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Radiometric dating , radioactive dating or radioisotope dating is a technique which is 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. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.
When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.
Radiometric dating steps
Cosmic ray exposure calibrations must take into. Nevertheless, terrestrial cosmic-ray exposure dating has been shown to be useful in many cases. We have covered a lot of convincing evidence that the Earth was created a very long time ago. The agreement of many different dating methods, both radiometric and non-radiometric, over hundreds of thousands of samples, is very convincing.
Yet, some Christians question whether we can believe something so far back in the past. My answer is that it is similar to believing in other things of the past. It only differs in degree. Why do you believe Abraham Lincoln ever lived? Because it would take an extremely elaborate scheme to make up his existence, including forgeries, fake photos, and many other things, and besides, there is no good reason to simply have made him up.
Well, the situation is very similar for the dating of rocks, only we have rock records rather than historical records. Consider the following:. The last three points deserve more attention. Some Christians have argued that something may be slowly changing with time so all the ages look older than they really are. The only two quantities in the exponent of a decay rate equation are the half-life and the time. So for ages to appear longer than actual, all the half-lives would have to be changing in sync with each other.
One could consider that time itself was changing if that happened remember that our clocks are now standardized to atomic clocks! Beyond this, scientists have now used a "time machine" to prove that the half-lives of radioactive species were the same millions of years ago.
This time machine does not allow people to actually go back in time, but it does allow scientists to observe ancient events from a long way away. The time machine is called the telescope. Because God's universe is so large, images from distant events take a long time to get to us.
Telescopes allow us to see supernovae exploding stars at distances so vast that the pictures take hundreds of thousands to millions of years to arrive at the Earth. So the events we see today actually occurred hundreds of thousands to millions of years ago. And what do we see when we look back in time? Much of the light following a supernova blast is powered by newly created radioactive parents.
So we observe radiometric decay in the supernova light. The half-lives of decays occurring hundreds of thousands of years ago are thus carefully recorded! These half-lives completely agree with the half-lives measured from decays occurring today. We must conclude that all evidence points towards unchanging radioactive half-lives. Some individuals have suggested that the speed of light must have been different in the past, and that the starlight has not really taken so long to reach us.
However, the astronomical evidence mentioned above also suggests that the speed of light has not changed, or else we would see a significant apparent change in the half-lives of these ancient radioactive decays.
Some doubters have tried to dismiss geologic dating with a sleight of hand by saying that no rocks are completely closed systems that is, that no rocks are so isolated from their surroundings that they have not lost or gained some of the isotopes used for dating. Speaking from an extreme technical viewpoint this might be true-perhaps 1 atom out of 1, of a certain isotope has leaked out of nearly all rocks, but such a change would make an immeasurably small change in the result.
The real question to ask is, "is the rock sufficiently close to a closed system that the results will be same as a really closed system? These books detail experiments showing, for a given dating system, which minerals work all of the time, which minerals work under some certain conditions, and which minerals are likely to lose atoms and give incorrect results.
Understanding these conditions is part of the science of geology. Geologists are careful to use the most reliable methods whenever possible, and as discussed above, to test for agreement between different methods. Some people have tried to defend a young Earth position by saying that the half-lives of radionuclides can in fact be changed, and that this can be done by certain little-understood particles such as neutrinos, muons, or cosmic rays.
This is stretching it. While certain particles can cause nuclear changes, they do not change the half-lives. The nuclear changes are well understood and are nearly always very minor in rocks. In fact the main nuclear changes in rocks are the very radioactive decays we are talking about. There are only three quite technical instances where a half-life changes, and these do not affect the dating methods we have discussed.
Only one technical exception occurs under terrestrial conditions, and this is not for an isotope used for dating. According to theory, electron-capture is the most likely type of decay to show changes with pressure or chemical combination, and this should be most pronounced for very light elements. The artificially-produced isotope, beryllium-7 has been shown to change by up to 1.
Jun 17, Radiometric dating is based on an observable fact of science: unstable atoms will break down over a measurable period of time. Actually, it isn't really a decay process in the normal sense of the word, like the decay of rhodeshotel.net: Dr. Andrew A. Snelling. Radiometric dating. 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. Radiometric dating. First step: we need to identify a radioactive parent material that is present in our rock and that decays into daughter material at a rate that ensures enough of both parent and daughter in the rock to measure them.
