TIN ISOTOPY—A NEW METHOD FOR SOLVING OLD QUESTIONS

Authors

  • M. HAUSTEIN,

    1. Curt-Engelhorn-Zentrum für Archäometrie, D6, 3, D-68159 Mannheim, Germany
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  • C. GILLIS,

    1. Department of Classical Archaeology and Ancient History, University of Lund,
      Box 117, S-221 00 Lund, Sweden
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  • E. PERNICKA

    1. Eberhard-Karls-Universität Tübingen, Institut für Ur- und Frühgeschichte und Archäologie des Mittelalters, Schloss Hohentübingen, D-72070 Tübingen
      and
      Curt-Engelhorn-Zentrum für Archäometrie, D6, 3, D-68159 Mannheim, Germany
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Abstract

Tin was a vital commodity in times past. In central Europe, the earliest finds of tin-bronze date to about 2200 bc, while in Greece they are c. 400–500 years earlier. While there is evidence for prehistoric copper mining—for example, in the Alps or mainland Greece, among other places—the provenance of the contemporary tin is still an unsolved problem. This work deals with a new approach for tracing the ancient tin via tin isotope signatures. The tin isotope ratios of 50 tin ores from the Erzgebirge region (D) and 30 tin ores from Cornwall (GB) were measured by MC–ICP–MS. Most ore deposits were found to be quite homogeneous regarding their tin isotope composition, but significant differences were observed between several deposits. This fact may be used to distinguish different tin deposits and thus form the basis for the investigation of the provenance of ancient tin that has been sought for more than a century. Furthermore, the tin-isotope ratio of the ‘Himmelsscheibe von Nebra’ will be presented: the value fits well with the bulk of investigated tin ores from Cornwall.

INTRODUCTION

Tin was an extremely important but rare commodity in Europe during the Bronze Age and after. The provenance of prehistoric tin, which was used for the production of bronze, is still an unsolved problem in archaeological research. The first bronze finds in central Europe date to 2200 bc, while tin-bronze appears some c. 400–500 years earlier in Greece. The Aunjetitz Culture (2100–1600 bc), a very common prehistoric culture group in central Europe, is very rich in bronze finds. The fact that the Erzgebirge is the only region in this area with a notable tin mineralization has interested and inspired archaeologists and others for some time. However, evidence for prehistoric tin mining in the Erzgebirge region has not yet been found (Niederschlag et al. 2003). In Greece, for example, there are no tin ores, yet tin-bronze becomes common in the late Early Bronze Age: the tin was obviously imported. While several sources ranging from Turkey, Egypt, the former Yugoslavia, Italy and as far away as Afghanistan, Kirgistan and Uzbekistan, as well as the Erzgebirge region and Cornwall, have been suggested, there is no proof that these sources were used in Greece in the Greek Bronze Age (see, e.g., Muhly 1973; Penhallurick 1986; Pernicka 1987; Gillis 1991; Gillis and Clayton 2008). If the sources of the tin were known, this knowledge would provide much information about the mechanisms of ancient trade, mining, contacts and so on. Questions such as how many tin sources can be found in a single site, whether there are diachronic changes and how they relate to a greater contextual understanding of the site, what the range of the trading sphere was at any one time or place; and so on, could be answered. Thus if a method could be developed for sourcing tin, it would be invaluable not only in itself but also in a much wider, social context.

For Cornwall, the well-known old mining region in western Europe, tin mining over the past 3500 years is assumed by several authors (see, e.g., Penhallurick 1986). This assumption is founded on a small number of tin-containing slag finds and prehistoric remains in tin streamers. The oldest of the slags, excavated near St Austell, is dated to the 16th to 14th century bc. Otherwise the old tin ingots, which have been found mostly along the coast of Cornwall, can be dated to the Middle Ages rather than to prehistoric times (Rhoden 1985). In fact, there is no evidence that one prehistoric bronze find was made from Cornish tin.

The reason for the relatively scanty knowledge concerning ancient tin mining, smelting and trade is obvious: there is no scientific method to trace the tin to its source. Due to the fact that the lead contents, which could be found in ancient bronze artefacts, come mostly from the copper ore, lead isotope analyses cannot be applied for determining the provenance of the tin (Begemann et al. 1999). Hence, this present work deals with the use of tin isotopes for this purpose.

Tin has 10 stable isotopes, more than any other element, that cover a mass range of 12 amu (112Sn–124Sn). In the past it was assumed that there is no, or only a very small, variation of tin isotopes in nature (see, e.g., De Laeter et al. 1965, 1967; Rosman et al. 1984). McNaughton and Rosman (1991) analysed nine cassiterite samples from different locations and types of ore deposits. By using a multicollector TIMS and a double-spike procedure, they found that only one cassiterite sample (#1) with +0.15 ± 0.02‰ (2 s.d.) per mass unit differed significantly from a laboratory standard. Apart from this, the external reproducibility (2 s.d.) from seven of the nine samples was much poorer (e.g. #3, #7 and #9, 0.32‰ per mass unit). Thus it was not possible to detect any other significant variations. In 1998, the IUPAC Commission on Atomic Weights and Isotopic Abundances reported an atomic weight of 118.710 ± 0.007 for tin. Isotopic variations were considered insignificant.

The first published suggestion for using tin isotope ratios for solving an archaeological problem was made by Gale in 1997. This suggestion dealt with the idea that re-melted bronze must have a different tin-isotopic composition than primary metal. Thus it should be possible to detect recycling rates of bronze by measuring the tin isotopes. Unfortunately, Gale could not find significant isotopic variations.

Further investigations into tin isotopes for the purpose of creating a method for tracing archaeological tin were carried out between 1997 and 2002. By using a TIMS (VG Isolab 54) equipped with seven Faraday detectors, Gale (1997) achieved a reproducibility of 0.28‰ (2 s.d.) for 122Sn/116Sn, while Clayton et al. (2002) achieved 0.23‰ (n = 14) by using a MC–ICP–MS (Micromass IsoProbe) for the same isotope ratio. The measurements were carried out on an internal standard, Johnson Matthey Puratronic tin metal (Batch W14222). By measuring a cassiterite sample (Pen388/2) from Malaysia in comparison with the standard, Clayton et al. (2002) detected isotopic differences that were significantly larger than the analytical uncertainty. Furthermore, Clayton et al. (2002) presented tin isotope data of single cassiterite samples from Cornwall, the Erzgebirge, Egypt and Kirgizstan. Also, some archaeological material from the Aegean Bronze Age was analysed. The investigations showed that significant isotopic differences occur both in the ores and in the artefacts and between object types, ingots versus foils. Due to the fact that only a small number of samples were analysed, further conclusions could not be drawn. Further investigations in the field of tin isotopes with an archaeological background were also performed by Gillis and Clayton (2008), Nowell et al. (2002), Gillis et al. (2001) and Yi et al. (1999). For the TIMS or MC–ICP–MS measurements, the same standard material (Johnson Matthey Puratronic tin, as mentioned before) was always used, making the investigations comparable. An exception is Begemann et al. (1999), where a SnCl2 solution was applied. A more detailed description of this research is given by Clayton et al. (2002).

