Magnetite grain-size analysis and sourcing of Mediterranean obsidians

The potential of magnetic grain-size variations as an obsidian source characteristic is investigated using geological and archaeological obsidians from five islands of the Mediterranean Sea: Lipari, Sardinia, Palmarola, Pantelleria, Melos. Four parameters are used: magnetic (χ) and anhysteretic (χ a) susceptibilities, saturation isothermal remanent magnetizations at room (SIRM 293) and liquid nitrogen (SIRM 77) temperature. The ratio S T = SIRM 77/SIRM 293, which depends on the superparamagnetic grains relative abundance, varies little in each individual site, with the exception of Lipari which is characterized by large variations and the highest content of superparamagnetic grains. The χ a vs. χ plot (King et al., 1982) shows some within-site dispersion of the samples; but the ratio Q a = χ a/χ, which is strongly influenced by the single domain grains content, is characteristic for each site. The combined use of the King and Q a vs. S T plots discriminates the samples from most of the sites and suggests that the grain-size analysis is a promising approach in sourcing obsidian archaeological artefacts. Moreover, the measurements of the four parameters used are simple, quick and feasible with no or little damage to archaeological finds.