Lead–lead dating - Wikipedia
Oct 24, Uranium-Lead dating methods are the pará-iso isotope geology laboratory, it the age is called the decay of uranium dating, pb-pb isochrons. Adapted from The Age of the Earth, by the Branch of Isotope Geology, United The "best" age for the Earth is based on the time required for the lead isotopes in . There are a number of isotopes of interest in U-Pb dating. U (uranium) decays to Pb (lead) by a complex decay.
Patterson also analyzed terrestrial sediment collected from the ocean floor, which was believed to be representative of the Bulk Earth composition.
Historical Geology/U-Pb, Pb-Pb, and fission track dating - Wikibooks, open books for an open world
Because the isotope composition of this sample plotted on the meteorite isochron, it suggested that earth had the same age and origin as meteorites, therefore solving the age of the Earth and giving rise to the name 'geochron'. Lead isotope isochron diagram used by C. Patterson to determine the age of the Earth in Animation shows progressive growth over million years Myr of the lead isotope ratios for two stony meteorites Nuevo Laredo and Forest City from initial lead isotope ratios matching those of the Canyon Diablo iron meteorite.
Precise Pb—Pb dating of meteorites[ edit ] Pb—Pb isochrons for the oldest known material in the solar system. Hence precise dating of these objects is important to constrain the early evolution of the solar system and the age of the earth. The U—Pb dating method can yield the most precise ages for early solar-system objects due to the optimal half-life of U. However, the absence of zircon or other uranium-rich minerals in chondrites, and the presence of initial non-radiogenic Pb common Pbrules out direct use of the U-Pb concordia method.
Therefore, the most precise dating method for these meteorites is the Pb—Pb method, which allows a correction for common Pb.Radiometric or Absolute Rock Dating
This makes it difficult to determine the analytical uncertainty on the age. The most accurate ages are produced by samples near the y-axis, which was achieved by step-wise leaching and analysis of the samples.
This supports the idea that CAIs crystallization and chondrule formation occurred around the same time during the formation of the solar system. However, chondrules continued to form for approximately 3 My after CAIs. Hence the best age for the original formation of the solar system is This date also represents the time of initiation of planetary accretion.
Dalrymple cites examples of lead isotope dating that give an age for the earth of about 4. Lead isotopes are important because two different lead isotopes Pb and Pb are produced from the decay series of two different uranium isotopes U and U.
Since both decay series contain a unique set of intermediate radioactive isotopes, and because each has its own half-life, independent age calculations can be made from each Dalrymple The presence of a stable lead isotope that is not the product of any decay series Pb allows lead isotopes to be normalized, allowing for the use of isochrons and concordia-discordia diagrams as dating tools. Two other characteristics of lead isotope measurements make it superior to other methods.
First, measuring the isotope ratio of a single element can be done much more precisely than measuring isotope ratios of two differing elements.
Second, using two isotopes of the same element makes the sample immune to chemical fractionation during a post-crystallization disturbance Dalrymple The commonly accepted 4. This model, which describes the accumulation of lead isotopes in meteorites, the Earth, and the Solar System, was proposed independently by E. Gerling, Arthur Holmes, and Fritz G.
Houtermans in the s Dalrymple This model ultimately led to the development of isochrons, in which two isotopes are plotted against each other to calculate an age for the mineral or rock. Those who developed the method utilized Pb and Pb, lead isotopes that are the product of radioactive decay, normalized to Pb. The amount of Pb will remain constant throughout the history of a rock because it is a stable isotope that is not the product of any decay series, thus allowing for the normalization Dalrymple Two requirements of the Gerling-Holmes-Houtermans model make it difficult to use.
The first is that it requires single-stage leads, which are systems that begin at some initial lead composition and remain on the same growth curve throughout their histories Dalrymple The second requirement is that assumptions about the genetic relationship between the Earth and meteorites must be made. Although single-stage leads are difficult to find on Earth due to the constant recycling of Earth's crust, Pb-Pb isochrons remain powerful tools in making age of the Earth calculations.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
The resulting line drawn through the plotted points will pass through a point representing the initial lead composition of the system. Although this point cannot be determined, the isochron will rotate about it as the rock ages because the initial amount of lead is constant regardless of age.
An example isochron from Dalrymple is shown in Figure 4. The slope of the line gives the age of the rock.
Unlike other isochrons, the slope of the Pb-Pb isochron decreases with increasing age. This is because U has a half-life of million years, while U has a half-life of 4.
The rate at which daughter isotopes accumulate is dependent on the amount of parent isotope present. Since U has a much shorter half-life, a larger fraction of the initial U present in the rock will have decayed compared to U. Therefore, Pb will accumulate at a slower rate than Pb, causing the isochron to decrease in slope with increasing age. The use of lead isotope ratios makes this isochron self-checking. A large scattering of measurements would indicate the sample is multi-stage rather than single-stage, making the isochron unreliable.
Another situation in which single-stage systems give unreliable information is the extraction of lead from uranium to form lead ore.
It is possible that a system could undergo a geological process that extracts lead, leaving the new system without any uranium. If that system were dated at that point in time, it would fall on the isochron and give the correct age of the mineral.
However, without any uranium present, accumulation of daughter isotopes ceases even though time continues to pass. Such events produce a frozen record, giving the amount of time from crystallization to extraction of lead to form lead ore.