In another experiment, a half-life change of a small fraction of a percent was detected when beryllium-7 was subjected toatmospheres of pressure, equivalent to depths greater than miles inside the Earth Science, All known rocks, with the possible exception of diamonds, are from much shallower depths. In fact, beryllium-7 is not used for dating rocks, as it has a half-life of only 54 days, and heavier atoms are even less subject to these minute changes, so the dates of rocks made by electron-capture decays would only be off by at most a few hundredths of a percent.
Physical conditions at the center of stars or for cosmic rays differ very greatly from anything experienced in rocks on or in the Earth. Yet, self-proclaimed "experts" often confuse these conditions. Cosmic rays are very, very high-energy atomic nuclei flying through space.
The electron-capture decay mentioned above does not take place in cosmic rays until they slow down. This is because the fast-moving cosmic ray nuclei do not have electrons surrounding them, which are necessary for this form of decay. Another case is material inside of stars, which is in a plasma state where electrons are not bound to atoms.
In the extremely hot stellar environment, a completely different kind of decay can occur. This has been observed for dysprosium and rhenium under very specialized conditions simulating the interior of stars Phys. All normal matter, such as everything on Earth, the Moon, meteorites, etc.
As an example of incorrect application of these conditions to dating, one young-Earth proponent suggested that God used plasma conditions when He created the Earth a few thousand years ago. This writer suggested that the rapid decay rate of rhenium under extreme plasma conditions might explain why rocks give very old ages instead of a young-Earth age. This writer neglected a number of things, including: a plasmas only affect a few of the dating methods.
More importantly, b rocks and hot gaseous plasmas are completely incompatible forms of matter! The material would have to revert back from the plasma state before it could form rocks. In such a scenario, as the rocks cooled and hardened, their ages would be completely reset to zero as described in previous sections. That is obviously not what is observed. The last case also involves very fast-moving matter. It has been demonstrated by atomic clocks in very fast spacecraft.
These atomic clocks slow down very slightly only a second or so per year as predicted by Einstein's theory of relativity. No rocks in our solar system are going fast enough to make a noticeable change in their dates. These cases are very specialized, and all are well understood.
None of these cases alter the dates of rocks either on Earth or other planets in the solar system. The conclusion once again is that half-lives are completely reliable in every context for the dating of rocks on Earth and even on other planets. The Earth and all creation appears to be very ancient. It would not be inconsistent with the scientific evidence to conclude that God made everything relatively recently, but with the appearance of great age, just as Genesis 1 and 2 tell of God making Adam as a fully grown human which implies the appearance of age.
The idea of a false appearance of great age is a philosophical and theological matter that we won't go into here. The main drawback-and it is a strong one-is that this makes God appear to be a deceiver. However, some. Certainly whole civilizations have been incorrect deceived? Whatever the philosophical conclusions, it is important to note that an apparent old Earth is consistent with the great amount of scientific evidence.
As Christians it is of great importance that we understand God's word correctly. Yet from the middle ages up until the s people insisted that the Bible taught that the Earth, not the Sun, was the center of the solar system.
It wasn't that people just thought it had to be that way; they actually quoted scriptures: "The Earth is firmly fixed; it shall not be moved" Psalmor "the sun stood still" Joshua ; why should it say the sun stood still if it is the Earth's rotation that causes day and night? I am afraid the debate over the age of the Earth has many similarities.
But I am optimistic. Today there are many Christians who accept the reliability of geologic dating, but do not compromise the spiritual and historical inerrancy of God's word. While a full discussion of Genesis 1 is not given here, references are given below to a few books that deal with that issue.
There are a number of misconceptions that seem especially prevalent among Christians. Most of these topics are covered in the above discussion, but they are reviewed briefly here for clarity. Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered. This is not at all true, though it is implied by some young-Earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes.
These half-lives have been measured over the last years. They are not calibrated by fossils. No one has measured the decay rates directly; we only know them from inference. Decay rates have been directly measured over the last years. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed.
In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector. These instruments count the number of decays over a long time. If the half-lives are billions of years, it is impossible to determine them from measuring over just a few years or decades. The example given in the section titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter.
