C. Dupasbruzek et al., MECHANISMS OF TRANSFORMATION AND DEFORMATION IN MG1.8FE0.2SIO4 OLIVINE AND WADSLEYITE UNDER NONHYDROSTATIC STRESS, Physics of the earth and planetary interiors, 108(1), 1998, pp. 33-48
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.