Studying Obsidians from Milos by Complementary Techniques: An Application of Ion Beam and Instrumental Neutron Activation Analysis

The natural volcanic glass obsidian was an important raw material for tools and arms during prehistoric time and has been found by researchers at great distances from potential natural sources. Reliable provenancing can provide evidence of contacts over a certain dis-tance and information about exchange patterns and mobility of prehistoric people.
Instrumental Neutron Activation Analysis (INAA) is one of the major analytical techniques to solve the problem of obsidian provenancing by means of its highly specific chemical composition, the “chemical fingerprint” [1-4]. The advantages of bulk INAA are the absence of matrix effects, the large number of ele-ments simultaneoulsy detectable, its sensitivity and accuracy. However, INAA is limited as being destruc-tive and inclusions cannot be distinguished from the matrix applying routine analytical practice.
Additional application of non-destructive Ion Beam Analysis (IBA), consisting of Proton Induced X-ray Emission (PIXE), Proton Induced Gamma-ray Emission (PIGE) and Rutherford Backscattering Spectrometry (RBS), supplement INAA measurements by enabling systematic spatially resolved surface investigations and adding a complementary element spectrum [5-9]. Furthermore, a comparison of chemical compositions obtained by different analytical methods provide the actual degree of the reliability of the analytical results [10].
For this study, both INAA and IBA measurements have been applied to the same samples to gain a more complete set of elements and to check the self-consistency of the analytical results. The samples originate from the obsidian sources Demenegakion and Agia Nychia (Cape Bombarda) on the island of Melos (Greece) (Fig. 1). Our INAA studies have been performed at the TRIGA MkII 250 kW research reactor of the Atominstitut in Vienna, where 150 mg of ground aliquots have been irradiated for 1 min at a thermal neutron flux of 3.3x10¹²cm^-2s^-1. IBA measurements have been carried out using the 4 MeV proton beam in-air of the 5 MV tandem accelerator of the Ion Beam Centre of FZD [9,11].

Fig. 1: Geographical situation of obsidian sources on the island of Melos (after [12]).

Our investigations are part of a joint project to apply selected analytical methods, in particular INAA, IBA and Inductively Coupled Plasma-Mass Spec-trometry (ICP-MS), to reveal a maximum of compositional differences between easily available samples of the natural obsidian sources in Europe. This knowledge should enable to decide, which least invasive analytical method should be chosen for the analysis of a specific highly valuable archaeological artefact, on a case by case basis.

References: [1] Meloni S. et al. J. Radioanal. Nucl. Chem. 271 (2007) 533-539. [2] Arias A. et al. J. Radioanal. Nucl. Chem. 268 (2006) 371-386. [3] Kilikoglou V. et al. J. Archaeol. Sci. 23 (1996) 343–349. [4] Aspinall. A. et al. Nature 237 (1972) 333–334. [5] Bellot-Gurlet L. et al. C. R. Palevol 7 (2008) 419–427. [6] Calligaro T. X-Ray Spectrometry 37 (2008) 169–177. [7] Lugliè C. et al. C. R. Palevol 5 (2006) 995–1003. [8] Kim J.C. et al. IPPA Bull. 27 (2005) 122–128. [9] Bugoi R. and Neelmeijer C. NIMB 226 (2004) 136-146. [10] Hancock R.G.V. and Carter T. J. Archaeol. Sci. 37 (2010) 243–250. [11] Saminger S. et al. J. Radioanal. Nucl. Chem. 245 (2000) 375-383. [12] Higgins M. and Higgins R. (1996) A geological companion to Greece and the Aegean Cornell University Press.