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The rheology of the transition zone of the Earth's mantle is largely controlled by the plastic properties of the high-pressure polymorphs of olivine: wadsleyite and ringwoodite. We have thus undertaken an investigation of the deformation mechanisms of these two minerals. The first results obtained on wadsleyite are presented here.

We have taken care to perform synthesis and plastic deformation in separate experiments to ensure that the dislocation microstructures observed are not a result of imperfect growth. Mg₂SiO₄ wadsleyite was synthesised from a forsterite powder, loaded in a rhenium capsule, fitted in a 10/5 multianvil assembly and annealed for 240 min at 18 GPa and 1600-1800°C. After the run, the specimen was recovered and put into a second high-pressure (10/5) assembly designed to induce plastic deformation. The deformation experiment was run for 90 min at 15 GPa and 1000°C.

At the beginning of this study we focused on the determination of the dislocation Burgers vectors that are activated during plastic deformation. Complete characterisation of Burgers vectors in wadsleyite by TEM is difficult because relatively few diffraction vectors exhibit a good structure factor. The sensitivity to electron irradiation render this work even more difficult. Large Angle Electron Diffraction (LACBED) has proved to be a very valuable technique for determining Burgers vectors in such conditions.

As expected on energetic grounds, [100] is the dominant Burgers vector. Indeed, this corresponds to the shortest lattice repeat (5.69 Å) in the wadsleyite structure. The other types of dislocation found are 1/2<111> (7.61 Å), <101> (10.03 Å) and [010] (11.46 Å) with comparable occurrences. An example of Burgers vector determination by LACBED is illustrated on Figure 3.1-19 with a [010] dislocation. Such large Burgers vectors usually result in a dissociation to stabilise the dislocation core. This is confirmed by weak-beam dark-field examination of the dislocation's fine structures (Fig. 3.1-20). Simultaneous occurrence of 1/2<111>, <101> and [010] dislocations might be the result of the low resistance of {10} planes to plastic shear.

Fig. 3.1-19: Example of a Burgers vector characterisation performed on wadsleyite by Large Angle Convergent Beam Electron Diffraction. Case of a [010] dislocation crossed with, , , and Bragg lines.

Fig. 3.1-20: Transmission electron micrographs (weak-beam dark-field) of deformed wadsleyite. (a) Gliding dislocations (b) Evidence for dissociation of [010] dislocations.