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EID-02 Properties and dynamics of mantle and core
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New Frontiers in the laboratory study of seismic-wave dispersion and attenuation: The roles of dislocations and water
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Yoshitaka Aizawa, Okayama University (Japan)
Auke Barnhoorn, Utrecht University (Netherlands)
Robert Farla, Australian National University (Australia)
Ulrich Faul, Boston University (United States)
Ian Jackson, Australian National University (Australia)
John Fitz Gerald, Australian National University (Australia)
Harri Kokkonen, Australian National University (Australia)
Istvan Kovacs, Australian National University (Australia)
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Here we report progress in extending seismic-frequency forced-oscillation measurements of high-temperature viscoelastic relaxation in olivine towards new frontiers: dislocation damping and the role of water. Dislocations introduced into mantle minerals by tectonic deformation may account for much of the observed seismic-wave attenuation. In preparation for laboratory measurements of dislocation damping, we have made a study of the high-temperature stability of dislocation microstructures in pure synthetic Fo90 material. Cores from a specimen previously deformed in compression by dislocation creep were statically annealed under controlled furnace atmosphere within the olivine stability field. Dislocations were decorated by oxidation, imaged with backscattered electrons in a FESEM operated at 5 kV, and automatically counted. The reduction in dislocation density follows a second-order kinetic rate law with a well-defined activation energy for the recovery rate. It follows that dislocation microstructures can be preserved during prolonged measurements of seismic properties at temperatures as high as 1100°C. However, preliminary torsional forced-oscillation data for a pre-deformed specimen fail to reveal significantly enhanced viscoelastic relaxation. Possible explanations for this null result include the difference between stress states governing dislocation motion during prior compression of the specimen and its subsequent testing in torsion. The feasibility of laboratory studies of the role of water in seismic-wave attenuation in the upper mantle has been tested on a natural dunite specimen containing accessory hydrous phases at high temperature and confining pressure. It has been demonstrated that a new assembly involving a welded Pt capsule retains aqueous fluid during prolonged high-temperature exposure - allowing the first high-temperature forced-oscillation measurements under high aqueous pore-fluid pressure. At T 1000°C, a marked reduction in shear modulus, without concomitant increase in strain energy dissipation Q-1 is attributed to widespread wetting of grain boundaries resulting from grain-scale hydro-fracturing and the maintenance of conditions of low differential pressure. Staged cooling from 1000°C is accompanied by decreasing pore pressure and progressive restoration of significantly positive differential pressure resulting in a microstructural regime in which the fluid on grain boundaries is increasingly restricted to arrays of pores. The more pronounced viscoelastic behaviour observed within this regime for the wet specimen may reflect both water-enhanced solid-state relaxation and the direct influence of the fluid phase. This work has established the feasibility of planned experiments with smaller amounts of dissolved hydroxyl. Meanwhile, the overpressurised fluids and hydro-fracturing of the Pt-encapsulated dunite specimen may be relevant to high attenuation and low wave speeds observed in subduction zones.
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