Provenance investigation using Re–Os isotopes and trace element analyses of Hallstatt C–D and La Tène A bipyramidal ingots from the Speyer depot (upper Rhine region, Germany)

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Although bi-pyramidal ingots are the most abundant type of iron ingots from the Hallstatt C–D to La Tène A time in Central Europe, comprising more than 4 tonnes of iron in total, their provenance remains unresolved. This study addresses this question through an integrated approach combining measurement 187 Re/ 188 Os ratios, determined here for the first time in archaeological iron, with 187 Os/ 188 Os of isotope ratios, and trace element analyses of slag inclusions and the metal matrix by (LA) ICP-MS. The study focuses on 20 bi-pointed ingots of the BLS4 type from the “Speyer depot” in the Upper Rhine region. In addition, 40 iron-ore samples from southern Germany were analysed and combined with published data from the CIPIA project (Dillmann et al., 2017) to establish a comparative reference dataset. Two AMS radiocarbon measurements of the metal, obtained for the first time for BLS4-type ingots, indicate an age range of 800–400 BCE, consistent with previously published dates. The results indicate a similar provenance for the Speyer and CIPIA ingots. The combined use of Re–Os isotope systematics and trace element analysis proved effective for characterizing the types of iron ore represented in archaeological iron. Most of the bi-pointed ingots probably derive from the broader group of ooidal iron ores of the North Alpine region. Ores and slag from the Haye Plateau provide the closest match in terms of lithophile trace element patterns. However, because two known smelting sites from this production area do not match the ingots in their 187 Os/ 188 Os ratios, the iron used for most of the bi-pointed ingots may have been smelted from a geochemically similar ore source in Lorraine or southern Luxembourg. This would have major implications for reconstructing iron supply networks of the Celtic elites during the Early Iron Age.