This is because a all decay curves have exactly the same shape Fig. Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives. Most of the decay rates used for dating rocks are known to within two percent. Such small uncertainties are no reason to dismiss radiometric dating.
Whether a rock is million years or million years old does not make a great deal of difference. A small error in the half-lives leads to a very large error in the date. Since exponents are used in the dating equations, it is possible for people to think this might be true, but it is not.
This is not true in the context of dating rocks. Radioactive atoms used for dating have been subjected to extremes of heat, cold, pressure, vacuum, acceleration, and strong chemical reactions far beyond anything experienced by rocks, without any significant change.
The only exceptions, which are not relevant to dating rocks, are discussed under the section, "Doubters Still Try", above. A small change in the nuclear forces probably accelerated nuclear clocks during the first day of creation a few thousand years ago, causing the spuriously old radiometric dates of rocks. Rocks are dated from the time of their formation. For it to have any bearing on the radiometric dates of rocks, such a change of nuclear forces must have occurred after the Earth and the rocks were formed.
To make the kind of difference suggested by young-Earth proponents, the half-lives must be shortened from several billion years down to several thousand years-a factor of at least a million.
But to shorten half-lives by factors of a million would cause large physical changes. As one small example, recall that the Earth is heated substantially by radioactive decay. If that decay is speeded up by a factor of a million or so, the tremendous heat pulse would easily melt the whole Earthincluding the rocks in question! No radiometric ages would appear old if this happened. The decay rates might be slowing down over time, leading to incorrect old dates.
There are two ways we know this didn't happen: a we have checked them out with "time machines", and b it doesn't make sense mathematically. We should measure the "full-life" the time at which all of the parent is gone rather than the half-life the time when half of it is gone.
Unlike sand in an hourglass, which drops at a constant rate independent of how much remains in the top half of the glass, the number of radioactive decays is proportional to the amount of parent remaining.
A half-life is more easy to define than some point at which almost all of the parent is gone. Scientists sometimes instead use the term "mean life", that is, the average life of a parent atom. For most of us half-life is easier to understand. To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there.
It is very easy to calculate the original parent abundance, but that information is not needed to date the rock. All of the dating schemes work from knowing the present abundances of the parent and daughter isotopes. There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to anomalously old ages.
A good part of this article is devoted to explaining how one can tell how much of a given element or isotope was originally present.
Important steps in a, stable isotopes 41k and accelerator mass. By alpha steps will have both radioactive dating is unstable, she compares conventional and spontaneously decays to lead by a method would be older than. C four steps until all decay rhodeshotel.net As with all dating, the agreement of two or more methods is highly recommended for confirmation of a measurement. If the samples are beyond the range of radiocarbon (e.g., > 40, years), a second method for confirmation of thorium ages may need to be a non-radiometric method such as ESR or TL, mentioned below.
Usually it involves using more than one sample from a given rock. It is done by comparing the ratios of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope. From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount it would not change if there were no parent isotope to decay.
Figures 4 and 5, and the accompanying explanation, tell how this is done most of the time. This article has listed and discussed a number of different radiometric dating methods and has also briefly described a number of non-radiometric dating methods. There are actually many more methods out there. Well over forty different radiometric dating methods are in use, and a number of non-radiogenic methods not even mentioned here.
Radiometric Dating Step By and Free way to Bang Local Girls. Let's be honest, you're here because you're tired of jerking off, swiping endlessly on regular dating apps, and wasting your hard-earned money at bars and clubs/ Radiometric dating steps The age of radiometric dating. After all true, she compares conventional and disappear within a radioactive isotopes. Introduction to date sedimentary rocks and seek you. Start studying radiometric dating is a cave in metamorphic rocks usually indicates the internal. For radiometric dating generally yields the unstable and dating for your team for radioactive isotope to date. To how old the steps help remove as a complex series of steps and non-radioactive isotopes.
This refers to tiny halos of crystal damage surrounding spots where radioactive elements are concentrated in certain rocks. Halos thought to be from polonium, a short-lived element produced from the decay of uranium, have been found in some rocks. A plausible explanation for a halo from such a short-lived element is that these were not produced by an initial concentration of the radioactive element. Rather, as water seeped through cracks in the minerals, a chemical change caused newly-formed polonium to drop out of solution at a certain place and almost immediately decay there.
