HYDRATION OF PACHUCA OBSIDIAN: IMPLICATIONS FOR ODDSIMS ARCHAEOCHRONOMETRY

Obsidian hydration dating has long been high in promise but short on results. Unfortunately, most of our mechanistic understanding of this problem is based on experiments on quenched high temperature melts. Needed polytemporal-polythermal and spectroscopic studies detailing the time evolution of the diffusion profile at low temperatures are rare. Thus, few useful data are available. We have experimentally hydrated obsidian from the Pachuca source (Basin of Mexico) at temperatures from ~20 - 150°C for times ranging from 1 day to over three years, and compared these results with data obtained from the Chalco site in the Basin of Mexico. Samples have been analyzed using SIMS to provide concentration-depth data. Results clearly demonstrate that a simple square-root-of-time model of the evolution of the diffusion profile is not adequate and that the low-temperature hydration process is qualitatively and quantitatively different from that at higher temperatures. Diffusion profile shapes show the effects of concentration- and stress-dependent, non-Fickian diffusion. Extrapolation of high temperature data for hydroxyl and molecular water concentrations suggests that hydroxyl groups should be essentially nonexistent in this temperature range, but their presence is clearly indicated by FTIR. Total oxygen and hydrogen analyses show that the increase in hydrogen in the rim is not accompanied by a detectable increase in oxygen content, and obsidian hydration thus appears to be a hydrogenation process. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but at T * 75°C an intermediate plateau is observed. The latter is associated in glassy polymer systems with the build-up and relaxation of self-stress caused by the influx of diffusing material. An intrinsic hydration chronometric technique must, therefore, account for more than simple diffusive processes to be effective.

Research sponsored by the Archaeometry Program, National Science Foundation, and by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.