UPb

UPb dating, or uranium-lead dating, is a radiometric method used to determine the ages of rocks and minerals by measuring the decay of uranium isotopes to stable lead isotopes. The method exploits two independent decay chains: 238U decays to 206Pb with a half-life of about 4.47 billion years, and 235U decays to 207Pb with a half-life of about 704 million years. Because these isotopes decay at known rates, the ratios of lead to uranium provide an elapsed time since the mineral closed to U and Pb mobility, typically at crystallization.

The technique is widely applied to minerals that incorporate uranium but exclude lead at formation, most notably zircon, as well as monazite and baddeleyite. Zircon is favored due to relatively high uranium content, chemical durability, and a high closure temperature for Pb loss, which helps preserve a reliable record of the formation age. In practice, measurements must correct for initial or common lead using available isotopic information, such as 204Pb, and for potential Pb loss or inheritance.

Analytical approaches include thermal ionization mass spectrometry (TIMS), isotope-dilution TIMS (CA-TIMS), SIMS (secondary ion mass spectrometry), and laser ablation ICP-MS (LA-ICP-MS). Each method has trade-offs in precision, spatial resolution, and sample preparation. Data are commonly interpreted with the concordia diagram, where ages plot along a curve defined by the two decay schemes. If a system remains closed, measurements lie on the concordia curve; lead loss or disturbance yields discordia lines from which ages can be inferred.

UPb dating is central to geological timescale construction, crustal growth studies, and planetary differentiation analyses, and it remains a cornerstone technique in geochronology due to its robustness and cross-checking potential between decay systems.