Intrasource Chemical Variability of Artefact-Quality Obsidians from the Casa Diablo Area, California

Abstract Obsidian from the Casa Diablo area of Mono County, California, was an important source for artefacts and raw material in prehistoric California, although little work has been devoted to investigating the potential geochemical variability within the many quarry areas it contains. In this study, 200 obsidian source samples from 20 collection locations in the Casa Diablo area were analysed by energy dispersive X-ray fluorescence spectrometry. Laboratory analyses of solid flake and powdered obsidian samples are presented in quantitative measurement units (parts per million and/or weight percent composition) calibrated to international rock standards. The results of this research indicate that two—possibly three—geochemically distinct varieties of artefact-calibre obsidian exist within the source area once believed by archaeologists to be a single geochemical entity. These new findings carry implications for obsidian trade/exchange systems studies in California and the Great Basin, and for obsidian sourcing research in other areas of the world.