Microstructure evolution and grain boundary mobility during creep deformation and annealing of anhydrite rocks.

硬石膏(CaSO4)作为构造板块边界主要断裂带的滑脱层、油气藏的盖层、二氧化碳的封存和放射性废物的潜在储存库，在浅层地壳中具有重要的作用。此外，硬石膏是一种有用的类似硅酸盐的材料，其物理性质与其他同类矿物的流变性和重结晶有关。回复和再结晶过程发生在材料的塑性变形(位错蠕变)和退火(静态加热)过程中，通过晶界的形成和移动。在地壳和地幔中，岩石的同构造(动态)和后构造(静态)重结晶可以改变颗粒的大小、形状和晶体取向。这影响物理性质和各向异性，对解释沿板块边界的主要断裂带、山区地质地形和地震各向异性数据的岩石力学行为至关重要。硬石膏和矿物的再结晶行为和相关的边界性质(几何形状、流动性、扩散性和滑移性)一般都知之甚少。在具有双晶界(硬石膏、方解石、石英、斜长石)等特殊界面的矿物中，观察到的微观结构不能仅用亚晶旋转和晶界迁移重结晶来解释，并提出了另外两种机制，即晶界滑动伴随扩散导致物质弱化和考虑特殊(孪晶)晶界的重结晶机制。这发生在相对较高应力下的晶体塑性变形过程中。在自然和实验变形岩石的最终微结构中，微结构演化的详细证据以及驱动微结构演化的机制往往被抹去。非标准的变形和退火实验室实验，其中硬石膏骨料将分别以小的应变增量和时间增量，在每一增量后，将进行EBSD分析，以深入了解1.孪晶边界辅助的再结晶的动力学和运动学，2.这在硬石膏骨料和其他类似矿物的变形行为中所起的作用。这样的测试是非标准的，因为相同的样本，而不是传统的岩石变形测试中的不同样本，将被带到应变或时间的增量，并按顺序进行分析。这将允许跟踪在多晶材料的蠕变、变形和退火过程中单个颗粒和晶界的演变。在每次变形实验中，变形装置的高分辨率应变片将记录对特定微观结构变化的精细机械响应。将获得关于晶界几何、取向偏差、晶界变形、低角度和高角度晶界迁移的运动学、晶界迁移率、孪晶的作用以及对微观结构变化的力学响应的定量信息。在蠕变试验台上测量的边界迁移率将与在扫描电子显微镜中直接观察新的原位退火实验中的边界运动获得的迁移率数据进行比较，扫描电子显微镜将在硬石膏上进行这一实验计划。将进行高围压试验，并与蠕变试验台试验结果(室内压力)进行比较，以检验等静压对晶界迁移率的重要影响。由此收集到的关于硬石膏多晶微观结构变化的再结晶机制、迁移率和力学响应的证据将是建立更真实的再结晶模型的基础。这将支持我们对地壳和地幔中的同步性和后构造性过程的解释。

Anhydrite (CaSO4) is important in the shallow Earth's crust as a detachment horizon in major fault zones at tectonic plate boundaries, cap-rock for hydrocarbon reservoirs, CO2 sequestration, and potential repository for radioactive waste. Also anhydrite is a useful silicate-analogue material and its physical properties are relevant to the rheology and recrystallization of other comparable minerals. Recovery and recrystallization processes occur during plastic deformation (dislocation creep) and annealing (static heating) of materials, through the formation and movement of grain boundaries. In the Earth's crust and mantle syn-tectonic (dynamic) and post-tectonic (static) recrystallization of rocks can modify grain sizes, shapes and crystallographic orientations. This affects physical properties and anisotropies and is central to the interpretation of the mechanical behaviour of rocks in major fault zones along plate boundaries, geological terrains in mountain belts, and seismic anisotropy data. The recrystallization behaviour and relevant boundary properties (geometry, mobility, diffusivity and sliding) of anhydrite and minerals in general, are poorly understood. In minerals characterized by special boundaries such as twin boundaries (anhydrite, calcite, quartz, plagioclase), observed microstructures cannot be explained by sub-grain rotation and boundary migration recrystallization alone and two other mechanisms have been proposed, namely grain boundary sliding, accompanied by diffusion and resulting in material weakening and a recrystallization mechanism accounting for special (twin) boundaries. This occurs during crystal plastic deformation at relatively high stresses. In the final microstructures of naturally and experimentally deformed rocks detailed evidence of microstructural evolution, and the mechanisms that drive it, is often obliterated. Non-standard deformation and annealing laboratory experiments, where anhydrite aggregates will be taken to small increments of strain and time respectively and, after each increment, analysed using EBSD, will be performed to gain insight into 1. The dynamics and kinematics of recrystallization assisted by twin boundaries, 2. The role that this plays in the deformation behaviour of anhydrite aggregates and other comparable minerals. Such tests are non-standard because the same sample, rather than different ones as is conventional in rock deformation tests, will be taken to increments of strain or time and sequentially analysed. This will allow tracking the evolution of individual grains and grain boundaries during creep deformation and annealing of a polycrystalline material. During each deformation experiment the fine mechanical response to specific microstructural changes, will be recorded by the high resolution strain gauges of the deformation apparatus. Quantitative information on boundary geometry, misorientation, grain distortion, kinematics of low and high angle grain boundary migration, grain boundary mobility, the role of twinning, and mechanical response to microstructural change will be achieved. Boundary mobility measured in the creep rig will be compared with the mobility data obtained from direct observation of boundary motion in novel in-situ annealing experiments in the scanning electron microscope, which will be performed on anhydrite as part of this experimental program. The important effect of isostatic pressure on grain boundary mobility will be tested performing high confining pressure experiments and comparing results with those from creep rig tests (room pressure). The evidence thus gathered on recrystallization mechanisms, mobility and mechanical response to microstructural changes of anhydrite polycrystals will be the basis upon which more realistic recrystallization models can be constructed. This will underpin our interpretation of syn- and post-tectonic processes in the Earth's crust and mantle.