Isotopes could reveal ancient American turquoise trade
Isotopes could reveal ancient American turquoise trade
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Mines in the Halloran Hills of California’s San Bernardino County were one location where researchers collected samples of
turquoise to study the mineral’s lead and strontium isotopic compositions. Credit: Alyson Thibodeau.
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For centuries before the arrival of Europeans, turquoise was prized among pre-Hispanic cultures of North America. Caches of the distinctive, creamy-blue-green mineral have been unearthed in crypts and other ritually significant structures in what are now the southwestern United States and northern Mexico. Farther south, in Mesoamerica, archaeologists have found elaborate mosaic masks and ornamentation made of turquoise pieces. Despite multiple anthropological and historical hints, identifying where the turquoise used by different civilizations came from has proven difficult. But in a recent study, scientists have described a geochemical fingerprinting technique that may help parse the geographic origins of turquoise specimens and illuminate trade routes in ancient America.
Turquoise is a copper aluminum phosphate mineral that forms in the shallow underground from rain and groundwater that has percolated through and weathered bedrock near copper ore deposits. In addition to its main constituents, trace amounts of other elements from the bedrock, such as lead and strontium, also find their way into the turquoise, such that a given specimen’s isotopic composition “should essentially mirror … the isotopic composition of the surrounding geology,” says Alyson Thibodeau, an isotope geochemist at Dickinson College in Pennsylvania and lead author of the new study, published in the Geological Society of America Bulletin.
By examining the composition of a turquoise artifact — particularly the lead and strontium isotopes, which are powerful tracers because they vary extensively in nature — and comparing it to known geochemical signatures of the rock in and near copper deposits in the U.S. and Mexico, researchers can hopefully locate the origin of the turquoise, Thibodeau says.
To confirm that turquoise preserves isotopic signatures of its source rocks as expected, Thibodeau and her colleagues first analyzed lead and strontium isotopes in 137 pieces of turquoise with known origins in one of 19 different mining districts. Some samples were borrowed from museum collections; others were hand-collected from mining sites in Arizona, California and New Mexico. Overall, the researchers found that lead and strontium isotopic ratios in the samples varied widely, and that the samples reflected the compositions of the geologic settings in which they had formed.
More:
http://www.earthmagazine.org/article/isotopes-could-reveal-ancient-american-turquoise-trade