Obsidian provenance studies

Provenance studies are another field of application of the Sciences for Cultural Heritage. The main aim of these kind of studies is the identification of the origin of the raw materials used to made an artefact. 

These informations are essential to reconstruct the interactions, the exchanges and the movements of ancient communities, most likely related to human navigation. 

How can these investigation be done?

The study is based on chemical and petrochemical analyses. This article deals with obsidian, volcanic glass with generally acid composition, consisting mainly of silica, formed during the rapid cooling of highly viscous spindles. Thanks to its physico-chemical properties (such as the hardness around 5-5.5 in the Mohs scale, the perfectly conchoidal fracture and the glassy shine) the obsidian has been used since prehistoric times for the construction of weapons, tools, blades, jewellery, decorative objects. In particular, in jewellery the polychromatic varieties of the obsidians have been used. 

Therefore, it is one of the most sought and traded object since the prehistoric age, thanks to its availability in all continents. In Italy we find it in four Mediterranean islands: in Pantelleria, Lipari, Palmarola and Sardinia in the Monte Arci area. 

Since the obsidian structure is amorphous and devoid of phenocrystals, the investigation is mainly based on the chemical composition of the rock and, splitting its components into major, minor and trace elements. 

The trace elements are the most discriminating for the provenance studies because their concentration, expressed in parts per million (ppm), depends on the local characteristics of the magmatic sources. According to their load / radius ratio and the degree of compatibility / incompatibility against the solid that is forming, the trace elements are grouped into: LILE (Large Ion Lithophile Elements), REE (Rare Earth Elements), HFSE (High Fields Strenght Elements), TE (Transition Elements), PGE (Platinum Group Elements). Plotting the content of these elements, appropriately normalized, in specific diagrams, called "spider diagrams" and "rare earth patterns", it is possible to compare unknown findings with others of known origin, observing the trend, any positive and negative anomalies and the concentrations of trace elements.

 Martina

- Foresta Martin, F., Di Piazza, A., Doriano, C., Carapezza, M. L., Paonita, A., Rotolo, S. G., & Sagnotti, L. (in press). New insights into the provenance of the obsidian fragments of the Island of Ustica (Palermo, Sicily). Archaeometry. 

- Khalidi, Lamya, et al., 2016. The growth of early social networks: New geochemical results of obsidian from the Ubaid to Chalcolithic Period in Syria, Iraq and the Gulf." Journal of Archaeological Science: Reports 9: 743-757.