The U–Pb decay scheme is the most widely applied geologically robust geochronometer. Due to its unique attribute of containing two independent decay chains, starting from different parent isotopes and finishing with different daughter isotopes of the same elements, it is possible to cross-correlate the data obtained simultaneously to produce routine, high-precision (often ~0.1 per cent) geochronological data on ancient samples. In addition, U-series disequilibria can be employed to investigate very young processes; however, the principles behind these techniques are quite different and relevant to only a smaller number of mineralised settings, and hence are introduced separately below. Finally, the spontaneous fission products of 235U produce radiation damage trails in crystalline materials, such as apatite and zircon, and form the basis of fission track analysis (Section 8.4)

The utility and precision of U–Pb stems from a number of significant factors:

1. • The half lives of both the key decay schemes (235U to 207Pb = ~704 Myr and 238U to 206Pb = ~4.47 Gyr) are short enough to have been calibrated to high precision by direct observation measurements.

2. • Although each scheme decays via a series of short-lived intermediaries (238U undergoes eight α and six β decays, 235U seven α and four β decays) to reach their respective daughter products, provided closed system behaviour is maintained, each decay from U to Pb can be considered to take place geologically instantly (Figure 8.1).

3. • The existence of the ‘parallel’ schemes allows coupled ages to be calculated using the concordia method (see below), with a resulting inherent measure of closed system behaviour and increased precision due to cancellation of errors due to simultaneous acquisition of ages.

4. • Due to the significantly different compatibility of U and Pb, there is a wide range of common rock-forming minerals which will incorporate U (and Th) in their lattice and exclude Pb – notably zircon, monazite, titanite and baddeleyite.

5. • Geological understanding of U–Pb systematics are quite mature, with over 100 years worth of investigations into the applicability for geochronology commencing with Rutherford and Holmes in the first quarter of the twentieth century.

U–Pb isotopes can be applied to obtain high-temperature, ancient (>0.5 Ma) ages via one of three techniques, depending on the material being analysed and the problem being investigated:

1. (1) Either decay scheme (generally 238U to 206Pb, but 235U to 207Pb and even 232Th to 208Pb) can be used to produce isochrons (see Chapter 7).