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

利用聚焦离子束（FIB）方法和高分辨率透射电镜（HRTEM）技术，研究了不同变质岩、深成岩和火山岩中斜长石、钾长石、辉石、角闪石、方解石和石英等造岩矿物的颗粒和相界。这些边界部分开放至几百纳米，部分至全部充满次生矿物，如放线石、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减压相关的体积膨胀的相互作用，可以在二维中半定量地解释开口。为了深入了解开放边界网络的时间演化，将接触力学与有限元方法相结合，对开放边界网络进行了三维数值模拟。与天然石英岩的晶界开度数据比较表明，抗拉屈服强度在= 25 MPa时拟合最佳。此外，开放晶界的比例值为55%，与早期调查和当前项目的最新调查中获得的天然岩石微观结构观测结果一致。温度和压力降低约80℃和MPa后，石英晶界开始打开。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，在项目所要求的延长期内，将取得执行数值模型的困难和耗时的成果。详细地说，定义了以下目标。(i)以能量为基础的断裂扩展标准将用于模拟晶界断裂，并将导致对模拟第一阶段的预测进行严格的检验和改进，利用现有的石英岩断裂韧性的实验室实验。将对晶粒尺寸对晶界打开的影响进行系统的研究。将对石英颗粒的实验推导的晶界流变学进行一次关键试验。天然石英岩微观结构的三维模型将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自然样品中观察到的损伤模式相匹配。

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.