Minerals used in isotopic dating

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All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus.Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus.A particular isotope of a particular element is called a nuclide. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide.This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles) and beta decay (electron emission, positron emission, or electron capture).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.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.

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.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.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.

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