A geological perspective on the use of Pb isotopes in archaeometry

Abstract

Lead isotope ratios in ore bodies and magmatic rocks depend in a complex way on several a priori independent parameters, including the geological age of the tectonic province in which the ores and magmas formed and its U/Pb (μ) and Th/U (κ) ratios, two very sensitive parameters characteristic of metal sources. All these parameters are entangled in hard-to-read Pb isotopic ratios. With respect to the commonly used fingerprinting techniques, which rely on the comparison of raw isotope ratios, the main motivation for the present work is to provide a method for making geologically and geochemically educated guesses about metal provenance even in the absence of isotopic data on reference ores. It shows how to unscramble a geological model age and μ and κ information from isotopic measurements. This approach brings to light a new organization of the Pb isotope database and an untapped wealth of information that can be used for provenance studies and other archaeometric purposes. We provide expressions with which to calculate these parameters and, using literature data, demonstrate how Pb isotopes in ores and magmas define clear zones in the silver-rich provinces of the Central Andes. We further show how the geological model age and μ and κ values fingerprint production areas in 16th-18th century silver coins minted in Mexico and South America. Finally, we use Pb isotopes to illustrate how the Reconquista of the Emirate of Granada (1482-91) and the seizure of the Betic silver mines are reflected in the silver coins of the Catholic Monarchs. © 2011 University of Oxford.