Qu, Tongzhang1, Jackson, Ian1, David, Emmanuel C.1,2, Faul, Ulrich H.1,3
1Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia, 2Now at Department of Earth Sciences, University College London, London, United Kingdom, 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
The Earth’s upper mantle is being imaged with increasing fidelity with the various methods of modern seismology. However, such images are of limited value without laboratory-based models for their robust interpretation in terms of the many factors (e.g., chemical composition, temperature, and partial melting) that influence the seismic properties of mantle materials. Over decades, we have developed a unique capability for measurement of seismic properties of rock cylinders by forced oscillation at seismic periods (1-1000 s) rather than the much shorter periods (ns-ms) of laboratory wave-propagation methods. Sustained application of these methods has documented grain-size sensitive viscoelastic relaxation, responsible for frequency dependence (dispersion) of the shear modulus and hence wavespeed and related strain-energy dissipation in polycrystalline olivine, as well as the influence of partial melting and dislocation density, and more recently, of oxidizing/hydrous conditions.
Here we report new measurements on olivine-orthopyroxene mixtures. For temperatures reaching 1200°C and seismic periods, the strain-energy dissipation and shear modulus dispersion are generally similar to those for essentially pure olivine, but somewhat diminished and slightly more temperature-sensitive with increasing orthopyroxene concentration from 5% to 95%. The viscoelastic behaviour is of the high-temperature-background type without any evidence of the superimposed dissipation peak reported by others for a melt-bearing specimen of otherwise similar composition. It is concluded that our olivine-based model for seismic wave dispersion and attenuation will require only modest modification to accommodate the role of orthopyroxene.
In order to further refine our methodology, we are also addressing the effect of uncertainty in the mechanical behaviour of the enclosing mild-steel jacket arising from the transition between the austenite and ferrite phases on cooling across the interval 900-700ºC. Variations of its microstructure and hence viscoelastic behaviour have the potential to mask the seismologically important onset, within this temperature range, of appreciably viscoelastic behaviour of upper-mantle materials. Accordingly, we have previously conducted a study in which a specimen of polycrystalline olivine is jacketed instead within a copper sleeve which retains its face-centred-cubic (fcc) structure throughout the range of the measurements – limited to 1050ºC maximum by the proximity of the melting point. Here we report new measurements to higher temperature (1200ºC) in which we employed austenitic (fcc) stainless steel (SS) as an alternative jacket material. Prior measurement of the mechanical properties of the SS jacket material allowed subtraction of its contribution to the torsional stiffness of the SS-jacketed specimen. The resulting dissipation spectrum for the olivine specimen consists of a monotonic background dissipation with a superimposed peak located within the 1-1000 s period range for temperatures between 900 and 1050ºC. The dissipation peak and associated shear modulus dispersion, potentially attributable to elastically accommodated grain-boundary sliding, display an Arrhenius dependence upon temperature – moving systematically to shorter periods with increasing temperature. Comparison of the results obtained for synthetic Fo90 olivine specimens enclosed within the alternative mild-steel, copper, and stainless-steel jackets is providing new insight into the nature of the seismologically important transition between the elastic and anelastic regimes.
Tongzhang Qu is currently a PhD student in Rock Physics at ANU, following undergraduate studies at Jilin and Pennsylvania State Universities.