Experimental investigation of grain-scale microstructure evolution during olivine-wadsleyite phase transformation under “dry” conditions

Abstract:

We investigate the evolution of grain-scale microstructure during the olivine to wadsleyite transformation through high pressure and temperature experiments. The grain-size evolution and the spatial distribution of newly formed fine grains during the phase transformation in the mantle transition zone have potentially large influence on the strength of a slab in the transition zone that has an important control on the slab deformation. However, most of previous experimental studies on the processes of phase transformations have focused on the kinetics of phase transformation and no experimental studies have been published on these microstructural issues. The key issues that we investigate include (i) the size of new grains and (ii) spatial distribution of new grains (critical conditions for percolation).

We conduct high-pressure, temperature annealing experiments and investigate the grain-scale microstructure evolution. We find that olivine transforms to wadsleyite mainly via grain boundary nucleated transformation mechanism: New grains are formed on pre-existing olivine-olivine grain-boundaries in all cases. In some runs, we identified the time for site saturation on grain-boundaries and together with the grain-size at site saturation we calculated both nucleation and growth rate. During early stages of transformation a grain boundary percolated microstructure develops and this may be very crucial in decreasing the overall strength of composite during this step. The grain size at the site saturation seems to decrease with overpressure. We also find that inadequate annealing of defects may give rise to apparent kinetic parameters interpretation of which may not be straightforward. We report inferred functional forms of nucleation and growth rate and discuss possible implications of these experimental observations on the weakening of a slab in the mantle transition zone.