Uranium-lead Dating - Details of The Process

Details of The Process

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:

(1)

and

(2)

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.

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.

Fig. 1: Concordia Diagram for data published by Mattinson 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.