The application and associated problems of EBSD technique in fabric analysis

在过去的二十年里,EBSD(Electron Backscattered Diffraction),即电子背散射衍射测试技术,已广泛应用于韧性组构分析,成为变形运动学、流变学分析的常规手段。该方法主要应用于流变条件下矿物晶轴组构定向性分析,以判定流变剪切指向、对比应变强度、估算变形温度。理论上讲,EBSD法适用于所有矿物的全部晶轴定向的分析测试。然而鉴于天然变形的复杂性,笔者建议EBSD分析应以石英,特别是经历了动态重结晶的石英条带为组构分析的主要对象。长期以来,石英晶轴组构的不对称性被视作独立的剪切指向标志。然而,近年来基于天然变形和一般剪切实验的研究结果表明,塑性流变的剪切指向含义应为多重流变剪切指向标志综合判别比对的结果。尽管在提出之初,石英的轴组构开角被视作独立可靠的变形温度计(Kruhl,1998)。然而限于天然变形的复杂性,特别是对变质与变形阶段的对应、耦合的认识;尽管石英变形滑移系及石英晶轴组构开角可为动力变质温度提供重要的参考,但是石英晶轴组构开角并非独立的变形温度计。;In the last two decades, EBSD (Electron Backscattered Diffraction) has been widely used as a routine approach in ductile fabric analyses, focusing on flow kinematics and rheology of ductile deformation. It is predominantly applied to determining the shear-sense of the bulk flow, evaluating strain strength, and assessing deformation temperature via analyzing Crystallographic Preferred Orientation (CPO) and deformation mechanisms of recrystallized minerals. Theoretically, EBDS retains the potential to recover the crystal-axis orientation of all minerals. However, dynamically recrystallized quartz grains, particularly, quartz ribbons are recommended for EBDS analysis due to the complexities of natural deformation. Conventionally, quartz c-axis obliquity is used as a reliable and independent shear-sense indicator, while the c-axis opening angle and recrystallization mechanisms of quartz (i.e., bulging, subgrain rotation, and grain boundary migration) have been treated as deformation thermometers. However, recent studies on naturally deformed rocks and general-shear experiments indicate that cautions must be exercised when using EBSD results to extract flow kinematics and multi-criteria are suggested for a proper interpretation of flow kinematics in ductile shear zones. Similarly, limited to our understanding of the process of natural deformation, particularly the coupling between metamorphism and ductile deformation, crystal fabric-based deformation thermometers have their own caveats and limitations that EBSD-derived thermal data, the quartz c-axis opening-angles, may also be subject to multiple interpretations although it was originally proposed as an independent deformation thermometer (Kruhl, 1998).