Musings of a Political Scientist

Uranium-lead Dating

Uranium Lead Dating the Earth

Uranium-lead Dating

Uranium-lead dating computes the age of the earth at 4.54 billion years old with less than 1% margin of error.  It 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 precisions in the 0.1- 1 percent range. The method relies on the coupled chronometer provided by the decay of 238U to 206Pb, with a half-life of 4.46 billion years and 235U to 207Pb, with a half-life of 704 million years. One of the advantages of uranium-lead dating is the two separate, chemically identical chronometers and is accepted as the most reliable measurement of the age of the Earth.

Loss of Lead Discordance Important for Reliability

Loss (leakage) of lead within the sample will result in a discrepancy in the two decay schemes, resulting in a different age determined by each decay scheme. This effect is referred to as discordance, and provides a check on the reliability of the age. The presence of minerals or zones within minerals, older than the rock being dated can also cause 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.

Uranium-lead Dating Usually Performed on Zircon

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 Chemically Inert and Resistant to Weathering

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 with for example, an ion microprobe (SIMS) or laser ICP-MS.

Uranium Lead Dating Concordia Chart