LEAD AND STRONTIUM ISOTOPES AS TRACERS OF TURQUOISE

Turquoise [CuAl₆(P0₄)₄(OH)₈·4H₂0] is a mineral that was widely traded among prehistoric societies in the American Southwest and Mexico. Since the 1960's, there have been numerous efforts to link turquoise found in the archaeological record to specific geologic sources (e.g. Mathien and Olinger 1992, Sigleo 1975, Weigand et al. 1977). Attempts to chemically source turquoise have been hindered both by the chemical variability of individual deposits, abundant impurities, and the susceptibility of turquoise to alteration when exposed to surface conditions (Hull 2006).

This study was designed to determine if the isotopic signatures of lead and strontium can provide a basis for discriminating between turquoise sources and identifying the provenance of prehistoric turquoise artifacts. We analyzed samples of turquoise from deposits in Arizona, New Mexico, and Colorado for lead and strontium isotopes by solution MC-ICPMS and TIMS, respectively. Initial results confirm that the isotopic signature of lead and strontium varies significantly between sources and show that inter-source variation is greater than the intra-source variation. Using these isotopic measurements, we suggest it is also possible to distinguish between individual mines within larger mining districts.

Because southwestern turquoise occurs as a cryptocrystalline material and can contain many impurities, both leaching experiments and petrographic examination of samples are necessary to fully understand isotopic variation within deposits. Analyses of leached fractions of turquoise show that variable isotopic domains are contributing to the “bulk” isotopic signature of individual samples. This variability may be the result of a) impurities, b) transient nonstoichiometric release c) surficial weathering processes or d) long-term formation of turquoise from different sources. Despite this observed variation, it is still possible to resolve isotopic differences among all the turquoise sources examined in this study. We conclude that lead and strontium isotopes can be used to differentiate between turquoise deposits, and may provide a way to trace archaeological turquoise back to its geologic source.