Abstract text (incl. figure legends and references)

TEM/FIB-based investigations have shown that grain and phase boundaries of various minerals in different metamorphic and magmatic rocks are lastingly open on the nanometer scale and partly to totally filled with secondary minerals [1, 2]. The most likely reason for opening is the exhumation of rocks and the associated temperature and pressure decrease, which leads to volume reduction and internal stresses on the grain scale and causes fracturing and opening of the boundaries. This is confirmed by 3D numerical modelling, combining contact mechanics and finite-element method [3]. It is assumed that opening starts below the temperature threshold for dislocation creep in the different minerals.

TEM investigations are restricted to small material volumes. Therefore, scanning electron microscopy (SEM) is applied to cm-large thin-section areas. The drastically enlarged data set provides statistically reliable information about conditions of opening and filling of boundaries in grain aggregates with different size and crystallographic orientation of grains. Investigated rocks and minerals include contact-metamorphic quartzite, deformed vein quartz, metagabbro, lherzolite, garnet-sillimanite rock, felsic granulite, calcsilicate rock, jadeite quartzite, syntectonic granitoid, and marble.

Over entire thin-section areas, roughly 90% of the boundaries are continuously open or partly filled with secondary minerals, independently of mineral type, and grain size, shape and crystallographic orientation. This confirms previous TEM investigations on a much broader data basis. The two crystal faces of an open boundary are mostly strictly parallel to each other. The width of opening remains generally constant between triple junctions, i.e., typically over tenth of micrometers. However, boundary kinks show that movement directions are often not perpendicular but oblique to the boundary. Open space is also developed between filling and crystal faces. This points to more than one stage of opening. Particularly in quartz, micrometer-large, often conical voids frequently occur along open boundaries indicating dissolution-precipitation processes. For quartz, a weak correlation exists between grain size and boundary opening width, however, is absent for the crystallographic orientation of boundaries and opening width. This confirms that cracking and opening of boundaries are controlled in a complex way by the development of stress on the grain scale and movements in the grain aggregate.

In general, these findings demonstrate that grain and phase boundaries are completely or partly open in all types of magmatic and metamorphic rocks and form pathways for fluid migration on thin-section scale but most likely also on larger scales. This represents a ubiquitous structural phenomenon of crystalline materials under at least upper-crust to surface conditions. It controls not only various physical properties of crystalline material but also governs its behavior during different natural and technical conditions as well as in experiment.

[1] Kruhl, J.H., Wirth, R., Morales, L.F.G., 2013. JGR Solid Earth. https://doi.org/10.1002/jgrb.50099

[2] Wirth, R., Kruhl, J.H., Morales, L.F.G., Schreiber, A., 2020. J. Metam. Geol. https://doi.org/10.1111/jmg.12610

[3] Raghami, E., Schrank, C., Kruhl, J.H., 2020. Tectonophysics. https://doi.org/10.1016/j.tecto.2019.228242