Isothermal Time‐Series Determination of the Rate of Diffusion of Water in Pachuca Obsidian*

Abstract

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.

Experimental investigation of rates and mechanisms of isotope exchange (O, H) between volcanic ash and isotopically-labeled water

“…1) with volume element equal to original volume minus new volume, etc., b) convert volume element to mass by way of density, c) assuming total water is evenly distributed calculate weight water (convert to moles) for each "layer" from known total water %, d) compute number of sites in each water "layer" necessary to come to equilibrium with labeled water (by moles), and e) sum deuterium values for layer by added layer. These incrementally calculated δ 2 H can be compared to times for experimental values of δ 2 H. By way of comparison of diffusion coefficients a value of around 8 x 10 -8 at 75°C is reported for obsidian (Anovitz et al, 2004 in figure 5) …”

“…Concluding that overprinting occurs with the new solvent front, they also note the apparent alteration of "intrinsic" water in the obsidian. This work is in addition to several low temperature diffusion studies employing SIMS (Anovitz et al, 2004;Anovitz et al, 2008;Riciputi 2002) providing useful perspectives on environmental fates of glasses as well as diffusion data.…”

The significance of obsidian hydration dating in assessing the integrity of Holocene midden, Hokkaido, northern Japan

“…Direct measurement of EHT is the method to measure the soil temperatures from study sites and regions. It includes methods that bury artifact-embedded cells into study site (Ambrose, 1976;Anovitz et al, 2004;Ridings, 1991Ridings, , 1996Stevenson et al, 1998), and thermal cells that periodically record subsurface temperatures (Fredrickson et al, 2006;Jones et al, 1997). Indirect measurements of EHT, on the other hand, use temperatures recorded in surrogate station that is expected to have proximate climatic regime to study site (e.g., Friedman and Long, 1976;Lee, 1969;Rogers, 2007).…”

“…The default model of time calculation using temperature and measured rim thickness is the Arrhenius equation (e.g., Long, 1976, 1978;Rogers, 2007). Through induced hydration experiments (e.g., Michels 1986;Michels et al 1983;Rogers and 4 Duke, 2011;Stevenson et al 1998) and application of new measurements of hydration rims, notably SIMS, the secondary ion mass spectrometry (e.g., Anovitz et al 1999Anovitz et al , 2004Liritzis, 2006;Stevenson et al, 2001), precise measurements of hydration rims quantified by the amount of hydration ions are provided. Thorough examinations of diffusion profiles have suggested that the diffusion of water molecules to create hydration rims is accurately described by the Fick"s law (e.g., Anovitz et al, 1999Anovitz et al, , 2004Liritzis, 2006).…”

Archaeological Age Estimation Based on Obsidian Hydration Data for Two Southern Andean Sources

“…Liritzis and Laskaris 2011). However, other experiments challenge the square root of time dependence (Anovitz et al 1999(Anovitz et al , 2004Riciputi et al 2002), and some indicate that t 0.6 and t 0.7 are better statistical fits for experimental samples with varying proportions of intrinsic water (Stevenson et al 2013). "The diffusion of water is a complex and dynamic process" and the standard, constant model of diffusion (t 0.5 ) "may not be the best descriptor of the hydration process" (Stevenson et al 2013:3021).…”