Uranium-lead dating
Uranium-lead dating is the most precise and versatile method of radiometric dating, applied primarily to the mineral zircon. The method exploits the two independent decay chains of uranium-238 and uranium-235, which decay to lead-206 and lead-207 respectively. Because the two parent isotopes have different half-lives (4.468 billion years for U-238 and 704 million years for U-235), the ratio of the two lead isotopes increases predictably with time, providing an internal consistency check that few other dating methods can match.
The power of U-Pb dating lies in the mineral zircon's geochemical behavior: it strongly incorporates uranium but almost entirely excludes lead during crystallization. This means the lead measured in a zircon crystal is overwhelmingly radiogenic, minimizing the correction required for initial lead. The result is a method capable of dating rocks from the early Earth — the oldest known terrestrial material, a 4.4-billion-year-old zircon from the Jack Hills of Western Australia, was dated by U-Pb.
Uranium-lead dating is not merely precise. It is structurally redundant: two decay chains converging on the same conclusion. This redundancy is the feature that makes it trustworthy, not the precision of any single measurement.