Lead-lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium-lead dating U-Pb dating , but in certain specialized situations such as dating meteorites and the age of the Earth it is more important than U-Pb dating. There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U and Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This can be expressed by the following decay equations:.
Third, zircon is widespread in igneous rocks as a primary mineral. This makes it especially valuable for dating these rocks, which have no fossils to indicate their age. Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.
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Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite. However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating.
But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zirconsthen assessing the quality of the data.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool.
Consider the concordia: as zircons age, they move outward along the curve.
But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram. The straight line takes the zircons off the concordia.
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This is where data from many zircons is important. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.
The results from these zircons therefore plot along that straight line, establishing what is called a discordia. Now consider the discordia. If a million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance. This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.
The oldest zircon yet found dates from 4.
With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's " Earliest Piece of the Earth " page, including the paper in Nature that announced the record-setting date.
Lead-lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite.
For most dating requirements it has been superseded by uranium-lead dating U-Pb datingbut in certain specialized situations such as dating meteorites and the age of the Earth it is more important than U-Pb dating. There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb.
These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U and Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate.
This can be expressed by the following decay equations:. The concept of common Pb-Pb dating also referred to as whole rock lead isotope dating was deduced through mathematical manipulation of the above equations. This rearranged equation formed:.
As evident by the equation, initial Pb isotope ratios, as well as the age of the system are the two factors which determine the present day Pb isotope compositions.
This was first established by Nier et al. The Pb ratios of three stony and two iron meteorites were measured. By dating meteorites Patterson was directly dating the age of various planetesimals.
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As planetesimals collided, various fragments were scattered and produced meteorites. Iron meteorites were identified as pieces of the core, while stony meteorites were segments of the mantle and crustal units of these various planetesimals.
Samples of iron meteorite from Canyon Diablo Meteor Crater Arizona were found to have the least radiogenic composition of any material in the solar system. Therefore, troilite found in Canyon Diablo represents the primeval lead isotope composition of the solar system, dating back to 4.
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Together, these samples define an isochron, whose slope gives the age of meteorites as 4. Patterson also analyzed terrestrial sediment collected from the ocean floor, which was believed to be representative of the Bulk Earth composition.
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.
Age of earth uranium lead dating
Chondrules and calcium-aluminium-rich inclusions CAIs are spherical particles that make up chondritic meteorites and are believed to be the oldest objects 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.
Radiometric Dating: Carbon-14 and Uranium-238
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. When the abundance of Pb is relatively low, this isotope has larger measurement errors than the other Pb isotopes, leading to very strong correlation of errors between the measured ratios.
Dalrymple cites examples of lead isotope dating that give an age for the earth of about billion years. 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). The oldest zircon yet found dates from billion years ago. With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's "Earliest Piece of the Earth" page, including the paper in . Before , ages for the Earth based on uranium/thorium/lead ratios were generally about a billion years younger than the currently popular billion years. The radiometric evidence for a b.y. old Earth is reviewed and deficiencies of the uranium/lead method are discussed. The basic theory of radiometric dating is briefly reviewed.
This makes it difficult to determine the analytical uncertainty on the age. To avoid this problem, researchers  developed an 'alternative Pb-Pb isochron diagram' see figure with reduced error correlation between the measured ratios.