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3.1 d. Deformation of transition zone minerals in the multianvil press. (S.I. Karato, J.D Lawlis, D. Yamazaki, K-H Lee, D.J. Frost and D.C. Rubie)

A series of deformation experiments on wadsleyite, and wadsleyite-olivine pairs have been carried out in the multi-anvil press in order to determine the rheology of these minerals and the fabrics that may result from their deformation (i.e. the lattice preferred orientation). We use a simple shear deformation technique in which a thin slice of sample is sandwiched at 45o between two pistons. The sample strain is measured by rotation of a Pt strain marker vapour deposited onto cut surfaces of the sample (see Annual Reports 1998, 1999). Upon pressurization, compressional stress develops along the sample column due to elastic anisotropy of the sample assembly. This stress is relaxed by plastic flow of the sample when heated. By placing two samples on top of each other in the deformation assembly the relative strengths of both materials can be assessed from the difference in rotation of the strain markers in each sample. This technique has been successfully applied to olivine in the dislocation creep regime and preliminary results on transformation and deformation mechanisms were obtained for ringwoodite.

The application of this technique, however, has also revealed some complications.

(1) The deformation experiments are stress-relaxation tests and hence the stress magnitude changes during a single experiment. Deformation mechanisms and the dominant slip systems may therefore change as the stress changes. This causes complication in the interpretation of the deformation mechanisms and microstructures (such as the lattice preferred orientation) as some observations may be remnant from earlier stages of the experiment.

(2) Chemical reactions can occur between the Al2O3 pistons and samples at high temperatures and consequently deformation solely along the reaction interface can occur.

We have tried to overcome these problems with a series of experiments designed to determine the relative strengths of wadsleyite and olivine, the grain-size sensitivity of wadsleyite deformation and the dominant slip systems in wadsleyite. To achieve these goals, we prepared samples with different grain-sizes by synthesizing them at different temperatures. Samples of wadsleyite synthesized from San Carlos olivine at T~1200 K had an average grain-size of 2-3 microns, whereas samples synthesized at T~1500 K had an average grain-size of 5-8 microns. Using these samples, we performed deformation experiments at different temperatures for different durations. The idea was to identify changes in deformation mechanism with time (i.e., stress), grain size and temperature.

In order to address problem (1) a series of experiments were conducted on single slices of hot pressed wadsleyite at identical pressures and temperatures but for different periods of heating. After heating for 1 hour 60% shear strain was recorded in the sample and an identical sample deformed for 8 hours showed 73% shear strain. The lowering of the strain rate is expected for these experiments because the applied stress is relaxing with time. TEM investigation of these samples showed evidence for the same three glide systems in both experiments (see following contribution by Cordier et al.). The consistency of the glide systems between samples, even though the stress on the second sample must have been considerably reduced after 8 hours, suggests at least a 'microstructural steady state' is reached in these experiments and it is unlikely that these features are remnant from the cold compression or initial heating stages of the experiments.

To address problem (2) we performed deformation experiments using sintered diamond pistons (SYNTEK). Even at temperatures of ~1900 K, no significant reactions were detected between the deformed samples and the diamond pistons and the separation of the two pistons from the sample for polishing was possible after an experiment. This is a promising observation which may allow us to investigate deformation behaviour at high temperature, the results of which may be more relevant to Earth than low temperature results.

We have also performed some new analyses of deformation fabrics using electron backscattering patterns (EBSP). The pole figures (Fig. 3.1-4) show an alignment of <100> close to the experimental shear direction, whereas poles to {001} and, less strongly, poles to {010} cluster close to the pole of the macroscopic shear plane. The alignment of <100> close to the shear direction appears to be consistent with the observation of <100> as the Burgers vector (following contribution by Cordier et al.). By combining evidence from the measured experimental strain with TEM observations of slip systems and fabric analyses we hope to develop a consistent picture of the rheology of minerals from the transition zone and lower mantle.

Fig. 3.1-4. Pole figures (a-c) of deformed wadsleyite in the dislocation regime at T~1500 K and P~15 GPa to strain of ~100%. The shear direction (Sd) and the trace of the shear plane (SP) are indicated, shear sense is dextral; m.u.d. = multiples of uniform distribution; (d) shows an electron backscattering pattern (EBSP) from a wadsleyite grain.

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