COMPARING ANALYTICAL DATA FROM A HANDHELD ENERGY-DISPERSIVE XRF INSTRUMENT WITH A LABORATORY-BASED WAVELENGTH-DISPERSIVE XRF INSTRUMENT, WITH IMPLICATIONS FOR FINGERPRINTING OF OBSIDIAN ARTIFACTS IN THE AMERICAN SOUTHWEST

Obsidian is a volcanic glass valued for its strength and conchoidal fracture pattern, making it ideal for early stone tools. Obsidian geochemistry is useful as a window into the volcanic eruption from which it formed. When analyzed for its major and trace elemental concentrations, it is possible to create a “chemical fingerprint” for each unique volcano and eruption. A 2014 Keck Research project looked at samples from a variety of locations surrounding the Mt. Taylor and Mule Creek regions of New Mexico using a portable energy-dispersive X-Ray Fluorescence (pXRF). A goal of our project at F&M over the past year has been to compare data previously produced by pXRF (Roth, 2014) with the data obtained here by the lab-based wavelength-dispersive XRF (WD-XRF) at F&M. WD-XRF is more accurate and precise compared to pXRF, due to the basic analytical principles behind the method compared to ED-XRF, and because the WD-XRF is calibrated against international standards. We note that the pXRF elemental concentrations tend to be systematically lower than concentrations measured by WD-XRF. The average percent difference between the two methods was around 80-90%. These results indicate that the previously reported pXRF data might not be as useful as initially thought. pXRF is a useful instrument in the field due to its speed, and the fact that it preserves the original sample, but the data produced cannot be reliably used for absolute elemental concentrations. This comparison may be useful when deciding which instrument to use and also for understanding the accuracy of data produced by pXRF. Additionally, quantifying the differences between these two analytical techniques can hopefully yield a correction factor—specific to a pXRF device and determined via standard reference material—to allow for improved pXRF accuracy. The evaluation of these new and previously collected analyses will further expand the geochemical dataset used for sourcing obsidian artifacts in the American Southwest.