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Application Most suitable for dating very recent sediments suspected of being less than years old. Cost and turnaround time Results are usually available within weeks.follow
Historical Geology/U-Pb, Pb-Pb, and fission track dating
Technical Information Method Decay of uranium in the earth's crust releases the gas radon which produces Pb by decay in the atmosphere. Method Decay of uranium in the earth's crust releases the gas radon which produces Pb by decay in the atmosphere. As a result, we expect speleothems when they are first formed to contain some uranium but little or no lead — just like zircons. So we can apply the same technique to speleothems as we do to zircons. We can exploit our double system of U- Pb and U- Pb in another way.
It can be shown mathematically that if the rock has been undisturbed, so that the isotope ratios reflect nothing but the passage of time, then just as with the isochron diagrams we've already discussed though for a different reason the minerals so plotted will lie on a straight line on the graph; and the age of the rock can be calculated from the slope of the line.
Unlike the ordinary isochron methods such as Rb-Sr , the Pb-Pb method does not allow us to deduce the original proportions of the various lead isotopes from the data acquired from the sample. Instead, we need to find this out some other way. We can do this by finding minerals that contain lead but never contained any uranium, or only ever contained it in negligible quantities. Troilite Fe S from iron-rich meteorites fits the bill: You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth. Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook.
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Another reason for believing it is that if we calculate Pb-Pb dates on this basis, the dates we get are in agreement with dates produced by other methods where they can be applied: So taking the figure derived from the troilite as an "anchor" for our calculations, we can then go ahead and apply the Pb-Pb method to rocks which do contain significant quantities of uranium.
Now, recalling that I began this article by explaining that the isochron method is no use for U-Pb, you may wonder why this Pb-Pb isochron should be any better.
However, recall that one of the major problems with the U-Pb isochron is that uranium compounds are highly soluble and are easily removed from the rock by weathering. But when that occurs, the lead will still remain and can be used for Pb-Pb dating. What's more, even if some lead is also removed, then since all the lead isotopes are the same element, having the same merely chemical properties, there will be no tendency for one isotope to be lost in a greater proportion to the others, and so the isotope ratios will remain the same.
It is true, of course, that the removal of the uranium by weathering will slow down and even, if all the uranium is removed, completely stop the radiometric clock, so that we will not have an accurate measurement of the time after the weathering began, and Pb-Pb dating will therefore tell us that the rock is a little younger than it is.
Pb Dating - Geochronology Services by StrataData Ltd
But only a little younger, because a typical chunk of igneous rock will only have spent a relatively short amount of time being exposed to chemical weathering compared to the time when it was not. As with the isochron methods we've already met, the Pb-Pb isochron method carries its own built-in check on its correctness: Finally, I should mention fission track dating. The decay chain by which uranium decays involves the emission of alpha particles , and as these particles travel through the rock they produce microscopic scars fission tracks in the minerals they pass through, which can be revealed by cutting and polishing the minerals and inspecting them through a microscope.
The Pb-Pb dating method could be applied to date uraniferous bitumens representing hydrocarbon migration in diverse geologic environments.
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Volume 18, Number Previous Article Next Article. Research Article October 01, Ian Swainbank Ian Swainbank.
Geology 18 Abstract Previous attempts at U-Pb dating of uraniferous bitumens have had limited significance because of radioelement migration. You do not currently have access to this article.