Geosciences

Geological site characterization is essential to prevent natural hazards, improve urban infrastructures and access to water, energy and mineral resources. Site investigations are important to establish a geological history and require researchers to characterize and understand geotechnical complexities that contribute to the formation of the material, subsequent modification by deformation and diagenesis, as well as weathering processes. This knowledge not only enables researchers to recover or understand these natural resources but allows them to perform this with minimal impact on the environment. Useful analyses to elucidate these geotechnical complexities include:

• Analyze overgrowth, cementation, microfractures, and fluid flow in metamorphic samples
• Measure trace elements, internal structures, and textural features in small sample areas
• Classify authigenic and detrital (mineral) phases and their distribution
• Reconstruct the chronology for metamorphic processes, such as mineral and phase formations

For more in-depth information, please see our Cathodoluminescence techniques for the geosciences application note.

To adequately characterize and understand geological materials, you must first ensure each specimen is of the highest quality to resolve the material interface and properly controlled so you manipulate it, when necessary, under environmental stimuli. Once prepared, several techniques are available to better understand the relationship between microstructure, defects, and the optical properties of materials.

Reveal overgrowth and cementation processes

Color cathodoluminescence images reveal cementation processes in reservoir rocks. In the left image, you can see low-temperature quartz cement (bluish) and a later (hotter) overprint (reddish) along grain boundaries in the overgrowth. In the right image, you can see well-developed overgrowth cement with multiple zoning. Image courtesy of Dr. Juergen Schieber, Indiana University Shale. Acquired on the ChromoCL2™ system.

Measure trace elements, internal structures, and zoning

Here, you can see a large collection of zircon grains that were imaged to reveal a full zonation structure and late rims that are only observable with cathodoluminescence. Field stitching software was used to create a 70 million-pixel image with a field of view that is several millimeters in size. Results demonstrate that exact pixel registration between all signals allows you to directly correlate between topography, composition, and trace element chemistry over large areas. Sample courtesy of Geological Survey Finland.

Analyze microfractures and fluid flow

The distribution of microfractures in geological materials can provide insight into emplacement events, such as damage associated with tectonic faulting. The image below shows how cathodoluminescence can be a valuable tool to reveal microfractures under high spatial resolution conditions (low accelerating voltage, low CL signal). Image courtesy of After Laubach et al., Journal of Structural Geology 2005. 

Cathodoluminescence provides unrivaled sensitivity to trace element composition, so you can observe chemical overprinting where fluid flow along cracks has caused alteration in a polycrystalline diamond sample.

Identify minerals, phases, and their distribution

These results demonstrate that cathodoluminescence can reveal mineral attributes unavailable through alternative techniques. When compared, an EDS/BSE analysis (left) shows a uniform distribution of quartz across the sample, while the secondary electron image (right) shows that there are a few large grains of authigenic and detrital quartz.

Reconstruct geological processes

To better understand the genesis of polycrystalline diamonds, you can compare the cathodoluminescence spectral characteristics to determine the cause of irradiation or point defects. For this sample, (a) a composite cathodoluminescence image of polycrystalline diamond was created using red, green, and blue spectrally filtered cathodoluminescence images. When cross-correlated with spectroscopy results, individual elements or substances were identified within the sample. Results showed a radioactive fluid ingress along grain boundaries, which led to the radiation halo effects that were observed (yellow). Data courtesy of Dr. E. Vicenzi, Smithsonian Institute.