MECHANISMS OF TRANSFORMATION AND DEFORMATION IN MG1.8FE0.2SIO4 OLIVINE AND WADSLEYITE UNDER NONHYDROSTATIC STRESS

We have studied the effect of non-hydrostatic stress on the mechanisms of the olivine-wadsleyite-ringwoodite (alpha-beta-gamma) phase transf ormations and deformation mechanisms of olivine and wadsleyite at high pressure. Experiments were performed at 900 degrees C in the beta-sta bility field (15 GPa) for 0.5 h and in the beta + gamma stability fiel d (16 GPa) for 11 h using a multianvil apparatus with San Carlos olivi ne as the starting material, A sample assembly designed to produce non -hydrostatic stress was used. The deformed samples have been character ised using optical and transmission electron microscopy, Remnant olivi ne contains high densities of mostly c dislocations and deformation oc curs by dislocation glide involving the slip systems (010)[001] and (1 00)[001]. In wadsleyite, dislocations are in a climb, configuration, w hich suggests that self diffusion of Si and/or O is much faster in wad sleyite than in olivine at similar to 900 degrees C. Wadsleyite also c ontains (010)(beta) stacking faults which are interpreted to be growth defects that anneal out with time. During the olivine-wadsleyite tran sformation, non-hydrostatic stress results in anisotropic reaction tex tures. Wadsleyite nucleates preferentially on olivine grain boundaries that are oriented at a high angle to the direction of principal compr essive stress and/or the growth of wadsleyite occurs preferentially in this direction. Wadsleyite transforms to ringwoodite by two competing mechanisms: (i) coherent intracrystalline nucleation on a dislocation s, probably where they intersect (010)(beta) stacking faults, and (ii) partially-coherent nucleation at wadsleyite grain boundaries. Non-hyd rostatic stress enhances transformation rates by increasing the densit y of dislocations which act as nucleation sites for ringwoodite. Altho ugh the samples were partially reacted under non-hydrostatic stress, t here is no evidence for transformational faulting. (C) 1998 Elsevier S cience B.V. All rights reserved.