uraniumlead

Uranium-lead dating, often abbreviated as U-Pb dating, is a geochronological method that determines the age of rocks and minerals by measuring the radioactive decay of uranium isotopes to lead. The method relies on two decay chains: uranium-238 decays to lead-206 with a half-life of about 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of about 0.704 billion years. Because both processes operate in parallel, U-Pb dating can provide robust ages over a wide timescale, particularly for ancient rocks.

The technique is commonly applied to minerals that incorporate uranium but exclude initial lead, most notably zircon. Other minerals include monazite and titanite. Precise isotope ratios are measured using instruments such as thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Samples are prepared to separate uranium and lead for accurate ratio determinations. Initial lead that is present at formation is addressed through methods that use the two decay schemes and by analyzing concordia diagrams.

In a concordia diagram, the ratio 206Pb/238U is plotted against 207Pb/235U. If the system remained closed since formation, data lie on a single concordia curve, providing a precise age. Lead loss or later disturbance yields discordant data forming a line (discordia) that intersects the concordia curve at two points, giving minimum and maximum bounds for the formation age.

U-Pb dating is among the most reliable tools for dating early Earth materials and other ancient rocks, with typical uncertainties on the order of a few million years for younger samples and sub-percentage precision for very old specimens.