  To characterize the role of the microstructure and crystal-preferred
orientation (CPO) on the deformation of mantle rocks, we performed axial
extension experiments at 1200C and 300 MPa confining pressure on three natural
upper mantle peridotites: a fine-grained mylonitic harzburgite with a weak
olivine CPO and two coarse-grained well-equilibrated dunites with olivine CPO
of variable intensity. Despite the contrasting initial microstructure and
olivine CPO, yield stresses show a limited range of variation (115-165 MPa).
Fine and coarse-grained peridotites show nevertheless different evolutions of
both mechanical behavior and microstructure. In the fine-grained harzburgite
necking produced decrease of the apparent differential stress. The olivine CPO
intensity and recrystallized fraction increase and the recrystallized grain
size decreases towards the neck. Analysis of the stresses and strains in the
neck points to softening due to the evolution of the microstructure and CPO. In
contrast, necking of the coarse-grained dunite samples was not systematically
accompanied by decrease in apparent differential stress. This implies
hardening, consistently with the increase in bulk intragranular misorientation
with increasing strain in these samples. All coarse-grained dunites displayed a
heterogeneous deformation. Crystals well oriented to deform by dislocation
glide became elongated and developed marked undulose extinction, whereas
crystals in hard orientations remained almost undeformed. In the neck, stress
and strain concentration (up to 365 MPa and 240%) resulted in formation of
kinks in hard crystals, dynamic recrystallization in soft crystals and finally
development of extensional fractures. We interpret the more effective softening
of the fine-grained sample as due to easier dynamic recrystallization, probably
due to the higher proportion of grain boundaries acting as nucleation sites.
