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Quantitative Analysis of Obsidian and Determination of Source Provenance Using an Analytical Dual Beam SEM

Obsidian artifact discoveries cover a wide range of geography and timescales dating from hundreds of years before present (BP) to ≈ 320,000 BP [1, 2]. The general utility and widespread use of obsidian make it an archaeologically important material that has helped unravel the history and prehistory of humans, including its use to create permanent artistic expression on skin as tattoos [3]. Obsidian is a volcanic rock that contains trace volumes of crystals relative to a glassy matrix, and when fractured, particularly in a controlled manner, results in the formation of a razor sharp edge [4]. Artifacts created from obsidian comprise perhaps the sharpest-edged cutting tools manufactured and compare favorably to 21 st century surgical stainless steel scalpel knife edges [5]. This typically black volcanic glass also possesses a useful geochemical trait in that ratios of minor to trace elements can often be linked to its geographical origin [6]. We have demonstrated the use of a new tool to measure these elements through co-located scanning electron microscope (SEM)-based micro-X-ray fluorescence spectrometry (mXRF) and electron beam induced X-ray microanalysis. A rigorous evaluation of this dual microbeam analysis was performed by examining the major, minor, and trace elements within a well-characterized rhyolitic glass reference material to provide the measurement science foundation for understanding the method’s capabilities and limitations. This coupled X-ray spectrometry method was recently used to identify the source locations for two varieties of highly polished, kilogram-sized Mesoamerican obsidian “mirrors”/tablets from Central Mexico that are housed in the collections of the Smithsonian’s National Museum of the American Indian (NMAI) [7], demonstrating the utility of this method for archaeological science. The subject analysis and electron beam