Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
Basics of Uranium-Lead
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U). Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb). The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb. What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay). The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1). In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth. With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same. If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column).
Zircon in Uranium-Lead Dating
The favorite mineral among U-Pb daters is zircon (ZrSiO4), for several good reasons.
First, its chemical structure likes uranium and hates lead. Uranium easily substitutes for zirconium while lead is strongly excluded. This means the clock is truly set at zero when zircon forms.
Second, zircon has a high trapping temperature of 900°C. Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.
Third, zircon is widespread in igneous rocks as a primary mineral. This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.
Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.
Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite. However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."
But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool.
Concordia and Discordia
Consider the concordia: as zircons age, they move outward along the curve. But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram. The straight line takes the zircons off the concordia.
This is where data from many zircons is important. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched. The results from these zircons therefore plot along that straight line, establishing what is called a discordia.
Now consider the discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance. This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.
The oldest zircon yet found dates from 4.4 billion years ago. With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's "Earliest Piece of the Earth" page, including the 2001 paper in Nature that announced the record-setting date.