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

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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.