Obsidian homogeneity study for provenancing using Ion Beam- and Neutron Activation Analysis

Obsidian is a natural volcanic glass, which was one of the most appreciated materials of ancient man for cutting tools and has been found in many locations far away from any natural source. Reliable provenancing can provide evidence of contacts over certain distances and information about exchange patterns and mobility of prehistoric people.
The application of analytical methods can assist to solve the problem of obsidian provenancing by means of its highly specific chemical composition, the “chemical fingerprint”. Ion Beam Analysis (IBA) measurements, combining Particle Induced X-ray Emission (PIXE), Particle Induced Gamma-ray Emission (PIGE) and Rutherford Backscattering Spectrometry (RBS), are frequently used because of their high sensitivity and the non-destructive external beam mode [1-5]. Our studies have been carried out using the 4 MeV proton beam in-air of the 5 MV Tandem accelerator of the FZD. For comparative reasons and in order to obtain additional information, all obsidian samples were analysed by Neutron Activation Analysis (NAA) at the Atominstitut in Vienna, where previous investigations of volcanic rocks and glasses have been performed successfully [6-8].
Obsidian is generally described as a relatively homogeneous material [9]. Therefore, samples from the obsidian source Demenegakion (Milos, Greece) have been analysed in order to check the actual variation range of their chemical compositions. Special attention was paid to banded obsidians to clarify, if these bands show differences in the chemical composition or if these changes in the optical properties are related to inclusions of gas bubbles, microphenocrysts or similar features without significant compositional influence. Furthermore, both the influence of the surface quality and alteration by weathering has been studied.
This study is part of a joint project to apply selected analytical methods, in particular IBA, NAA and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), to reveal a maximum of compositional differences between (geological) samples of obsidian sources available in Europe. This knowledge is essential to decide, which least invasive analytical method should be chosen for the analysis of a specific archaeological artefact, on a case by case basis.

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