Skip Navigation | ANU Home | Search ANU | Directories
The Australian National University
Research School of Earth Sciences
Printer Friendly Version of this Document
Untitled Document

Viscoelasticity, poroelasticity and seismic properties

Ian Jackson, John Fitz Gerald, Robert Farla, Harri Kokkonen, Hayden Miller
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Figure 1


At sufficiently high temperatures in the Earth’s interior, the mechanical behaviour changes from elastic to viscoelastic with profound implications for mantle rheology and also seismic wave speeds and attenuation. Such viscoelastic behaviour results from the stress-induced migration of vacancies and dislocations (extended defects reflecting prior or current deformation: see also Farla et al.).  Equally, the stress-induced flow of fluids within cracked/porous media results in departures from elastic behaviour, termed poroelasticity. The following are highlights for 2008 of our ongoing study of rheology and seismic properties:

Hot-pressing and high-temperature deformation of titaniferous olivines (with U.H. Faul of Boston University):  Work has continued this year towards an understanding of the influence of trace impurities on the rheology of fine-grained polycrystalline olivine. Specimens have been hot-pressed at 1300°C from sol-gel-derived Fo90 olivine precursors containing 0.1 wt % each CaO and TiO2, and deformed at 1200-1300°C in compressive creep tests at progressively higher stress reaching ~300 MPa. Preliminary indications are that these materials (Fig. 1) undergo much more rapid grain growth than their Ti-free counterparts and are significantly weaker.

Seismic-wave dispersion and attenuation (with U.H. Faul, S.J.S. Morris of UC Berkeley, and D.R. Schmitt of the Univ. of Alberta):  Our torsional forced-oscillation method for the study of high-temperature viscoelastic relaxation has recently been refined to take account of (i) compliance associated with frictional coupling between the specimen and neighbouring torsion rods, and (ii) significantly viscoelastic behaviour of the alumina control specimen [1]. 

Our published data concerning the shear modulus G and dissipation 1/Q for fine-grained melt-free and melt-bearing olivine have been re-processed with this improved strategy.  Allowance for the compliant frictional coupling results in systematically higher G and lower 1/Q  - especially for relatively coarse-grained (low-loss) materials tested at the highest temperatures (31200°C) and longest periods (>100 s).  These effects are offset to some degree by allowance for the appreciably viscoelastic behaviour of the high-grade polycrystalline alumina control specimen. The interim result is an enhanced grain-size sensitivity of the viscoelastic relaxation (Fig. 2) meaning higher wavespeeds and lower attenuation on extrapolation to upper-mantle grain sizes. Additional experimental data for medium-coarse-grained materials are needed to underpin more robust extrapolation. Planned changes to the experimental procedure involving more active gripping of the cylindrical specimen and use of a single-crystal alumina control specimen may increase the signal/noise ratio for such low-loss materials. In a new initiative, our 'attenuation apparatus' is currently being modified to allow forced-oscillation measurements in extension/flexure, as well as torsion.  Such measurements will allow the probing of poroelastic effects in cracked and fluid-saturated media that are analogues for upper-crustal rocks.

Figure 2

Modelling of elastic properties and equation of state (with B.L.N. Kennett):  The thermodynamically consistent finite-strain model of Stixrude and Lithgow-Bertelloni (GJI, 2005) provides an attractive framework for the assessment and assimilation of experimental data concerning elastic properties and equation of state. The model requires the specification of 9 parameters: molar volume, (isotropic) bulk and shear moduli and their pressure derivatives, the effective Debye temperature, and the Grüneisen parameter and its volume and shear strain derivatives. We have explored the feasibility of using Sambridge's Neighbourhood Algorithm strategy (GJI, 1999) to undertake a guided search of the model space that is constrained simultaneously by diverse experimental datasets as an alternative to iterative least-squares fitting. This approach has been tested on data for MgO including measurements of specific heat and thermal expansion, static and shock compression, and the pressure and temperature dependence of elastic wavespeeds. The search converged on a unique model that adequately represents most of the experimental data, but not before revealing tensions between marginally incompatible datasets.



[1] Jackson, I,  A. Barnhoorn, Y. Aizawa and C. Saint. Improved experimental procedures for the study of high-temperature viscoelastic relaxation, Phys. Earth Planet. Interiors (in press).