DETERMINING THE ORIGIN OF ARCHAEOLOGICAL OBSIDIAN SAMPLES FROM CALIFORNIA USING LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)

Because archaeological materials can provide important information about the timing, strength, and longevity of prehistoric trading patterns, there is a strong interest in using multi-element chemical analysis to identify the provenance, i.e. geological source, of archaeological materials. A problem of particular interest is the classification of obsidian artifacts found across the western and southwestern United States. Much of the past effort to determine obsidian provenance has focused on XRF, INAA, and ICPMS - laboratory-based techniques that are both time consuming and expensive. Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy technique that is simultaneously sensitive to all elements, can provide the 'geochemical fingerprint' of a material in real time with a single laser shot, and has the potential for the in-situ analysis of artifacts in the field. Obsidian has been erupted at a large number of locations across California, with many of these locations being sources for tools used by the indigenous populations for more than 12,000 years. This obsidian was traded widely, so that determining the provenance of obsidian artifacts present at archaeological sites in California remains a priority activity for archaeological community. For example, ‘Coso-type’ obsidian artifacts are found throughout the southwestern United States. California obsidians are quite similar in bulk composition, but abundance differences for several trace elements have been used as the basis for distinguishing obsidian from different local sources. For this study, sets of broadband LIBS spectra were acquired for more than 100 obsidian samples from important obsidian sites across the California using both laboratory and field-portable LIBS systems and the data analyzed using advanced multivariate statistical signal processing techniques. Although all obsidians exhibited quite similar broadband LIBS spectra, a PLS-DA approach was able to clearly discriminate major obsidian sources and correctly assign obsidian artifacts to the putative sources established through prior archaeological analysis.