Volume 40, Number Previous Article Next Article. Research Article November 01, Rock uplift rates in South Africa from isochron burial dating of fluvial and marine terraces Erica D. Geology 40 Abstract The high plateau of southern Africa is considered to be either uplifting due to mantle-driven dynamic topography, or to have been stable since Mesozoic rifting.
You do not currently have access to this article. You could not be signed in. Librarian Administrator Sign In. Email alerts New issue alert. If all data points lie on a straight line, this line is called an isochron. The better the fit of the data points to a line, the more reliable the resulting age estimate. Since the ratio of the daughter and non-radiogenic isotopes is proportional to the ratio of the parent and non-radiogenic isotopes, the slope of the isochron gets steeper with time.
The change in slope from initial conditions—assuming an initial isochron slope of zero a horizontal isochron at the point of intersection intercept of the isochron with the y-axis—to the current computed slope gives the age of the rock. Whole rock isochron dating uses the same ideas but instead of different minerals obtained from one rock uses different types of rocks that are derived from a common reservoir; e. It is possible to date the differentiation of the precursor melt which then cooled and crystallized into the different types of rocks. One of the best known isotopic systems for isochron dating is the rubidium—strontium system.
Other systems that are used for isochron dating include samarium—neodymium , and uranium—lead. Some isotopic systems based on short-living extinct radionuclides such as 53 Mn , 26 Al , I , 60 Fe and others are used for isochron dating of events in the early history of the Solar System.
However, methods using extinct radionuclides give only relative ages and have to be calibrated with radiometric dating techniques based on long-living radionuclides like Pb-Pb-dating to give absolute ages. Isochron dating is useful in the determination of the age of igneous rocks , which have their initial origin in the cooling of liquid magma. It is also useful to determine the time of metamorphism, shock events such as the consequence of an asteroid impact and other events depending of the behaviour of the particular isotopic systems under such events.
From Wikipedia, the free encyclopedia. In contrast, P is a different element with different chemical properties. This results in a range of X-values for the data points representing individual minerals. Since the data points have the same Y-value and a range of X-values, they initially fall on a horizontal line:.
isochron burial dating | National Academy of Sciences
As more time passes and a significant amount of radioactive decay occurs, the quantity of P decreases by a noticeable amount in each sample, while the quantity of D increases by the same amount. This results in a movement of the data points to the left decreasing P and upwards increasing D. Since each atom of P decays to one atom of D , the data point for each sample will move along a path with a slope of As a result, the data points with the most P the right-most ones on the plot move the greatest distance per unit time.
The data points remain colinear as time passes, but the slope of the line increases:. The slope of the line is the ratio of enriched D to remaining P.
When a "simple" dating method is performed, the result is a single number. There is no good way to tell how close the computed result is likely to be to the actual age. 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 analysis , see:. 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. Where the simple methods will produce an incorrect age, isochron methods will generally indicate the unsuitability of the object for dating. 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.
The amount of initial D is not required or assumed to be zero. The greater the initial D -to- D i 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-axis , then 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 D i. For each sample, it would be possible to measure the amount of the D i , 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. In order to make the figures easy to read and quick to draw , the 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.
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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 D i:. In the special case where the isochron line has a zero slope indicating zero age , then 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.
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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:. There are two exceptions, where it is possible for migration of D to result in an isochron with reasonably colinear data points:. These exceptions should be of little comfort to young-Earthers, for 1 they are uncommon extremely uncommon in the case of partial resetting ; and 2 the result in both cases is an isochron age which is too young to represent the time of formation. Young-Earthers necessarily insist that all ancient isochron ages are really much too old.
In the real world, nothing is perfect. There are some isochron results which are obviously incorrect. The significance of isochron plots is a bit counter-intuitive in some cases. And there are known processes which can yield an incorrect isochron age. Does this leave room to discard isochron dating as entirely unreliable? One of the requirements for isochron dating is that the samples be cogenetic , meaning that they all formed at about the same time from a common pool of material in which the relevant elements and isotopes were distributed reasonably homogeneously. As described in Figure 4 , this is how the data are caused to be colinear.
Usually it is easy to determine whether or not this requirement is met. The check is not just the isochron plot itself which can in most cases indicate such a problem by failure of the data to fall on a line , but in addition the physical location and geological relationships of the samples selected for dating. If this requirement is violated, it is sometimes still possible to obtain an isochron plot with reasonably colinear data points. The significance of the computed age, however, will likely not be the last time of crystallization of each sample.
It might instead be the original time at which the samples became separated from a common pool of matter, or the age of that source material itself. The resulting age is meaningful , but it does not have the meaning which one might expect for the dating result i. Consider an old body of rock as evidenced by its good fit to an isochron with distinctly non-zero slope with minerals which melt at different temperatures.
In this example, the minerals with the lowest melting-point having the lowest P -to- D i and D -to- D i ratios:. The rock is heated slowly, and at various times the molten portions are moved to the surface in a series of lava flows. The earliest flows will have an isotopic composition close to that of the minerals with the lowest melting points; the latest flows will have an isotopic composition close to that of the minerals with the highest melting points.
The individual lava flows are not cogenetic. They did not separate at about the same time from an isotopically homogeneous pool of matter. For the sake of simplicity, we will assume three lava flows each with a composition matching the data points of the previous figure:.
It is likely that at least a small amount chemical differentiation will have occurred in each melt, and that as a result the minerals of each individual lava flow will exhibit a much younger isochron the actual age of each flow:. The data points for the overall composition of each flow fall on an isochron line representing the original crystallization time of the source material, which is much greater than the age of any of the flows. This sort of inherited age is well-understood, discussed thoroughly in the literature, and usually easily avoided by proper selection of samples.
Note also that chemical differentiation at the time of the latest melting resulting in the round data points in Figure 17 induces significant scatter into the isochron plot if any measure other than whole-rock is made:.