Scott Michlin welcomed me back to his radio show last month, and I came bearing tales of turquoise (click here to listen to our discussion). Sharon Hull (University of Manitoba, Department of Geological Sciences) and her colleagues recently finished a study of turquoise using a new sourcing method. Turquoise has been notoriously difficult to source, for a number of reasons, but I think it is fair to say that these researchers have cracked the turquoise code. Here’s Sharon:
North Americans have mined turquoise for more than 1,000 years! The procurement and trade of turquoise was as important to the inhabitants of ancient societies in the Greater Southwest and Mexico as gold is today. Archaeologists have recovered one million pieces of turquoise from archaeological sites throughout the Greater Southwest and Mesoamerica, but researchers have had difficulty reconstructing the spatial and temporal patterns of turquoise trade and procurement strategies.
For decades, archaeologists have sought to develop a technique that could identify the origins of turquoise artifacts using trace and rare earth element concentration patterns. These studies have met with limited success, however, due to the intrinsic limitations of these types of analyses for minerals such as turquoise, which varies in color and is chemically variable within a single sample or mine.
I am happy to report that my colleagues—Mostafa Fayek, Joan Mathien, and Heidi Roberts—and I have found a way to address this problem. To link turquoise artifacts to their geological source, we developed a technique using the isotope ratios of hydrogen and copper and the microanalytical abilities of a secondary ion mass spectrometer (SIMS). We then developed a comparative database consisting of more than 800 analyses from 22 turquoise resource areas in the western United States and northern Mexico. We also maintain a digital archive that contains the isotopic fingerprint of each turquoise artifact we analyzed, allowing us to examine turquoise procurement strategies from multiple perspectives and research focuses. For example, we can focus on particular sites (e.g., Chaco Canyon) or on a particular deposit (e.g., Halloran Springs), or we can compare the procurement patterns of contemporaneous sites or changes in these patterns through time.
This breakthrough technique is successful for several reasons. First, the hydrogen and copper isotopic signatures of turquoise are dictated by the geography and the geology of the turquoise deposits. Secondly, we use the in situ microanalytical capabilities of the secondary ion mass spectrometer (SIMS) to measure the isotopic ratios of the hydrogen and copper in turquoise. SIMS is capable of performing accurate isotopic measurements on very small samples. The small sample size required for analysis is particularly advantageous for analyzing precious archaeological artifacts. Thus, the technique is relatively nondestructive, and we can return turquoise artifacts to their original collections without apparent damage to their physical appearance.
In our most recent publication, we identified the geological source of turquoise artifacts from sites in Chaco Canyon, the northern San Juan basin, and Virgin Puebloan sites in Utah and Nevada. By comparing the procurement and exchange patterns of these sites, we were successful in identifying:
- Multiple trade networks in Chaco Canyon
- Different turquoise procurement strategies between Pueblo Bonito and the small sites at Marcia’s Rincon (on the south side of Chaco Canyon, opposite the visitors’ center)
- Similar procurement patterns for Pueblo Bonito and Aztec Ruins, and for the sites in Marcia’s Rincon and Salmon Ruin
- Links between turquoise found in the San Juan Basin and western turquoise deposits
- A possible link between the inhabitants of Marcia’s Rincon and the Virgin Pueblo region
- Virgin Pueblo sites that may be linked to the San Juan Basin via long-distance trade routes
Now we’re examining data from Mogollon sites located in the Hueco-Tularosa Basin, Cañada Alamosa, and the Villa Ahumada site in northern Mexico, where we have identified two possible turquoise trade routes—one from the west, and one that appears to follow along or near the Rio Grande Rift. We also plan to do additional work on samples from Mexico and southwestern New Mexico, and we will ultimately expand our research further south into Mexico, and possibly Chile.
For further reading:
Hull, S., M. Fayek, F. J. Mathien, and H. Roberts
2014 Turquoise Trade of the Ancestral Puebloan: Chaco and Beyond. Journal of Archaeological Science 45:187–195.