207Pb

207Pb is a stable isotope of lead (Pb) with mass number 207 and atomic number 82. It is one of the four stable isotopes of lead, the others being 204Pb, 206Pb, and 208Pb. In nature, 207Pb occurs as a radiogenic daughter product of the decay of 235U, and it accumulates with time in rocks and minerals. Because 235U decays to 207Pb with a known decay constant, the 207Pb/235U (and 207Pb/206Pb) ratios can be used to determine the age of geological materials, a method known as uranium–lead dating.

U–Pb dating is widely applied to metamorphic and igneous rocks and is especially valuable for dating very old materials, including meteorites and the early Earth. In practice, researchers measure both the 206Pb/238U and 207Pb/235U decay pairs, and plot concordia diagrams to identify ages and potential disturbance of the system. 207Pb is stable, so once formed it remains unchanged; the isotope provides a robust chronometer when the rock has remained closed to uranium and lead mobility.

Analytical techniques for determining 207Pb/206Pb ratios include thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), often after chemical separation of lead from the sample. Beyond dating, the isotopic composition of lead, including 207Pb, is used in geochemistry and planetary science to study crustal evolution and the timing of solidification in the solar system.