Realising the potential of portable XRF for the geochemical classification of volcanic rock types

The ability to geologically classify stone artefact materials is crucial because it is the first step in provenance allocation and geochemical grouping that inform archaeological models used for understanding exchange systems within prehistoric social networks. Geochemical analyses have revolutionised exchange studies by making it possible to match stone artefacts back to quarry sources, sometimes hundreds or thousands of kilometres away. Most archaeological basalt characterisation and provenance studies have relied on destructive sampling techniques to obtain the major trace elements NaO2, K2O and SiO2 (wt%) to identify volcanic rock types on the Total Alkali Silica (TAS) diagram. To overcome this limitation, this paper presents an alternative method, the Pearce W-F diagram, that determines volcanic rock type and alkalinity using elements TiO2, Zr, Y and Nb that are accurately detected by non-destructive portable X-ray fluorescence (pXRF). This new method is proven to be effective in a comparison with recent XRF archaeological basalt studies in the Pacific. The power to classify volcanic rock types non-destructively with pXRF significantly increases the number of artefacts and museum objects available for inclusion in archaeological exchange studies.