SOURCING TURQUOISE USING O AND H ISOTOPES

The evolution of the turquoise trade played a crucial role in the cultural intensification of social systems along the northern frontier of Mesoamerica and southwestern portions of the United States. Reconstruction of prehistoric southwestern turquoise trade networks depends on accurate knowledge of the sources of materials. Therefore, a method that can distinguish between turquoise source areas is important because once signatures for source areas have been established, artifacts from prehistoric archaeological sites can then be tested and inferences about the organization of their socio-political organization can be evaluated. Despite numerous trace element studies and a few isotopic studies, none have satisfactorily differentiated between all the turquoise source areas that exist in southwestern United States. We use a combination of geologic and a relatively non-destructive isotope analysis method (O and H isotope analyses by ion microprobe) to characterize and differentiate between seven turquoise mining districts in southwestern United States, which are potential source areas. The isotopic composition of O and H atoms in precipitation varies with latitude as well as other factors. Preliminary analyses of turquoise from four mining regions in the southwestern United States indicate that this regional isotopic variation is reflected in the water molecules bound into turquoise. Turquoise from each mining district has a unique O and H isotopic signature. The theoretical basis for this regional variation in the O and H isotopes of turquoise is that rainwater was the solvent forming the turquoise deposit. Thus, the O and H isotopic composition of turquoise is potentially a powerful tool for characterizing and distinguishing between source regions. The analysis of additional turquoise samples from a wide range of sources promises to provide a method to differentiate among the various turquoise sources. If preliminary results are confirmed, a 'source fingerprint' will permit archaeologists to identify the sources of turquoise found at sites across the continent and providing new insight into pre-contact trade patterns in North America.