An Archaeologist in the Laboratory
Mass spectrometry may resolve a lingering archaeological debate.
Most of the researchers who operate a secondary ion mass spectrometer at ORNL are trained in the physical sciences. Sharon Hull is an exception. A graduate student in archaeology at Eastern New Mexico University, Hull has quickly learned how to operate the SIMS instrument in a new way of value to archaeology. That's the assessment of her proud supervisor, Professor Mostafa Fayek, a University of Tennessee– ORNL joint faculty appointee.
"Sharon is one of the few archaeologists who has experience getting isotope signatures on turquoise using a SIMS instrument," he says. "She will be a go-to person in archaeology."
"Lots of turquoise artifacts have been found in Mexico where very few turquoise mines exist," Hull says. "We would like to link with confidence the turquoise in high-quality artifacts with specific turquoise deposits. Then we can address a current archaeological debate: ‘Were the American Southwest peoples in towns like Chaco Canyon, New Mexico, mining turquoise and trading it to the peoples of Mexico?' We believe the isotope signatures in turquoise that we obtain with mass spectrometry will enable us to connect the artifacts to the mines of origin."
Fayek says that Hull helped develop the SIMS technique for measuring how much more copper-65 than copper-63 is present in turquoise samples only 100 microns in diameter. Copper ratios have never been measured on a SIMS instrument. SIMS is ideal for archaeological studies because the technique is nondestructive and uses very small samples.
Turquoise is a chemical combination of copper, aluminum, phosphorus, and oxygen that forms only in the presence of nonacidic copper. Fayek thinks that rainwater plays an important role in the formation of turquoise. He reasons that rainwater dissolves the elements in rock fissures, allowing them to form turquoise when conditions are right. Rainwater falling at particular latitudes and longitudes has specific signatures revealed in turquoise as distinct ratios of hydrogen and deuterium isotopes, which also can be measured using SIMS.
Because SIMS can measure many different isotopes, it can provide archaeologists with a more definitive fingerprint of turquoise in artifacts that can be compared with signatures of raw turquoise samples from mines in Arizona, Colorado, Nevada, and New Mexico. In SIMS, a primary ion beam knocks out "secondary" ions from the sample surface, and a magnet separates selected ions according to their masses.
"Using SIMS Sharon completed 600 turquoise analyses in a month," Fayek says. "Using conventional techniques, other groups have required 20 years to conduct 3000 analyses. We're catching up."
Southwest archaeology was greatly influenced by a neutron activation study of turquoise conducted between 1979 and 1992 at Brookhaven National Laboratory. The findings established that most of the turquoise sources in North America are located in the Southwest, not Mesoamerica. The study suggested that several trade networks operated at various times using turquoise from mines in Arizona, Nevada, and New Mexico, and outlined three networks tied to the Cerrillos Hills near Santa Fe.
Armed with new data, Mathien disputes the 1992 study's claim that people in the Southwest did not value the mineral until they became aware of the demand for turquoise by royalty in central Mexico, where the ever more elaborate turquoise mosaics and masks conferred prestige on their owners. She also challenges the theories that Chaco Canyon turquoise is exclusively from Cerrillos Hills and that Chaco controlled Cerillos mines. Mathien is counting on innovative mass spectrometry at ORNL to resolve a lingering debate and help us better understand the peoples and culture of the ancient American Southwest.
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