PROVENANCE DETERMINATION OF CALIFORNIA OBSIDIAN SOURCES BY LASER-INDUCED BREAKDOWN SPECTROSCOPY

Multi-element chemical analysis has become a common means for attributing the provenance of archaeological materials and assessing prehistoric trading patterns. The Coso Volcanic Field (CVF) in California contains at least 38 high-silica rhyolite domes, many of which contain obsidian glass quarried for tools by the indigenous population for more than 12,000 years. Coso-Type obsidian artifacts are found throughout the southwestern United States. Geochemical analysis indicates a strikingly similar major element character, but abundance differences for several trace elements can be used to distinguish obsidian from four major CVF source areas.

Based upon this success, laser-induced breakdown spectroscopy (LIBS) was applied to the obsidian source discrimination problem. LIBS is an atomic emission spectroscopy technique that is simultaneously sensitive to all elements which, with a single laser shot, captures the plasma broadband emission spectrum and provides chemical fingerprint of any material (solid, liquid or gas). Sets of single-shot broadband LIBS spectra were collected for obsidians from major sites across the CVF and from other regional obsidian locations, with the data analyzed using advanced multivariate statistical techniques. Although all obsidians exhibited quite similar broadband LIBS spectra, a PLS-DA approach was able to clearly discriminate CVF obsidians from other regional sources. Within the CVF, it is possible to distinguish the sub-sources defined on the basis of ICP-MS analysis. This study demonstrates the potential of LIBS, which has an attractive potential for real-time man-portable geochemical analysis in the field and therefore the in-situ chemical analysis of obsidian artefacts, as a viable analytical technique for provenance determination.