Grain Boundary Characterization Using Atomic Force Microscopy: Investigating Possible Percolation Pathways in Rocks

Abstract:

Studies on the nature of grain boundaries, and the role they play in assisting or resisting fluid flow offer crucial insights into mechanisms by which fluids percolate and move through the lower crust. While previous studies have documented grain boundary widths and their variation with depth using Transmission Electron Microscopy (TEM), lateral variations in grain boundary widths have not been documented in detail. Our studies focus on determining lateral variations in grain boundary widths in seven quartzite samples deformed and metamorphosed at different metamorphic grades using Atomic Force Microscopy (AFM).

Using the AFM, the maximum grain boundary width (387.1 nm) has been observed in a mylonitised quartzite deformed under greenschist facies conditions. The minimum is recorded in a quartzite deformed under granulite facies conditions (33.5 nm). This suggests a correlation between the grade of metamorphism and grain boundary width, with higher grade rocks having lower grain boundary widths. Additionally, some grain boundaries appear to contain ‘bridge’-like structures across which the grains on either side appear joined together. These bridge structures may correspond to the periodic overlap of lattices developed along Coincident Site Lattice (CSL) boundaries.

Force Distance (FD) Spectroscopy using AFM was used as an indirect measure of the hardness of the surface being analysed. From the FD curve obtained from the AFM, a ‘Plastic deformation’ parameter was obtained from which it is possible to qualitatively argue that if the intermolecular force is stronger, then plastic deformation should typically be less when we are using same AFM tip and at the same ambient conditions (all analyses are carried out at room temperature and 1 atmosphere pressure). FD spectroscopy of the grain boundary domains reveal that segments of the grain boundaries which contain the bridge structures are typically ‘harder’ than those without the bridges, which might be related to special (CSL) boundaries in quartz. Therefore, at higher metamorphic grades, fluid percolation along a grain boundary might be impeded due to decreased width of grain boundaries as well as the development of bridge structures which are a probable manifestation of CSL boundaries.