What are the principles of radiometric dating

In nature, however, each mineral in the rock is likely to have a different amount of 87 Rb.

Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time. The Concordia curve can be calculated by defining the following: The discordia is often interpreted by extrapolating both ends to intersect the Concordia. Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay.

U would have been stable in the crystallographic site, but the site is now occupied by by Pb. An event like metamorphism could heat the crystal to the point where Pb will become mobile. Another possible scenario involves U leakage, again possibly as a result of a metamorphic event. U leakage would cause discordant points to plot above the cocordia.

The Age of the Earth A minimum age of the Earth can be obtained from the oldest known rocks on the Earth. So far, the oldest rock found is a tonalitic Gneiss metamorphic rock rock from the Northwest Territories, Canada, with an age of 3.

This gives us only a minimum age of the Earth. Is it likely that we will find a rock formed on the Earth that will give us the true age of the Earth? From the Pb-Pb isochron equation 11 we can make some arguments about meteorites. First, it appears that meteorites have come from somewhere in the solar system, and thus may have been formed at the same time the solar system and thus the Earth formed. If all of the meteorites formed at the same time and have been closed to U and Pb since their formation, then we can use the Pb-Pb isochron to date all meteorites.

First, however, we need to know the initial ratios of the Pb isotopes. We recognize two major types of meteorites: Fe- meteorites and stony or chondritic meteorites The Fe meteorites contain the mineral troilite FeS that has no U. Since the mineral troilite contains no U, all of the Pb present in the troilite is the Pb originally present, and none of it has been produced by U decay.

We can then determine the Pb ratios in other meteorites and see if they fall on a Pb-Pb isochron that passes through the initial ratios determined from troilite in Fe-meteorites. The slope of this isochron, known as the Geochron, gives an age of 4. K-Ar Dating 40 K is the radioactive isotope of K, and makes up 0.

Thus the ratio of 14 C to 14 N in the Earth's atmosphere is constant. Living organisms continually exchange Carbon and Nitrogen with the atmosphere by breathing, feeding, and photosynthesis. When an organism dies, the 14 C decays back to 14 N, with a half-life of 5, years. Measuring the amount of 14 C in this dead material thus enables the determination of the time elapsed since the organism died.

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Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells. Because of the short half-life of 14 C, it is only used to date materials younger than about 70, years. Other Uses of Isotopes Radioactivity is an important heat source in the Earth. Elements like K, U, Th, and Rb occur in quantities large enough to release a substantial amount of heat through radioactive decay.

Thus radioactive isotopes have potential as fuel for such processes as mountain building, convection in the mantle to drive plate tectonics, and convection in the core to produce the Earth's magnetic Field. Initial isotopic ratios are useful as geochemical tracers. Such tracers can be used to determine the origin of magmas and the chemical evolution of the Earth.

Short-lived isotopes Isotopes made during nucleosynthesis that have nearly completely decayed away can give information on the time elapsed between nucleosynthesis and Earth Formation. Ratios of stable, low mass isotopes, like those of O, S, C, and H can be used as tracers, as well as geothermometers, since fractionation of light isotopes can take place as a result of chemical process.

We can thus use these ratios of light isotopes to shed light on processes and temperatures of past events. Radioactivity is a source of energy and thus can be exploited for human use - good and bad. Examples of questions on this material that could be asked on an exam Which isotopic systems are most useful for radiometric dating and what are the limitations of each? What is an isochron and what information can be obtained from an isochron?

Why is zircon the preferred mineral for obtainting U - Pb dates? Review 'Well organized, beginning with the basic physics nuclear structure, radioactivity, nucleosynthesis then proceeding to how geochronometry actually works, including technical details of mass spectrometry and error analysis, then tackling the ways in which various isotope systems have been used to help understand the history of the earth and the solar system. Cambridge University Press; 1 edition May 4, Language: Be the first to review this item Amazon Best Sellers Rank: Try the Kindle edition and experience these great reading features: Share your thoughts with other customers.

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Description The time-dependent decay of naturally occurring radioactive isotopes or in-growth of their radioactive or stable daughter products form the basis of radiometric dating of several natural processes. Developed in the beginning of the last century mainly to determine the absolute ages of rocks and minerals, radiometric chronology now plays a central role in a broad range of Earth and planetary sciences - from extra-solar-system processes to environmental geoscience. With the prerequisite of only college-level knowledge in physics, chemistry and mathematics, this concise book focuses on the essential principles of radiometric dating in order to enable students and teachers belonging to diverse fields of studies to select, understand and interpret radiometric dating results generated and published by professionals.

Table of contents Preface; Acknowledgements; 1. Radioactivity and radiometric dating; 6. Mass spectrometry in isotope geochemistry; 7. Chronology of meteorite history;

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