[论文解读] Deformation of upper mantle rocks with contrasting initial fabrics in axial extension

Walid Ben Ismaı̈l, Andréa Tommasi+4 more|Research Explorer (The University of Manchester)|Jan 9, 2021

High-pressure geophysics and materials参考文献 58被引用 10

一句话总结

本研究探讨了在1200 °C和300 MPa条件下，上地幔橄榄岩轴向拉伸过程中初始晶粒尺寸和晶体优选取向（CPO）对微结构演化和力学行为的影响。细粒方辉橄榄岩因动态再结晶而出现应变软化，而粗粒纯橄榄岩则因应变集中和晶内取向偏差增加而出现硬化，凸显了晶界密度在变形机制中的关键作用。

ABSTRACT

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.

研究动机与目标

• 理解初始晶粒尺寸和晶体优选取向（CPO）对上地幔岩石微结构演化和力学行为的影响。
• 研究在大应变变形过程中，细粒与粗粒橄榄岩在应变局部化和应力演化方面的差异。
• 评估动态再结晶和晶界密度在控制变形软化或硬化中的作用。
• 确定初始CPO强度和晶粒尺寸是否影响缩颈和应变集中现象的发展。

提出的方法

• 在1200 °C和300 MPa围压下，以恒定位移速率进行轴向拉伸实验。
• 测试了三种天然橄榄岩：一种细粒糜棱岩化方辉橄榄岩（CPO微弱）和两种晶粒较粗的纯橄榄岩（CPO强度不同）。
• 利用电子背散射衍射（EBSD）分析微结构演化，追踪晶粒尺寸、CPO和晶内取向偏差。
• 通过监测应力-应变演化，评估变形过程中的力学软化或硬化行为。
• 通过应力-应变曲线和缩颈区域的显微结构分析，研究应变局部化和缩颈现象。
• 通过比较不同样品中再结晶分数和晶粒尺寸，评估晶界作为动态再结晶形核位点的作用。

实验结果

研究问题

• RQ1 初始晶粒尺寸在上地幔岩石轴向拉伸过程中如何影响动态再结晶和微结构演化？
• RQ2 初始晶体优选取向（CPO）强度对橄榄岩中应变局部化和应力演化有何影响？
• RQ3 为何细粒方辉橄榄岩在相似变形条件下表现出软化，而粗粒纯橄榄岩则表现出硬化？
• RQ4 晶界在多大程度上促进再结晶并影响高温地幔岩石的力学行为？
• RQ5 不同橄榄岩类型在缩颈区域的应力和应变集中如何影响晶粒形态和晶内取向偏差？

主要发现

• 细粒方辉橄榄岩初始流变应力最高（165 MPa），但在缩颈区域因动态再结晶而显著软化。
• 在细粒样品中，缩颈区再结晶晶粒尺寸减小，再结晶分数增加，表明应变诱发软化有效。
• 粗粒纯橄榄岩在缩颈过程中未出现明显表观偏应力下降，表明存在力学硬化。
• 粗粒样品中的应力和应变集中导致强取向晶粒形成弯曲，而弱取向晶粒则发生动态再结晶。
• 粗粒样品中整体晶内取向偏差的增加与硬化相关，因为最终缩颈区的应力与初始值相当或更高。
• 细粒橄榄岩中更有效的软化归因于更高的晶界密度，后者作为动态再结晶的形核位点。