The Dome Fire obsidian study: Investigating the interaction of heat, hydration, and glass geochemistry (New Mexico)

This research integrates the analysis of obsidian geochemistry with investigation of the effects of forest fires on obsidian artifacts in surface assemblages. The first component of this project investigated the nature of heat altered obsidian at a prehistoric quarry site following the 1996 Dome Fire in the Jemez Mountains of northern New Mexico. Burned artifacts were examined to discern indicators of heat alteration in obsidian and to measure the impact of the fire on obsidian hydration (OH) bands. Descriptive categories developed to encompass a range of fire effects are provided here as a tool for identifying heat-altered obsidian in archaeological contexts. Measurement of obsidian hydration in artifacts collected from across the site demonstrate substantial loss and alteration of OH information, as well as positive correlation of OH loss/alteration with degree of burn severity.; The second component of the project investigated the role of obsidian geochemistry in fire effects, especially obsidian vesiculation. Intrasource and intersource geochemical analyses of obsidian trace element composition were integrated with analysis of major/minor elements, and with analysis of the water content as a volatile constituent. Results show elemental homogeneity among the Dome area Obsidian Ridge/Rabbit Mountain obsidians of the Cerro Toledo Rhyolite (CTR), demonstrate a cogenic relationship among geographically separate CTR deposits, and confirm that these obsidians are chemical distinct from Valles Rhyolite (VR) glasses at Cerro del Medio. Analyses of obsidian water content using loss-on-ignition (LOI) and infrared spectroscopy (FTIR) demonstrate the efficacy of both techniques for archaeological applications. Water contents were found to be low in VR samples, but high and variable in CTR samples. These results accord with the expectation of higher and more variable water contents in glasses from pyroclastic deposits, as compared with lower water contents in obsidians from extrusive volcanic contexts. This study provides a new example in which obsidian water content is high and variable within a single chemical type. This example provides support for the inclusion of water content as a compositional variable in the OH dating model and demonstrates the utility of integrating analysis of both elemental and volatile composition into archaeological practices of obsidian geochemical analyses.