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How does uranium-lead dating determine the age of zircon crystals?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

Uranium-lead dating determines the age of zircon crystals by measuring the decay of radioactive uranium isotopes into stable lead isotopes. Zircon is ideal for this process because its crystal structure naturally incorporates uranium while excluding lead during formation, effectively starting the "clock" with an empty box.12

The method relies on two primary decay systems:
* $^{238} U$ decays to $^{206} Pb$
* $^{235} U$ decays to $^{207} Pb$

Because zircon is highly resistant to chemical and physical changes, it retains these isotopes over geological time. Geochronologists analyze the ratio of lead to uranium in the crystal to determine its age. By plotting these ratios on a concordia diagram, scientists can identify the timing of the primary crystallization event and any subsequent reheating or secondary events.1

Modern techniques, such as sensitive high-resolution ion microprobe (SHRIMP) or laser ablation mass spectrometry, allow researchers to date specific spots within a single grain, providing precise insights into the geological history of the rock.3