Uranium-lead dating

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.^[1] The method relies on the coupled chronometer provided by the decay of ²³⁸U to ²⁰⁶Pb, with a half-life of 4.47 billion years and ²³⁵U to ²⁰⁷Pb, with a half-life of 704 million years. This decay occurs through a series of alpha decays, of which ²³⁸ U undergoes seven total alpha decays whereas ²³⁵U only experiences six alpha decays. ^[2]

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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. Therefore we can assume that the entire lead content of the zircon is radiogenic. Where this is not the case, a correction must be applied.

During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas.^[3] Fission tracks and micro cracks within the crystal will further extend this radiation damage network. These inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of Pb isotopes from the zircon crystal. ^[4]

Under conditions where the system has remained closed, and therefore no lead loss has occurred, the age of the zircon can be calculated independently from the two equations:

²⁰⁶Pb / ²³⁸U = e ^λ238t– 1

And

²⁰⁷Pb / ²³⁵U = e ^λ235t– 1,

These are said to yield concordant ages. It is these concordant ages, plotted over a series of time intervals, that result in the concordant line.^[5]

Loss (leakage) of lead from the sample will result in a discrepancy in the ages determined by each decay scheme. This effect is referred to as discordance and is demonstrated in Fig.1. If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line. The upper intercept of the Concordia and the Discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages.^[6]

Fig. 1: Concordia Diagram for data published Mattinson ^[7] for zircon samples from Klamath Mountains in Northern California. Ages for the Concordia increase in increments of 100 million years.

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), and thus are said to demonstrate inherited characteristics. 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.