UraniumLeaddatering

Uranium–lead dating, commonly abbreviated as U-Pb dating, is a radiometric dating method used to determine the ages of rocks and minerals by measuring the relative abundances of uranium and lead isotopes. It relies on two independent decay chains: 238U decays to 206Pb with a half-life of about 4.468 billion years, and 235U decays to 207Pb with a half-life of about 0.704 billion years. Because both decay chains originate at crystallization, the resulting lead isotopes provide a robust clock for age determination.

The method is widely applied to zircon crystals because zircon incorporates uranium when it forms but typically excludes lead, making it highly resistant to chemical and physical changes. Other minerals used include monazite and titanite. U-Pb dating can yield ages from roughly 1 million to over 4.5 billion years, depending on the mineral, its history, and the quality of the sample.

Measurements are made with mass spectrometry techniques such as TIMS, SIMS, or laser ablation ICP-MS. Ages are derived from the ratios 206Pb/238U and 207Pb/235U, or via the concordia method, which uses both decay schemes simultaneously. Lead that is non-radiogenic (initial or common lead) and lead loss during metamorphism or alteration can cause discordant results, which are interpreted using discordia diagrams or correction models.

Applications of U-Pb dating include constraining the timing of crust formation, magmatic differentiation, and metamorphic or planetary events. Limitations include potential lead loss, lead inheritance from older components, and assumptions about the initial lead isotope composition. When applied carefully, U-Pb dating provides some of the most reliable radiometric ages in geology.