DETERMINATION OF THE RATE OF "H2O" DIFFUSION FROM 30 TO 150 C IN OBSIDIAN FROM THE PACHUCA SOURCE, SIERRA DE LAS NAVAJAS, MEXICO

The importance of the interaction between water and rhyolitic glasses and melts is a well-known problem in the geosciences, dating back at least to the work of Bowen (1913). Archaeological applications begin with the work of Friedman and Smith (1960) who proposed using the width of the hydration rim on obsidian artifacts for chronometry. Most of our understanding of this problem is, however, based on experiments performed at high temperatures. Thus, little useful data is available on the low-temperature diffusive hydration of silicate glasses. We have experimentally hydrated obsidian from the Pachuca source (Basin of Mexico) at temperatures from 30 to 150 C for times ranging from 1 day to over two years. Samples have been analyzed using SIMS to provide concentration/depth data, and the results modeled using a finite-difference approach. Results show 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 KD data for hydroxyl groups and molecular water suggests that hydroxyl groups should be essentially non-existent in this temperature range, but their presence is clearly indicated by FTIR. Comparison of total oxygen and hydrogen analyses across the hydration profiles shows that the increase in hydrogen in the rim is not accompanied by a detectable increase in oxygen content. Thus, obsidian hydration appears to be a hydrogenation process - hydrogen, not water, is what appears to enter the glass. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but at 75 C £ T an intermediate plateau is observed in its time evolution. Both of these effects are associated in glassy polymer systems with the build-up and relaxation of self-stress caused by the influx of diffusing material.

Research sponsored 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, and by the Archaeometry Program, National Science Foundation.