207Pb235U

207Pb235U refers to the radiogenic relation between lead-207 and uranium-235 used in uranium–lead dating. In this system, uranium-235 decays to lead-207 with a half-life of about 703.8 million years. The decay proceeds through a chain that includes seven alpha decays and four beta decays, ultimately producing stable lead-207. As 235U atoms decay over time, 207Pb accumulates in the mineral lattice, providing a clock that records elapsed geological time.

Uranium–lead dating relies on measuring the current abundances of 235U and 207Pb (and often 238U and 206Pb as well) to calculate age. The 207Pb/235U ratio and the 206Pb/238U ratio provide two independent age assessments for the same sample, enabling cross-checks and increases in reliability. The method is particularly valuable for dating ancient rocks and earth’s earliest crust, with ages spanning from millions to well over four billion years. Zircon and other minerals that incorporate uranium but exclude initial lead when they crystallize are commonly used, and measurements are typically performed by mass spectrometry or laser ablation techniques. Concordia diagrams are used to interpret the data and to identify closed-system behavior or lead loss.

Limitations include the potential for lead loss, metamorphic disturbance, or an initial lead component that must be accounted for. Robust ages often require concordant results from multiple isotopic systems or minerals. Overall, the 207Pb235U system remains a foundational tool in geochronology, contributing to the dating of rocks, meteorites, and the timing of planetary differentiation.