Obsidian hydration profiles and dating

Obsidian hydration dating (or more generally, glass hydration dating) has been investigated as a method to determine the age of archeological samples (e.g., Lee et al., 1974; Freidman and Long, 1976; Anovitz et al., 1999; Liritzis and Diakostamatiou, 2002; Riciputi et al., 2002). This presentation attempts to lay the theoretical foundation for understanding glass hydration. Treating hydration as diffusion into a semi-infinite medium, a theoretical foundation requires an understanding of the following: (1) water concentration at the surface (water solubility in glass and whether equilibrium is reached); (2) water diffusion in glass; (3) diffusion behavior for periodically changing temperature and periodic boundary condition; (4) possible dissolution of obsidian in water, and (5) the effect of long-term trends in temperature (global warming) and humidity (such as drying or uplift of the region, etc.). Previous experimental data and models are extrapolated to room temperature on water speciation, solubility and diffusivity. Such information is compared with that extracted from measured obsidian hydration profiles. The effect of periodic T and surface conditions on diffusion is analytically examined. We conclude: (i) molecular H2O is the dominant species at room T; (ii) the surface water concentration in obsidian hydration is roughly consistent with extrapolated water solubility at room T and a humidity of 1; (iii) high-T water diffusion data cannot be extrapolated to room T (Leschik et al., 2003); and (iv) the most reliable part of a hydration profile is at depth greater than 0.6 micrometers. Because the total hydration depth is proportional to the square root of the integration of diffusivity with respect to time, it is sensitive to not only age (for dating), but also to the variation of diffusivity, which depends on mean annual T and humidity. Only when the annual mean diffusivity is constant, and when there was no dissolution of obsidian, would it be reliable to determine the age. On the other hand, the sensitivity of the diffusivity and hence the hydration depth to climate conditions (especially humidity) means that obsidian hydration profiles may be a good climate indicator if age of obsidian can be independently determined by, e.g., C-14 dating (Riciputi et al., 2002).