Elements in radiometric dating
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Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating 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. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists.
Then, in , radioactivity was discovered. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently. 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.
The energies involved are so large, and the nucleus is so small that physical conditions in the Earth i. T and P cannot affect the rate of decay. The rate of decay or rate of change of the number N of particles is proportional to the number present at any time, i. So, we can write. After the passage of two half-lives only 0.
This can only be done for 14 C, since we know N 0 from the atmospheric ratio, assumed to be constant through time. For other systems we have to proceed further. The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. We can see how do deal with this if we take a particular case.
The neutron emits an electron to become a proton. We still don't know 87 Sr 0 , the amount of 87 Sr daughter element initially present. Thus, 86 Sr is a stable isotope, and the amount of 86 Sr does not change through time. So, applying this simplification,. The reason for this is that Rb has become distributed unequally through the Earth over time.
For example the amount of Rb in mantle rocks is generally low, i. Thus we could tell whether the rock was derived from the mantle or crust be determining its initial Sr isotopic ratio as we discussed previously in the section on igneous rocks. Two isotopes of Uranium and one isotope of Th are radioactive and decay to produce various isotopes of Pb. The decay schemes are as follows. Note that the present ratio of. If these two independent dates are the same, we say they are concordant.
We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates. The Concordia curve can be calculated by defining the following:. Zircon has a high hardness 7. Zircon can also survive metamorphism. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.
Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene. Discordant dates will not fall on the Concordia curve. Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. Such a chord is called a discordia.
We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8. Since we know that the , and assuming that the Pb and Pb dates are the same, then equation 11 is the equation for a family of lines that have a slope. The answer is about 6 billion years. This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. It does, however, give a maximum age of the Earth.
Is this the age of the Earth? Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Modern Oceanic Pb - i. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon. Thus, our best estimate of the age of the Earth is 4.
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. Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40 K is important in dating rocks. But this scheme is not used because 40 Ca can be present as both radiogenic and non-radiogenic Ca.
Since Ar is a noble gas, it can escape from a magma or liquid easily, and it is thus assumed that no 40 Ar is present initially. 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. Atmospheric Argon. Some 40 Ar could be absorbed onto the sample surface. This can be corrected for. Metamorphism or alteration. Most minerals will lose Ar on heating above o C - 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. Examples of questions on this material that could be asked on an exam. Radiometric Dating. 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. To see how we actually use this information to date rocks, consider the following: To account for this, we first note that there is an isotope of Sr, 86 Sr, that is: 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 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.
Radiometric dating is mostly used to determine the age of rocks, though a particular One key assumption is that the initial quantity of the parent element can be. Isotopes Commonly used for Radiometric Dating. Isotopes, Half-life (years), Effective Dating Range (years). Dating Sample, Key Fission Product. Lutetium-
Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms. When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate.
Petrology Tulane University Prof. Stephen A.
Science in Christian Perspective. Radiometric Dating. A Christian Perspective.
Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements. By "age" we mean the elapsed time from when the mineral specimen was formed. Radioactive elements "decay" that is, change into other elements by "half lives. The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives. If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula.
See also Counterexamples to an Old Earth. Radiometric dating is a method of determining the age of an artifact by assuming that on average decay rates have been constant see below for the flaws in that assumption and measuring the amount of radioactive decay that has occurred. Because radiometric dating fails to satisfy standards of testability and falsifiability , claims based on radiometric dating may fail to qualify under the Daubert standard for court-admissible scientific evidence. It is more accurate for shorter time periods e. There are a number of implausible assumptions involved in radiometric dating with respect to long time periods. One key assumption is that the initial quantity of the parent element can be determined. With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample. One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect too old. In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C 14 to C 12 , but the same principle otherwise applies.
Radiometric dating , radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.
Он поздравил меня с обнаружением черного хода в Попрыгунчике, - продолжал Хейл. - И назвал это победой в борьбе за личные права граждан всего мира. Ты должна признать, Сьюзан, что этот черный ход был придуман для того, чтобы ввести мир в заблуждение и преспокойно читать электронную почту. По мне, так поделом Стратмору.
Хейл очень опасен. Он… Но Стратмор растворился в темноте. Сьюзан поспешила за ним, пытаясь увидеть его силуэт. Коммандер обогнул ТРАНСТЕКСТ и, приблизившись к люку, заглянул в бурлящую, окутанную паром бездну. Молча обернулся, бросил взгляд на погруженную во тьму шифровалку и, нагнувшись приподнял тяжелую крышку люка. Она описала дугу и, когда он отпустил руку, с грохотом закрыла люк.
Шифровалка снова превратилась в затихшую черную пещеру. Скорее всего Северная Дакота попал в ловушку. Стратмор опустился на колени и повернул тяжелый винтовой замок. Теперь крышку не поднять изнутри. Подсобка компьютера надежно закрыта.
В свете ламп дневного света он сумел разглядеть под красноватой припухлостью смутные следы каких-то слов, нацарапанных на ее руке. - Но глаза… твои глаза, - сказал Беккер, чувствуя себя круглым дураком. - Почему они такие красные. Она расхохоталась. - Я же сказала вам, что ревела навзрыд, опоздав на самолет. Он перевел взгляд на слова, нацарапанные на ее руке. Она смутилась.
ТРАНСТЕКСТ вскрыл ключ, состоявший из шестидесяти четырех знаков, за десять с небольшим минут, в два миллиона раз быстрее, чем если бы для этого использовался второй по мощности компьютер АНБ. Тогда бы время, необходимое для дешифровки, составило двадцать лет. Производственное управление АНБ под руководством заместителя оперативного директора коммандера Тревора Дж. Стратмора торжествовало победу. ТРАНСТЕКСТ себя оправдал. В интересах сохранения в тайне этого успеха коммандер Стратмор немедленно организовал утечку информации о том, что проект завершился полным провалом.
Слишком уж удобная версия. Стратмор пожал плечами. - Слабое сердце… да к тому же еще испанская жара. Не забывай и о сильнейшем стрессе, связанном с попыткой шантажировать наше агентство… Сьюзан замолчала. Какими бы ни были обстоятельства, она почувствовала боль от потери талантливого коллеги-криптографа. Мрачный голос Стратмора вывел ее из задумчивости.
- Единственный луч надежды во всей этой печальной истории - то, что Танкадо путешествовал. Есть шанс, что его партнер пока ничего не знает.
Код страны - 1. Действительно хорошая новость. ГЛАВА 54 - Пусти. А потом раздался нечеловеческий крик. Это был протяжный вопль ужаса, издаваемый умирающим зверем.
Он ведь даже не знал, что она задействовала Следопыта. А если и знал, подумала Сьюзан, то зачем ему мешать ее поискам парня по имени Северная Дакота. Вопросы, не имеющие ответов, множились в голове. А теперь все по порядку, - произнесла она вслух. К Хейлу можно вернуться чуть позже. Сосредоточившись, Сьюзан перезагрузила Следопыта и нажала клавишу ВВОД.How Do We Know How Old the Earth Is?