  To explore the role of the olivine grain size and crystal preferred
orientation (CPO) on the evolution of the microstructure and the mechanical
behaviour of upper mantle rocks up to large strains, we performed axial
extension experiments at 1200C, 300MPa confining pressure, and constant
displacement rate on three natural peridotites: a fine-grained mylonitic
harzburgite with a weak CPO and two coarse-grained well-equilibrated dunites
with CPO of variable intensity. Initial flow stresses show a limited range of
variation (115-165MPa), with the fine-grained sample showing the highest
initial strength. However, the evolution of both mechanical behavior and
microstructure differs between fine and coarse-grained peridotites. In the
fine-grained harzburgite, necking resulted in decrease in the apparent
differential stress. Focusing of strain and stress produced increase of the
olivine recrystallized fraction and decrease of the recrystallized grain size
in the neck. Analysis of the final stress and strain in the neck indicates
softening due to evolution of the microstructure and CPO. In contrast, necking
of the coarse-grained samples produced weak or no decrease in the apparent
differential stress. This implies hardening, consistently with the increase in
bulk intragranular misorientation and final stresses in the neck similar or
higher than initial ones. Coarse-grained dunites deformed heterogeneously.
Crystals well oriented to deform by dislocation glide became elongated, whereas
those in hard orientations remained almost undeformed. In the neck, stress and
strain concentration resulted in formation of kinks in hard crystals and
dynamic recrystallization in soft crystals. We interpret the more effective
strain-induced softening of the fine-grained peridotite as due to easier
dynamic recrystallization, probably due to the higher proportion of grain
boundaries acting as nucleation sites.
