USE OF THE PORTABLE XRF IN CLASS PROJECTS: PERKS AND PITFALLS

Geology students who struggle with the concepts of chemistry in geological applications can benefit from hands-on experience with collecting and analyzing geochemical data. During the fall of 2011, students in the RU Sed Pet & Strat class incorporated the use of a handheld XRF into end-of-semester research projects in which they investigated sediments from Mountain Lake, SW VA. Mountain Lake periodically experiences extreme water loss, giving rise to an extraordinary opportunity to study modern lacustrine sediments in a restricted basin. In 2011 the water dropped to nearly its lowest level in many years, allowing the students to extract several sediment cores from different locations across the dry lakebed. Five cores, each approximately one meter in length, were collected, and each one was assigned to a student group for evaluation. In addition to standard sedimentological methods of analysis (e.g., core logging, grain size analysis), the students collected geochemical data at intervals along the length of their core using the handheld XRF in the ‘soils’ mode. As a first-time experiment using this technique for sediment analysis, we hoped to find systematic variations in major or trace elements that could be correlated with grain size and mineralogy. Given the limited range of elements the instrument is capable of analyzing, however, such variations turned out to be quite subtle. Two of the groups were able to correlate changes in a few elements (e.g., Fe, Zr) with grain size and sediment color, whereas the others simply noted changes but weren’t able to decipher systematic variations. Despite the mixed results, the project had its perks: most of the students learned to use the instrument as directed, evaluate data for elements below detection limit, enter data into an Excel spreadsheet and graph the results, and begin to pick out trends in particular elements. There were also pitfalls, including problems with data presentation, acceptance of spurious results without consideration of limitations of the instrument and methodology, and general application of geochemical concepts from previous classes. In hindsight, we learned some valuable lessons for the second time around, including better instruction on the instrument’s limitations, and more time given to discussion of results with individuals and groups.