The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. K-Ar Dating 40K is the radioactive isotope of K, and makes up 0. Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40K is important in dating rocks. But this scheme is not used because 40Ca can be present as both radiogenic and non-radiogenic Ca. Note that this is not always true.
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.
If only partial loss of Ar occurs then the age determined will be in between the age of crystallization and the age of metamorphism. If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event. The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred. Thus the ratio of 14C to 14N 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 14C decays back to 14N, with a half-life of 5, years.
Measuring the amount of 14C in this dead material thus enables the determination of the time elapsed since the organism died. Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells.
Because of the short half-life of 14C, 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? What is the Concordia, how is it used, and what information can be obtained from discordant dates? How does radiocarbon dating differ from the other methods of radiometric dating?