Dating - Rubidium–strontium method | afrocolombianidad.info
The Rb-Sr method is commonly used to date. Rb-rich We usually use the isochron method to Rb-Sr isochron diagram for a series of rock samples formed. What is isochron dating? a method, an equation, a graph. 87Rb, 87Sr, 86Sr, Rubidium-strontium dating, method of estimating the age of rocks, minerals, the stable isotope strontium formed by the decay of the unstable isotope (Rb-Sr ) dating was the first technique in which the whole-rock isochron method was.
Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in igneous rocks. So we have every reason to think that rocks when they form do incorporate strontium, and 87Sr in particular.
The isochron diagram[ edit ] However, there is still a way to extract a date from the rock. In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar methodalthough the two are not exactly alike.
The reasoning, then, goes like this. When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium. We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: Now consider the distribution of the two strontium isotopes 87Sr and 86Sr.
Because they are chemically indistinguishable, they will appear in the same ratio in every mineral at the time when it is formed: Starting position of an isochron diagram. The initial state of the rock may therefore be schematically represented by the graph to the right, which shows the initial states of four minerals imaginatively named A, B, C, and D.
Now consider what will happen to this system over time, as the 87Rb decays to 87Sr. An additional nice feature of isochron ages is that an "uncertainty" in the age is automatically computed from the fit of the data to a line.
A routine statistical operation on the set of data yields both a slope of the best-fit line an age and a variance in the slope an uncertainty in the age. The better the fit of the data to the line, the lower the uncertainty. For further information on fitting of lines to data also known as regression analysissee: Yorka short technical overview of a technique specially designed for assessing isochron fits. Note that the methods used by isotope geologists as described by York are much more complicated than those described by Gonick.
This will be discussed in more detail in the section on Gill's paper below. The "generic" method described by Gonick is easier to understand, but it does not handle such necessities as: Unfortunately, one must wade through some hefty math in order to understand the procedures used to fit isochron lines to data. General comments on "dating assumptions" All radiometric dating methods require, in order to produce accurate ages, certain initial conditions and lack of contamination over time.
The wonderful property of isochron methods is: This topic will be discussed in much more detail below.
- Rubidium–strontium dating
- Isochron Dating
- Historical Geology/Rb-Sr dating
Where the simple methods will produce an incorrect age, isochron methods will generally indicate the unsuitability of the object for dating. Avoidance of generic dating's problems Now that the mechanics of plotting an isochron have been described, we will discuss the potential problems of the "simple" dating method with respect to isochron methods.
Initial daughter product The amount of initial D is not required or assumed to be zero. The greater the initial D-to-Di ratio, the further the initial horizontal line sits above the X-axis. But the computed age is not affected. If one of the samples happened to contain no P it would plot where the isochron line intercepts the Y-axisthen its quantity of D wouldn't change over time -- because it would have no parent atoms to produce daughter atoms.
Whether there's a data point on the Y-axis or not, the Y-intercept of the line doesn't change as the slope of the isochron line does as shown in Figure 5. Therefore, the Y-intercept of the isochron line gives the initial global ratio of D to Di.
For each sample, it would be possible to measure the amount of the Di, and using the ratio identified by the Y-intercept of the isochron plot calculate the amount of D that was present when the sample formed.
That quantity of D could be subtracted out of each sample, and it would then be possible to derive a simple age by the equation introduced in the first section of this document for each sample. Each such age would match the result given by the isochron.
Contamination - parent isotope Gain or loss of P changes the X-values of the data points: Gain or loss of P. In order to make the figures easy to read and quick to drawthe examples in this paper include few data points. While isochrons are performed with that few data points, the best ones include a larger quantity of data. If the isochron line has a distinctly non-zero slope, and a fairly large number of data points, the nearly inevitable result of contamination failure of the system to remain closed will be that the fit of the data to a line will be destroyed.
For example, consider an event which removes P. The data points will tend to move varying distances, for the different minerals will have varying resistance to loss of P, as well as varying levels of Di: Loss of P in all samples The end result is that the data are nearly certain not to remain colinear: Loss of P destroys the fit to a line.
Even in our simple four-data-point example isochron, a change to two of the samples Migration of parent in two data points. Specific loss of P required to yield a different colinear plot.
The two samples must each change by the indicated amount -- no more and no less -- if the data are to remain colinear. In the special case where the isochron line has a zero slope indicating zero agethen gain or loss of P may move the data points, but they will all still fall on the same horizontal line. In other words, random gain or loss of P does not affect a zero-age isochron. This is an important point. If the Earth were as young as young-Earth creationists insist, then the "contamination" which they suggest to invalidate dating methods would have no noticeable effect on the results.
Moreover, the daughter atoms produced by decay in a mineral are isotopes of different elements and have different ionic charges and radii compared with their parents. The energy released during the decay may produce dislocations or even destroy the crystal lattice locally, thus making it all the more easy for the radiogenic daughters to escape.
This will change the vertical position of the data points: Gain or loss of D. As with gain or loss of P, in the general case it is highly unlikely that the result will be an isochron with colinear data points: Exceptions for loss of daughter There are two exceptions, where it is possible for migration of D to result in an isochron with reasonably colinear data points: If the D is completely homogenized, then the isochron age is reset to zero.
When this happens, any later dating attempt will yield the age of that metamorphic event rather than the original time of crystallization: Complete homogenization of radiogenic daughter resets the isochron age to zero.
If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped. If this happens, then the date obtained will be older than the date at which the magma erupted. For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar. Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.
The dating equation used for K-Ar is: Some of the problems associated with K-Ar dating are Excess argon. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals should not contain any excess Ar because Ar should not enter the crystal structure of a mineral when it crystallizes.
Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende.
If none of these are present, then the only alternative is to date whole rocks. Some 40Ar could be absorbed onto the sample surface. This can be corrected for. Most minerals will lose Ar on heating above oC - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock.