13th GRC International Frontier Seminar

THE LABORATORY STUDY OF SEISMIC WAVE DISPERSION AND ATTENUATION IN UPPER-MANTLE MATERIALS: PROGRESS AND PROSPECTS

Prof. IAN JACKSON
Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia

　　Novel techniques for torsional forced oscillation/microcreep measurements under conditions of simultaneously high temperature and pressure have recently been applied in a study of polycrystalline olivine (Mg1.8Fe0.2SiO4). Fine-grained specimens have been prepared with and without added basaltic glass with melt fractions ranging from < 0.01 to 4%. Forced oscillation tests performed during slow staged cooling following annealing at 1200-1300℃ reveal qualitatively different behaviour of the melt-free and melt-bearing specimens. For the melt-free specimens (< 0.01 % melt), the strain-energy dissipation is of the ‘high-temperature background’ type-varying monotonically with period and temperature. Specimens containing as little as 0.01% melt at high temperature exhibit a broad dissipation peak superimposed upon a melt-enhanced background. The peak height scales with melt fraction, and its breadth implies a distribution of relaxation times about two decades wide. The peak moves to markedly shorter oscillation period with increasing temperature requiring an activation energy of about 720 kJ/mol. The peak cannot plausibly be explained by stress-induced grain-scale redistribution of melt (‘melt squirt’). Instead, it is attributed to elastically accommodated grain-boundary sliding apparently facilitated by the rounding of grain edges at triple-junction melt tubules. Elastically accommodated sliding is evidently suppressed in the melt-free materials by the presence of tight (~ nm) grain-edge intersections. In both classes of material the background dissipation and associated modulus dispersion are attributed to diffusionally accommodated grain-boundary sliding. 　　
　　The observed viscoelastic behaviour is best parameterised with a Burgers-type creep function. This approach provides an internally consistent description of the variations of both the shear modulus G and the strain-energy dissipation 1/Q with frequency, temperature, grain size and potentially, melt fraction. Such models provide a robust framework for application of the insights gained in the laboratory to teleseismic wave propagation in the Earth’s mantle. Our model for the behaviour of melt-free olivine has recently been used to assess variations of shear-wave speed and attenuation in the upper mantle. Away from mid-ocean ridges and subduction zones, the first-order features of representative wave speed (and attenuation) versus depth models for both oceanic and continental upper mantle can be explained by sub-solidus temperature variations, without recourse to widespread partial melting and/or compositional variations. In ongoing work we are seeking to identify other factors with a potentially important influence on upper-mantle seismic-wave dispersion and attenuation-particularly the presence of dislocations reflecting prior deformation by dislocation creep and trace amounts of water. Progress in parallel experiments on synthetic olivine polycrystals and natural peridotites will be described.　

　

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