A molecular model for obsidian hydration dating

Abstract

Water diffusion in high silica volcanic melts results in the formation of rhyolite glasses where hydroxyls are linked to the silica tetrahedra. Space between these linkages form pathways for the diffusion of molecular water into the glass surface during secondary hydration. The accumulation of water in the near surface forms a water-rich hydration layer that can be converted to time by the application of experimentally derived diffusion coefficients. In this paper, we summarize mechanisms for water diffusion in obsidian and use a diffusion pathway model as the basis for a more detailed understanding of the parameters that control the magnitude of the water diffusion coefficient, its constancy over time, and how the properties of the glass reflect the thermal history experienced during secondary hydration. We evaluate hypotheses about water diffusion in obsidian with new low temperature (140–200 °C) accelerated hydration experiments and present a calibration for the age determination of obsidian artifacts.