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.     

A combined visual-geochemical approach to establishing provenance for pegmatite quartz artifacts

A technique for establishing source provenance for pegmatite quartz artifacts was developed using a quarry sample from the Churchill River basin of northern Manitoba and Saskatchewan. Secondary ion mass spectrometry (SIMS) was used to quantify Ti, Ge, Th, and U trace element concentrations and Pb isotope ratios, coupled with qualitative visual characteristics to distinguish between pegmatite quartz from different archaeologically exploited sources in the Churchill River basin region. The technique was also applied to a small sample of quartz artifacts recovered from sites in and around the study area in a preliminary attempt to assign or rule out raw material provenance to characterized sources.     

A chronometric tool for Hawaiian archaeology: the hydration dating of Pu'u Wa'awa'a trachytic glass

The Pu'u Wa'awa'a trachytic glass deposit located on the Big Island of Hawai'i was a source of lithic tools for much of Hawaiian prehistory. Routinely encountered at habitation sites, the glass has the potential to provide absolute dates based upon the degree of surface hydration. Infrared photoacoustic spectroscopy and secondary ion mass spectrometry were used to monitor the diffusion of water in archaeological and experimental samples. Accelerated laboratory hydration experiments show that water diffusion in trachytic glass containing a structural water content of 0.16% ± 0.03% is linear with time. Application of the calibration to trachytic glass artifacts from the Kahalu'u habitation cave have produced chronometric dates in the 17th–18th centuries. These age estimates are in agreement with radiocarbon dates and artifactual data that bracket the site occupation. However, the glass dates do not correlate well with site stratigraphy and this suggests that slight sample imperfections that retain water may be one factor that results in ages that are too early for later occupation levels.     

THE APPLICATION OF TIME‐OF‐FLIGHT SECONDARY ION MASS SPECTROMETRY (ToF‐SIMS) TO THE CHARACTERIZATION OF OPAQUE ANCIENT GLASSES*

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) has been used, for the first time, for the characterization of opaque ancient glasses. Isotope‐specific chemical imaging with sub‐micron resolution enabled the separate analysis of opacifiying inclusions and the surrounding glass matrix. Phase identification has been demonstrated and quantification of the matrix composition has been investigated by use of Corning Glass Standard B as a model. Trace element detection limits are typically in the range 0.5–5.0 ppm atomic—in favourable cases down to 0.01 ppm. For the analysis of inclusions in particular, this has the potential to provide new information of use in establishing provenance and trade routes by ‘fingerprinting’ as well as the investigation of manufacturing techniques, as demonstrated by comparisons between glasses and with EDX data from the same samples.     

A NEW DATING METHOD FOR HIGH‐CALCIUM ARCHAEOLOGICAL GLASSES BASED UPON SURFACE‐WATER DIFFUSION: PRELIMINARY CALIBRATIONS AND PROCEDURES*

The first European settlers came to North America in the early 17th century using glass in the form of containers and decorative objects. Thus, glass is a horizon marker for all historic period settlements and a potential source of chronometric dates at archaeological sites belonging to the historic period in the Americas. We have developed a new absolute dating method based upon water diffusion into the surface of manufactured glasses that predicts diffusion coefficients based upon variation in glass chemical constituents. Low‐temperature (< 190°C) hydration experiments have been performed on a set of five high‐calcium (21.7–28.3%) glasses that were used to manufacture wine bottles from the 17th?19th centuries. Infrared spectroscopy and secondary ion mass spectrometry was used to model the water diffusion/alkali exchange process. The ability of the model to accurately predict archaeological ages was evaluated with artefacts recovered from ceramic‐dated contexts at Thomas Jefferson's plantation known as Monticello.     

ANALYSIS OF THE NOTCHES OF ANCIENT SERRATED DENARS*

A number of serrated silver denars of the Roman Republic and a Greek bronze coin were investigated, paying special attention to the notches, in order to reveal their production technique. Particular interest was devoted to three contemporary forgeries of serrated denars, because the official pure silver issues were also available for inspection. Several microbeam analytical techniques were applied, such as scanning electron microscopy (SEM), electron probe micro‐analysis (EPMA) and secondary ion mass spectrometry (SIMS). The surfaces of the notches, which show traces of the tools used, were investigated by SEM. In the case of the forged coins, the thickness of the silver layer (inside the notches as well as on the surface of the coin) was determined by SEM and SIMS. The main components of the surfaces were similar in both cases as measured by EPMA. Combining the results, it is possible to reconstruct the steps in the production of the serrated denars. The investigations also permit a review of different opinions about the purpose of the notches.     

SIMS‐SS,A NEW OBSIDIAN HYDRATION DATING METHOD: ANALYSIS AND THEORETICAL PRINCIPLES*

The water diffusion mechanism in obsidian has been revisited with the advent of a new dating approach employing secondary ion mass spectrometry (SIMS). The water diffusion SIMS(+) profile and the concept of a surface saturation (SS) layer in obsidians both provide a sound basis for the new diffusion age equation (SIMS‐SS) and are supported by dated world examples ( Liritzis et al. 2004 ). Here, we present the basic physical–chemical analysis of the mass transport phenomenon on which the new dating method is based. The crucial age parameter of the SIMS‐SS dating approach for archaeological obsidians—that is, the surface saturation (SS) layer—is supported by both theoretical and experimental data.     

CHANGES IN THE METAL CONTENT OF HUMAN HAIR DURING DIAGENESIS FROM 500 YEARS,EXPOSURE TO GLACIAL AND AQUEOUS ENVIRONMENTS

Scanning electron microscopy, inductively coupled plasma mass spectroscopy and time‐of‐flight secondary ion mass spectrometry have been used to examine the extent and possible mechanisms by which the metal content of human hair is altered by exposure to aqueous environments. The results, using both modern hair and samples from 500‐year‐old hair associated with glacier‐entombed remains, show that the metal content has been altered sufficiently so that the interpretation of the metal signature in terms of diet or disease is problematic. While endogenous information is difficult to glean from these data, interesting observations have been made of possible early stages of mineral authigenic deposition. The chemistry of the outer hair surface was found to be consistent with deposition of Fe and Al silicates, as well as other mineral phases. The ancient hair was analysed at the root region and included a comparison of the internal versus external composition to assist in identifying the diagenetic processes.     

Characterizing Quartz Artefacts: A Case Study from Manitoba's Northern Boreal Forest

Visual and geochemical analyses of 30 pre‐contact quartz tools from the Churchill River basin of northern Manitoba were used to provisionally assess source provenance based on comparison to local quartz quarry data. The results indicate that tools made from quartz acquired from these quarries were transported and eventually deposited into archaeological sites up to 200 km away. Transport of this quartz, which is of relatively moderate quality, over such distances suggests that stone tool–using hunter–gatherer populations that lived in the Churchill River basin of northern Manitoba were highly mobile. It also indicates that higher‐quality raw materials were unavailable in the immediate area.