Baron Sandrine, Tamas Calin, Le Carlier Cécile (2013) How mineralogy and geochemistry can improve the significance of Pb isotopes in metal provenance studies, Archaeometry. (original) (raw)

Lead isotopes combined with trace element data represent a powerful tool for non-ferrous metal provenance studies. Nevertheless, unconsidered geological factors and archaeological data, as well as ignored analytical procedures, may substantially modify the interpretation of the isotopic and trace element signature obtained as a potential ore candidate. Three archaeological examples, accompanied by high-resolution lead isotopic measurements (MC-ICP-MS), are presented here to discuss the above-mentioned criticisms and to propose some solutions. The first example deals with prehistoric/historical gold/silver-mining activity from Romania (the Baia Borşa and Roşia Montanȃ ore deposits). The second one regards the lead/silver metallurgical activity from the Mont-Lozère massif (France) during medieval times. The third example focuses on the comparison between two batches of lead isotope data gathered on Roman lead ingots from Saintes-Maries- using different SRM 981 Pb values. geochronology: more details are reported elsewhere . We only recall that three stable Pb isotopes out of four, 206 Pb, 207 Pb and 208 Pb, are radiogenic (time dependent) and that they are produced by the radioactive decay of 238 U, 235 U and 232 Th, respectively. The 204 Pb isotope is stable (time independent) and its abundance has been constant since the Earth's formation. The abundances of the four lead isotopes are approximately 52.4%, 22.1%, 24.1% and 1.4% for 208 Pb, 207 Pb, 206 Pb and 204 Pb respectively. Since the introduction of multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) in the mid-1990s, it has been possible to measure several isotopes simultaneously, with a high mass resolution (Rehkämper and Halliday 1998). These new spectrometers permit notable advances in geochemistry, and more recently in archaeology too. The presentday precise and accurate measurements improve the tracing by increasing the analytical precision by a factor of 10 over that obtainable with routine thermal ionization mass spectrometry (TIMS), and comparable to the precision of TIMS with double or triple spiking. MC-ICP-MS also allows the measurements of new isotopic tracers such as Cu, Fe, Sn and so on (called 'non-traditional isotopes') . The isotopic data measured nowadays by MC-ICP-MS are more significant and relevant, and they can be more accurately interpreted.