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| '''Uranium–lead (U–Pb) dating''' is one of the oldest<ref name="Boltwood">Boltwood, B.B., 1907, On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium: American Journal of Science 23: 77-88.</ref> and most refined of the [[radiometric dating]] schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1–1 percent range.<ref>Parrish, Randall R.; Noble, Stephen R., 2003. Zircon U-Th-Pb Geochronology by Isotope Dilution – Thermal Ionization Mass Spectrometry (ID-TIMS). In Zircon (eds. J. Hanchar and P. Hoskin). Reviews in Mineralogy and Geochemistry, Mineralogical Society of America. 183-213.</ref>
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| The uranium–lead dating method relies on two separate [[decay chain]]s, the [[uranium series]] from <sup>238</sup>U to <sup>206</sup>Pb, with a half-life of 4.47 billion years and the [[actinium series]] from <sup>235</sup>U to <sup>207</sup>Pb, with a half-life of 704 million years.
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| These [[uranium]] to [[lead]] decay routes occur via a series of [[Alpha decay|alpha]] (and [[Beta decay|beta]]) decays, in which <sup>238</sup>U with daughter [[nuclide]]s undergo eight total alpha and six beta decays whereas <sup>235</sup>U with daughters only experience seven alpha and four beta decays.<ref name="Romer">Romer, R.L. 2003. Alpha-recoil in U-Pb geochronology: Effective sample size matters. Contributions to Mineralogy and Petrology 145, (4): 481-491.</ref>
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| The existence of two 'parallel' uranium-lead decay routes (<sup>238</sup>U to <sup>206</sup>Pb and <sup>235</sup>U to <sup>207</sup>Pb) leads to multiple dating techniques within the overall U–Pb system. The term ''U–Pb dating'' normally implies the coupled use of both decay schemes in the 'concordia diagram' (see below).
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| However, use of a single decay scheme (usually <sup>238</sup>U to <sup>206</sup>Pb) leads to the U–Pb isochron dating method, analogous to the [[rubidium-strontium dating]] method.
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| Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. This is termed the [[lead-lead dating]] method. [[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 accurate estimates of the [[age of the Earth]].
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| ==Mineralogy==
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| Uranium-lead dating is usually performed on the mineral [[zircon]] (ZrSiO<sub>4</sub>), though it can be used on other minerals such as [[monazite]], [[titanite]], and [[baddeleyite]].
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| The [[zircon]] mineral incorporates [[uranium]] and [[thorium]] [[atom]]s into its crystalline structure, but strongly rejects [[lead]]. Therefore we can assume that the entire lead content of the zircon is [[radiogenic]].
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| Where zircon like uranium and thorium inclusions are not the case, a better, more inclusive, model of the data must be applied. Uranium-lead dating techniques has also been applied to other minerals such as [[calcite]]/[[aragonite]] and other [[carbonate mineral]]s. These types of minerals often produce lower precision ages than [[igneous]] and [[Metamorphic rock|metamorphic]] minerals traditionally used for age dating, but are more common in the geologic record.
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| ==Interaction between mineralogy and radioactive breakdown==
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| 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.
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| 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.<ref name="Romer" /> [[Fission track]]s and micro-cracks within the crystal will further extend this radiation damage network.
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| 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.<ref name="Mattinson">Mattinson, J.M., 2005. Zircon U-Pb Chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step dissolution analysis for Improved precision and accuracy of zircon ages. Chemical Geology. 220, 47-66.</ref>
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| ==Chemical details==
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| Under conditions where the system has remained closed, and therefore no lead loss has occurred, the age of the zircon can be calculated independently from the two equations:
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| {{NumBlk|::|<math>{{^\text{206}\,\!\text{Pb}^*}\over{^\text{238}\,\!\text{U}}}=e^{\lambda_{238}t}-1</math>|{{EquationRef|1}}}}
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| and
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| {{NumBlk|::|<math>{{^\text{207}\,\!\text{Pb}^*}\over{^\text{235}\,\!\text{U}}}=e^{\lambda_{235}t}-1</math>|{{EquationRef|2}}}}
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| These are said to yield concordant ages. It is these concordant ages, plotted over a series of time intervals, that result in the concordant line.<ref name="Dickin">Dickin, A.P., 2005. Radiogenic Isotope Geology 2nd ed. Cambridge: Cambridge University Press. pp. 101.</ref>
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| 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.<ref name="Dickin" />
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| [[Image:ConcordiaDiagram.jpg|thumb|500px|center|Figure 1: Concordia diagram for data published by Mattinson<ref name="Mattinson" /> for zircon samples from Klamath Mountains in Northern California. Ages for the concordia increase in increments of 100 million years.]]
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| Undamaged zircon retains the lead generated by radioactive decay of uranium and thorium until 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. 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 ([[Secondary ion mass spectrometry|SIMS]]) or laser [[Inductively coupled plasma mass spectrometry|ICP-MS]].
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| ==Geographic Details==
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| Has been used for [[Lagerstätte|Lagerstätten]]:
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| * [[Yilgarn Craton]]
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| ==See also==
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| *[[lead-lead dating]] (Pb-Pb dating)
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| ==References==
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| {{reflist}}
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| {{Chronology}}
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| {{DEFAULTSORT:Uranium-Lead Dating}}
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| [[Category:Radiometric dating]]
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