However, a clear idea of the scale of the isotope variations of tin in nature could not be gained from the existing publications. The question of whether tin isotope ratios can be used as a tool for provenance analyses of ancient tin still awaits an answer for two main reasons: first, as already mentioned, the number of analysed samples so far is much too small, so that one cannot consider them as systematic investigations. Secondly, not enough attention was paid to the purification of the measurement solutions—for example, by ion chromatography—especially when the MC–ICP–MS technique was applied, which in principle provides the possibility to generate a large number of isotope measurements within a short time. With the present work, this gap may be closed. Systematic investigations of ores from several deposits in the Erzgebirge region and Cornwall, as reported here, demonstrate that the tin isotope ratio of a source is widely homogeneous. On the other hand, we found significant differences between ores from different sources. This provides the foundation for a useful procedure to be used for tracing the ancient tin via tin isotopes.

SAMPLING AND SAMPLE PREPARATION

The first step on the way to developing a useful procedure for tracing the ancient tin through its own isotope ratios is the investigation of tin ores from several occurrences and regions. Thus a database can be created, which allows a comparison of tin isotope data from data of the ores to that of archaeological artefacts.

Although about 20 minerals with tin as the major component are presently known, only cassiterite (SnO2) and, to a much lesser extent, stannite (Cu2FeSnS4) are of commercial importance. Cassiterite can be found in placer deposits, which today provide more than 50% of the world tin production. The primary deposits are always related to intrusive igneous rocks, especially to granite. Cassiterite can be concentrated in pegmatites and in pneumatolytic (high-temperature) or hydrothermal (low-temperature) quartz veins. A special case is ‘tin greisen’, which is the term for cassiterite-impregnated granite. Such mineralization can be found in the traditional tin-mining districts of Europe, such as Cornwall (GB) and the Erzgebirge (D). In rare cases, cassiterite can also be found in hydrothermal veins associated with stannite (cassiterite-sulphide type) and other minerals, such as tourmaline, topaz, wolframite and arsenopyrite. Examples of such deposits are, for example, the modern tin mines of Bolivia and the historical mining regions of St Agnes, Illogan and Redruth in Cornwall, Altenberg and Zinnwald in the eastern part of the Erzgebirge, and Rooiberg in South Africa. In contrast to pneumatolytic tin deposits, which are characterized by high Ta, Nb and low W concentrations, hydrothermal tin ores contain minor Ta and Nb, but are rich in W (Taylor 1979; Guilbert and Park 1986; Grant 1999).

For meaningful information about the variance of the tin isotope ratios in nature, samples from deposits of different geographical locations are required. The investigated samples derive from various locations in the Erzgebirge tin district (50 samples) and from Cornwall (30 samples). The Erzgebirge tin district includes the Erzgebirge low mountain range in Saxony (D) and Bohemia (CZ) and the area around it, especially the Vogtland (south-west Saxony) and Schlaggenwald (Horni Slavkov, Bohemia). The material came mostly from the mineralogical collection of the TU Bergakademie Freiberg. Only 10 samples were taken from placers in the Erzgebirge. One of the primary ore samples consists of stannite, two are greisens and the rest are cassiterite, often associated with quartz, granite, tourmaline, wolframite or gilbertite. A detailed description of the samples is given in Table 1.

