Uranium-Lead dating

The isochron method Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists. These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. In this case, it is a big advantage to present the data in a form in which the abundance of both the parent and daughter isotopes are given with respect to the abundance of the initial background daughter. The incremental additions of the daughter type can then be viewed in proportion to the abundance of parent atoms. In mathematical terms this is achieved as follows. This term, shown in Figure 1, is called the initial ratio. The slope is proportional to the geologic age of the system. In practice, the isochron approach has many inherent advantages. When a single body of liquid rock crystallizes, parent and daughter elements may separate so that, once solid, the isotopic data would define a series of points, such as those shown as open circles designated R1, R2, R3 in Figure 1. With time each would then develop additional daughter abundances in proportion to the amount of parent present.

Clocks in the Rocks

The four isotopes are uranium , uranium , lead , and lead The process of dating finds the two ratios between uranium and lead ; and uranium and lead The radiometric dater then uses the half-life of all four isotopes to find an age range the rock should be in. The half-lives of the cascade from uranium to lead has been been extrapolated to about million years and the cascade form uranium to lead has been calculated to about 4.

This data is compared to a curve called the Concordia diagram. This diagram has been made by using the ratio of uranium to lead of all the rocks dated with this method and their assumed age.

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Wikimedia Commons To produce fuel-grade uranium, the uranium has to be processed to produce uranium dioxide and to enrich or concentrate the U in the fuel pellets. During this processing, depleted uranium DU , enriched in U and depleted in U , is produced. DU and enriched uranium have numerous civilian and military uses. Since U is the most radioactive isotope of uranium, the removal of it to makes DU the least radioactive phase of uranium, but it still has heavy metal toxicity issues.

Despite any processing, enriched, depleted or natural uranium all behave the same chemically. U , when bombarded by neutrons, fissions or splits into two smaller nuclei and releases energy and starts nuclear chain reaction. Because of the energy released, U is efficient for power generation and the only isotope of uranium that can sustain these reactions.

For electrical power plants, this reaction is controlled so that it will not produce too many neutrons or heat and become explosive. Go to The Basics of Nuclear Reactions to find out more about nuclear reactions.

1. Rate of Decay

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. 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.

The dating method is usually performed on the mineral zircon. The mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead.

A commonly used radiometric dating technique relies on the breakdown of potassium (40 K) to argon (40 Ar). In igneous rocks, the potassium-argon “clock” is set the .

This is what archaeologists use to determine the age of human-made artifacts. But carbon dating won’t work on dinosaur bones. The half-life of carbon is only 5, years, so carbon dating is only effective on samples that are less than 50, years old. Dinosaur bones, on the other hand, are millions of years old — some fossils are billions of years old.

To determine the ages of these specimens, scientists need an isotope with a very long half-life. Some of the isotopes used for this purpose are uranium , uranium and potassium , each of which has a half-life of more than a million years. Unfortunately, these elements don’t exist in dinosaur fossils themselves. Each of them typically exists in igneous rock, or rock made from cooled magma.

Fossils, however, form in sedimentary rock — sediment quickly covers a dinosaur’s body, and the sediment and the bones gradually turn into rock. But this sediment doesn’t typically include the necessary isotopes in measurable amounts.

The Age of the Earth

Updated 8 January c Introduction In a related article on geologic ages Ages , we presented a chart with the various geologic eras and their ages. In a separate article Radiometric dating , we sketched in some technical detail how these dates are calculated using radiometric dating techniques. As we pointed out in these two articles, radiometric dates are based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas.

Dating schemes based on rates of radioactivity have been refined and scrutinized for several decades. The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.

Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form.

While there are numerous natural processes that can serve as clocks, there are also many natural processes that can reset or scramble these time-dependent processes and introduce uncertainties. To try to set a reasonable bound on the age, we could presume that the Earth formed at the same time as the rest of the solar system. If the small masses that become meteorites are part of that system, then a measurement of the solidification time of those meteorites gives an estimate of the age of the Earth.

The following illustration points to a scenario for developing such an age estimate. Some of the progress in finding very old samples of rock on the Earth are summarized in the following comments. It is a compound of zirconium, silicon and oxygen which in its colorless form is used to make brilliant gems. Samples more than 3.

Older ages in the neighborhood of 4. The graph below follows the treatment of Krane of Rb-Sr studies of meteorite samples from Wetherill in order to show the nature of the calculation of age from isochrons.

Why I Reject A Young Earth View: A Biblical Defense of an Old Earth

Which age is correct? Sample was dated by five different sources with nineteen different results. Here is how one of those sources tried to spin the results. The 40K Ar ages are for No.

Scientists used many different isotopes for dating rocks in Radiometric Dating, uranium/lead, potassium/argon and others are used. The half-life in some of these isotopes is .

Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. The final decay product, lead Pb , is stable and can no longer undergo spontaneous radioactive decay. All ordinary matter is made up of combinations of chemical elements , each with its own atomic number , indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes , with each isotope of an element differing in the number of neutrons in the nucleus.

A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture.

Dating rocks

Outlook Other Abstract U-Pb radioisotope dating is now the absolute dating method of first choice among geochronologists, especially using the mineral zircon. A variety of analytical instruments have also now been developed using different micro-sampling techniques coupled with mass spectrometers, thus enabling wide usage of U-Pb radioisotope dating. However, problems remain in the interpretation of the measured Pb isotopic ratios to transform them into ages.

Among them is the presence of non-radiogenic Pb of unknown composition, often referred to as common or initial Pb.

why is uranium used for dating rocks. Uranium has a half life of billion e of the huge differences in the half lives of carbon 14 and uranium they cannot be used 14 can only be used to date fossils of a very recent m can only be used to date volcanic rocks of a very old weariness of an uneventful issued a quick statement.

These are K-Ar data obtained on glauconite, a potassium-bearing clay mineral that forms in some marine sediment. Woodmorappe fails to mention, however, that these data were obtained as part of a controlled experiment to test, on samples of known age, the applicability of the K-Ar method to glauconite and to illite, another clay mineral. He also neglects to mention that most of the 89 K-Ar ages reported in their study agree very well with the expected ages.

Evernden and others 43 found that these clay minerals are extremely susceptible to argon loss when heated even slightly, such as occurs when sedimentary rocks are deeply buried. As a result, glauconite is used for dating only with extreme caution. The ages from the Coast Range batholith in Alaska Table 2 are referenced by Woodmorappe to a report by Lanphere and others

Why is U-238 used for the purpose of dating rocks?

Decay routes[ edit ] The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays. The term U—Pb dating normally implies the coupled use of both decay schemes in the ‘concordia diagram’ see below.

However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method. 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 estimates of the age of the Earth.

The oldest rocks on Earth, found in western Greenland, have been dated by four independent radiometric dating methods at billion years. Rocks billion years in age have been found in southern Africa, western Australia, and the Great Lakes region of North America.

Unfortunately, the situation is quite different in the case of thin layers of calcite that overlie Palaeolithic cave drawings. The conditions under which calcite forms depend largely on the hydrologic activity, which has greatly varied over the course of the Upper Palaeolithic and Holocene. In many cases, we can see that the growth of speleothems stopped during much of the Upper Palaeolithic. Consequently the ages obtained are minimum ages terminus ante quem which are frequently much younger than the real ages of the underlying artworks.

Moreover, a much more serious but rarely considered source of error contradicts the assumption of a closed system. In thin layers of carbonate deposits and in damp media, the uranium incorporated into the calcite during its crystallization may be partially eliminated because of its solubility in water. Uranium leaching causes an artificial increase of the age that may reach considerable proportions e.

Radioactive Dating and Half-Life with animation