Altar to Attic to Analysis: Geochemical Authentication of a Rediscovered Victorian Thin Section of Stonehenge’s Altar stone

Abstract

Authenticating purported fragments of the Altar Stone, the central six-tonne megalith at Stonehenge, underpins ongoing archaeological investigations and helps preserve valuable material by limiting the need for further destructive sampling. Here, using automated mineralogy and U–Pb zircon–apatite isotopes we examine Victorian thin-section S45 rediscovered in 2021, to evaluate its provenance. Pb-contamination from antique coverslip balsam resin complicates obtaining primary zircon U–Pb ratios. To address this issue, we subset time-resolved zircon isotopic ratio integrations from laser ablation signals and calculate internal grain-level regressions, maximising data recovery. This approach yields twelve zircon dates spanning 389–1850 Ma, while apatite yields components at ca. 1043 and 449 Ma. These ages are consistent with a provenance from northeast Scotland’s Upper Old Red Sandstone Orcadian Basin. Our findings support S45 as an Altar Stone fragment and demonstrate a framework for extracting reliable U–Pb ages from historic thin sections.