Minerals used in isotopic dating
Undamaged zircon retains the lead generated by radioactive decay of uranium and thorium up to 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.
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 Figure 1.
and most refined of the radiometric dating schemes.
It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range. This mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead when forming.
Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. 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.
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.
Although zircon (Zr Si O) is most commonly used, other minerals such as monazite (see: monazite geochronology), titanite, and baddeleyite can also be used.
Where crystals such as zircon with uranium and thorium inclusions do not occur, uranium-lead dating techniques have also been applied to other minerals such as calcite/aragonite and other carbonate minerals.
Search for minerals used in isotopic dating:
If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line.