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Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements. By "age" we mean the elapsed time from when the mineral specimen was formed. Radioactive elements "decay" that is, change into other elements by "half lives. The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives. If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula.

Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock generally during potassic alteration or calcic albitisation alteration.

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Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement.

A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock. The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites.

Potassium-Argon Dating. Potassium-Argon (K-Ar) dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. Precise dating has been accomplished since Rubidium Strontium billion years Potassium Argon billion years and shale are related to the radiometric time scale by bracketing them within time zones that are determined by dating appropriately selected igneous rocks. Carbon dating: See Carbon 14 Dating in this web site. Rubidium-Strontium dating: The nuclide rubidium decays, with a half life of billion years, to strontium Strontium is a stable element; it does not undergo further radioactive decay. (Do not confuse with the highly radioactive isotope, strontium).

The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. Although this is a potential source of error for terrestrial rocks, it is irrelevant for lunar rocks and meteorites, as there are no chemical weathering reactions in those environments. The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined.

This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not completely understood for older sequences. In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers for example U-Th leads to greater uncertainties in the exact shape of the Sr isotope seawater curve.

From Wikipedia, the free encyclopedia. Redirected from Rubidium-strontium dating.

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Categories : Radiometric dating Rubidium Strontium. Hidden categories: Articles with short description Articles with long short description Commons category link is on Wikidata.

Rubidium-strontium dating is based on the beta decay of rubidium to strontium, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Radiometric dating can be performed on . Thermal ionization mass spectrometer used in radiometric dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon, or a long-life radioactive element plus its decay product, e.g., potassium/argon

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In other projects Wikimedia Commons. By using this site, you agree to the Terms of Use and Privacy Policy. Wikimedia Commons has media related to Rb-Sr dating. Therefore the amount of argon formed provides a direct measurement of the amount of potassium present in the specimen when it was originally formed. Because argon is an inert gasit is not possible that it might have been in the mineral when it was first formed from molten magma.

Radiometric Age Dating

Any argon present in a mineral containing potassium must have been formed as the result of radioactive decay. F, the fraction of K40 remaining, is equal to the amount of potassium in the sample, divided by the sum of potassium in the sample plus the calculated amount of potassium required to produce the amount of argon found.

The age can then be calculated from equation 1.

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In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape. Creationists claim that argon escape renders age determinations invalid. However, any escaping argon gas would lead to a determined age younger, not older, than actual. The creationist "argon escape" theory does not support their young earth model.

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The argon age determination of the mineral can be confirmed by measuring the loss of potassium. In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon.

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The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed. Carbon dating: See Carbon 14 Dating in this web site. Rubidium-Strontium dating: The nuclide rubidium decays, with a half life of Strontium is a stable element; it does not undergo further radioactive decay.

Do not confuse with the highly radioactive isotope, strontium Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically.

Note that this does not mean that the ratios are the same everywhere on earth.

Rubidium-87 radiometric dating

It merely means that the ratios are the same in the particular magma from which the test sample was later taken. As strontium forms, its ratio to strontium will increase. Strontium is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process.

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The amount of strontium in a given mineral sample will not change. It turns out to be a straight line with a slope of The corresponding half lives for each plotted point are marked on the line and identified.

It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different mineralsif the plotted half life points are connected, a straight line going through the origin is produced. These lines are called "isochrons". The steeper the slope of the isochron, the more half lives it represents.

When the fraction of rubidium is plotted against the fraction of strontium for a number of different minerals from the same magma an isochron is obtained. If the points lie on a straight line, this indicates that the data is consistent and probably accurate.

An example of this can be found in Strahler, Fig However, if strontium 87 was present in the mineral when it was first formed from molten magma, that amount will be shown by an intercept of the isochron lines on the y-axis, as shown in Fig Thus it is possible to correct for strontium initially present.

Comparing figures The age of the sample can be obtained by choosing the origin at the y intercept.

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In Fig Note that the amounts of rubidium 87 and strontium 87 are given as ratios to an inert isotope, strontium However, in calculating the ratio of Rb87 to Sr87, we can use a simple analytical geometry solution to the plotted data. Again referring to Fig. Since the half-life of Rb87 is Therefore: log.

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When properly carried out, radioactive dating test procedures have shown consistent and close agreement among the various methods. If the same result is obtained sample after sample, using different test procedures based on different decay sequences, and carried out by different laboratories, that is a pretty good indication that the age determinations are accurate.

Of course, test procedures, like anything else, can be screwed up. Mistakes can be made at the time a procedure is first being developed.

Radiometric dating / Carbon dating

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