Geochronologists are real detectives able to unravel the age of minerals and rocks on Earth. One of the widespread methods within geochronology is the radiometric dating technique based on the radioactive decay of Uranium U into Lead Pb. With this technique, geochronologists can date rocks of million to billions of years old.
It works like a clock that starts ticking as soon as the rock is formed.
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Rocks often contain traces of the element uranium and some of the uranium U decays to lead Pb. During the life of a rock, the amount of uranium decreases and the amount of lead increases.
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Young rocks have very high amounts of uranium and low amounts of lead content, whereas very old rocks have very little uranium and high lead amounts. Since the half-life is known and one can measure the uranium and lead contents in the rock, one can calculate the age of a rock. As rocks contain of various minerals, geochronologists need to select the minerals that contain the most uranium.
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One of the mostly dated minerals is zircon ZrSiO 4. In order to get the age of the rock with precisions better than 0. This is done by crushing the rock and separating the zircon crystals. Those get dissolved by chemical dissolution, followed by chemical separation procedure to separate the uranium from the lead.
Do you tell your age? – High-precision U–Pb dating
Clair Cameron Patterson , an American geochemist who pioneered studies of uranium—lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth. Although zircon ZrSiO 4 is most commonly used, other minerals such as monazite see: Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied. 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.
These fission tracks inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal. Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium.
These are said to yield concordant ages. 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. 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.
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.
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The disintegration products of uranium: American Journal of Science Retrieved 7 January Alpha-recoil in U-Pb geochronology: Effective sample size matters. Contributions to Mineralogy and Petrology , 4: Combined annealing and multi-step dissolution analysis for Improved precision and accuracy of zircon ages. Radiogenic Isotope Geology 2nd ed. Canon of Kings Lists of kings Limmu.