Tin isotope fractionation during cassiterite smelting and its implication for tracing the tin sources of the Bronze Age (original) (raw)
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Determination of the tin stable isotopic composition in tin-bearing metals and minerals by MC-ICP-MS
2017
This study uses MC-ICP-MS for the precise analysis of the stable tin isotopic composition in ore minerals of tin (cassiterite, stannite), tin metal and tin bronze. The ultimate goal is to determine the provenance of tin in ancient metal objects. We document the isotope compositions of reference materials and compare the precision of different isotope ratios and the accuracy of different procedures of mass fractionation correction. These data represent a base with which isotopic data of future studies can be directly compared. The isotopic composition of cassiterite and stannite can be determined after reduction to tin metal and bronze, respectively. Both metals readily dissolve in HCl, but while the solutions of tin metal can be directly measured, the bronze solutions must be purified with an anion exchanger. The correction of the mass bias is best performed with an internal Sb standard and an empirical regression method. A series of Sn isotope determinations on commercially available mono-element Sn solutions as well as reference bronze materials and tin minerals show fractionations ranging from about-0.09‰ to 0.05‰/amu. The combined analytical uncertainty (2s) was determined by replicate dissolutions of reference materials of bronze (BAM 211, IARM-91D) and averages at about 0.005‰/amu.
Journal of Archaeological Science, 2018
Provenance studies of metal artefacts are well-established in the interdisciplinary field of science-based archaeology primarily using the chemical and isotopic composition. In the last decades, tin isotopes became gradually more important as a fingerprinting tool for the provenance of tin, but many questions especially regarding the behaviour of tin isotopes during pyrometallurgical processes are still not satisfactorily answered. This paper is a contribution to the understanding of tin isotope fractionation on tin ore smelting under prehistoric conditions and discusses the consequences for tin provenance studies. It presents the results of smelting experiments that were carried out with cassiterite in the laboratory and in the field, respectively. Besides chemical characterisation with XRF, SEM-EDX and Q-ICP-MS, tin isotope composition of tin ores and smelting products (tin metal, tin vapour, slag) were determined using solution MC-ICP-MS. Although tin recovery on smelting in the field was low (20-30%) due to tin losses to fuming and slag formation, the results indicate that the tin isotope composition is less affected than anticipated from theoretical considerations (Rayleigh fractionation). If cassiterite is completely reduced during the smelting reaction the tin metal becomes enriched in heavy tin isotopes with a fractionation of D124 Sn = 0.09-0.18‰ (0.02-0.05‰ u-1) relative to the original cassiterite. An estimate of the provenance of the original cassiterite and the potential ore source would still be possible because the variability of tin isotope ratios in tin ore provinces is much larger. If the cassiterite becomes incompletely reduced, however, then fractionation increases significantly up to D 124 Sn = 0.88‰ (0.22‰ u-1) and conclusions on tin sources are limited. Similarly, condensed tin vapours (D 124 Sn = 1.13‰ (0.28‰ u-1)) and slags (D 124 Sn = 0.42-1.32‰ (0.11-0.33‰ u-1)) that are by-products of the smelting process show large fractionation with respect to the original tin ore as well, which makes them unsuitable for provenance studies.
