Massaro, Enrico

This work is an extension of previous research in which we successfully tested the effectiveness of chlorine (Cl) versus sodium (Na) diagrams to unambiguously identify the provenance of obsidian artifacts attributable to Italian outcrops widely exploited during prehistory, i.e., Lipari, Pantelleria, Palmarola, and Monte Arci. In this further work, we found that the ratio R = Cl/Na decreases with the age of emplacement (t) of the obsidian outcrops and can be well described by the inverse law: . Obsidian samples were analyzed both by electron probe micro-analyzer (EPMA) and by Instrumental Neutron Activation Analysis (INAA). Radiometric dates have been obtained both from the literature and through 40Ar/39Ar dating. Data processing confirmed the time evolution of the Cl/Na ratio through the same equation, indicating that it can be used as an empirical estimator of the obsidian formation age. A best-fit analysis of the collected data gives the relation in units of ka. To verify the validity of this relation for non-Italian obsidians, we applied it to Sierra de Las Navajas (State of Hidalgo, Mexico) obsidians, estimating an age of 1.75 Ma, in agreement with the upper limit of 1.8–2 Ma generally accepted for these rocks. We propose that the Cl/Na ratio changes with time because of the differential loss of chlorine and sodium as a consequence of the micro-fracturing of the obsidian glass after the emplacement. If future tests on different obsidians can confirm the validity of this approach, a rough estimate of the age of emplacement of the obsidian outcrops could be derived from their geochemical compositions.