Rubidium-strontium Dating | uzveli.info
Step-by-step description of sample preparation. 16 involved. Application of the Rb-Sr dating technique. The first studies using Rb-Sr technique on. 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. In this article I shall introduce the Rb-Sr dating method, and explain how it sort of family resemblance to the reasoning behind step heating in the Ar-Ar method, .
Which minerals and rocks can be dated with the Rb-Sr method? The minerals must contain Rb, which is a rather rare element. Examples include the mica family biotite and muscovite and the feldspar family plagioclase and orthoclase. Select a fresh, unweathered rock sample.
Sample Selection A geologist collects a fresh, unweathered hand sample for age dating. Fresh is the key word here, and means that the chemistry of the sample has NOT been changed since the sample formed.
Weathering alters the chemistry of rocks including their isotopic compositions. Therefore, a highly weathered rock may yield unreliable age information.
Crush the rock and separate the Rb-bearing minerals. Getting a Rock Sample Ready for the Mass Spectrometer For reliable age determination, careful sample preparation is an important and often tedious process. The rock is mechanically crushed into small fragments. Fragments of the Rb-bearing minerals are then separated from the whole rock using a variety of methods, such as a magnetic separator. These materials are then used to prepare a "whole-rock" sample and several "mineral separate" samples.Decay scheme of K-Ar, U-Pb and Sm-Nd, petrogenetic implications-part B
The whole rock sample will yield the weighted average isotopic composition of all the minerals in the rock.
Each mineral separate will yield the composition of that particular mineral. Other Steps There are other steps that must be carried out to prepare a sample for analysis by a mass spectrometer, such as converting the sample to a solution by dissolving the mineral separates in selected acids, using techniques of column chemistry to increase the concentration of the small amounts of Rb and Sr in the solution and then precipitating the concentrated solution as a "salt" compound.
It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis. Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer. A Mass Spectrometer is used to Measure Isotopic Ratios The gas source mass spectrometer includes three fundamental parts, 1 a "source" of positively charged ions or molecular ions, 2 a magnetic analyzer, and 3 ion collectors.
Once formed, the ions are accelerated and focused by charged plates into a beam that enters a flight tube. Multiple ion detectors are arranged to collect the ion beams of interest. These collectors measure each beam as a current that can be amplified and determined with high precision.
A Mass Spectrometer is a very powerful and sophisticated instrument. Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument.
Understanding how a mass spectrometer functions is beyond the level of this activity. But you should know that it measures the amounts of various isotopes present in specially prepared samples of rocks and minerals as well as other materials. Understanding the isochron diagram is the key to determining the age of a rock using the Rb-Sr method. Thermoluminensnce Thermoluminescence TL dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated lava, ceramics or exposed to sunlight sediments.
- Historical Geology/Rb-Sr dating
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As a crystalline material is heated during measurements the process of thermoluminescence starts. Thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating. 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.
Rubidium–strontium dating - Wikipedia
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.
Now consider what will happen to this system over time, as the 87Rb decays to 87Sr. If you have difficulty seeing this, try considering the extremal case of a mineral which contains no rubidium at all. The isochron diagram changes over time.
The effect of the decay process on the isotope ratios can again be plotted on a graph, as shown to the right. So now we can find a date for the rock. This one additional piece of information about the initial state of the rock allows us to calculate its age. Confounding factors[ edit ] As with the other methods we've discussed so far, the Rb-Sr method will only work if nothing but the passage of time has affected the distribution of the key isotopes within the rock.
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 mineralswhich 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: