As part of SCRAP 2015, we have been using a newer technology called portable X-ray fluorescence (pXRF) in order to characterize the geochemistry of local granite outcrops and granite architecture and tools (manos and metates) without having to destroy the artifacts themselves. This is the main focus of project member Tawny Tibbits’ dissertation research. So far no other research has taken place on such coarse-grained rocks using pXRF and Tawny is particularly interested in determining if the machine can produce data that is as accurate as the destructive methods that are more commonly used.
The Maya Mountains provide a nice case study on granite. There are three petrographically (but not necessarily visually) distinct plutons: Mountain Pine Ridge, Hummingbird Ridge, and Cockscomb Basin. During the past few years, and this year with SCRAP, Tawny has sampled outcrops from throughout these plutons in order to develop a database that archaeological materials could be compared to in order to determine the point of origin for the actual raw material. Each of these plutons has variations within them, and she has attempted to collect samples from throughout each pluton in order to capture this variability. Using specific ratios, she can then plot the variability within and between plutons.
Tawny has visited several archaeological projects in Belize to analyze their ground stone tools. Generally the only granite tools present are manos and metates. From this work she is able to determine which pluton the artifact likely originated from. She takes no fewer than five data points per sample in order to get an average geochemical composition. It is necessary to take multiple points since granite has such large crystals. The diameter of the beam on the pXRF is so small that it is possible to only shoot a single mineral at a time, which would not give the overall composition of the rock and would lead to a misinterpretation of the data. So she makes sure to take plenty of data points to get the best bulk geochemical signature possible. She has compared it to the results of powdering the outcrop rocks and has found that about 5-6 data points can approximate the powdered results with the added benefit of not destroying the tool. With this information we hope to be able better understand the movement of ground stone tools to different sites throughout Belize at varying times in the archaeological record. This could shed light on some potential exchange networks as well as their shifts as different sites come into and decline from power.
At SCRAP we have been focusing on locating outcrop areas around Alabama in order to further characterize this area of the Cockscomb pluton, and analyze whatever granite tools and architecture we find during survey and surface collections. Tawny will be crunching all the numbers and coming up with some conclusions when she makes it home next week!
Cheers from Belize!