Petrographically similar but chemically different: the application of LA-ICP-MS on the mineral inclusions in the sourcing of ceramics raw materials
Résumé
he analysis of archaeological ceramics can be realised from multiple methods: the petrographic analysis with polarising microscope on thin sections, by X-ray diffraction (XRD), the global chemical analysis of paste generally by spectrometric methods (XRF, INAA, ICP-OES...). However, in some cases, these analyses do not allow us to determine the geographical origin of the raw material used to shape the potteries. It is the case for pots mounted with clays coming from the weathering of granitic and granito-gneissic rocks (eg. granites, gneiss, gabbros...), because of the similarity of the mineral inclusions assemblies present in the pastes of the ceramics. Indeed, the lack of distinctive features prevents accurate identification of the existence of different petrographic groups. Furthermore, the global chemical compositions of this type of ceramic may vary due to the presence of larger numbers of certain mineral species or large differences in the size of inclusions. It was therefore necessary to develop a new approach to distinguish the sources of raw materials used by potters.
Therefore, we have developed a method based on the analysis by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) of mineral inclusions (eg. biotite, opaque minerals, amphibole) present in ceramic pastes. We compare the chemical composition of the mineral with the same mineral from the supposed mother rock and, as a result, discriminate different raw materials sources. In this communication, we will present our research and development we have made in the field of ceramic sourcing, through several case studies distributed over a large period (from the Neolithic to the Late Iron Age), located on different islands of Brittany in western France.