And of course this is not necessarily the case. Hydrothermal or metasomatic events may have added or subtracted rubidium and strontium to or from the rocks since their formation; or a metamorphic event may have redistributed the rubidium or strontium among its constituent minerals , which would also interfere with the method. However, barring an extraordinary coincidence, the result of such events will be that when we draw the isochron diagram, the minerals will no longer lie on a straight line. A small deviation from a straight line tells us that there is some uncertainty about the date, and this degree of uncertainty can be calculated; and if we get something which is nothing like a straight line, then the method simply doesn't supply us with a date.
So just as step heating in Ar-Ar dating protects us from error, so too does the isochron method in Rb-Sr dating: There is, however, one potential source of error which will not show up on the isochron diagram, since it is expected to produce a straight line. Suppose that the original source of the rock was two different magmas call them X and Y imperfectly mixed together so that some parts of the rock will be all X, some all Y, some part X and part Y in varying proportions.
Then these different parts of the rock, when analyzed for their isotopic composition, will plot in a straight line on the isochron diagram; and the slope of this line, and the point at which it intercepts the vertical axis, will have nothing to do with the age of the rock, and everything to do with the compositions of X and Y. About half the time this will produce a straight line with negative slope: Such a line must necessarily be produced by mixing, since a real isochron will always have positive slope: We can also test for mixing using what is known as a mixing plot: It can happen that if we produce a mixing plot for a perfectly good isochron, it will by some statistical fluke produce a straight line on the mixing plot; we would then be throwing out a perfectly good date.
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However, this is worth it: From Wikibooks, open books for an open world. Starting position of an isochron diagram. Since potassium is usually added by alteration, the daughter—parent ratio and the age might be too low. A method designed to avoid such complexities was introduced by American geochronologist Craig M. Merrihue and English geochronologist Grenville Turner in In this technique, known as the argon—argon method, both parent and daughter can be determined in the mass spectrometer as some of the potassium atoms in the sample are first converted to argon in a nuclear reactor.
In this way, the problem of measuring the potassium in inhomogeneous samples is eliminated and smaller amounts of material can be analyzed. An additional advantage then becomes possible.
The sample can be heated in stages at different temperatures and the age calculated at each step. If alteration is evident, the invalid low-temperature age can be eliminated and a valid high-temperature age determined. In some cases, partly reset systems also may be detected. As in all dating systems, the ages calculated can be affected by the presence of inherited daughter products. In a few cases, argon ages older than that of Earth which violate local relative age patterns have even been determined for the mineral biotite. Such situations occur mainly where old rocks have been locally heated, which released argon into pore spaces at the same time that new minerals grew.
Under favourable circumstances the isochron method may be helpful, but tests by other techniques may be required. For example, the rubidium—strontium method would give a valid isotopic age of the biotite sample with inherited argon. As techniques evolved, argon background levels have been reduced and the method has become more and more sensitive.
Capitalizing on this, it is now possible to measure the minute amount of argon released when a single spot on a crystal is heated by an intense laser beam. For geologically old potassium-rich materials, a single spot may produce sufficient gas for analysis, whereas single millimetre-sized grains 1 mm equals 0. Progressive refinement of the method has made new areas of research possible, and the ability to understand complexities encountered in earlier investigations has increased.
This was done by melting single millimetre-sized grains with a laser and measuring individual argon—argon ages with a highly sensitive gas mass spectrometer. It has been instrumental, for example, in determining the ages of the stripes of alternating normally and reversely magnetized volcanic rocks that parallel the axis of the mid-oceanic ridges.
In ancient shield areas large segments of crust that were uplifted and cooled at the same time—i. The technique is highly responsive to thermal events in a relatively predictable fashion, so the cooling history of a region may be established.
Historical Geology/Rb-Sr dating
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Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Rubidium—strontium method The radioactive decay of rubidium 87 Rb to strontium 87 Sr was the first widely used dating system that utilized the isochron method.
Dating simple igneous rocks The rubidium—strontium pair is ideally suited for the isochron dating of igneous rocks. Dating minerals Potassium -bearing minerals including several varieties of mica, are ideal for rubidium—strontium dating as they have abundant parent rubidium and a low abundance of initial strontium.
Rubidium-strontium dating | ajypeges.tk
Dating metamorphic rocks Should a simple igneous body be subjected to an episode of heating or of deformation or of a combination of both, a well-documented special data pattern develops. Samarium—neodymium method The radioactive decay of samarium of mass Sm to neodymium of mass Nd has been shown to be capable of providing useful isochron ages for certain geologic materials.
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- Meaning of "rubidium-strontium dating" in the English dictionary.
- Historical Geology/Rb-Sr dating - Wikibooks, open books for an open world.
- Rubidium-strontium Dating | ajypeges.tk.
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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. Potassium—argon methods The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method.
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He was knighted in Dating Greek writing In calligraphy: The rubidium-strontium dating method is a radiometric dating technique used by scientists to determine the age of rocks and minerals from the quantities they contain of specific isotopes of rubidium 87 Rb and strontium 87 Sr, 86 Sr. Development of this process was aided by German chemists Otto Hahn and Fritz Strassmann , who later went on to discover nuclear fission in December The utility of the rubidium — strontium isotope system results from the fact that 87 Rb one of two naturally occurring isotopes of rubidium decays to 87 Sr with a half-life of In addition, Rb is a highly incompatible element that, during partial melting of the mantle, prefers to join the magmatic melt rather than remain in mantle minerals.
As a result, Rb is enriched in crustal rocks. The radiogenic daughter, 87 Sr, is produced in this decay process and was produced in rounds of stellar nucleosynthesis predating the creation of the Solar System. During fractional crystallization , Sr tends to become concentrated in plagioclase , leaving Rb in the liquid phase.
Highest ratios 10 or higher occur in pegmatites. For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar , K-feldspar , hornblende , biotite , and muscovite. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt. The ideal scenario according to Bowen's reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende i. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate.
The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different.