A dual beam SEM-based EDS and micro-XRF method for the analysis of large-scale Mesoamerican obsidian tablets

Abstract

X-ray fluorescence spectrometry (XRF) is commonly used as a non-destructive method to identify the geochemical source of obsidian artifacts. We examine in detail three decimeter-scale, polished obsidian “mirrors” housed in the Smithsonian Institution’s (SI) National Museum of the American Indian (NMAI) collections that have previously had minor and trace element components analyzed by portable XRF. Two of the tablets have raw material provenance to the Ucareo source in Michoacán, Mexico, a location within the Los Azufres volcanic complex, while the source for the third tablet, made of mahogany (or meca) obsidian, remained unassigned. We provide a complete analysis of the major, minor, and trace element compositions for each tablet using co-located SEM-based energy dispersive X-ray spectrometry and micro-XRF spectrometry with fundamental parameters quantification. Source assignments for the tablets were made by comparing minor and trace element ratios to obsidian reference samples using canonical discriminant analysis. The reference specimens analyzed expand upon archaeological sources in Central Mexico relevant during Colonial times and include new data for obsidian locations within the Los Azufres complex. Our analyses confirm the previous source assignment for Ucareo for the black tablets and indicate a geochemical match for the mahogany obsidian from a nearby location within the Los Azufres complex that has not previously been associated with archaeological artifacts. Finally, we provide a detailed comparison of the portable and micro-XRF methods and discuss disparities in data and their implications.