All absolute isotopic ages are based on radioactive decaya process whereby a specific atom or isotope is converted into another specific atom or isotope at a constant and known rate. Most elements exist in different atomic forms that are identical in their chemical properties but differ in the number of neutral particles— i. For a single element, these atoms are called isotopes. Because isotopes differ in mass, their relative abundance can be determined if the masses are separated in a mass spectrometer see below Use of mass spectrometers.
Radioactive decay can be observed in the laboratory by either of two means: The particles given Dating using uranium-238 isotopes during the decay process are part of Dating using uranium-238 isotopes profound fundamental change in the nucleus. To compensate for the loss of mass and energythe radioactive atom undergoes internal transformation and in most cases simply becomes an atom of a different chemical element.
In terms of the numbers of atoms present, it is as if apples changed spontaneously into oranges at a fixed and known rate.
In this analogythe apples would represent radioactive, or parent, atoms, while the oranges would represent the atoms formed, the so-called daughters. Pursuing this analogy further, one would expect that a new basket of apples would have no oranges but that an older one would have many. In fact, one would expect Dating using uranium-238 isotopes the ratio of oranges to apples would change in a Dating using uranium-238 isotopes specific way over the time elapsed, since the process continues until all the apples are converted.
In geochronology the situation is identical. A particular rock or mineral that contains a radioactive isotope or radio-isotope is analyzed to determine the number of parent and daughter isotopes present, whereby the time since that mineral or rock formed is calculated.
Of course, one must select geologic materials that contain elements with long half-lives— i.
The age calculated is only as good as the existing knowledge of the decay rate and is valid only if this rate is constant over the time that elapsed. Fortunately for geochronology the study of radioactivity has been the subject of extensive theoretical and laboratory investigation by physicists for almost a century.
The results show that there is no known process that can alter the rate of radioactive decay. By way of explanation it can be noted that since the cause of the process lies deep within atomic nucleus, external forces such as extreme heat and pressure have no effect.
The same is true regarding gravitational, magnetic, and electric fields, as well as the chemical state in which the atom resides. In short, the process of radioactive decay is immutable under all known conditions.
Although it is impossible to predict when a particular atom will change, given a sufficient number of atoms, the rate of their decay is found to be constant.
The situation is analogous to the death rate among human populations insured by an insurance company. Even though it is impossible to predict when a given policyholder will die, the company can count on paying off a certain number of beneficiaries every month.
The recognition that the rate of decay of any radioactive parent atom is proportional to the number of atoms N of the parent remaining at any time gives rise to the following expression:. Converting Dating using uranium-238 isotopes proportion to an equation incorporates the additional observation that different radioisotopes have different disintegration rates even when the same number of atoms are observed undergoing decay.
Solution of this equation by techniques of the calculus yields one form of the fundamental equation for radiometric age determination. Two alterations are generally made to "Dating using uranium-238 isotopes" 4 in order to obtain the form most useful for radiometric dating. In the first place, since the unknown term in radiometric dating is obviously tit is desirable to rearrange equation 4 so that it is explicitly solved for t.
Half-life is defined as the time period that must elapse in order to halve the initial number of radioactive atoms. The half-life and the decay constant are inversely proportional because rapidly decaying radioisotopes have a high decay but a short half-life. With t made explicit and half-life introduced, equation 4 is converted to the following form, in which the symbols have the same meaning:.
Alternatively, because the number of daughter atoms is directly observed rather than Nwhich is the initial number of parent atoms present, another formulation may be more convenient. Since the initial number of parent atoms present at time zero N 0 must be the sum of the parent atoms remaining N and the daughter atoms present Done Dating using uranium-238 isotopes write:.
Substituting this in equation 6 gives. If one Dating using uranium-238 isotopes to use P to designate the parent atom, the expression assumes its familiar form:. This pair of equations states rigorously what might be assumed from intuitionthat minerals formed at successively longer times in the past would have progressively higher daughter-to-parent ratios. This follows because, as each parent atom loses its identity with time, it reappears as a daughter atom.
Equation 8 documents the simplicity of direct isotopic dating. The time of decay is proportional to the natural logarithm represented by ln of the ratio of D to P. In short, one need only measure the ratio of the number of radioactive parent and daughter atoms present, and the time elapsed since the mineral or rock formed can be calculated, provided of course that the decay rate is known.
