Lead isotopes primordial composition

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

The geochron The geochron represents an isochron for f = 0 drawn through the composition of the appropriate isotope ratio at the time of the formation of the Earth — the primordial composition. In principle a geochron can be defined for any Isotopic system although in practice it is most commonly used in the interpretation of lead... 

The common-lead method looks at the isotopic evolution of lead in systems with U/Pb and Th/Pb ratios similar to or less than the ratios in bulk solar system materials. The original formulation, by Holmes and Houtermans, is a single stage model that accounts for the isotopic composition of any sample of common lead in terms of primordial lead plus radiogenic lead produced in the source up to the time that lead was separated from uranium and thorium. Multistage models that more accurately describe the evolution of natural systems have been developed. The common-lead method is used in cosmochemistry primarily to study the time of differentiation and reservoir evolution in differentiated bodies... 

Equation (8.47), with t = 0 and the composition of lead from meteoritic troilite used for the initial isotopic ratio of lead, was used by Clair Patterson (1955,1956) to determine the age of the Earth. In the 1950s, the largest uncertainty in determining the age of the Earth was the composition of primordial lead. In 1953, Patterson solved this problem by using state-of-the-art analytical techniques to measure the composition of lead from troilite (FeS) in iron meteorites. Troilite has an extremely low U/Pb ratio because uranium was separated from the lead in troilite at near the time of solar-system formation. Patterson (1955) then measured the composition of lead from stony meteorites. In 1956, he demonstrated that the data from stony meteorites, iron meteorites, and terrestrial oceanic sediments all fell on the same isochron (Fig. 8.20). He interpreted the isochron age (4.55+0.07 Ga) as the age of the Earth and of the meteorites. The value for the age of the Earth has remained essentially unchanged since Patterson s determination, although the age of the solar system has been pushed back by —20 Myr. 

The isotopic composition of lead in some meteorites is given in Table 13 and plotted in Fig. 12. The selection of meteorites is taken from the evaluation of Murthy and Patterson (1962). Also given are the newly recommended values determined by Oversby (1970) where the ratios have been corrected for mass spectrometer bias, and the attempt has been made to eliminate samples that have suffered terrestrial lead contamination. Stacey et al. (1969) have pointed out that there is little change in derived values such as age of the earth or primary 2 8u/204pb if all samples are in absolute ratios. Care should therefore be taken to make sure that all samples are in absolute ratios when using the values of primordial lead recommended by Oversby (1970). Other papers of interest are by Starik et al. (1962a), Sobotovich et al. (1964), and Marshall (1968). 

Age of Earth. All leads from terrestrial rocks except those from uranium and thorium ores are enclosed in the cross-hatched areas of Fig. 12. If the earth and meteorites were totally unrelated in time of origin or in the isotopic composition of primordial lead, there is no reason why terrestrial lead should cluster about the primary isochron for meteorites. The proximity of terrestrial leads to the primary meteorite isochron has been used by Patterson (Patterson et al, 1955 Patterson, 1956 Murthy and Patterson, 1962) in estimating the age of the earth as well as the isotopic composition of terrestrial primordial lead. Little change has been made in the estimate since Patterson s first work nevertheless, detailed studies on terrestrial materials do indicate that the earth may be as much as 200-m.y.-older (Tilton and Steiger, 1965, 1969) however, if the earth is even that much older, either there must have also been differentiation of U relative to lead in the early history (Patterson and Tatsumoto, 1964) or the values of primordial lead for the earth and for meteorites must differ (Doe et al, 1965). 

---