POWDERED OBSIDIAN FOR DETERMINING HYDRATION RATES AND SITE THERMOMETRY

Abstract

Obsidian hydration dating shares the disadvantage of all the chemical reaction dating systems in its reliance on experimentally determined short-term rate constants for long-term age extrapolation. In archaeological applications the extrapolation may need to encompass local edaphic variation and longer range environmental changes. Before the contribution of these varying field related rate determinants are considered the initial trajectory of the extrapolation needs to be determined. In many studies the extrapolation is circumvented by adopting radiometric systems that provide reference-age frameworks that are then used to calibrate hydration depth readings. The relative hydration ages that are attached to the reference age still require -that the hydration rate constants be experimentally determined. This is to ensure that any discrepancies between the observed artefact hydration readings and the experimentally found hydration rate can be identified and, if possible, explained. Given that deposit perturbation and artefact weathering are factors in many archaeological sites it is therefore still necessary to know the inherent hydration rate of the obsidian being dated. This paper seeks to develop obsidian powder systems to derive short-term rate constants that can apply to ambient conditions at archaeological sites. Three cases employing obsidian powders for hydration dating research are presented here in pursuit of this aim. The first case employs rate constants estimated from the powder data and optically measured hydration; the second combines obsidian powder data and SIMS measurement on solid flakes; the third case attempts to apply simple powder diffusion data to two sites near Talasea in Papua New Guinea (Figure 1). The results of the powder experiments support the view that ‘water’ rather than H alone is responsible for the hydration process.