Uranium-lead dating
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Uranium-lead is one of the oldest and most refined radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1- 1 percent range.
Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it can be used on other minerals such as monazite, titanite, and baddeleyite. Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Undamaged zircon retains the lead generated by radioactive decay of uranium and thorium until very high temperatures (about 900 °C), though accumulated radiation damage within zones of very high uranium can lower this temperature substantially. 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. Zircon crystals with prolonged and complex histories can thus contain zones of dramatically different ages (usually, with the oldest and youngest zones forming the core and rim, respectively, of the crystal). Unraveling such complications (which, depending on their maximum lead-retention temperature, can also exist within other minerals) generally requires in situ micro-beam analysis via, say, ion microprobe (SIMS) or laser ICP-MS.
One of the great advantages of uranium-lead dating is that any sample provides two clocks, one based on the decay of uranium-235 to lead-207 with a half-life of about 700 million years, and one based on the decay of uranium-238 to lead-206 with a half-life of about 4.5 billion years. If the sample is old enough – more than about 400 million years – significant loss (leakage) by the sample of its uranium-generated lead will cause discordance between the two uranium-lead ages. The presence of minerals (or zones within minerals) older than the rock being dated can also cause such age-discordance. In either case, the geochronologist is warned that such uranium-lead ages cannot be taken at face value. When such discordant ages are encountered,. a suite of several samples must be analyzed, and one of several mathematical methods (depending on the nature and complexity of the age discordance) applied to arrive at a reliable age-estimate.
