relatively faster grain-boundary diffusion of the slowest moving species through the thin (<2 nm) impurity films and larger impurity segregations present in these samples. The ultrasonic measurements, combined with those of Kung [Kung, J., 1997. Systematics among the elastic properties of perovskite-structured compounds and geophysical implications. PhD, Australian National University, Canberra, 112 pp.] for ScAlO3 perovskite, indicate relatively tightly clustered high-frequency (anharmonic) values of |(∂KS/∂T)P| and |(∂G/∂T)P| within the ranges 0.023-0.033 and 0.015-0.021 GPa K-1, respectively, neither of which includes the most recent determinations for MgSiO3 perovskite. The diffusion controlled viscoelastic rheology observed here results in the magnitude of the highest temperature shear moduli determined for fine-grained CaTiO3 and SrTiO3 perovskite at seismic frequencies being less than one-quarter that of the ultrasonic moduli, and the temperature dependence of the seismic-frequency moduli being larger by a factor of 10. An extrapolation of the rheology of fine-grained SrTiO3 and CaTiO3 perovskites to 5 mm grainsize shows that a ~20% dispersion in wavespeed is expected for the period range 1-1000 s, with dissipation (Q-1) ~10-2 for 1 s period waves at 1300°C. Viscoelastic behaviour related to diffusional creep in the silicate perovskites in the lower mantle may result in a similar reduction in seismic shear-wave speed accompanied by an increase in shear-wave dissipation, and an apparent increase in the temperature dependence of seismic shear-wave speed.
Fitz Gerald John
Jackson Ian
Webb Stephen
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