Surface Analysis of Archaeological Obsidian by SIMS

Abstract

Secondary ion mass spectrometry (SIMS) is the mass spectrometry of atomic species that is emitted when a solid surface is bombarded by an energetic primary ion beam. The surface composition and possibility of dating archaeological obsidian using SIMS has been investigated in this paper. It has been previously suggested that obsidian could be dated by measuring the penetration depth of nitrogen into weathered surfaces using SIMS. Here, it is shown that this method cannot be used as the nitrogen distribution is not uniform across the surface. Nitrogen is found to be localized in pits on weathered surfaces of most obsidian samples studied. It is shown that an apparent nitrogen penetration profile is measured because one or more of these nitrogen rich pits often lie within the SIMS analysed area. Analysis of freshly cleaved surfaces has shown that the pits on weathered surfaces are likely to have been formed during a cleaving process, possibly during tool making, rather than a weathering process during burial, but that long term burial is required for nitrogen to accumulate.