ADVANCES IN TURQUOISE SOURCING

The amount of turquoise artifacts recovered from archaeological sites in the American Southwest and Mesoamerica suggests that turquoise was an important commodity in pre-Columbian trade networks. To improve upon previous sourcing methods that utilized trace element analysis and refine prehistoric turquoise trade network models, we developed a method to “fingerprint” turquoise sources using hydrogen (D/H) and copper (⁶⁵Cu/(⁶³Cu) stable isotopes. We analyzed thirteen deposits identifying ten turquoise source areas. We tested the technique by analyzing seventeen artifacts from Chaco Canyon and Guadalupe Community. Thirteen of the artifacts fell into the distribution patterns of the turquoise deposits. Recently, we improved our analytical protocol and analyzed a number of samples from different localities within a single turquoise mine to determine the variation in H and Cu isotope ratios at the deposit-scale. We also studied, in detail, the effects of alteration on the H and Cu isotopic composition of turquoise. Our results show that the Cu isotopic composition within a single mine varies by only 1.5‰, whereas the H isotopic composition varies by 13‰. This narrow range in isotopic composition at the deposit scale provides a unique isotopic fingerprint for each deposit. The alteration of turquoise can vary between deposits. For example, green, iron-rich turquoise from the Castillian Mine in New Mexico alters to pyrophyllite (Al₄[Si₈O₂₀](OH)₄) whereas blue, Fe-poor turquoise from the Sleeping Beauty Mine in Arizona alters to gibbsite (Al(OH)₃). This is likely due to differences in the activity of silica during turquoise alteration. The alteration of turquoise causes the H and Cu isotopic composition to vary by as much as 80‰ and 4‰, respectively. Our isotopic technique promises to provide a robust method for Archaeologist to source turquoise and develop a detailed model for prehistoric trade of turquoise in the southwestern United States and Mesoamerica.