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 °C, 300 MPa confining pressure, and constant displacement rate leading to strain rates ~10⁻⁵ s⁻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 ± 5 MPa), 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 ± 15 MPa 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.

为了探索橄榄石晶粒尺寸和晶体择优取向 (CPO) 对上地幔岩石微观结构演化和大应变力学行为的影响，我们在 1200 °C、300 MPa 围压下进行了轴向拉伸实验, 和恒定位移率导致应变率 ~10 ^-5  s ^-1在三种天然橄榄岩上：一种具有弱橄榄石 CPO 的细粒糜棱斜方辉石和两种具有不同强度橄榄石 CPO 的粗粒良好平衡的单质岩。尽管初始微观结构不同，但初始流动应力显示出有限的变化范围 (115–165 ± 5 MPa)，细粒菱镁矿显示出最高的初始强度。然而，细粒橄榄岩和粗粒橄榄岩的力学行为和微观结构的演变明显不同。在细粒菱镁矿中，颈缩与表观微分应力的降低有关。应变和应力的集中导致橄榄石CPO强度和再结晶分数的增加以及颈部再结晶晶粒尺寸的减小。颈部最终应力和应变的分析表明响应于微观结构和 CPO 的演变而软化。相比之下，粗粒纯晶样品的颈缩与表观微分应力较弱或没有减少有关。这意味着硬化，与（1）随着在这些样品中观察到的应变的增加，体积晶粒内取向错误的增加和（2）颈部的最终应力与初始应力相似或更高。所有粗粒砂粒都表现出高度不均匀的变形。通过位错滑移良好定向变形的晶体被拉长并形成明显的波状消光，而硬定向的晶体几乎保持不变形。在脖子上，应力和应变集中（局部应力和应变分别达到 365 ± 15 MPa 和 240%）导致在“硬”晶体中形成扭结，在“软”晶体中形成动态再结晶，并且轴向应力克服了围压, 伸展性骨折的发展。我们将细粒橄榄岩更有效的应变诱导软化解释为更容易动态再结晶，这可能是由于作为成核点的晶界比例较高。