Review "Contested Bones" (Part 13 - Chapter 12 "Dating Methods" [Part 2 - U-Th/U-Pb/14C]) 5-5-2018Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence. Uranium comes in two common isotopes with atomic weights of and we'll call them U and U. Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead Pb. The two cascades are different—U becomes Pb and U becomes Pb.
They are 12 C, 13 C and 14 C. C being the symbol for carbon and the isotopes having atomic weights 12, 13 and The three isotopes don't occur equally either, The radiocarbon dating method is based on the rate of decay of the radioactive or unstable 14 C which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen After formation the three carbon isotopes combine with oxygen to form carbon dioxide.
The carbon dioxide mixes throughout the atmosphere, dissolves in the oceans, and via photosynthesis enters the food chain to become part of all plants and animals. In principle the uptake rate of 14 C by animals is in equilibrium with the atmosphere. As soon as a plant or animal dies, they stop the metabolic function of carbon uptake and with no replenishment of radioactive carbon, the amount of 14 C in their tissues starts to reduce as the 14 C atoms decay.
Libby and his colleagues first discovered that this decay occurs at a constant rate. They found that after years, half the 14 C in the original sample will have decayed and after another years, half of that remaining material will have decayed, and so on. This became known as the Libby half-life. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 to 60 years, the limit of the technique is reached beyond this time, other radiometric techniques must be used for dating.Decay scheme of K-Ar, U-Pb and Sm-Nd, petrogenetic implications-part B
By measuring the 14 C concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the number of decay events per gram of Carbon. By comparing this with modern levels of activity wood corrected for decay to AD and using the measured half-life it becomes possible to calculate a date for the death of the sample.
As a result of atomic bomb usage, 14 C was added to the atmosphere artificially. This affects the 14 C ages of objects younger than Any material which is composed of carbon may be dated. Herein lies the true advantage of the radiocarbon method.
U-Th-Pb Decay Systems 9/4/ Lecture outline: U-Th-Pb 3) Discordant U-Pb dates and open . endorses radiometric dating as the only promising approach. U-Pb radioisotope dating is now the absolute dating method of first choice Two New Studies Revise Key Parameters in Radiometric Dating. Radiometric measurements of time discusses how geological time can be measured Uranium-Lead (U-Pb) dating is the most reliable method for dating.
Potassium-Argon Dating. Potassium-Argon K-Ar dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral.
Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.
Some examples of isotope systems used to date geologic materials. If we divide equation 4 through by the amount of 86 Sr, then we get:. Note also that equation 5 has the form of a linear equation, i.
How can we use this? 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 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.
U pb radiometric dating
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.
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed. 1 The isotopes; 2 Isochron dating and U-Pb; 3 Zircons; 4 The method igneous rocks, it is sufficiently common to be used for the purposes of radiometric dating. ity and prolificacy of the U–Pb system. Though zircon is by far the most commonly utilized mineral for U–Pb dating. (Hanchar and Hoskin, ), monazite.
Measuring the amount of 14 C 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 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. Now, compounds of uranium are often highly soluble in water this, indeed, is one of the major problems with U-Pb isochron dating whereas compounds of lead are stubbornly insoluble.
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-Srthe 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.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
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: its present ratio of uranium to lead is so tiny that either the solar system and indeed the universe is many many times older than cosmologists think, or, given the long half-life 4. 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. 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: this would hardly be possible if we were using the wrong figures for the initial lead isotope ratios.
Of all the isotopic dating methods in use today, the uranium-lead method is the The favorite mineral among U-Pb daters is zircon (ZrSiO4). High-precision U–Pb dating of complex zircon from the Lewisian also use Ar-Ar radiometric dating to get age information, for example. Principles of Radiometric Dating. Radioactive decay is Daughter. ?1/2. Useful Range. Type of Material. U. Pb. b.y. >10 million.
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. 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: if the rock has been seriously disturbed, so that the isotope ratios depend on something other than the passage of time, then when we plot the minerals on our graph, they will almost certainly not lie on a straight line, and we will not obtain a date. Finally, I should mention fission track dating.