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 (SIRM₂₉₃) and liquid nitrogen (SIRM₇₇) temperature. The ratio S_T = SIRM₇₇/SIRM₂₉₃, 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.