Tin isotope fingerprints of ore deposits and ancient bronze
The tinworking landscape of Dartmoor in a European context - Prehistory to 20th century. Papers presented at a conference in Tavistock, Devon, 6-11 May 2016 to celebrate the 25th anniversary of the DTRG, 2017
The sources and origin of tin, and the dispersion of bronze technology in the 3rd and 2nd millennium BC, are the central research topics of our multi-disciplinary research project, funded by an Advanced Grant of the European Research Council (ERC). It has the general goal to establish the tin isotopic composition of tin ores and tin-bearing artefacts, and considers the infl uence of anthropogenic processes on the isotope ratios. We discuss the tin isotopic composition of cassiterite from two major tin provinces in Europe: from Cornwall and Devon (Southern England), and from the Erzgebirge (Germany and Czech Republic). The samples from both tin provinces show a very large variation of isotopic compositions with δ124/120Sn-values ranging overall from -0.28 to 0.85‰. Although there is large overlap, on average, cassiterite from the Erzgebirge (δ124/120Sn = 0.09‰) is isotopically lighter than that of southwest England (δ124/120Sn = 0.18‰). This is due to a higher proportion of heavy isotope compositions in the samples from Cornwall and Devon. In addition, we compare the ore data with preliminary tin isotopic systematics in Early Bronze Age metal artefacts from the Únětice Culture in Central Germany and from several ancient settlements in Mesopotamia belonging to the Early Dynastic III and the Akkadian Periods. Bronze artefacts of the Únětice Culture containing more than 3 wt.% tin have rather constant isotopic compositions (δ124/120Sn = 0.2 to 0.31 ‰), despite having highly variable trace element concentrations and tin contents. This suggests the intentional addition of an isotopically homogeneous tin raw material (metal or cassiterite) to the copper ore or melt. In contrast, the tin isotopic composition of artefacts from Mesopotamia (>3 wt. % Sn) show a much larger δ124/120Sn variation from -0.2 to +0.4‰. This is even observed in single settlements such as Ur. Since there is no sizeable tin mineralization in the vicinity, this implies that the tin demand of the ancient metallurgist was covered by trading tin from different ore sources.
2019
Tin isotope ratios may be a useful tool for tracing back the tin in archaeological metal artefacts (tin metal, bronze) to the geological source and could provide information on ancient smelting processes. This study presents the results of laboratory experiments, which reduced (smelted) synthetic stannic oxide, natural cassiterite and corroded archaeological tin and bronze objects. The overall aim of the study is to find a reliable method for the decomposition of tin ores and corrosion products in order to determine their tin isotopic composition, and to explore possible effects on the tin isotope ratios during pyrometallurgy. We focused on five methods of reduction at high temperatures (900–1100 °C): reduction with CO (plain smelting), reduction with KCN/CO (cyanide reduction), reduction with Na2CO3/CO, reduction with Cu/CO (‘cementation technique’) and reduction with CuO/CO (‘co-smelting’). The smelting products are analysed by means of optical and scanning electron microscopy as well as X-ray diffraction, while their isotope composition is determined with a high-resolution multi-collector mass spectrometer with inductively coupled plasma ionisation. The results show that all five methods decompose synthetic stannic oxide, cassiterite and corrosion products. Ultimately, reduction with KCN is the best solution for analysing tin ores and tin corrosion because the chemical processing is straightforward and it provides the most reproducible results. Reduction with Na2CO3 and copper is an alternative, especially for bronze corrosion, but it requires laborious chemical purification of the sample solutions. In contrast, evaporation of tin and incomplete alloying during plain smelting and co-smelting can cause considerable fractionation among smelting products (Δ124Sn = 0.10 ‰ (0.03 ‰ u−1)). A less precise and even inaccurate determination of the tin isotopic compositions of the tin ores would be the consequence. However, the results of this study help to evaluate the possible influence of the pyrometallurgical processes on the tin isotope composition of tin and bronze artefacts.
