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In uranium-lead U-Pb dating of zirconthe zircon is found to exclude initial lead almost completely. The incremental additions of the daughter type can then be viewed in proportion to the abundance of parent atoms.

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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. In practice, the isochron approach has many inherent advantages. The uncertainty in determining the slope is reduced because it is defined by many points.

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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. This term is called the initial ratio.

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The slope is proportional to the geologic age of the system. Load Next. Videos Images. If a simple mineral is widespread in the geologic recordit is more valuable for dating as more units can be measured for age and compared by the same method.

Radiometric time scale

In rubidium-strontium datingmicas exclude strontium when they form but accept much rubidium. 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. Fast Facts. Should a point plot below the line, it could indicate that a particular sample was open to migration of the dating elements or that the sample was contaminated and lay below the isochron when the rock solidified. 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.

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.

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Rubidium-strontium Rb-Sr dating was the first technique in which the whole-rock isochron method was extensively employed. 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. Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not. A rock, on the other hand, may contain minerals formed at more than one time under a variety of conditions.

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An ideal mineral is one that has sufficient parent and daughter isotopes to measure precisely, is chemically inert, contains little or no ificant initial daughter isotopes, and retains daughter products at the highest possible temperatures. 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.

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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. With time, each would then develop additional daughter abundances in proportion to the amount of parent present. If a of samples are analyzed and the are shown to define a straight line within error, then a precise age is defined because this is only possible if each is a closed system and each has the same initial ratio and age. In this case, a valid age can still be obtained, provided that they remain within the rock.

By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed.

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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. Additional Info. Under such circumstances the isolation and analysis of certain minerals can indicate at what time these conditions prevailed.

A specific datable mineral like rutilewhich can be linked to a specific event such as the formation of a mineral depositis especially important.

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

When some daughter atoms are initially present deated D 0the total D is the sum of radiogenic and initial atoms, so that. A second advantage of the method relates to the fact that under high-temperature conditions the daughter isotopes may escape from the host minerals. Some studies have shown that rubidium is very mobile both in fluids that migrate through the rock as it cools and in fluids that are present as the rock undergoes chemical weathering.

These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. 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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Similar studies have shown that the samarium - neodymium Sm-Nd parent-daughter pair is more resistant to secondary migration but that, in this instance, sufficient initial spread in the abundance of the parent isotope is difficult to achieve. Load. To establish the condition that both parent and daughter abundances should be relative to the initial background, a stable isotope S of the daughter element can be chosen and divided into all portions of this equation; thus.

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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. In mathematical terms this is achieved as follows. In some cases the discovery of a rare trace mineral in a major breakthrough as it allows precise ages to be determined in formerly undatable units.