Table 1. The list of samples, with locations and macroscopic descriptions of tin ores that were applied for isotope measurements
ItemSample numberLocationDescription
 0StandardModern high-purity tin-metalIn-house standard material
 1FG-011414Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with quartz
 2FG-011416Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with quartz and gilbertite
 3FG-011494Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite in quartz vein
 4FG-011501Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite in quartz vein
 5FG-011506Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite in quartz vein
 6FG-050664Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with quartz, fluorite and gilbertite
 7FG-050665Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite in quartz vein with gilbertite
 8FG-050666Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with quartz, gilbertite and granite
 9FG-050667Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with gilbertite
10FG-050668Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with gilbertite, feldspar and granite
11FG-050672Erzgebirge, Ehrenfriedersdorf, Vierung MineCassiterite with gilbertite and apatite
12MA-081325Erzgebirge, placer, north-west of Ehrenfriedersdorf townCassiterite as placer-tin
13MA-081326Erzgebirge, placer, north-east of Geyer townCassiterite as placer-tin
14MA-081327Erzgebirge, placer, north-east of Geyer townCassiterite as placer-tin
15MA-081328Erzgebirge, placer, east of Geyer townCassiterite as placer-tin
16FG-050678Erzgebirge, Geyer, Hohes neues Jahr MineTin-greisen ore
17FG-011014Schlaggenwald-SchönfeldCassiterite with quartz
18FG-011019Schlaggenwald-SchönfeldCassiterite with quartz
19FG-011020Schlaggenwald-SchönfeldCassiterite with quartz
20FG-011406Schlaggenwald-SchönfeldCassiterite with quartz
21FG-011417Schlaggenwald-SchönfeldCassiterite with quartz
22FG-050671Schlaggenwald, Wilhelmschacht Mine, MariengangCassiterite with quartz
23FG-050673Schlaggenwald, Wilhelmschacht Mine, SchnödenstockCassiterite with molybdenite, fluorite and chalcopyrite
24FG-011392Erzgebirge, Zinnwald, Zwitterfeld Fundgrube MineCassiterite with quartz
25FG-011482Erzgebirge, Zinnwald, Zwitterfeld Fundgrube MineCassiterite with quartz
26FG-011489Erzgebirge, Zinnwald, Zwitterfeld Fundgrube MineCassiterite with quartz
27FG-050680Erzgebirge, Zinnwald, Zwitterfeld Fundgrube Mine, Flötz bCassiterite-graupen in quartz
28FG-050681Erzgebirge, Zinnwald, Reichentroster Schacht MineCassiterite with scheelite and zinnwaldite
29FG-050682Erzgebirge, Zinnwald, Zwitterfeld Fundgrube Mine, Flötz bCassiterite in quartz
30FG-050683Erzgebirge, Zinnwald, Reichentroster Weitung MineTin-greisen ore with wolframite
31FG-011490Erzgebirge, Zinnwald, Zwitterfeld Fundgrube MineStannite
32FG-050685Erzgebirge, Altenberg, Paradies Fundgrube MineCassiterite in granite
33FG-050669Erzgebirge, Auersberg, Grüne Tanne Fundgrube MineCassiterite with quartz
34FG-050674Erzgebirge, Auersberg, Sechs Brüder Zeche MineCassiterite in quartz vein
35FG-050675Erzgebirge, Auersberg, Sechs Brüder Zeche MineTin ore, massive
36FG-050677Erzgebirge, Auersberg, Hohenmaas Fundgrube MineTin ore in tourmaline-slate
37MA-081320Erzgebirge, placer ‘Sauschwemme’ AuersbergCassiterite as placer-tin
38MA-081321Erzgebirge, placer ‘Sauschwemme’ AuersbergCassiterite as placer-tin
39MA-081322Erzgebirge, placer ‘Sauschwemme’ AuersbergCassiterite as placer-tin
40MA-081323Erzgebirge, placer ‘Sauschwemme’ AuersbergCassiterite as placer-tin
41MA-081324Erzgebirge, placer ‘Sauschwemme’ AuersbergCassiterite as placer-tin
42FG-050687Vogtland, Gottesberg, Geyer-PingeCassiterite as placer-tin
43FG-050692Vogtland, GottesbergCassiterite in quartz-granite matrix
44FG-050693Vogtland, Gottesberg, Geyer-PingeCassiterite
45MA-081329Vogtland, placer in the vicinity of GottesbergCassiterite as placer-tin
46FG-050688Vogtland, Mühlleiten, Tannenberg Fundgrube MineCassiterite with quartz and topaz
47FG-050689Vogtland, Mühlleiten, Tannenberg Fundgrube MineCassiterite
48FG-050690Vogtland, Mühlleiten, Tannenberg Fundgrube MineCassiterite
49FG-050695Erzgebirge, Bohemia, Graupen, Luxer-Gang MineCassiterite with fluorite and mica
50FG-050696Erzgebirge, Bohemia, Graupen, Luxer-Gang MineCassiterite in quartz
51MA-080863Cornwall, East Pool MineCassiterite with quartz
52MA-080513Cornwall, Botallack MineCassiterite
53MA-080518Cornwall, Botallack MineCassiterite
54MA-080519Cornwall, Botallack MineCassiterite-greisen
55MA-080520Cornwall, Botallack MineCassiterite in quartz
56MA-080521Cornwall, Botallack MineTin ore
57MA-080522Cornwall, Botallack MineCassiterite
58MA-080523Cornwall, Botallack MineCassiterite
59MA-080881Cornwall, Carnan-placer near TruroCassiterite as placer-tin
60MA-080871Cornwall, Penhalls MineCassiterite with quartz, pyrite and chalcopyrite
61MA-080867Cornwall, Wheal Kitty MineCassiterite with quartz
62MA-080874Cornwall, Wheal Kitty MineCassiterite with chalcopyrite and pyrite
63MA-080879Cornwall, Wheal Kitty MineCassiterite and wolframite in quartz
64MA-080504Cornwall, Redruth MineCassiterite with quartz
65MA-080862Cornwall, Dolcoath MineCassiterite with quartz, fluorite and chlorite
66MA-080508Cornwall, St Agnes MineCassiterite with gangue
67MA-080510Cornwall, St Agnes MineCassiterite, fine-grained
68MA-080511Cornwall, St Agnes MineCassiterite, coarse-grained
69MA-080507Cornwall, St Agnes MineCassiterite
70MA-081297Cornwall, Penderves MineCassiterite
71MA-080878Cornwall, Scarrier MineCassiterite with chlorite
72MA-080509Cornwall, St Just MineCassiterite, coarse-grained with quartz
73MA-080512Cornwall, St Just MineCassiterite
74MA-080880Cornwall, Wheal Martin MineCassiterite
75MA-080876Cornwall, West Wheal Eliza MineCassiterite as impregnation in tourmaline-slate
76MA-081292Cornwall, Pentowan MineCassiterite as placer-tin
77MA-080872Cornwall, South Crofty MineCassiterite with quartz
78MA-080505Cornwall, Redruth MineStannite
79MA-080516Cornwall, Illogan MineStannite with wolframite, chalcopyrite, arsenopyrite and quartz
80MA-080517Cornwall, Illogan MineStannite

Sample dissolution

Cassiterite is very resistant to acids and other chemicals, thus making it difficult to obtain a measuring solution for ICP–MS. This problem was never satisfactorily solved in previous investigations referred to in the introduction. Therefore, one important aim of this study was to develop a reliable and reproducible dissolution procedure for tin ore. On the one hand, as much tin as possible should be dissolved but, on the other, the dissolution of trace elements that occurs in cassiterite, and that can cause interference effects during the isotope measurements, has to be avoided.

McNaughton and Rosman (1991) reduced the crushed ore with ultrapure graphite to tin metal, which can be dissolved by 6 M HCl. This procedure was also applied by Clayton et al. (2002) and Nowell et al. (2002). However, for the reduction process, high temperatures of about 1200°C are required and the metal obtained is more or less contaminated with graphite or crucible material.

It seems to be more practical to use KCN (potassium cyanide)—as is mentioned, for example, by Jander and Wendt (1957)—as a reducing agent. For the reduction, which can be carried out in graphite crucibles, following the equation

image

a temperature of around 800°C is required. At this relatively low temperature, the possibility of isotope fractionation can be excluded. The tin metal obtained by this process is very pure. Possible residues of KOCN can be removed by rinsing the metal with water. Furthermore, other elements such as Cu, Fe, Se, Te, W, Mo, Ta, Nb, Hf and Zr, which occur in natural cassiterite, were completely or partly removed by the KCN procedure. Elements such as Cd and In cannot be removed, because they are taken up by the molten tin. Thus, with this procedure a quasi-complete extraction of tin from cassiterite is possible. As a secondary effect, the obtained tin and the measurement solution, respectively, are relatively pure. Table 2 shows concentrations of tin and some trace elements in cassiterite (sample FG-011416) and in the obtained tin metal. Furthermore, possible interferences that need to be avoided when making isotope measurements are listed. It is clearly demonstrated that the reduction with KCN is the best method for dissolving the cassiterite for tin isotope measurements. It must be noted that potassium cyanide is very toxic, and so it must be handled with care. The work has to be carried out in a hood. In contrast, the obtained potassium cyanate is non-toxic. After reduction, the water-rinsed tin regulus can be easily dissolved with 6 M HCl.

Table 2. The results of cassiterite reduction with KCN at 800°C. In columns 2 and 3, the tin (%) and the trace element concentrations (ppm) of the cassiterite sample FG-011416 and the obtained tin metal are compared. Important interferences of the analysed trace elements on tin isotopes are given in columns 4 and 5
 CassiteriteReduced tinIsobars to
112Sn114Sn115Sn116Sn117Sn118Sn119Sn120Sn121Sb122Sn123Sb124Sn
Sn69.1%97.3%  
Fe91001300            
Cu11001000            
Ag3.8<1.0      12C107Ag 14N107Ag, 12C109Ag 14N109Ag 
Cd3538112Cd114Cd 116Cd 12C106Cd 14N106Cd 12C110Cd, 16O106Cd 14N108Cd 12C112Cd 14N110Cd 16O108Cd
Se21<10 40Ar74Se 40Ar76Se40Ar77Se40Ar78Se 40Ar80Se 40Ar82Se  
Te6017       120Te 122Te123Te124Te
Sb21.7        121Sb 123Sb 
As3436  40Ar75As         
W83<0.3            
Mo9 (LA)<0.3 (LA)14N98Mo, 16O96Mo16O98Mo, 14N100Mo 16O100Mo        
In1110  115In         
Ge<1 (LA)<1 (LA)40Ar72Ge40Ar74Ge 40Ar76Ge        
Ta1190.7            
Nb92 (LA)1 (LA)            
Hf55<0.2            
Zr115<6616O96Zr           

Sample purification by ion chromatography

To avoid interferences, it is necessary to check the measurement solution for signals at masses 112–124. The possible elemental isobaric interferences are as follows: 112Cd, 114Cd, 115In, 120Te, 122Te and 124Te. Due to the fact that the solutions for isotope measurements are doped with Sb for mass-bias correction (see the section on isotope measurements below), it is necessary to make sure that all antimony in the sample solution is removed. Furthermore, molecular interferences on the tin isotope ratios, as shown in Table 2, also have to be considered. For this purpose, the use of ion chromatography is an elegant way to obtain pure tin solutions, which is the basic requirement for reproducible isotope measurements. If tin isotopes from bronze objects are to be measured, the removal of the copper content of the alloy by ion chromatography is essential.

For the selective separation of tin, the anion-exchange resin TRU-Spec from EICHROM was used. TRU-Spec resin consists of 13% ocytl(phenyl)-N,N-diisobutylcarbomolmethyl-phosphinoxide and 27% tributylphosphate coated on a copolymeric support. This resin is suitable for the separation of Fe, Sn, U and some actinide elements, while most of the other metals are not retained on the column (Huff and Huff 1993). In Figure 1, the elution diagram of TRU-Spec resin with a test solution containing 20 μg of Sn, Cu and Pb as well as 2 μg of Sb, Te, Cd, In and Bi is shown. The volume of the solution was 10 ml, while the volume of the column was 0.5 ml. The first step was the conditioning of the resin with 5 ml 1 M HCl. The test solution was then put into the column, followed by elution with 10 ml 1 M HCl. In this fraction, Cu, Pb, Sb, Te, Cd and In were recovered. With 6 ml 5 M HNO3, Bi can be eluted while Sn can be obtained with 4 ml 1 M HNO3. If Bi is absent or occurs only at low concentrations in the measurement solution, which is mostly the case, this step can be dropped. The recovery rates for tin were always between 94 and 97%. At this good recovery rate, an isotope fractionation of tin by ion chromatography is unlikely; nevertheless, this important point was checked. A tin solution was measured both before and after ion chromatography: no isotope changes could be detected.

Figure 1.

The application of TRU-Spec resin for purification of the measurement solution. An elution diagram of a tin-containing solution doped with some trace elements. Note that the concentrations of the Sn, Cu and Pb contents must be multiplied by 10.

ISOTOPE MEASUREMENTS WITH MC–ICP–MS

Standard materials

A commercial tin isotope standard is not yet available. Thus Puratronic Grade 1 tin metal foil, supplied by Johnson Matthey, was used as an ‘in-house’ standard (PSn). This material was also applied in other investigations (see the Introduction). For mass-bias correction an antimony solution, Specpure ICP–AES, supplied by Fisher Chemicals, was used. The unknown ratio 123Sb/121Sb was calculated after an exponential law by the 122Sn/116Sn ratio of 0.318634, which was determined by Clayton et al. (2002).

Mass-bias correction

The ICP source causes a mass bias that is much larger than the isotope differences that are to be measured. Therefore a precise mass-bias correction procedure must be applied. Antimony doping, as used for example by Clayton et al. (2002), seems to be the most suitable method. The mass bias can be corrected following the exponential law

image

where Ri are the true and ri the measured isotopic ratios, and Mi are the isotopic masses. The β-exponent is expressed as follows:

image

Detailed descriptions of the applied or further mass-bias correction methods are given in Clayton et al. (2002).

Adjustment of the mass spectrometer and measurement conditions

Isotope measurements were carried out with a VG Axiom MC–ICP–MS equipped with eight adjustable and one fixed axial Faraday cups. The mass range that could be covered was 10 amu. This means that it was already theoretically impossible to collect masses 112Sn and 124Sn simultaneously. The axial position was used for 119Sn (natural abundance 8.59%), while the principal isotopes 120Sn (32.59%) and 118Sn (24.23%) were avoided. The complete multicollector configuration of the instrument and the relative abundances of tin isotopes are given in Table 3. The advantage of this configuration is that measurement solutions with relatively high concentrations (1 or max. 2 ppm Sn) can be used, which leads to a high measurement precision for ratios such as 122Sn/116Sn.

Table 3. The multicollector settings of the VG Axiom and the relative abundances of tin isotopes
 Low 4Low 3Low 2Low 1AxialHigh 1High 2High 3High 4
Collector position (mm)24.7719.6514.329.7609.2414.1318.4622.75
Isotope114Sn115Sn116Sn117Sn119Sn121Sb122Sn123Sb124Sb
Abundance0.650.3414.537.688.5957.34.6342.75.79

For precise isotope measurements, it is also necessary to suppress the memory effect of tin in the sample introduction system of the MS. The application of special wash mixtures containing HF, as has been suggested by McGinnis et al. (1997), or surface-active agents like Triton-X (Wilbur 2006) did not yield satisfactory results. Furthermore, any acid mixtures with an oxidizing component, such as nitric acid in aqua regia, must be strictly avoided because of the precipitation of SnO2. For rinsing the tubing, chambers and the nebulizer of the MS, 1 M HCl was used. However, the aspiration of the solutions by a microconcentric nebuliser such as the Cetac MCN 6000 should be avoided, because it operates at temperatures about 90°C or more, which leads to the precipitation of SnO2. When a MCN 6000 was used, the rinsing time between each run increased to 30 min, which is far from ideal as it causes a very long analysis time (Clayton et al. 2002).

The concentration of the measurement solutions was adjusted to 1 ppm Sn and 0.25 ppm Sb in 1 M ultrapure HCl, which led to a signal intensity of approximately 4 × 107 cps for 119Sn. For the measurements, a glass Micro-Mist nebulizer (0.1 ml min−1) as well as a cyclonic and a Scott spray chamber (both borosilicate glass) were applied, and samples were changed manually. The peristaltic pump of the VG Axiom was used only for rinsing and washing out any remaining sample material: the nebulizer was operating in free uptake mode. This configuration requires an uptake time of 3 min. The nebulizer Ar flow was 0.7 l min−1, the auxiliary gas flow was 1.05 l min−1 and the cooling gas flow was 12.5 l min−1. The RF power of the plasma was adjusted to 1280 W. Rinsing was carried out with 1 M ultrapure HCl for 5 min; after 3–4 min, the remaining signal intensity was suppressed to less than 1% of the measurement intensity. This condition could be achieved at the beginning of the measurement, but equally well after the MS had been in operational mode for 10 h. For data-collection, 100 measurements (10 blocks of 10 cycles, with a 20 s integration time) of each sample were taken. The required volume of measurement solution for one isotope measurement is about 8 ml, which contains 8 μg of Sn. It should be noted that nickel or platinum can be used as cone material, while aluminium produces an observable memory effect.

Repeatability of the isotope measurements

The repeatability of a 1 ppm Puratronic tin-standard solution (PSn) and cassiterite sample FG-011068 (purified by IC), both doped with 0.25 ppm Sb, is illustrated in Figure 2. Each symbol represents one run, consisting of one block with 10 cycles. For the ratio 122Sn/116Sn, a repeatability of 0.17‰ (2 s.d.) for the standard and 0.23‰ (2 s.d.) for sample FG-011014 was found. Clayton et al. (2002), who used an unpurified measuring solution, achieved a significantly lower repeatability of 0.36‰ (2 s.d.) for a cassiterite sample (Pen388/2). Figure 2 shows clearly that the signal does not change with time. A differentiation between the standard and the cassiterite sample with the achieved measurement precision and repeatability is possible. Reproducibility has been proved by repeating the measuring of 10 sample solutions on different days.

Figure 2.

The repeatability of tin isotope measurements in the PSn standard and a sample solution (FG-011068) purified by ion chromatography. Each symbol represents one run, which consists of one block with 10 cycles: 30 runs are equivalent to a time sequence of about 10 h. The concentrations were adjusted to 1 ppm Sn and 0.25 ppm Sb. The errors of the measurement values are in the size range of the symbols.

RESULTS AND DISCUSSION

Isotope variations of tin in nature

In Table 4, the 122Sn/116Sn and 117Sn/119Sn ratios of 50 tin ore samples and the PSn standard are listed. The absolute values are given in columns 4 and 6, while in columns 5 and 7 the fractionation limits in comparison to the standard are reported. Both data are given with 2 s.d. errors. For greater clarity in Figures 3 and 4, the isotopic values were plotted without errors.

