Dating - Rubidium–strontium method | mawatari.info
«Rubidium-Strontium dating» The rubidium-strontium dating method is a radiometric dating Examples of use in the English literature, quotes and news about. rubidium—strontium dating* A radiometric dating  method based on the age determinations from the same sample (see POTASSIUM—ARGON DATING). For example, consider the case of an igneous rock such as a granite Each of these minerals has a different initial rubidium/strontium.
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.
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. See Carbon 14 Dating in this web site. 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. 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. The amount of strontium in a given mineral sample will not change. Therefore the relative amounts of rubidium and strontium can be determined by expressing their ratios to strontium These curves are illustrated in Fig 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 minerals, if 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 If the strontium isotope was not present in the mineral at the time it was formed from the molten magma, then the geometry of the plotted isochron lines requires that they all intersect the origin, as shown in figure 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.
The age of the sample can be obtained by choosing the origin at the y intercept. 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. Their presence in certain minerals in water-deposited gold veins, however, does suggest mobility under certain conditions. In addition, their behaviour under high-temperature metamorphic conditions is as yet poorly documented.
Rubidium-strontium Dating | mawatari.info
The exploitation of the samarium—neodymium pair for dating only became possible when several technical difficulties were overcome. Procedures to separate these very similar elements and methods of measuring neodymium isotope ratios with uncertainties of only a few parts inhad to be developed.
In theory, the samarium—neodymium method is identical to the rubidium—strontium approach. Both use the isochron method to display and evaluate data. In the case of samarium—neodymium dating, however, the chemical similarity of parent and daughter adds another complication because fractionation during crystallization is extremely limited. This makes the isochrons short and adds further to the necessity for high precision.
With modern analytical methods, however, uncertainties in measured ages have been reduced to 20 million years for the oldest rocks and meteorites. Mineral isochrons provide the best results.
The equation relating present-day neodymium isotopic abundance as the sum of the initial ratios and radiogenic additions is that of a straight line, as discussed earlier for rubidium—strontium. Other successful examples have been reported where rocks with open rubidium—strontium systems have been shown to have closed samarium—neodymium systems.
In other examples, the ages of rocks with insufficient rubidium for dating have been successfully determined. There is considerable promise for dating garneta common metamorphic mineral, because it is known to concentrate the parent isotope. In general, the use of the samarium—neodymium method as a dating tool is limited by the fact that other methods mainly the uranium—lead approach are more precise and require fewer analyses.
In the case of meteorites and lunar rocks where samples are limited and minerals for other dating methods are not available, the samarium—neodymium method can provide the best ages possible.
Rhenium—osmium method The decay scheme in which rhenium is transformed to osmium shows promise as a means of studying mantle—crust evolution and the evolution of ore deposits. Osmium is strongly concentrated in the mantle and extremely depleted in the crustso that crustal osmium must have exceedingly high radiogenic-to-stable ratios while the mantle values are low. In fact, crustal levels are so low that they are extremely difficult to measure with current technology.
Most work to date has centred around rhenium- or osmium-enriched minerals. Because rhenium and osmium are both siderophilic having an affinity for iron and chalcophilic having an affinity for sulfurthe greatest potential for this method is in studies concerning the origin and age of sulfide ore deposits.
Potassium—argon methods The radioactive decay scheme involving the breakdown of potassium of mass 40 40K to argon gas of mass 40 40Ar formed the basis of the first widely used isotopic dating method.
Since radiogenic argon was first detected in by the American geophysicist Lyman T.
Nierthe method has evolved into one of the most versatile and widely employed methods available. In fact, potassium decays to both argon and calciumbut, because argon is absent in most minerals while calcium is present, the argon produced is easier to detect and measure.
Argon dating involves a different technology from all the other methods so far described, because argon exists as a gas at room temperature. Thus, it can be purified as it passes down a vacuum line by freezing out or reacting out certain contaminants.
It is then introduced into a mass spectrometer through a series of manual or computer-controlled valves. Technical advances, including the introduction of the argon—argon method and laser heating, that have improved the versatility of the method are described below.
In conventional potassium—argon datinga potassium-bearing sample is split into two fractions: After purification has been completed, a spike enriched in argon is mixed in and the atomic abundance of the daughter product argon is measured relative to the argon added.
The amount of the argon present is then determined relative to argon to provide an estimate of the background atmospheric correction. In this case, relatively large samples, which may include significant amounts of alteration, are analyzed. Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock. Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock generally during potassic alteration or calcic albitisation alteration.
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.
Geochronology[ edit ] The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites.