Uranium–lead dating

This mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead when forming.

[4] The existence of two 'parallel' uranium–lead decay routes (238U to 206Pb and 235U to 207Pb) leads to multiple feasible dating techniques within the overall U–Pb system.

Clair Cameron Patterson, an American geochemist who pioneered studies of uranium–lead radiometric dating methods, used it to obtain one of the earliest estimates of the age of the Earth in 1956 to be 4.550Gy ± 70My; a figure that has remained largely unchallenged since.

[5] Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of uranium.

[6] Loss (leakage) of lead from the sample will result in a discrepancy in the ages determined by each decay scheme.

Zircon is very chemically inert and resistant to mechanical weathering – a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium–lead age intact.

Unraveling such complexities (which can also exist within other minerals, depending on their maximum lead-retention temperature) generally requires in situ micro-beam analysis using, for example, ion microprobe (SIMS), or laser ICP-MS.