Partially open grain- and phase boundaries as fluid pathways in metamorphic rocks: new observations and modelling

部分开放的晶界和相界作为变质岩中的流体路径：新的观察和建模

• 批准号: 273065999
• 负责人: Professor Dr. Wolfgang W. Schmahl
• 金额: --
• 依托单位: Arbeitsgebiet für anorganische und biogene Geomaterialien
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/273065999?language=en
• 关键词: Partially; open; grain; phase; boundaries

Based on the Focused Ion Beam (FIB) method and High-Resolution Transmission Electron Microscopy (HRTEM), grain and phase boundaries between various rock-forming minerals, such as plagioclase, K-feldspar, pyroxene, amphibole, calcite and quartz, from a variety of metamorphic, plutonic and volcanic rocks were investigated. These boundaries are partially open up to several hundred nanometre and partially to totally filled with secondary minerals, such as actinolite, biotite, sheet silicates, and quartz. In relation to quartz, the opening has been explained semi-quantitatively in 2D by interaction of cooling-induced volume reduction and decompression-related volume expansion.In order to insight into the time evolution of open boundary networks, 3D numerical modelling has been performed combining contact mechanics with the finite-element method. Comparison with grain-boundary opening data from natural quartzite indicates that the tensile yield strength show best-fit results for  = 25 MPa. Moreover, values of 55% for proportion of open grain boundaries are indiated, in agreement with microstructural observations of natural rocks, obtained during earlier investigations and latest ones as part of the current project. In addition, it implies that quartz grain-boundary opening initiates after a temperature and pressure decrease of ca. 80 °C and MPa. All this confirms the validity of the numerical modelling and its essential assumptions. Based on these findings, during the requested extension period of the project the fruits of the difficult and time-consuming implementation of the numerical models shall be reaped. In detail, the following goals are defined. (i) An energy-based fracture propagation criterion will be used for the simulation of grain-boundary cracking and will lead to a critical testing and refinement of the predictions of the first phase of modelling with available laboratory experiments on the fracture toughness of quartzites. (ii) A systematic study of the effect of grain size on grain-boundary opening will be performed. (iii) A critical test of experimentally derived grain-boundary rheology for quartz grains will be conducted. 3D models of a natural quartzite microstructure will be compared with our numerically determined grain-boundary rheology to test if the latter can match the damage pattern observed in the natural sample.

用聚焦离子束(FIB)方法和高分辨电子显微镜(HRTEM)研究了不同变质岩、深成岩和火山岩中的各种造岩矿物(斜长石、钾长石、辉石、角闪石、方解石和石英)之间的颗粒和相界。这些边界部分开放到几百纳米，部分或全部充满次生矿物，如阳起石、黑云母、片状硅酸盐和石英。对于石英，通过冷却引起的体积缩小和与减压相关的体积膨胀的相互作用，在二维中半定量地解释了开口。为了深入了解开放边界网络的时间演化，将接触力学和有限元方法相结合进行了三维数值模拟。与天然石英岩的晶界张开数据对比表明，当K=25 Mpa时，拉伸屈服强度的拟合效果最好。此外，还提出了55%的开放晶界比例的值，这与在早期调查和作为当前项目一部分的最新调查中对天然岩石的微观结构观察相一致。此外，它还表明，在温度和压力降低约80°C和Mpa后，石英晶界开始张开。所有这些都证实了数值模拟及其基本假设的有效性。根据这些调查结果，在要求的项目延长期内，将收获实施数值模型的困难和耗时的成果。具体来说，确定了以下目标。(I)基于能量的断裂扩展准则将用于模拟晶界开裂，并将利用现有的关于石英岩断裂韧性的实验室实验，对模拟的第一阶段的预测进行关键检验和改进。(2)系统地研究了晶粒度对晶界张开的影响。(3)对实验得出的石英颗粒的晶界流变学进行关键测试。将天然石英岩微结构的3D模型与我们的数值确定的晶界流变学进行比较，以测试后者是否能与在天然样品中观察到的损伤模式相匹配。