A Microscale Investigation of a 1000km Metamorphic Belt

How electron microscopy reveals details about large-scale metamorphic zones.
EDS map of schist
Geologists combine both field and laboratory analysis to unravel the geological history of a region. In the field, they identify the different rocks types and geological structures which then get used to create a geological map. In the lab, geologists can use a wide range of techniques to further examine the rocks they have collected. This can include slicing the rocks into thin sections only 30 µm thick for use in optical microscopy where individual minerals can be identified, isotope analysis to work out how old certain minerals are, and electron microscopy analysis to investigate microscale structures as well as detailed elemental compositions.
This case study features a sample of a schist from Omeo in east Victoria, Australia, which sits at the southern end of the Wagga-Omeo Metamorphic Belt and is part of the 200,000 km2 Lachlan Fold Belt. Schists are metamorphic rocks that are produced when sedimentary rocks experience low-temperature and low-pressure metamorphism. This causes the platy minerals in them to align and wrap around larger, more rigid crystals. However, because these platy minerals are often very small it is difficult to fully understand the features of the rock without microscopic analysis.

After cutting a small piece of rock from the main sample and mounting it in an SEM-safe resin, it was polished flat to allow for good quality compositional data to be collected using our JEOL IT-800 FEG-SEM.
Backscattered electron (BSE) imaging creates a greyscale that reflects the atomic number contrast of the sample, where high atomic number elements show as white and light greys whilst low atomic number elements are darker. This means that different minerals appear as different shades of grey, allowing for the first stages of characterisation to begin.
In particular, BSE imaging of this sample reveals the distribution of key minerals such as biotite mica (light grey) and quartz (dark grey), and gives a much clearer idea as to the texture of the schist than is possible through optical microscopy. The Omeo schist shows a number of bands of minerals that have been deformed into waves, likely reflecting larger-scale regional metamorphic deformation.
Schist BSE
BSE image of the Omeo Schist
Schist EDS
Layered element map of the Omeo schist.
Electron microscopy can also be used to analyse the composition of minerals at an elemental scale through either energy dispersive spectroscopy (EDS) or wavelength dispersive spectroscopy (WDS). Here, EDS was used to create coloured element maps of the sample which can be layered together to show the distribution of different minerals. In this colour scheme, blue is quartz, green is muscovite mica, and pink and yellow are different compositions of biotite mica.
Element maps like this can be used to guide further research by revealing areas of interest. For example, the composition and location of a particular mineral in a sample can then be applied to dating techniques such as laser ablation or more advanced electron microscopy techniques including electron backscatter diffraction (EBSD) which allows researchers to investigate strain and deformation of a crystal.
During metamorphism, schists can grow "porphyroblasts" - a large mineral crystal that has grown within a fine-grained matrix that often wraps around the newly-formed porphyroblast. In the field, these porphyroblasts were identified as an aluminum-silicate mineral called andalusite. However, analysis through electron microscopy reveals that these porphyroblasts are actually an aggregate of quartz (blue), mica (pink and yellow), and apatite (green) with only tiny pieces of <40 µm andalusite (red) found, suggesting that the schist has undergone more alteration than originally thought.
Electron microscopy has revealed significantly more detail about the low-grade metamorphism and alteration that occurred in the Wagga-Omeo Metamorphic Belt than would be possible through optical microscopy. Future EBSD and compositional analysis should provide even greater insights into the deformation and alteration recorded by this schist.
Schist knot eds
Layered element map of an altered porphyroblast in the Omeo schist.

More information

If you would like more information on how electron microscopy - particularly EDS and WDS - could benefit your research, please contact Dr Jen Mitchell .