Likewise, the conditions that must be met to make the calculated age precise and meaningful are in themselves simple:. The rock or mineral must have remained closed to the addition or escape of parent and daughter atoms since the time that the rock or mineral system formed. It must be possible to correct for other atoms identical to daughter atoms already present when the rock or mineral formed.
The measurement of the daughter-to-parent ratio must be accurate because uncertainty in this ratio contributes directly to uncertainty in the age.
Different schemes have been developed to deal with the critical assumptions stated above. In uranium—lead datingminerals virtually free of initial lead can be isolated and corrections made for Dating using uranium-238 isotopes trivial amounts present.
In whole rock isochron methods that make use of the rubidium—strontium or samarium—neodymium decay schemes see belowa series of rocks or minerals are chosen that can be assumed to have the same age and identical abundances of their initial isotopic ratios. The results are then tested for the internal consistency that can validate the assumptions.
In all cases, it is the obligation of the investigator making the determinations to include enough tests to indicate that the Dating using uranium-238 isotopes age quoted is valid within the limits stated. In other words, it is the obligation of geochronologists to try to prove themselves wrong by including a series of cross-checks in their measurements before they publish a result. Such checks include dating a series of ancient units with closely spaced but known relative ages and replicate of different parts of the same rock body with samples collected at widely spaced localities.
The importance of internal checks as well as interlaboratory comparisons becomes all the more apparent when one realizes that geochronology laboratories are limited in number. Because of the expensive equipment necessary and the combination of geologic, chemical, and laboratory skills required, geochronology is usually carried out by teams of experts. Most geologists must rely on geochronologists for their results. In turn, the geochronologist relies on the geologist for relative ages.
In order for a radioactive parent—daughter pair to be useful for dating, many criteria must be met. This section examines these criteria and explores the ways in which the reliability of the ages measured can be assessed. Because geologic materials are diverse in their origin and chemical content and datable elements are unequally distributed, each method has its strengths and weaknesses. When the elements in the Earth were first created, many radioactive isotopes were present.
Of these, only the radioisotopes with extremely long half-lives remain. The table lists a number of such isotopes and their respective daughter products that are used in various forms of rock dating. It should be mentioned in passing that some of the radioisotopes present early in the history of the solar system and now completely extinct have been recorded in meteorites in the form of the elevated abundances of their daughter isotopes.
Analysis of such meteorites makes it possible to estimate the time that elapsed between element creation and meteorite formation. Natural elements that are still radioactive today produce daughter products at a very slow rate; hence, it is easy to date very old minerals but difficult to obtain the age of those formed in the "Dating using uranium-238 isotopes" geologic past.
This follows from the fact that the amount of daughter isotopes present is so small that it is difficult to measure. The difficulty can be overcome to some degree by achieving lower background contamination, by improving instrument sensitivity, and by finding minerals with abundant parent isotopes. Geologic events of the not-too-distant past are more Dating using uranium-238 isotopes dated by using recently formed radioisotopes with short half-lives that produce more daughter products per unit time.
Two sources of such isotopes exist. In one case, intermediate isotopes in the uranium or thorium decay chain can become isolated in certain minerals due to differences in chemical properties and, once fixed, can Dating using uranium-238 isotopes to new isotopes, providing a measure of the time elapsed since they were isolated. To understand this, one needs to know that though uranium U does indeed decay to lead Pbas indicated in the table, it is not a one-step process.
In fact, this is a multistep process involving the expulsion of eight alpha particles and six beta particles, along with a considerable amount of energy. There exists a series of different elements, each of them in a steady state where they form at the same rate as they disintegrate. The number present is proportional to their decay rate, with long-lived members being more abundant. Because all of these isotopes have relatively short half-lives, none remains since the creation of the elements, but instead they are continuously provided by the decay of the long-lived parent.
This type of dating, known as disequilibrium dating, will be explored below Dating using uranium-238 isotopes the section Uranium-series disequilibrium dating. The amounts produced, although small, provide insight into many near-surface processes in the geologic past. The most widely used radioactive cosmogenic isotope is carbon of mass 14 14 Cwhich provides a method of dating events that have occurred over roughly the past 50, years. This time spans much of the historic and prehistoric record of mankind.
Cosmogenic isotopes and those used for disequilibrium dating are listed in the table. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our "Dating using uranium-238 isotopes," and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions.
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