Table 4. The isotope ratios of the measured samples and their fractionation factors in comparison to the in-house PSn standard. Both sets of data are given with 2 s.e. errors
ItemSample numberLocation (see Table 1)122Sn/116SnFractionation (‰)117Sn/119SnFractionation (‰)
 0Standard 0.318634 ± 0.0000560.00 ± 0.170.893110 ± 0.0000510.00 ± 0.06
 1FG-011414Erzgebirge, Ehrenfriedersdorf0.318715 ± 0.0000590.25 ± 0.190.893045 ± 0.000055−0.07 ± 0.06
 2FG-011416Erzgebirge, Ehrenfriedersdorf0.318751 ± 0.0000480.37 ± 0.150.893010 ± 0.000042−0.11 ± 0.05
 3FG-011494Erzgebirge, Ehrenfriedersdorf0.318720 ± 0.0000600.27 ± 0.190.893000 ± 0.000059−0.12 ± 0.07
 4FG-011501Erzgebirge, Ehrenfriedersdorf0.318648 ± 0.0000650.04 ± 0.200.893090 ± 0.000062−0.02 ± 0.07
 5FG-011506Erzgebirge, Ehrenfriedersdorf0.318660 ± 0.0000500.08 ± 0.160.893140 ± 0.0000540.03 ± 0.06
 6FG-050664Erzgebirge, Ehrenfriedersdorf0.318742 ± 0.0000530.34 ± 0.170.893043 ± 0.000050−0.08 ± 0.05
 7FG-050665Erzgebirge, Ehrenfriedersdorf0.318706 ± 0.0000440.23 ± 0.140.893070 ± 0.000040−0.04 ± 0.04
 8FG-050666Erzgebirge, Ehrenfriedersdorf0.318764 ± 0.0000450.41 ± 0.140.892950 ± 0.000043−0.18 ± 0.05
 9FG-050667Erzgebirge, Ehrenfriedersdorf0.318880 ± 0.0000420.77 ± 0.130.893030 ± 0.000044−0.09 ± 0.05
10FG-050668Erzgebirge, Ehrenfriedersdorf0.318744 ± 0.0000550.35 ± 0.170.893048 ± 0.000049−0.07 ± 0.05
11FG-050672Erzgebirge, Ehrenfriedersdorf0.318787 ± 0.0000580.48 ± 0.180.893040 ± 0.000057−0.08 ± 0.06
12MA-081325Erzgebirge, placer0.318720 ± 0.0000390.27 ± 0.120.893010 ± 0.000042−0.11 ± 0.05
13MA-081326Erzgebirge, placer0.318750 ± 0.0000470.36 ± 0.150.893025 ± 0.000050−0.10 ± 0.05
14MA-081327Erzgebirge, placer0.318710 ± 0.0000440.24 ± 0.140.893005 ± 0.000049−0.12 ± 0.05
15MA-081328Erzgebirge, placer0.318740 ± 0.0000580.33 ± 0.180.892984 ± 0.000055−0.14 ± 0.06
16FG-050678Erzgebirge, Geyer0.319180 ± 0.0000431.71 ± 0.140.892630 ± 0.000045−0.54 ± 0.05
17FG-011014Schlaggenwald-Schönfeld0.318823 ± 0.0000660.59 ± 0.210.892950 ± 0.000061−0.18 ± 0.07
18FG-011019Schlaggenwald-Schönfeld0.318717 ± 0.0000540.26 ± 0.170.893030 ± 0.000056−0.09 ± 0.06
19FG-011020Schlaggenwald-Schönfeld0.318798 ± 0.0000600.51 ± 0.190.892940 ± 0.000055−0.19 ± 0.06
20FG-011406Schlaggenwald-Schönfeld0.318725 ± 0.0000480.29 ± 0.150.893020 ± 0.000044−0.10 ± 0.05
21FG-011417Schlaggenwald-Schönfeld0.318608 ± 0.000052−0.08 ± 0.160.893160 ± 0.0000500.06 ± 0.05
22FG-050671Schlaggenwald, Wilhelmschacht0.318620 ± 0.000061−0.04 ± 0.190.893140 ± 0.0000580.03 ± 0.07
23FG-050673Schlaggenwald, Wilhelmschacht0.318583 ± 0.000040−0.16 ± 0.130.893150 ± 0.0000420.04 ± 0.05
24FG-011392Erzgebirge, Zinnwald0.318694 ± 0.0000410.19 ± 0.130.892967 ± 0.000042−0.16 ± 0.05
25FG-011482Erzgebirge, Zinnwald0.318665 ± 0.0000670.10 ± 0.210.892980 ± 0.000064−0.14 ± 0.07
26FG-011489Erzgebirge, Zinnwald0.318771 ± 0.0000460.44 ± 0.140.893013 ± 0.000050−0.11 ± 0.05
27FG-050680Erzgebirge, Zinnwald0.318704 ± 0.0000530.22 ± 0.160.893068 ± 0.000055−0.05 ± 0.06
28FG-050681Erzgebirge, Zinnwald0.318630 ± 0.0000490.01 ± 0.150.893153 ± 0.0000460.05 ± 0.05
29FG-050682Erzgebirge, Zinnwald0.318737 ± 0.0000540.32 ± 0.170.893024 ± 0.000053−0.10 ± 0.06
30FG-050683Erzgebirge, Zinnwald0.318901 ± 0.0000580.84 ± 0.180.892913 ± 0.000060−0.22 ± 0.07
31FG-011490Erzgebirge, Zinnwald0.318460 ± 0.000041−0.55 ± 0.130.893300 ± 0.0000450.21 ± 0.05
32FG-050685Erzgebirge, Altenberg0.318782 ± 0.0000520.46 ± 0.160.893027 ± 0.000047−0.09 ± 0.05
33FG-050669Erzgebirge, Auersberg0.318753 ± 0.0000410.37 ± 0.130.892940 ± 0.000043−0.19 ± 0.05
34FG-050674Erzgebirge, Auersberg0.318798 ± 0.0000600.51 ± 0.190.892910 ± 0.000062−0.22 ± 0.07
35FG-050675Erzgebirge, Auersberg0.318651 ± 0.0000560.55 ± 0.180.893050 ± 0.000055−0.07 ± 0.06
36FG-050677Erzgebirge, Auersberg0.319012 ± 0.0000631.19 ± 0.200.892690 ± 0.000065−0.47 ± 0.07
37MA-081320Erzgebirge, placer0.318720 ± 0.0000380.26 ± 0.120.893040 ± 0.000043−0.08 ± 0.05
38MA-081321Erzgebirge, placer0.318700 ± 0.0000450.21 ± 0.140.893022 ± 0.000048−0.10 ± 0.05
39MA-081322Erzgebirge, placer0.318650 ± 0.0000440.05 ± 0.140.893085 ± 0.000043−0.03 ± 0.05
40MA-081323Erzgebirge, placer0.318677 ± 0.0000400.14 ± 0.130.893090 ± 0.000048−0.02 ± 0.05
41MA-081324Erzgebirge, placer0.318690 ± 0.0000500.18 ± 0.160.893065 ± 0.000051−0.04 ± 0.06
42FG-050687Vogtland, Gottesberg0.318625 ± 0.000046−0.03 ± 0.140.893040 ± 0.000043−0.08 ± 0.05
43FG-050692Vogtland, Gottesberg0.318580 ± 0.000042−0.17 ± 0.130.893080 ± 0.000042−0.03 ± 0.05
44FG-050693Vogtland, Gottesberg0.318560 ± 0.000056−0.23 ± 0.180.893130 ± 0.0000540.02 ± 0.06
45MA-081329Vogtland, placer0.318580 ± 0.000049−0.17 ± 0.150.893180 ± 0.0000470.08 ± 0.05
46FG-050688Vogtland, Mühlleiten0.318631 ± 0.000045−0.01 ± 0.140.893060 ± 0.000044−0.06 ± 0.05
47FG-050689Vogtland, Mühlleiten0.318886 ± 0.0000610.80 ± 0.190.892797 ± 0.000059−0.35 ± 0.07
48FG-050690Vogtland, Mühlleiten0.318623 ± 0.000043−0.03 ± 0.140.893050 ± 0.000040−0.07 ± 0.04
49FG-050695Erzgebirge, Graupen0.318888 ± 0.0000540.80 ± 0.170.892832 ± 0.000050−0.31 ± 0.05
50FG-050696Erzgebirge, Graupen0.318885 ± 0.0000630.79 ± 0.200.892787 ± 0.000061−0.36 ± 0.07
51MA-080863Cornwall, East Pool Mine0.318750 ± 0.0000480.36 ± 0.150.892980 ± 0.000040−0.15 ± 0.04
52MA-080513Cornwall, Botallack Mine0.318940 ± 0.0000420.96 ± 0.130.892770 ± 0.000040−0.38 ± 0.04
53MA-080518Cornwall, Botallack Mine0.318660 ± 0.0000370.08 ± 0.120.893004 ± 0.000033−0.08 ± 0.04
54MA-080519Cornwall, Botallack Mine0.318940 ± 0.0000420.96 ± 0.130.892740 ± 0.000040−0.41 ± 0.04
55MA-080520Cornwall, Botallack Mine0.318950 ± 0.0000480.99 ± 0.150.892730 ± 0.000042−0.43 ± 0.05
56MA-080521Cornwall, Botallack Mine0.318800 ± 0.0000450.52 ± 0.140.892910 ± 0.000040−0.22 ± 0.04
57MA-080522Cornwall, Botallack Mine0.318920 ± 0.0000510.90 ± 0.160.892790 ± 0.000047−0.36 ± 0.05
58MA-080523Cornwall, Botallack Mine0.318890 ± 0.0000400.80 ± 0.130.892850 ± 0.000030−0.29 ± 0.03
59MA-080881Cornwall, Carnan-placer0.318740 ± 0.0000360.33 ± 0.110.892960 ± 0.000026−0.17 ± 0.02
60MA-080871Cornwall, Penhalls Mine0.318780 ± 0.0000420.46 ± 0.130.892940 ± 0.000043−0.19 ± 0.05
61MA-080867Cornwall, Wheal Kitty Mine0.318790 ± 0.0000380.49 ± 0.120.892950 ± 0.000040−0.18 ± 0.04
62MA-080874Cornwall, Wheal Kitty Mine0.318832 ± 0.0000280.62 ± 0.090.892903 ± 0.000033−0.23 ± 0.04
63MA-080879Cornwall, Wheal Kitty Mine0.318644 ± 0.0000410.03 ± 0.130.893100 ± 0.000047−0.01 ± 0.05
64MA-080504Cornwall, Redruth Mine0.318780 ± 0.0000330.46 ± 0.100.892941 ± 0.000038−0.11 ± 0.04
65MA-080862Cornwall, Dolcoath Mine0.318862 ± 0.0000470.71 ± 0.150.892870 ± 0.000050−0.27 ± 0.06
66MA-080508Cornwall, St Agnes Mine0.319100 ± 0.0000351.46 ± 0.110.892670 ± 0.000038−0.49 ± 0.04
67MA-080510Cornwall, St Agnes Mine0.318769 ± 0.0000300.43 ± 0.090.892970 ± 0.000032−0.16 ± 0.04
68MA-080511Cornwall, St Agnes Mine0.318883 ± 0.0000400.77 ± 0.130.892852 ± 0.000045−0.29 ± 0.05
69MA-080507Cornwall, St Agnes Mine0.318844 ± 0.0000220.65 ± 0.070.892911 ± 0.000023−0.22 ± 0.03
70MA-081297Cornwall, Penderves Mine0.318852 ± 0.0000340.68 ± 0.110.892891 ± 0.000035−0.25 ± 0.04
71MA-080878Cornwall, Scarrier Mine0.318804 ± 0.0000250.52 ± 0.080.892959 ± 0.000028−0.17 ± 0.03
72MA-080509Cornwall, St Just Mine0.318828 ± 0.0000310.62 ± 0.100.892882 ± 0.000030−0.26 ± 0.03
73MA-080512Cornwall, St Just Mine0.318851 ± 0.0000280.68 ± 0.090.892861 ± 0.000033−0.28 ± 0.04
74MA-080880Cornwall, Wheal Martin Mine0.318630 ± 0.000047−0.01 ± 0.150.893130 ± 0.0000500.02 ± 0.06
75MA-080876Cornwall, Wheal Eliza Mine0.318130 ± 0.000032−1.58 ± 0.100.893622 ± 0.0000350.57 ± 0.04
76MA-081292Cornwall, Pentowan Mine0.318811 ± 0.0000370.55 ± 0.120.892970 ± 0.000040−0.16 ± 0.04
77MA-080872Cornwall, South Crofty Mine0.318710 ± 0.0000390.24 ± 0.120.892870 ± 0.000043−0.27 ± 0.05
78MA-080505Cornwall, Redruth Mine0.318720 ± 0.0000400.24 ± 0.130.893050 ± 0.000043−0.09 ± 0.05
79MA-080516Cornwall, Illogan Mine0.318711 ± 0.0000440.24 ± 0.140.893030 ± 0.000047−0.09 ± 0.05
80MA-080517Cornwall, Illogan Mine0.318690 ± 0.0000450.18 ± 0.140.893082 ± 0.000045−0.03 ± 0.05
Figure 3.