Mixed lead sources in tin metal: Implications for using lead isotopes to study tin artifacts
Journal of Archaeological Science, 2024
Methods for provenancing copper, lead, and silver using the isotopic composition of lead are well-established. Lead isotope analysis holds promise for the study of tin metal as well, as long as one accounts for the U-Th-Pb systematics of cassiterite (SnO 2) and chaîne opératoire of tin production. Although Precambrian cassiterite may contain 10s of ppm Pb or more (predominantly radiogenic 206 Pb), Phanerozoic examples typically contain only a few parts per million Pb. However, all but one of the 133 raw tin ingots excavated from European Bronze Age shipwrecks contains more Pb than could have come from cassiterite alone, as do six of the twelve analyzed tin objects interpreted to have been derived from the ores of southern Africa. Accordingly, almost all tin objects must contain Pb from external cassiterite sources and interpretation of LIA must account for this contamination. The nature of the contaminant (sulfides, U-Th-bearing minerals, silicates) can be inferred from patterns in Pb concentration and LI values. The 3 major sources of Pb that can typically be identified in tin artifacts are original Pb from the tin ore, radiogenic Pb produced insitu due to U decay, and external Pb added during the cassiterite smelting and ingot production. As cassiterite has high U/Pb but low Th/Pb, the 208 Pb/ 204 Pb may be representative of the initial Pb incorporated in the mineral. This is assuming either that no external Pb is added during the ore processing or that the added Pb is from coeval sulfides from the same Pb ore provenance. In such cases 208Pb/204 Pb can be used to estimate a Pb model age, which in turn can be used for provenance estimate of the ingots. If the addition of Pb is from U-Th-mineral contaminants to the ore concentrate, then this will also increase 208 Pb/ 204 Pb and point to erroneously young model Pb ages. In such cases, the problem would be evident in positively correlated values of 206 Pb/ 204 Pb and 208 Pb/ 204 Pb. If Pb concentrations are above a certain threshold (approximately 5 ppm). LIA typical common Pb isotope ratios will be clear indication that external, non-cassiterite Pb, is added to the tin artifact. This tin could be from impurities in the ore (e.g., inclusions in cassiterite, impurities in the ore concentrate, or added during ore smelting and/or metal processing. Overall, elevated Pb concentrations accompanied with non-radiogenic Pb isotopes typical for common Pb, is a clear indication that significant amount of external (contaminant) Pb is added to the tin artifact.
Journal of Archaeological Science, 2021
Lead isotope analysis (LIA) can be applied to tin provenance studies when both the U–Th–Pb characteristics of cassiterite and potential Pb contamination are taken into account. As cassiterite (SnO2) contains higher con-centrations of U than Pb and negligible Th, radioactive decay increases 206Pb/204Pb and 207Pb/204Pb ratios in cassiterite over time. However, 208Pb/204Pb ratio retains its primary value and can be correlated with LIA da-tabases. A majority of documented Bronze Age tin ingots from Europe have Pb in excess of the 5 ppm maximum that can be derived from Cenozoic to Late Paleozoic cassiterite. A minute mass of galena (PbS) in the ore concentrate is sufficient to mask the cassiterite-derived lead, as would the addition of any lead contaminant introduced in the smelting/casting process. If the galena is cogenetic with the cassiterite, then LIA will be un-affected. The inclusion of uranium-rich minerals in the tin ore concentrate is another potential source of excess lead. In this case, the additional Pb is uranogenic, and so 206Pb/204Pb and 207Pb/204Pb will reflect the age of the uranium minerals, but 208Pb/204Pb will retain its primary value. If the U-minerals are cogenetic or coeval with tin mineralization, then a Pb isochron age will indicate the age of the ore. Between 1984 and 1994, at least 117 ingots, or roughly one tonne, of tin was raised from the Late Bronze Age Uluburun shipwreck (ca. 1320 B.C.). Over half of the analyzed ingots from this wreck site have high Pb con-centrations (>100 ppm), indicative of contamination from non-radiogenic lead associated with lead metal or galena. LIA indicates that the Pb originated from the Pb–Ag-rich Bolkarda ̆g region of the south-central Taurus Mountains. A second group of approximately 28 tin ingots with lower Pb content (<100 ppm) contain additional uranogenic Pb but retain 208Pb/204Pb compositions that overlap with the ca. 300 Ma tin regions of Western Europe and Central Asia, with the most likely source being the Tienshan Mountains in Kyrgyzstan, Tajikistan, and Uzbekistan. By compensating for previous uncertainty around the use of Pb isotopes for sourcing tin objects, it is now possible to contextualize the Uluburun tin ingots more securely within the metallurgical systems of the Central Taurus-Cilicia-Amanus axis. Recent scholarship has shown that tin production in the South-Central Taurus region had taken place at a scale not previously anticipated. Two parallel production systems appear to have been in place serving elite and common consumption networks via markedly different technologies. The South-Central