FINGERPRINTING ARCHAEOLOGICAL AND GEOLOGIC QUARTZITE SAMPLES USING ICP-MS TECHNIQUES

In the Gunnison Basin, Southwest Colorado, archaeological assemblages of all ages are overwhelmingly dominated by chipped stone tools and debitage made of quartzite: 90% or more is a typical percentage of quartzite for a Gunnison Basin archaeological site, whether Paleoindian or late Prehistoric. Currently, no method exists for geochemically fingerprinting either the quartzite artifacts or their potential source areas on the landscape.

In a 2007 pilot study, Gunnison Basin artifacts, bedrock, and surficial deposits were sampled (n=20) and analyzed using numerous geochemical techniques; the goal being to evaluate each technique for its potential to discriminate among samples from different sources. Both AD-ICP-MS and LA-ICP-MS showed excellent potential for discriminating among source areas, whereas other techniques (e.g., XRF) did not (Pitblado et al., 2008a).

A successive, more extensive study was designed to (a) determine if the less-destructive LA-ICP-MS technique (which ablates only a minute amount of sample, rather than pulverizing it) could yield data of sufficiently high resolution to allow chemical differentiation among geologic sources; and (b) to sample geologic sources throughout the Gunnison Basin to test the results with a sample collection large enough to yield statistically meaningful results. Results thus far suggest that LA-ICP-MS is a viable analytical approach for discriminating among Gunnison Basin source areas (Pitblado et al., 2008b).

Recent field work in the Gunnison Basin has resulted in the mapping and collection of more than 400 samples from over 25 different quartzite sources—both bedrock and surficial deposits. ICP-MS analyses are currently underway at the IIRMES lab, California State University, Long Beach. Petrographic analysis is also underway to provide a physical context for the geochemical results.

This presentation will review the results of the pilot study of quartzite sourcing; overview results-to-date of comparisons of LA- vs. AD-ICP-MS analysis of quartzite samples; and discuss the methods used to collect the 400+ samples and how analysis of those samples is proceeding. We will also touch on the implications of the effort to geochemically fingerprint quartzite for Gunnison Basin archaeology and other archaeological undertakings across the globe.