Determining the geochemical variability of fine-grained basalt sources/quarries for facilitating prehistoric interaction studies in Polynesia

Abstract

Polynesian adze sourcing studies that rely on geochemical analyses to assign distant artefacts to a source or quarry have been undertaken for more than three decades. Understanding intra-source geochemical variability is essential for robust artefact source assignments, yet these data are not available for most quarries. We provide a seven-step protocol for sampling source rocks and associated artefacts at quarries and, using the large Eiao quarry (Marquesas Islands) as a case study, demonstrate the efficacy of the protocol. A "sampling to redundancy" statistical procedure documents that ∼7-11 samples are required to capture the variability of 10 oxides, 49 trace elements and seven isotope ratios. We advocate using the broadest array of fully quantitative geochemical values to characterise quarries and sources that will facilitate current and future sourcing studies. In the biological sense, this is analogous to the formal description of a holotype. Quarry samples should also be collected and curated to enable research when new protocols are established and more precise and accurate geochemical techniques develop