Isotope data of primary tin deposits in the Erzgebirge and the Vogtland in comparison with the associated placers. The figure also gives an impression of the homogeneity of a deposit (e.g., Ehrenfriedersdorf) and its distinction from other tin deposits (e.g., Cornwall, Botallack Mine).

Figure 4.

Isotope variations of tin in nature and a comparison with the isotope value of the ‘Himmelsscheibe von Nebra’. As can be seen, the value of the artefact fits well with the data of the ores from Cornwall and appears quite separate from those of the Erzgebirge.

Placers

Due to its high density of 6.8–7.1 g cm−3 as well as its remarkable resistance to weathering, cassiterite is a type of mineral that could be found in placers. In his book De natura fossilium, Agricola (1546) describes cassiterite grains of an ounce (about 27 g) or more as common finds. Large placers can be found in the Erzgebirge as well as in Cornwall. Due to its much easier minability in prehistoric times, the use of this secondary deposit type is more common than mining on primary cassiterite veins.

Tin placers can be formed by the weathering of different primary cassiterite occurrences and different ore types, respectively. Therefore it had to be tested whether the isotopic composition of cassiterite from the placer fits with the associated primary occurrence. The result of this test is shown in Figure 3; for the numerical data, see Table 4. The isotope composition of 11 primary ores from Erzgebirge-Ehrenfriedersdorf (dark circles) is shown in comparison to four placer samples (light circles) from the same location. It becomes clear that no significant differences occur between primary and secondary deposits. A similar conclusion can be drawn in the case of the tin deposits in the Vogtland (see the light squares in Fig. 3) and ores from the Auersberg-Erzgebirge (see the numerical data in Table 4). In the last two cases, it has to be considered that the placers are not clearly associated with all mentioned primary sources. However, the general assumption that no significant isotopic differences occur between placer tin and primary tin seems to be justified.

Homogeneity and distinction

The isotope homogeneity of a tin occurrence as well as the existence of significant differences with other tin deposits is the general supposition of a tin-isotope procedure for tracing the ancient tin. Figure 3 gives an illustration of the homogeneity of occurrences in the Vogtland (two samples from Mühlleiten and three from Gottesberg, plus one placer tin sample), the Erzgebirge (15 samples from Ehrenfriedersdorf, including placers) and Cornwall (six samples from the Botallack Mine). It is clearly shown that a differentiation between samples from different locations is possible. Please note that one value from the Botallack Mine samples (MA-080518) and one of the Mühlleiten samples (FG-050689) were not plotted because they are far away from the bulk.

An illustration of all results of isotope measurements is shown in Figure 4. The highest value for the ratio 122Sn/116Sn, at 0.31910, was measured for sample MA-080508 from the St Agnes Mine in Cornwall, while the lowest value for the same ratio, at 0.31813, was measured for sample MA-080876 from the West Wheal Eliza Mine in Cornwall (not plotted in Fig. 4: see Table 4). Between the two values there is a difference of 3.04‰ for 122Sn/116Sn, or 0.51‰ amu−1. This may be the largest isotope fractionation of tin in nature that has hitherto been observed. It demonstrates that relatively large isotope variations of tin in nature do exist. Furthermore, it may be speculated that even larger variations may be detected once many more samples have been measured. However, these two extreme values may be exceptions caused by a special composition of the ores (see Table 1).

