Investigating faults through EBSD

Supporting PhD research into Complex Fault Systems with Grain Orientation Analysis

Plymouth Electron Microscopy Centre works closely with the School of Geography, Earth and Environmental Sciences , supporting undergraduate and postgraduate projects as well as academic research. Our full suite of electron microscopes and analytical techniques are available for use, and gives students valuable hands-on experience.

The data in this case study was collected as part of Joseph Connolly 's multi-technique PhD project (supervised by Professor Mark Anderson ), investigating a complicated series of faulting events in south west England which takes full advantage of PEMC's live and automated analysis.

Introducing EBSD

Electron Backscatter Diffraction (EBSD) is a technique used to analyse the orientations of crystals in a crystalline or polycrystalline material, utilising unique crystallographic structures. In geology, EBSD is often used to investigate the orientation of crystals in context of the whole rock, to find evidence of mechanical deformation, to identify recrystallisation, and much more. It's also very useful in materials science.

EBSD is available on a range of instruments at PEMC, including the JEOL 7001 FE SEM , JEOL 6610 LV SEM , JEOL IT800 FE SEM and the ZEISS Crossbeam 550 FIB-SEM , and can be run in conjunction with Energy Dispersive Spectroscopy (EDS) analysis.

Analysis at PEMC

EBSD for geological samples at PEMC can be conducted on petrographic thin or thick sections, and on polished resin mounts. For this experiment, we used petrographic sections that had been previously been mapped with an optical microscope so we could easily identify the key areas once the sample was loaded into our JEOL IT-800 FE-SEM. The minerals in the samples were confirmed through optical microscopy and EDS prior to EBSD, and we were able to establish that the main phases of interest in this study were calcite (CaCO₃) and celestine (SrSO₄).

Working alongside Joe, we analysed the areas of interest using EBSD but also set up large area montages to collect data overnight (or over a weekend), generating data sets that cover the majority of sample.

Moving earth

Our EBSD analysis delved into the history of these fault rocks, revealing several stages of fault movement and mineral crystallisation. The images below are orientation maps of calcite and celestine respectively. In this figures, the colour reflects the orientation of the crystal and a gradient shows how the structure has been deformed within the crystal.

The calcite shows areas of small crystals that are the result of recrystallisation during faulting, as well as a large amount of twinning. In geology, crystal twinning is caused by adjacent crystals of the same mineral that share points of their crystal lattice in a symmetrical orientation. Celestine grows as fine fibrous crystals, and this is made very obvious in EBSD. The celestine here infills the newly opened fault.

Calcite orientation map

Celestine orientation map

Automated Analysis

As mentioned before, we also set up a series of automated analyses. This lets us cover a much larger area (almost the whole sample in some cases) that can help give more context as well as finding other interesting features. Setting up the automated analysis can take time and is not a replacement for live analysis where the user or PEMC staff can change conditions, react to instrument changes, and so on.

PEMC also offers automated imaging, automated EDS including both point analysis and mapping, automated WDS, and Automated Mineralogy through dedicated software.

Combining techniques

Electron microscopy is a powerful tool, especially when combined with other techniques, but it is important to ensure that the order in which experimental stages are done is appropriate. Namely, beginning with non-destructive analysis and moving into destructive analysis. In this case, the experiment began with optical microscopy and electron microscopy. The imaging and EBSD carried out at PEMC helped Joe to unravel the different stages and timings of fault events, which in turn adds more context for the subsequent destructive dating analysis.

The team at PEMC are happy to give guidance on how and when to slot electron microscopy in to your experiment.