Ores, lead isotopic analysis

Lead isotopic analysis (J, 2, 3, 4, 5) is a relatively new member of this expanding group of complementary methods. Like the others, it has its own peculiarities, advantages, and disadvantages which have been described. Lead isotope studies help to identify possible geographical origins of the ores from which ancient leads were smelted. Even where specific mines cannot be identified, the objects can still be classified as to which could or could not have had a common origin. The method requires only small sample sizes, and the results are independent of the histories of the objects studied. 

A.D. were found in old mines, and sizable slag deposits suggest at least some level of lead exploitation during Roman times (2). By the careful selection of Roman lead artifacts likely to have been manufactured in Carthage, and the use of lead isotope analysis, it may be possible to confirm the use of Tunisian lead ores during the Roman period. In this preliminary study, analysis of 22 curse tablets by electron microprobe analysis (EMPA) and six tablets by thermal ionization mass spectrometry (TIMS) suggest that these artifacts might be used to better understand Roman lead use and trade. 

To summarize the archaeological applications of lead isotope analysis, we note that despite the problems of ore body identification and metal recycling, it is becoming possible to build up the geographical details in the early history of metal making [28] for China [29] and for Bronze Age Europe, as well as later applications such as the mining of lead by the Romans in Germany [30]. 

Angelini, E., Atzeni, C., Bianco, P, Rosalbino, F, Virdis, PF. (1993) Lead isotope analysis of Nnragic bronzes and copper ores by ICP-MS. In Applications of Plasma Source Mass Spectrometry II, edited by Holland, G., Eaton, A.N. Cambridge, UK Royal Society of Chemistry, pp. 165-174. 

Habicht-Manche, J.A., Gleim, S.T., Schmidt, M.P, Franks, R., Milford, H., Flegal, A.R. (2002) Stable lead isotope analysis of Rio Grande glaze paints and ores nsing ICP-MS a comparison of acid dissolution and laser ablation techniques. Journal of Archaeological Science, 29,1043-1053. 

Stable lead isotope analysis of Rio Grande glaze paints and ores Laser type not indicated ICP-SF-MS Comparison of LA and solution-ICPMS for Pb isotopic analysis of glazes and ores Habicht-Mauche et al. [108]... 

Barnes, I. L., Shields, W. R. S., Murphy, T. J., and Brill, R. H. (1974). Isotopic analysis of Laurion lead ores. In Archaeological Chemistry, ed. Beck, C. W., Advances in Chemistry Series 138, Washington, DC, American Chemical Society, pp. 1-10. 

In the 15 years since Pintozzi s study, great improvements have been made in lead isotope studies. In particular, the instruments and techniques have greatly improved. Additionally, the foundation of a standardized archaeometry database (5) has greatly increased the required instrument precision required for acceptable sourcing of artifacts using the ore source data. As interesting as Pintozzi s results are, her method was not as precise as current TIMS analysis. 

In Figure 6, the curse tablet lead isotope ratios create an interesting pattern, with those tablets identified by EMPA analysis to contain the least number of inclusions plotting between two areas of Tunisian lead ores. The curse tablets identified as containing the greatest number of inclusions are those tablets plotting farthest away from the Tunisian lead ores. Additionally, the tablets appear to define a mixing line of ratios between two or more sources. 

LA-TOF-ICP-MS may be a useful tool for distinguishing different sources of lead found in low-lead, alkaline-based and lead-based glazes from Mesopotamia. Even with the limitations of the present study, lead-isotope ratios determined by TOF-ICP-MS indicate that multiple ore sources were utilized by makers of Islamic lead-glazed pottery. With the further refinements of Dudgeon et al. (this volume), LA-TOF-ICP-MS promises to be a rapid, cost-effective approach to lead-isotope ratio analysis of glazed pottery. 

A better representation of the three-dimensional lead isotope fingerprint can be made by using a multivariate discriminant analysis. Figure 3 shows the characteristic lead isotope composition of the same three ore deposits that was prepared by using multivariate discriminant analysis (Pollard, M., University of Cardiff, personal communication in 1986). It shows clearly, as the two-dimensional diagrams do not, that these fields may be resolved by using all three isotopic ratios. 

Certain ore deposits may partially overlap in their lead isotope composition. In such a case, it might not be possible to decide on the basis of the lead isotope data alone which of the ore deposits in question provided the ore for metal artifacts that have lead isotope compositions that fall into the overlapping space. Sometimes, one of these ore deposits can be ruled out by using trace element data, particularly from gold and silver analyses (6). Lead isotope analyses by themselves can make a negative statement with absolute certainty in a way which chemical analysis can never hope to do. If the lead isotope composition of an artifact falls well outside the lead isotope field characteristic of a particular ore deposit then it is certain that the metal... 

Figure 3. Stepwise discriminant analysis by M. Pollard of all three measured lead isotope ratios for ore samples from Cyprus, Kythnos, and Laurion ore...

In 1941, Nier used a mass spectrograph to measure the relative abundances of lead isotopes in samples of lead and uranium ores. He found that in the uraniiun ores, which also contained thorium, there were higher isotopic concentrations of Pb, Pb, and Pb than were found in lead ores not associated with uranium. The results of an analysis of Parry Sound uraninite are listed below. Each lead isotope is reported in terms of the amount in excess of that which would be expected from the natural lead content of the ore. 

The advent of laser ablation MC-ICP-MS technology allows the rapid in situ determination of the stable isotope ratios of heavy metals commonly found in sulfide ore deposits (e.g., Cu, Zn, Fe, Sb, Ag) providing important information on the source, transport, and depositional mechanisms of these metals. Pb, Pb, and Pb are formed as the end product of radioactive decay and the isotopic variability of lead results because the elements from which the isotopes form were not evenly distributed in ore bodies. Hence, the analysis of stable lead isotopes in annually laminated lake-sediments is a useful method to study lead pollution history as the relative contribution of pollution and natural lead in sediment samples can be calculated. The analysis of lead isotopes by SIMS has also been used to identify the geographical origin of bullets. 

Electrothermal vaporization has many appHcations relating to ICP analysis the abihty to vaporize solvents and to handle small soHd or viscous samples are particularly important. In addition, the measurement of the isotopic ratios of lead in various ores provides a unique identification of the ore and its source. Now that commercial systems are available, the technique will undoubtedly become popular. These techniques seem to have found favour, particularly in Japan. 

Lead curse tablets from Roman Carthage contain variable amounts of very small metallic inclusions. Electron microprobe analysis confirmed these metallic inclusions were bronze, brass, and a Sn-Sb alloy. This was interpreted as possible evidence of lead metal recycling. Six samples were chosen to represent a range of tablets containing the minimum to the maximum number of inclusions. Thermal ionization mass spectrometry of the Pb isotopes in the curse tablets appear to define a mixing line, with the tablets containing the least number of inclusions plotting closest to the Tunisian lead ore isotope ratios. 

At the time, Soddy was seeking evidence that lead from thorium ores had different atomic weights from normal lead. When Soddy announced the discovery of a sample of lead of atomic mass 207.74, he acknowledged the contribution of Hitchins for the separation and analysis work. Thus, Hitchins precise and accurate measurements on the atomic masses of lead from different sources were among the first evidence for the existence of isotopes.51 In addition, Hitchins took over the research on protactinium from Cranston when the latter was drafted for the First World War. 

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