The U-Th-Pb system

Uranium has two long-lived isotopes, 235U and 238U, that decay to 207Pb and 206Pb, respectively. Thorium has one long-lived isotope, Th, that decays to Pb. The isotopic abundances of uranium and thorium are summarized in Table 8.5. The isotopic abundances of terrestrial lead are given in Table 4.2. 

The principal decay mode of 238U is -decay, but a small fraction of the decays are by spontaneous fission. The emission of an a-particle initiates a series of decays known as the uranium series (Fig. 8.14), which ends at 206Pb. The uranium series can be summarized as 

Isotope Abundance Decay mode Half-life Decay constant 

The decay path splits at several points where decay can be either by a- or P-decay, but all 

The decay of 235U is also primarily by -decay through the actinium series (Fig. 8.15). 

---

Pb from h. The Pb systems are discussed last because of the complexities of the U-Th-Pb system in several aspects. One is that each of the nuclides and h undergoes a long chain of decay to the final stable nuclide decays... 

In contrast to refractory uranium and thorium, lead is a moderately volatile element. Uranium and thorium are lithophile, while lead can exhibit lithophile, siderophile, or chalcophile behavior. This means that in many cosmochemical situations, it is possible to strongly fractionate the daughter lead from parent uranium and thorium, a favorable situation for radiochronology. On the other hand, lead tends to be mobile at relatively low temperatures and can be either lost from a system or introduced at a later time. As already mentioned, uranium can also become mobile under oxidizing conditions. This means that the U-Th-Pb system is more susceptible to open-system behavior than several other commonly used dating techniques. However, as we discuss below, there are ways to recognize and account for the open-system behavior in many cases. 

Figure 1. Concordia diagram illustrating misbehavior in the U-Th-Pb system that is relevant to U-Pb dating. Arrows show how concordant data can be affected by inheritance, Pb loss, disequilibrium, and common Pb (Pb°) corrections.

ICP-MS Equipment The U/Th-Pb system is one of the few geochronological systems where the use of single-collector ICP-MS (see also Chapter 2) gives adequate results. The use of quadrupole-based and sector field single- and multicollector ICP-MS equipment of different makes is reported in the literature. 

General. Three independent ages may be obtained in the U-Th-Pb system or and... 

This method has been most widely used in (i) the °K- °Ar system, (ii) the U-Th-He system, (iii) the U-Pb dating of zircon, and (iv) the U-Th-Pb dating of monazite using electron microprobe measurements. The last is a developing method with large errors because of the low analytical precision and because isotopes are not measured. 

The U-Th-Pb isotope systems have half-lives ranging from 0.7 to 4.6 Ga, and can therefore be used to investigate variations in time-integrated U/Pb (pi) and Th/U (ki) ratios, and to constrain models for the evolution of the mantle and crust from early Archaean time to the present (Sinha Tilton 1973 Stacey Kramers 1977 Kramers Tolstikhin 1997). The Pb isotope compilation and model of Kramers Tolstikhin (1997) highlight the range in the initial Pb/ Pb ratios (and thus in time-integrated pi) in suites of Archaean to early Proterozoic rocks. Inferred differences between high-pi lower crust and... 

The transport balance model of Kramers and Tolstikhin (1997) for the U-Th-Pb isotope system is particularly sensitive to the extent to which crustal material can be recycled into the mantle and offers some constraints on the mechanism of crustal recycling. They concluded that the rate of recycling of crustal material back into the Earth s mantle has increased with time, particularly since 2.0 Ga. We know from Pb-isotope studies that some sediment recycling took place during the Archaean (Halla, 2005), but it would seem as though the proportion was small. 

The actinides, Th and U, also decay by a decay en route to the stable Pb isotopes. For each actinide nucleus, several a particles (" e nuclei) are produced. However, the same problems that make it difficult to use U,Th-He dating in terrestrial studies (Farley 2002, this volume), the recoil upon creation and the extremely low temperature at which it is lost, make it difficult to apply to meteorites as well. So the K-Ar system, particularly the " Ar- Ar version, is used far more often than is the U-Th-He system, and hence will be mentioned far more often in applications. 

Figure 4. Correlation of I-Xe ages with petrographic type for phosphates and feldspars from ordinary chondrites. Data are from Brazzle et al. (1999). Note that type 6 (more equil-ibrated) chondrites have later ages than type 4 chondrites. The very old age for one of the type 5 chondrites may only be apparent, an artifact of a shock event (Hohenberg et al. 2000). The absolute ages are calibrated by assuming the I-Xe and U-Th-Pb systems for Acapulco phosphates reflect the same time.

Figure 5. Summary of a variety of ages in the early solar system, as determined by I-Xe, Al-Mg, Mn-Cr and Pb-Pb. Absolute I-Xe are generated by assuming Acapulco phosphate (near bottom of figure) became closed to the I-Xe and U-Th-Pb systems at the same time, although an alternative calibration point is to assume that Ste. Marguerite phosphates are the same age in both the I-Xe and...

Lancelot, J. R. Allegre, C. J. (1974). Origin of carbonatitic magmas in the light of Pb-U-Th isotope system. Earth Planet. Sci. Letters, 22, 233-8. 

Silver, L. T. U-Th-Pb Isotope systems in Apollo 11 and 12 regolithic materials and a possible age for the Copernicus impact event. Paper presented at Amer. Geo-phys. Union 52nd Ann. Meeting. Washington, D. C. 1971. 

---