Review of Clarke et al. (2026) ‘Altar to Attic to Analysis: Geochemical Authentication of a Rediscovered Victorian Thin Section of Stonehenge’s Altar Stone’

Anthony J.I. Clarke, Christopher L. Kirkland, Arthur de Oliveira Vicentini, Lisa Brown, Altar to Attic to Analysis: Geochemical Authentication of a Rediscovered Victorian Thin Section of Stonehenge’s Altar stone, Journal of Archaeological Science: Reports, Volume 70, 2026, 105619, ISSN 2352-409X, https://doi.org/10.1016/j.jasrep.2026.105619. (https://www.sciencedirect.com/science/article/pii/S2352409X26000544)

Clarke et al. (2026), published in Journal of Archaeological Science: Reports, examine thin section S45 from the William Cunnington III collection (1876–1881), rediscovered in 2021 at the Wiltshire Museum. The study applies automated mineralogy (TIMA SEM-EDS) and laser-ablation ICP-MS U-Pb dating of zircon and apatite to assess the section’s authenticity as material from Stonehenge’s Altar Stone and to constrain its geological provenance.

The modal mineralogy of S45 comprises quartz (53.9 vol. %), calcite cement (19.2 vol. %), plagioclase (12.5 vol. %), white mica, chlorite, and trace heavy minerals (rutile, chromite, zircon, apatite), with fabric and phase abundances that align closely with previously verified Altar Stone fragments such as 2010 K 240 and MS-3. Apatite U-Pb analyses define two isochrons yielding lower-intercept ages of 1043 ± 29 Ma and 449 ± 24 Ma. Zircon data are affected by common-Pb contamination from residual Canada balsam resin; the authors address this by subdividing time-resolved ablation signals into short (typically 3–6 s) integrations and performing unanchored lower-intercept regressions on a grain-by-grain basis. Twelve regressions meet the stated acceptance criteria, producing dates between 389 Ma and 1850 Ma with main density peaks at approximately 435 Ma and 1021 Ma. These age populations, together with the mineral assemblage, are consistent with derivation from the Upper Old Red Sandstone of the Orcadian Basin in northeast Scotland.

The paper adds to knowledge of the Altar Stone by authenticating a historic thin section without requiring new sampling of the 6-tonne megalith, thereby supporting material preservation while reinforcing the Scottish provenance established in earlier work. It also presents a data-reduction procedure for extracting U-Pb information from resin-contaminated legacy thin sections.

Limitations arise from the small size of the analysed chip (~250 mm² surface area, ~20 % of a standard thin section), which restricted the dataset to 55 zircon analyses and only 12 acceptable ages. Conventional concordant ages could not be obtained, and the sub-set integration approach, although functional in this case, rests on bespoke acceptance thresholds (minimum three integrations, initial ²⁰⁷Pb/²⁰⁶Pb ≥ 0.837, MSWD 0.1–2.0, ≤10 % age uncertainty) that have not been validated against uncontaminated reference materials. The overall results remain confirmatory of prior mineralogical and isotopic studies rather than introducing new interpretive elements. Minor overdispersion in the apatite regressions is noted but attributed to natural variation in closure temperatures.

In balance, Clarke et al. (2026) demonstrate the continued utility of 19th-century thin-section collections for modern archaeometric questions, subject to the constraints of sample volume and preparation artefacts. The work contributes incremental but useful reinforcement to current models of Altar Stone provenance and to methodological options for heritage-science applications.