Iron isotopes as a new tool for ancient metal tracing: comparison with classical tracing methods
Channel: Recording Archaeology
Date: 2017-01-08

The development of precise and accurate analytical techniques over the last decades has allowed expanding the range of methods for ancient metal tracing. Elemental and isotopic analyses are now widely used for this purpose. For a relevant archaeological interpretation, metallurgical processes must be taken into account, and analyses must be performed on archaeological ore, slag and metal. So far, isotopic methods were rather employed for non-ferrous metal tracing [e.g. 1] whereas elemental analyses were used for ferrous metal tracing [e.g. 2]. However, currently used elemental and isotopic methods show limitations, which underline the need to develop new tracers to complement existing ones3.
We used Fe isotopes as a new tool for ancient metal provenance studies. Isotopic ratios 57Fe/54 Fe and 56Fe/54Fe were measured using a Multiple Collector Inductively Coupled Plasma Mass Spectrometer after sample dissolution and Fe purification4. We first developed this approach by analysing materials from archaeological experiments on iron ore reduction. In order to compare our results with classical tracing methods, we also analysed the Fe isotope compositions of archaeological materials whose provenance hypotheses have been established with trace elements analyses of slags. Furthermore, some materials coming from different regions of ferrous and non-ferrous metal production were analysed to assess the tracing potential of Fe isotopes compared to trace elements and Pb isotope analyses. Our first results show that the bloomery process does not induce Fe isotopic fractionation, i. e., the signature of metals and slags reflect that of their corresponding ores. Moreover, Fe isotopes analyses tend to confirm the provenance hypothesis of ancient artefact established with trace elements analysis of slags. The results obtained so far suggest that Fe isotopes are a relevant tracer for archaeological materials, which can be applied to ferrous metals. The combination of this new method and the more classical ones involving trace elements or Pb isotopes analysis could allow to refine previous provenance hypotheses of ancient metals.
1 S. Klein, C. Domergue, Y. Lahaye, G.P. Brey and H.-M. von Kaenel, 2009, The lead and copper isotopic composition of copper ores from the
Sierra Morena (Spain), Journal of Iberian Geology 35 (1), 59-68.
2 M.-P. Coustures, D. Béziat, F. Tollon, C. Domergue, L. Long and A. Rebiscoul, 2003, The use of trace element analysis of entrapped slag
inclusions to establish ore–bar iron links: examples from two Gallo-Roman ironworking sites in France (Les Martys, Montagne Noire and Les
Ferrys, Loiret), Archaeometry 45, 599–613.
3 S. Baron, M.P. Coustures, D. Béziat, M. Guérin, J. Huez and L. Robbiola, 2011, Lingots de plomb et barres de fer des épaves romaines des
Saintes-Maries-de-la-Mer (Bouches-du-Rhône, France): Questions de traçabilité comparée, Revue Archéologique de Narbonnaise 44, 71-98.
4 F. Poitrasson and R. Freydier, 2005, Heavy iron isotopes composition of granites determined by high resolution MC-ICP-MS, Chemical Geology
222, 132-147.

Author - PhD student Milot, Jean, Géosciences Environnement Toulouse, Toulouse, France (Presenting author)
Co-author(s) - Poitrasson, Franck, Géosciences Environnement Toulouse, Toulouse, France
Co-author(s) - Baron, Sandrine, TRACES Laboratory, Toulouse, France