Abstract
Abstract. To apprehend plate tectonics and the dynamics of the
lithosphere–asthenosphere boundary, composed principally of olivine, we
need to understand the mechanisms that control plastic deformation of
olivine in the relevant temperature domain. After more than 50 years of
laboratory studies and investigations on natural rocks, the interplay of
several key parameters (e.g. temperature, pressure, vacancy concentration,
dislocation densities, grain size, strain rate) controlling polycrystalline
olivine plasticity remains difficult to assess. Here, we study four olivine
polycrystals, which have been deformed in axial compression under a
confining pressure of 300 MPa, at 1273 or 1473 K. Despite significant
differences in mechanical properties (stress–strain curves), previous
characterization by scanning (SEM) and transmission electron microscopy
(TEM) did not reveal significant differences in dislocation microstructures
which could explain these contrasted behaviours. We have undertaken
automatic crystallographic orientation mapping (ACOM) analyses in TEM to
increase the spatial resolution of characterization compared to previously
obtained electron backscatter diffraction maps to further decipher the
microstructures at nanoscale. With this novel technique applied to olivine,
a noticeable difference in the onset of microstructural recovery has been
identified between specimens deformed at 1273 and 1473 K. The
microstructures of the olivine polycrystals deformed at 1473 K exhibit
numerous curved grain and subgrain boundaries, advocating for recovery by
boundary migration. In contrast, the microstructures of the olivine
polycrystals deformed at 1273 K have significantly fewer subgrain
boundaries and show more straight boundaries (i.e. closer to an equilibrium
microstructure) than in the specimen deformed at 1473 K. Characterization by
ACOM-TEM has permitted the identification of the onset of recovery, which is led by
boundary migration even for very low macroscopic finite strains.