Stony meteorites however, exhibited very high Pb ratios, indicating that these samples came from the crust or mantle of the planetesimal.
Together, these samples define an isochron, whose slope gives the age of meteorites as 4.55 Byr.
) is most commonly used, other minerals such as monazite (see: monazite geochronology), titanite, and baddeleyite can also be used.
By dating meteorites Patterson was directly dating the age of various planetesimals.
Assuming the process of elemental differentiation is identical on Earth as it is on other planets, the core of these planetesimals would be depleted of uranium and thorium, while the crust and mantle would contain higher U/Pb ratios.
As a result, newly-formed zircon deposits will contain no lead, meaning that any lead found in the mineral is radiogenic.
Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age.
These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record.
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.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.If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line.