Weak-Beam Dark-Field TEM Characterization of Dislocations and Slip Systems in Wadsleyite deformed in Simple Shear at Pressure-Temperature Conditions of the Mantle Transition Zone

Abstract:

Characterization of dislocations and slip systems in deformed wadsleyite is important in linking them to crystallographic preferred orientation (CPO) of wadsleyite, geophysical observation on seismic anisotropy and mantle convection at the mantle transition zone (MTZ). A [001](010)-type CPO pattern of wadsleyite was recently reported by deformation experiments on wadsleyite at pressure-temperature conditions of the MTZ (Kawazoe et al., 2013; Ohuchi et al., 2014). However, [001] dislocations have been rarely reported in wadsleyite in the literature (cf. Cordier, 2006). To reconcile the wadsleyite CPO pattern with its slip systems, dislocation microstructures of a [001](010)-textured wadsleyite have been investigated in weak-beam dark-field imaging in a transmission electron microscope. 1/2<101> partial dislocations on the (010) plane are characterized with [100] dislocations on the (010) plane and 1/2<111> dislocations. The former partial dislocations are extended on the (010) stacking fault as a glide configuration (i.e. Shockley-type stacking faults with 1/2<101> replacement vectors). The [001] slip on the (010) plane occurs by glide of the dissociated dislocations on a sub-oxygen close packing plane, which is consistent with the reported CPO in water-poor conditions.