Lead and oxygen isotopes in ancient objects

Abstract

During the past few years most archaeologists and museum curators have become familiar with the term isotope . Isotopes are atoms of the same chemical element which differ slightly from one another in mass. For instance, there are four stable isotopes of lead, 204 Pb, 206 Pb, 207 Pb and 208 Pb, having, respectively, approximate masses of 204, 206, 207 and 208 atomic mass units; and there are three isotopes of oxygen, 16 O, 17 O and 18 O, having masses of 16, 17 and 18. The isotopic composition of most elements is uniform throughout nature. That is to say, occurrences of a particular element in different places or in different chemical forms usually contain the same relative proportions of that element’s isotopes. There are, however, a few notable exceptions, and wherever they occur they are invariably of interest to the scientist. The example most familiar to the archaeologist is the variation in the proportion of 14 C in the carbon present in archaeological objects. This variation constitutes the basis of radiocarbon dating, the discovery and remarkable advances of which are being commemorated at this Symposium. In this paper we shall deal with two other elements, lead and oxygen, whose isotopic compositions vary in natural occurrences due to rather well defined mechanisms. Unlike the 12 C— 14 C relationship, which involves the decay of a radioactive species and leads directly to a dating method, the research reported here with lead and oxygen does not involve radioactive decay in the same way, and neither investigation at this stage is aimed towards developing a direct dating method for archaeological finds. With lead, the isotopic variations observed are associated with differences in the geological ages and geochemical origins of lead deposits. (The differences arise because three of the four lead isotopes are continually produced within the earth by radioactive decay of uranium and thorium.) In the case of oxygen, physical and chemical fractionation processes are responsible for the isotopic variations.