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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.

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The elements uranium and thorium gradually decay into lead, different isotopes of lead arising from the various isotopes of uranium and thorium; some isotopes of lead are, however, not produced by any radioactive decay process. History at your fingertips.

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When an igneous rock crystallizes, a wide variety of major and trace minerals may form, each concentrating certain elements and radioactive trace elements within the rock. By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed. In this case, a graph can be set up in which the slope of the line may be computed from an assumed value for the initial ratio, and it is usually possible to show that uncertainties related to this assumption are negligible.

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This is possible in potassium-argon K-Ar datingfor example, because most minerals do not take argon into their structures initially.

In rubidium-strontium datingmicas exclude strontium when they form but accept much rubidium.

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In uranium-lead U-Pb dating of zirconthe zircon is found to exclude initial lead almost completely. Minerals too are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered.

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A rock, on the other hand, may contain minerals formed at more than one time under a variety of conditions. Under such circumstances the isolation and analysis of certain minerals can indicate at what time these conditions prevailed. If a simple mineral is widespread in the geologic recor it is more valuable for dating as more units can be measured for age and compared by the same method.

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. Basics of Uranium-Lead. Uranium comes in two common isotopes with atomic weights Author: Andrew Alden. Here, a single uranium-lead isotopic analysis can provide an age more precise than can be obtained by the whole-rock isochron method involving many analyses. When single minerals are analyzed, each grain can be studied under a microscope under intense side light so that alterations or imperfections can be revealed and excluded.

However, if a single parent-daughter pair that is amenable to precise analysis can be measured in a variety of minerals, the ages of a wide variety of rock types can be determined by a single method without the need for intercalibration.

In some cases the discovery of a rare trace mineral results in a major breakthrough as it allows precise ages to be determined in formerly undatable units.

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For example, the minerals baddeleyitean oxide of zirconium ZrO 2and zirconolite CaZrTi 2 O 7have been shown to be widespread in small amounts in mafic igneous rocks i. Here, a single uranium-lead isotopic analysis can provide an age more precise than can be obtained by the whole-rock isochron method involving many analyses. When single minerals are analyzed, each grain can be studied under a microscope under intense side light so that alterations or imperfections can be revealed and excluded.

If minerals are used for dating, the necessary checks on the ages are achieved by analyzing samples from more than one location and by analyzing different grain sizes or mineral types that respond differently to disturbing events. It can be said that minerals provide a high degree of sample integrity that can be predicted on the basis of experience gained through numerous investigations under a variety of geologic conditions.

Uranium lead age dating

An ideal mineral is one that has sufficient parent and daughter isotopes to measure precisely, is chemically inert, contains little or no significant initial daughter isotopes, and retains daughter products at the highest possible temperatures. A specific datable mineral like rutilewhich can be linked to a specific event such as the formation of a mineral depositis especially important.

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). Since both decay series contain a unique set of intermediate. For practice, use the graph above to estimate the age of a rock sample that contains 10uranium and 90lead. The example above describes uranium/lead decay, which happens very slowly; however, different radioactive elements have different half-lives. Some are much shorter. This allows scientists to date events that are more or less ancient. Uranium-thorium-lead dating, method 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 rock or mineral that contains a large amount of lead and a small amount of the radioactive progenitors of .

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U-Pb Zircon Geochronology - for determining the age of a rock

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