Nuclear and Particle Physics. Submit Feedback. Lamarsh, A. The radioactive decay law states that the probability per unit time that a nucleus will decay is a constant, independent of time. In the series of unstable products from the radioactive decay of uranium, lead results from the decay of radon and is a precursor of the stable isotope lead Uranium-lead dating method is usually performed on the mineral zircon. The gas radon Rn escapes from the ground and decays rapidly in the atmosphere to lead Pb , which falls quickly to the surface where…. Namespaces Article Talk.
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence. Uranium comes in two common isotopes with atomic weights of and we'll call them U and U. Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead Pb. The two cascades are different—U becomes Pb and U becomes Pb. What makes this fact useful is that they occur at different rates, as expressed in their half-lives the time it takes for half the atoms to decay. The U—Pb cascade has a half-life of million years and the U—Pb cascade is considerably slower, with a half-life of 4. So when a mineral grain forms specifically, when it first cools below its trapping temperature , it effectively sets the uranium-lead "clock" to zero. 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. First, its chemical structure likes uranium and hates lead. Uranium easily substitutes for zirconium while lead is strongly excluded.
Uranium-thorium-lead datingalso called Common-lead Datingmethod of establishing the time of origin of a rock by means of the amount of common lead it contains; common lead is any lead from a leax or https://blackhills.xyz/action/element-used-in-dating-rocks-crossword-clue.php lead dating contains a large amount of lead and a small amount of the radioactive progenitors of lead—i.
The important characteristic of common lead is that it contains no significant proportion of radiogenic lead dating accumulated since the time that the mineral or rock phase was formed. Of the four isotopes of lead, two are formed from the uranium isotopes and one is formed from the thorium isotope; only lead is not known to have any long-lived radioactive progenitor.
Primordial lead is thought to have been formed by stellar nuclear reactions, released to space by supernovae explosions, and incorporated within the dust cloud that constituted the primordial solar system; the troilite iron sulfide phase of iron meteorites contains lead that approximates the primordial composition.
The lead incorporated within the Earth has been evolving continuously from primordial lead and from the radioactive decay of uranium and thorium isotopes. Thus, the lead isotopic composition of any mineral or rock depends upon its age and the environment from which it was formed; that is, it would depend upon the ratio of uranium plus thorium to lead in the parent material. The Earth can be assumed to be a very go here sample containing lead evolving from primordial lead by radiogenic increments.
Leadd modern lead, for example, from marine lead dating or modern basalts has the composition of daitng in the Earth and if the lead in the troilite phase click here iron meteorites has the composition of primordial lead, then a simple model yields datibg 4. This age is in good agreement with the see more of the meteorites and the age of the Moon as determined independently.
Uranium-thorium-lead dating. Article Media. Info Print Cite. Submit Leac. Thank you for your feedback. The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History.
Learn More in these related Britannica articles:. The elements uranium and thorium gradually decay into lead, different isotopes of lead dating arising from the various isotopes of uranium and thorium; some lead dating of lead are, however, not produced by any radioactive decay process.
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