Electron spin resonance of Fe3+ ion in obsidians from Mediterranean islands. Application to provenance studies

Archaeological artefacts made of obsidian are all the more precious for archaeologists as they are witnesses of cultural, social or ‘economic’ relationships between prehistoric populations. The main obsidian sources reachable in these times have been identified and characterized by chemical analysis and fission track dating. Because of iron presence (from 1% to 10% expressed in Fe2O3) it is possible to use Mössbauer spectroscopy and electron spin resonance (ESR) on geological samples to characterize different sources. We studied by electron spin resonance about seventy obsidians coming from six Mediterranean volcanic islands. The complex spectra are mainly due to iron in different states and site location (and sometimes isolated Mn2+ ions). X-band (9 GHz) spectra exhibit a signal at g=4.3 with a shoulder at 9.8 ascribed to isolated Fe3+ in the glassy matrix with a rhombic environment (C2v). Condensed clusters of Fe3+ ions give rise to a resonance line at g=2.0 whose position and width do not depend on temperature and in addition to this signal broad resonance lines are ascribed to micro-crystallites of mixed iron oxides (hematite, magnetite and related spinels, titanates, silicates) as inclusions into the amorphous matrix of obsidian. When temperature decreases from 300 to 5 K these signals shift towards weak fields. The contribution of every signal to ESR spectrum is different from one source to the other according to the different thermodynamic conditions that occurred during obsidian formation. It is thus possible to discriminate a priori one geological source using ESR spectra. At room temperature the first differences appear at X-band, low temperatures experiments may allow to remove some ambiguities.