Preliminary work conducted in the US Southwest demonstrated that turquoise presents regional patterning in lead (Pb) and strontium (Sr) isotopic ratios. A competing method proposes copper (Cu) and hydrogen (H) isotopes may also be of value (Hull et al. 2008). Though both methods are very promising, overall low sample numbers and the exclusive use of only two dimensions of variation in either approach has led to limited confidence in the discreteness of distinct groups and resultant source assignments. That is, for some sources, it is not clear that there is greater between group than within group variation in isotopic ratios. For example, a limited application of the H and Cu method to archaeological samples produced a high proportion of results outside of the stipulated ranges of known sources, suggesting the full range of these sources is not yet measured. However, it stands to reason on first principles that a combination of methods would add resolution to current rough sourcing regions. Also based on first principles of geology, we propose adding measurements of neodymium (Nd). We predict determining isotopic ratios for this suite of elements will not only allow determination to general source regions, but in some cases to individual prehistoric mines. At present, source determinations are only attempted at the scale of large geographic provinces.
As noted, the preliminary applications have applied these methods to a small subset of archaeological materials. Because sample sizes were relatively small only limited qualitative contributions were made, but these were nonetheless impactful. One application (Pb and Sr) demonstrated that the decades old assumption that turquoise was exported over long distances from the US Southwest to Mesoamerica seems highly unlikely (Thibodeau et al. 2018). Two other limited applications of this method confirmed long suspected relationships between specific archaeological sites and spatially proximate turquoise sources in southern Arizona (Hohokam region) (Thibodeau et al. 2012) and west central New Mexico (Ancestral Zuni region) (Thibodeau et al. 2015). A final application of the Pb and Sr method comes closest to our intended project output by sampling the archaeological assemblage from a dozen sites in eastern Arizona to determine the relationship to a well-known regional source (Hedquist et al. 2017).
To summarize, these preliminary studies indicate the potential of developing methods but are restricted in their scope and hindered by a lack of resolution, owing to reliance on only two isotopic systems each. They are also limited in their specific application in that they only consider source regions north of the international border. Our research would first focus on substantially widening the breadth of sampling to northern Mexico as well as understudied regions of arid North America. We would apply a variety of analytical techniques for the purposes of materially, mineralogically, and chemically for characterizing the geological samples from various turquoise sources. It would also leverage multiple isotopic systems to increase source determination precision and accuracy. The construction of this database in concert with published data would then facilitate a large-scale analysis of archaeological turquoise from across the region.
References:
Hedquist, Saul L., Alyson M. Thibodeau, John R. Welch, and David J. Killick. 2017. Canyon Creek Revisited: New Investigations of a Late Prehispanic Turquoise Mine, Arizona, USA. Journal of Archaeological Science 87:44-58.
Hull, S., M. Fayek, F.J. Mathien, P. Shelley, and K.R. Durand. 2008. A new approach to determining the geological provenance of turquoise artifacts using hydrogen and copper stable isotopes. Journal of Archaeological Science 35:1355-1369.
Thibodeau, Alyson M., John T. Chesley, Joaquin Ruiz, David J. Killick, and Arthur W. Vokes. 2012. An alternative approach to the prehispanic turquoise trade. In Turquoise in Mexico and North America: Science, Conservation, Culture and Collections, edited by J. C. H. King, Max Carocci, Caroline Cartwright, Colin McEwan, and Rebecca Stacey, pp. 65-74. Archetype Publications, London.
Thibodeau, Alyson M., David J. Killick, Saul L. Hedquist, John T. Chesley, and Joaquin Ruiz. 2015. Isotopic Evidence for the Provenance of Turquoise in the Southwestern United States. Geological Society of America Bulletin 127:1617-1631.
Thibodeau, Alyson M., Leonardo López-Luján, David J. Killick, Frances F. Berdan, and Joaquin Ruiz. 2018. Was Aztec and Mixtec Turquoise Mined in the American Southwest? Science Advances 4(6):eaas9370.
Thomas R. Fenn, Department of Anthropology
Matthew Pailes, Department of Anthropology
Robert L. White, Department of Chemistry and Biochemistry
Andrew Krug (PhD student)
Heidi Noneman (PhD student)
Rayhana Niyogi (Undergraduate)
A small collection, part of a pilot study for this research project, was subjected to compositional analysis utilizing a hand-held portable X-ray fluorescence analysis (pXRF) unit, housed in the Department of Anthropology's Fenn Laboratory for Archaeological Sciences and History (FLASH). An undergradaute student, Rayhana Niyogi, conducted preliminary pXRF analysis during the spring academic semester at OU and presented her preliminary results at the University of Oklahoma Undergradaute Research Day (URD) 2022 and in an OU First-Year Research Experiences (FYRE) end of the year showcase in spring 2022.
Fourier-Transform Infrared (FTIR) spectroscopy is another analytical method employed in our turquoise characterization program. This research was conducted by graduate students Andrew Krug (Anthropology) , Heidi Noneman (Dept. of Chemistry & Biochemistry), and Prof. Robert L. White (Dept. of Chemistry & Biochemistry). The research team utilized a novel sampling method developed in the Department. of Chemistry and Biochemistry by Dr. White, and it was used to determine both the feasibility of this sampling method and the potential for FTIR to discriminate between turquoises originating from different geological sources.
The research also is employing Scanning Electron Microscopy (SEM), with an attached energy-dispersive spectrometer (EDS), for analysis and characterization.
The turquoise research project also will conduct isotopic analysis of the geological samples to better constrain the detectible range of variation within and between potential geological sources for the selected archaeological samples.