There is no clear idea about the reasons for isotope differentiation of tin during the formation of the ore. It can be a result of prevalent temperatures and pressures or equilibrium fractionation between sulphide and oxide phases (McNaughton and Rosman 1991). In recent investigations (Polyakov et al. 2005), equilibrium isotope fractionation between di- and tetravalent tin compounds depending on the temperature was also found. However, our results show that local differences in the isotopic composition of tin do exist. Perhaps this effect is supported by different formation conditions at individual deposits.

In any case, 95% of the values can be found in a relatively small range of 0.31855–0.31900 for 122Sn/116Sn. Inside this range, differentiation between deposits from the Vogtland, the Erzgebirge and Cornwall is possible. The six cassiterite samples from the Vogtland (Fig. 4, dark squares) can be found between 0.31855 and 0.31865 for 122Sn/116Sn and bwtween 0.8930 and 0.8932 for 117Sn/119Sn, respectively. This means that these ores are enriched in light isotopes. Because of the fact that these ores are derived from three different occurrences (see Table 4), the homogeneity found is remarkable. Sample FG-050689 was not taken into account. A similar situation can be observed in the case of the ores from the Erzgebirge (Fig. 4, light circles). With the exception of the two samples from Graupen (Fig. 4, circles with a cross) and samples FG-050678 and FG-050677, about 85% of the values were found between 0.3186 and 0.3188 for 122Sn/116Sn and between 0.8929 to 0.89315 for 117Sn/119Sn, respectively (see Fig. 4, second ellipse from the left-hand margin). The importance of this result becomes clear if it is noted that ‘Erzgebirge’ means nine large deposits in this region. For the two samples from Graupen, which is an old tin district in the north-east Bohemian Erzgebirge, it is presently unclear whether it has to be considered as a group of its own. More measurements are necessary to answer this question.

The samples from Cornwall are derived from 17 old mining districts. Anyway, about 70% of the measurement data were located in the range of 0.31875–0.31895 for 122Sn/116Sn and 0.8927–0.8930 for 117Sn/119Sn (see Fig. 4, third ellipse from the left-hand margin). From this, the conclusion can be drawn that there is a probability of 70% that the tin that was mined in Cornwall can be found in this isotope window.

The tin of the ‘Himmelsscheibe von Nebra’

The ‘Himmelsscheibe von Nebra’ (Sky disc of Nebra), which is dated to 1600 bc, represents the earliest concrete image of the sky. The artefact was located in 1999 at the Mittelberg near the small town of Nebra, Sachsen-Anhalt, Germany. For detailed information about this famous artefact, see Meller (2004).The isotope composition of the ‘Himmelsscheibe von Nebra’ was measured three times on different days. The values found were 0.318885 ± 0.000040, 0.318839 ± 0.000027 and 0.318903 ± 0.000030 for 122Sn/116Sn, and 0.892860 ± 0.000039, 0.892910 ± 0.000034 and 0.892867 ± 0.000039 for 117Sn/119Sn, respectively. It can therefore be said that the measurements are of an excellent reproducibility. The mean isotope value of this famous artefact is plotted in Figure 4 (square with cross). It is seen that the Himmelsscheibe fits well with the ores from Cornwall. While more analyses and comparanda could perhaps refine the results somewhat, already at this stage it seems quite possible that the tin content of the Himmelsscheibe, which was measured as 2.5% (Pernicka and Wunderlich 2002) derives from Cornish ores rather than from the Erzgebirge or the Vogtland.

Fractionation during smelting

Due to the fact that cassiterite is rarely found in archaeological sites, tin metal was used with copper rather than cassiterite. However, a determination must be made regarding whether the smelting process can lead to significant changes in the isotope composition of tin. Investigations in this field were carried out by Gale (1997) and Yi et al. (1999). They heated different alloys of copper and tin up to 1200°C. The isotope composition of tin was measured before and after heating. No fractionation effect could be detected.

This study deals with very small isotope differences, so it was mandatory to exclude any fractionation during smelting. Therefore a bronze was heated up to 1200°C twice, for 30 and 60 min. Although there was an observable loss of tin, the experiment clearly shows that fractionation did not occur.

CONCLUSIONS

In this study, the isotope composition of 50 tin ores, determined by precise MC–ICP–MS measurements, was presented. Great attention was paid to the development of a practical protocol for the dissolution of cassiterite samples and purification of the measurement solutions. It could be shown that reduction by KCN and dissolution of the tin metal obtained with HCl followed by ion chromatography yields measurement solutions that can be used for obtaining precise and reproducible isotope values. To suppress the dominant memory effect, only rinsing with 1 M HCl could be recommended. Oxidizing agents, and also aspiration of solutions by microconcentric nebulizers such as the Cetac MCN 6000, should be avoided, because they lead to precipitation of SnO2.

However, with the achieved repeatability of 0.17‰ (2 s. d.) for the 122Sn/116Sn ratio of the PSn standard, the observed isotope variations in the range from −1.58‰ to 1.46‰ for the same ratio could be precisely determined. With the obtained isotope data, it is possible to distinguish between ores from the Vogtland, the Erzgebirge and Cornwall. Furthermore, the measured isotope composition of the tin isolated from the ‘Himmelsscheibe von Nebra’ allows one to suggest that this famous artefact was made with Cornish tin. It is thus possible that the new method of ‘tin isotopy’ might be a tool with which to solve the old question of the sources of ancient tin.

DEDICATION

Robin E. Clayton (1960–2005) was one of the pioneers in tin isotopy. He started his work in the mid-1990s, and had made great progress in the study of isotope ratios in tin both modern and ancient. His most untimely death in 2005 put an end to a brilliant career as an isotope geochemist. We who worked with him, and admired him both personally and professionally, wish to dedicate this paper to his memory. He is sorely missed.

ACKNOWLEDGEMENTS

This research was funded by the Deutsche Forschungsgemeinschaft Bonn (Project PE 405/18-1,2, as part of the research group FOR550 ‘Der Aufbruch zu neuen Horizonten. Die Funde von Nebra, Sachsen-Anhalt, und ihre Bedeutung für die Bronzezeit Europas’) and the Curt-Engelhorn-Zentrum Archäometrie, Mannheim. We would like to thank Karin Rank, from the mineralogical collection of the TU Bergakademie Freiberg, for providing sample material and Dr Harald Meller, from the Landesamt für Archäologie und Denkmalpflege Sachsen-Anhalt, for providing samples from the Himmelsscheibe.

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