Grain rotation during diffusion creep of forsterite + diopside

Abstract:

To understand the role of grain rotation on development of crystallographic preferred orientation (CPO) during diffusion creep (Miyazaki et al. 2013 Nature), we conducted high temperature deformation experiments on cylindrical polycrystalline samples of synthesized forsterite plus 5-20 vol. % diopside. We polished the lateral side of the sample where we imposed grooves parallel to the compression axis of the sample using a focused ion beam. These marker lines allow us to observe grain rotation due to a plastic deformation of the sample. After the high temperature compression creep, we observed the marker lines under scanning electron microscope. Numbers of evidences and characters of grain rotation were identified:

(1) Grain rotates individually.

(2) Both forsterite and diopside grains rotate.

(3) Rotation is significant and insignificant around the axis perpendicular and parallel to the compression axis, respectively.

(4) Rotation rate is larger for finer and anisotropic grains.

(5) Rotation rate is maximized when low-index plane grain boundaries are oriented at ~45 degree against the compressional axis and minimized when they are parallel and perpendicular to the compressional axis.

(6) Presence of domains of grains rotation to the same direction.

All these observations are well reproduced by grain rotation due to shear stress applied on the low-index plane grain boundaries, which explains the CPO development in these samples.