Isothermal time-series determination of the rate of diffusion of water in Pachuca obsidian

Friedman and Smith's (1960) article introducing an exciting, potentially precise and inexpensive method of dating obsidian artefacts has thus far failed to reach its potential. Numerous efforts to refine, improve and even redevelop the method since that time have similarly failed to achieve the original promise. Only within the last eight years have significant improvements been made, due to both improved analytical techniques and a better understanding of the hydration process. However, most of our mechanistic understanding of the interaction of water with rhyolitic glass is based on experiments performed on melts and glasses at temperatures above their glass transitions, conditions inappropriate for investigation of near-surface environmental conditions. Unfortunately, studies detailing the temporal evolution of the diffusion profile at low temperatures are rare, and few useful data are available on the low-temperature diffusive hydration of silicate glasses. This paper presents data on the experimental hydration of obsidian from the Pachuca source (a.k.a. Sierra de las Navajas, Basin of Mexico) at 75 °C for times ranging from 3 to 562 days, and compares these results with data for samples obtained from a stratigraphic excavation of the Chalco site in the Basin of Mexico. Samples have been analysed using secondary ion mass spectrometry (SIMS) to provide concentration/depth data. While 75 °C is still significantly above the temperatures at which archaeological obsidians hydrate, it is well below the glass transition temperature (approx. 400 °C) and thus processes are likely to be similar to those that occur in nature, but fast enough to be observed over a laboratory timescale. The results demonstrate that a simple square-root-of-time model of the evolution of the diffusion profile is not adequate to describe the diffusion process, as measured diffusion profiles exhibit the effects of concentration- and time-dependent, non-Fickian diffusion. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but an intermediate plateau is observed in its time evolution that is consistent with results obtained from the suite of Chalco samples. Both of these effects have been observed during diffusion in glassy polymer systems and are associated with the build-up and relaxation of self-stress caused by the influx of diffusing material.