Dating - Rubidium–strontium method | myhyundai.info
Calculated age of the formation, based on strontium-isotope stratigraphy, The Rb-Sr dating method is used in dating Paleozoic and Precambrian rocks. Chapter 14 Groundwater It was only in the early part of the 20th century, when isotopic dating methods were first applied, that we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic Rubidium- strontium, 47 Ga, 10 Ma – Ga, Less precision than other methods at old dates. Dating - Rubidium–strontium method: The radioactive decay of rubidium ( 87Rb) of the 87Sr/86Sr ratio in a modern volcano can be used to determine age if.
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. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
Calculating the age[ edit ] The age of a sample is determined by analysing several minerals within the sample. If these form a straight line then the samples are consistent, and the age probably reliable.
The slope of the line dictates the age of the sample. 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. Epidote, a low-temperature alteration mineral with a very high concentration of radiogenic strontium, has been found in rocks wherein biotite has lost strontium by diffusion.
The rock itself has a much lower ratio, so that it did not take part in this exchange.
Rubidium–strontium dating - Wikipedia
Although rubidium—strontium dating is not as precise as the uranium—lead method, it was the first to be exploited and has provided much of the prevailing knowledge of Earth history.
The procedures of sample preparationchemical separation, and mass spectrometry are relatively easy to carry out, and datable minerals occur in most rocks. Precise ages can be obtained on high-level rocks i. The mobility of rubidium in deep-level crustal fluids and melts that can infiltrate other rocks during metamorphism as well as in fluids involved in weathering can complicate the results.
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. Both parent and daughter belong to the rare-earth element group, which is itself the subject of numerous geologic investigations.
All members of this group have similar chemical properties and charge, but differ significantly in size. Because of this, they are selectively removed as different minerals are precipitated from a melt.
In the opposite sense, their relative abundance in a melt can indicate the presence of certain residual minerals during partial melting. Unlike rubidium, which is enriched over strontium in the crust, samarium is relatively enriched with respect to neodymium in the mantle.
Consequently, a volcanic rock composed of melted crust would have elevated radiogenic strontium values and depressed radiogenic neodymium values with respect to the mantle.
As a parent—daughter pair, samarium and neodymium are unique in that both have very similar chemical properties, and so loss by diffusion may be reduced. Their low concentrations in surface waters indicates that changes during low-temperature alteration and weathering are less likely.
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.
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. 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.