Mechanisms of transformation and deformation in Mg_1.8Fe_0.2SiO₄ olivine and wadsleyite under non-hydrostatic stress

We have studied the effect of non-hydrostatic stress on the mechanisms of the olivine - wadsleyite-ringwoodite (α-β-γ) phase transformations and deformation mechanisms of olivine and wadsleyite at high pressure. Experiments were performed at 900°C in the β-stability field (15 GPa) for 0.5 h and in the β + γ stability field (16 GPa) for 11 h using a multianvil apparatus with San Carlos olivine as the starting material. A sample assembly designed to produce non-hydrostatic stress was used. The deformed samples have been characterised using optical and transmission electron microscopy. Remnant olivine contains high densities of mostly χ dislocations and deformation occurs by dislocation glide involving the slip systems (010)[001] and (100)[001]. In wadsleyite, dislocations are in a climb configuration, which suggests that self diffusion of Si and/or O is much faster in wadsleyite than in olivine at ~900°C. Wadsleyite also contains (010)_β stacking faults which are interpreted to be growth defects that anneal out with time. During the olivine-wadsleyite transformation, non-hydrostatic stress results in anisotropic reaction textures. Wadsleyite nucleates preferentially on olivine grain boundaries that are oriented at a high angle to the direction of principal compressive stress and/or the growth of wadsleyite occurs preferentially in this direction. Wadsleyite transforms to ringwoodite by two competing mechanisms: 1) coherent intracrystalline nucleation on α dislocations, probably where they intersect (010)_β stacking faults, and 2) partially-coherent nucleation at wadsleyite grain boundaries. Non-hydrostatic stress enhances transformation rates by increasing the density of dislocations which act as nucleation sites for ringwoodite. Although the samples were partially reacted under non-hydrostatic stress, there is no evidence for transformational faulting.