High-Precision Chemical Characterization of Major Obsidian Sources in Guatemala

The important Mesoamerican obsidian source El Chayal has been carefully explored and sampled. Forty new source specimens have been subjected to neutron activation analysis, and the composition patterns are recorded and compared. The westernmost of the known Guatemalan obsidian sources, Tajamulco, has also been studied. Precise analytical data are also reported for the important Ixtepeque and Rio Pixcaya sources. In addition, the complete analytical data for a group of artifacts in Belize excavated by Norman Hammond are reported and compared with the source compositions. The establishment in the literature of a library or collection of precise and intercomparable composition data for the sources of this general region is suggested, and the present paper is offered as a first contribution toward this objective. Elemental analysis of obsidian has been shown to be useful in reconstructing ancient trade routes. A sizable amount of work has been done on Mesoamerican sites and sources (Cobean et al. 1971; Hammond 1972; Heizer et al. 1965; Stross et al. 1976a). In this paper we propose to discuss 2 problems that have naturally arisen in these studies and to suggest at least partial solutions. reliability of the results have tended to differ from author to author. The second problem concerns the cost of reliable analysis and sampling, which is perhaps far higher than necessary. Previous research has shown that in some cases a correlation of an artifact with its source can be made by analysis of only 2 or 3 elements, but in other instances a far greater number of elements must be determined to reach a firm conclusion with regard to the source of the glass. This is true not only for the obvious reason that the chance of overlapping patterns increases with the increasing number of supply sources and archaeological sites, but because a priori statements on the chemical composition pattern of an unanalyzed obsidian source are impossible to make. An obsidian source or supply area may have several outcrops, which may or may not be chemically similar. However, even an individual outcrop may show a systematic variation within its own boundaries (Bowman et al. 1973). A source thus has to be adequately sampled so that its homogeneity may be demonstrated, or if it is found to be heterogeneous, so that the character of the variation in its composition may be defined by analysis. It is thus desirable to develop libraries of composition patterns for all sources of archaeological obsidian in a given region. These composition records should be as complete, accurate, and precise as possible. They should be published in units and in a form that makes it convenient to compare them, in order to be of use to all interested in identifying artifacts according to source. It is also desirable that the data be published in journals that are generally known and accessible to those working on the subject. Comparison of the composition patterns of the obsidian of a given cultural area often reveals that determination of the abundances of a limited number of elements is sufficient to distinguish the sources within the region in question. The determination of these few elements in the artifacts collected within this area should be sufficient to assign provenience to these artifacts, and this would be far less expensive than a complete analysis of the samples. When a generally applicable, detailed catalog of composition patterns is available, it is not too difficult to determine the most economical method for matching artifacts with their respective sources. Existence of