Sourcing obsidian: a new optimized LA-ICP-MS protocol

Abstract

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry [LA-ICP-MS] is one of the most successful analytical techniques used in archaeological sciences. Applied to the sourcing of lithic raw materials, it allows for fast and reliable analysis of large assemblages. However, the majority of published studies omit important analytical issues commonly encountered with laser ablation. This research presents a new advanced LA-ICP-MS protocol developed at Southern Cross GeoScience (SOLARIS laboratory, Southern Cross University, Australia), which optimizes the potential of this cutting-edge geochemical characterization technique for obsidian sourcing. This new protocol uses ablation lines with a reduced number of assayed elements (specific isotopes) to achieve higher sensitivity as well as increased precision and accuracy, in contrast to previous studies working with ablation points and an exhaustive list of measured isotopes. Applied to obsidian sources from the Western Mediterranean region, the Carpathian basin, and the Aegean, the results clearly differentiate between the main outcrops, thus demonstrating the efficiency of the new advanced LA-ICP-MS protocol in answering fundamental archaeological questions. Statement of significance Our new LA-ICP-MS protocol, specifically tailored for the geochemical sourcing of obsidian artefacts in the Western Mediterranean area, was developed at SOLARIS (Southern Cross GeoScience, Southern Cross University, Australia) with a top-of-the-range Agilent 7700x ICP-MS coupled to a an ESI NWR 213 Laser Ablation System. Taking into account the common analytical issues encountered with the LA-ICP-MS technique, we focused on two parameters: the use of ablation lines instead of ablation points, and the development of a reduced list of measured isotopes. The use of ablation lines aims to compensate for any sample heterogeneity, achieve a higher count rate as well as a better signal stability, and also reduce laser-induced elemental fractionation. The measured isotopes have been carefully selected amongst the most efficient to discriminate between the different obsidian sources. This shortened list of isotopes achieves precise and accurate measurements with a higher sensitivity, and with the use of ablation lines, contributes to enhancing the potential of this geochemical characterization technique for obsidian sourcing. Data availability The LA-ICP-MS results for the obsidian geological samples from the Mediterranean area are available as supplementary data.