It is normally assumed that in any small part of the Earth's crust and underlying mantle which, at the time of formation, contained primordial lead together with uranium and thorium, no radiogenic lead could have been present. With the passage of time atoms of radiogenic Pb, Pb, and Pb gradually replaced uranium and thorium atoms. If, at one instant in time, all the lead in the area under discussion was removed in solution and deposited as a lead ore, then this would preserve a record of the isotopic balance of lead at the time. Given that this ore mineral would not contain any uranium or thorium, it would be preserved as a unique point on the lead growth curve. Using the Holmes-Houtermans model, and plotting Pb: Pb against Pb: Pb, a series of growth curves would be obtained based on the different isotopic ratios.
The above diagram from Patterson represented a major breakthrough in the use of lead isochrons when it was published in It presented the analysis of three stony meteorites and two iron meteorites and showed that they fell on the same isochron.
In addition, it showed that a sample of modern ocean sediment fell on the same isochron. Besides offering confirmation of the meteorite age that had been approached in many studies, it offered evidence that meteorites and the Earth are closely related and of the same age.
DefinitionUranium-Lead dating is the geological age-determination method that uses the radioactive decay of uranium (U) isotopes (U, U, and also in this entry Th) into stable isotopes of. uranium-lead dating A radiometric dating technique that uses the decay of U and U, which are present in all naturally occurring uranium in the ratio and both of which decay to lead in stages that involve 14 steps, but are different for each isotope. U (half-life Ma) decays to Pb; U (half-life Ma) decays to Pb. Lead-lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite.
Like most modern lead age studies, it used the troilite mineral from the Canyon Diablo meteorite as the standard. The isochron age was reported by Patterson as 4.
This is termed the lead-lead dating method. Clair Cameron Pattersonan 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.
Although zircon ZrSiO 4 is most commonly used, other minerals such as monazite see: monazite geochronologytitaniteand baddeleyite can also be used. 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 Thexpelling 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.
But if asked what is the most reliable and precise method for dating the Earth and meteorites, Brent Dalyrymple would point to lead isochrons. He calls the lead method "the hourglass of the solar system". The current approach to plotting Pb-Pb isochrons is referred to as the Holmes-Houtermans method. concentrate or deplete uranium or thorium relative to lead would have an influence on the radiometric ages computed by uranium-lead or thorium-lead dating. Also, the fact that there are two kids of magma could mean that the various radiometric ages are obtained by mixing of. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.
These fission tracks act as conduits deep within the crystal, 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.
lead-lead dating A dating method based on the comparison of isotopes of lead (Pb). Uranium-lead dating, abbreviated U-Pb dating, is one of the oldestand most refined of the radiometric datingschemes. It can be used to date rocks that formed and crystallised from about 1 million years to over billion years ago with routine precisions in the -1 percent range. The method is usually applied to zircon. In uranium-lead (U-Pb) dating of zircon, the zircon is found to exclude initial lead almost completely. Minerals too are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered.
That is. 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 crystaland 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.
From Wikipedia, the free encyclopedia. Type of radiometric dating. American Journal of Science. Bibcode : AmJS Retrieved 7 January Chemical Geology.
Bibcode : ChGeo. Pb geochronology: Effective sample size matters".
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Contributions to Mineralogy and Petrology. Bibcode : CoMP.
Radiogenic Isotope Geology.