Deformation of upper mantle rocks with contrasting initial fabrics in axial extension

Abstract

Abstract To explore the role of the olivine grain size and crystal preferred orientation (CPO), on the evolution of the microstructure and the mechanical behavior of upper mantle rocks up to large strains, we performed axial extension experiments at 1200\xa0°C, 300\xa0MPa confining pressure, and constant displacement rate leading to strain rates ~10−5\xa0s−1 on three natural 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 microstructures, initial flow stresses show a limited range of variation (115–165\xa0±\xa05\xa0MPa), with the fine-grained harzburgite displaying the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs markedly between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking is associated with decrease in the apparent differential stress. Focusing of strain and stress resulted in increase of the olivine CPO intensity and recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening in response to the evolution of the microstructure and CPO. In contrast, necking of the coarse-grained dunite samples is associated with either a weaker or no decrease in the apparent differential stress. This implies hardening, consistently with (1) the increase in bulk intragranular misorientation with increasing strain observed in these samples and (2) final stresses in the neck similar or higher than the initial ones. All coarse-grained dunites displayed a highly 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 (local stresses and strains attained up to 365\xa0±\xa015\xa0MPa and 240%, respectively) resulted in formation of kinks in “hard” crystals, dynamic recrystallization in “soft” crystals and, where the axial stress overcame the confining pressure, development of extensional fractures. 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.