A halo would build up over a long period of time even though the center of the halo never contained more than a few atoms of polonium at one time.
Other researchers have found halos produced by an indirect radioactive decay effect called hole diffusion, which is an electrical effect in a crystal.
These results suggest that the halos in question are not from short-lived isotopes after all.
At any rate, halos from uranium inclusions are far more common. Because of uranium's long half-lives, these halos take at least several hundred million years to form. Because of this, most people agree that halos provide compelling evidence for a very old Earth.
A young-Earth research group reported that they sent a rock erupted in from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years. This shows we should not trust radiometric dating. There are indeed ways to "trick" radiometric dating if a single dating method is improperly used on a sample. Anyone can move the hands on a clock and get the wrong time. Likewise, people actively looking for incorrect radiometric dates can in fact get them.
Geologists have known for over forty years that the potassium-argon method cannot be used on rocks only twenty to thirty years old. Publicizing this incorrect age as a completely new finding was inappropriate. The reasons are discussed in the Potassium-Argon Dating section above.
Be assured that multiple dating methods used together on igneous rocks are almost always correct unless the sample is too difficult to date due to factors such as metamorphism or a large fraction of xenoliths. Low abundances of helium in zircon grains show that these minerals are much younger than radiometric dating suggests. Zircon grains are important for uranium-thorium-lead dating because they contain abundant uranium and thorium parent isotopes.
Helium is also produced from the decay of uranium and thorium. However, as a gas of very small atomic size, helium tends to escape rather easily. Researchers have studied the rates of diffusion of helium from zircons, with the prediction from one study by a young- Earth creationist suggesting that it should be quantitatively retained despite its atomic size.
The assumptions of the temperature conditions of the rock over time are most likely unrealistic in this case. The fact that radiogenic helium and argon are still degassing from the Earth's interior prove that the Earth must be young. The radioactive parent isotopes, uranium and potassium, have very long half-lives, as shown in Table 1. These parents still exist in abundance in the Earth's interior, and are still producing helium and argon. There is also a time lag between the production of the daughter products and their degassing.
If the Earth were geologically very young, very little helium and argon would have been produced. One can compare the amount of argon in the atmosphere to what would be expected from decay of potassium over 4.
The waters of Noah's flood could have leached radioactive isotopes out of rocks, disturbing their ages. This is actually suggested on one website!
While water can affect the ability to date rock surfaces or other weathered areas, there is generally no trouble dating interior portions of most rocks from the bottom of lakes, rivers, and oceans.
Additionally, if ages were disturbed by leaching, the leaching would affect different isotopes at vastly different rates. Ages determined by different methods would be in violent disagreement. If the flood were global in scope, why then would we have any rocks for which a number of different methods all agree with each other?
In fact, close agreement between methods for most samples is a hallmark of radiometric dating. We know the Earth is much younger because of non-radiogenic indicators such as the sedimentation rate of the oceans. There are a number of parameters which, if extrapolated from the present without taking into account the changes in the Earth over time, would seem to suggest a somewhat younger Earth.
These arguments can sound good on a very simple level, but do not hold water when all the factors are considered. Some examples of these categories are the decaying magnetic field not mentioning the widespread evidence for magnetic reversalsthe saltiness of the oceans not counting sedimentation! While these arguments do not stand up when the complete picture is considered, the case for a very old creation of the Earth fits well in all areas considered.
The fact is that there are a number of Bible-believing Christians who are involved in radiometric dating, and who can see its validity firsthand. A great number of other Christians are firmly convinced that radiometric dating shows evidence that God created the Earth billions, not thousands, of years ago.
This is not true at all. The fact that dating techniques most often agree with each other is why scientists tend to trust them in the first place. Nearly every college and university library in the country has periodicals such as ScienceNatureand specific geology journals that give the results of dating studies. The public is usually welcome to and should! So the results are not hidden; people can go look at the results for themselves.
Over a thousand research papers are published a year on radiometric dating, essentially all in agreement. Besides the scientific periodicals that carry up-to-date research reports, specific suggestions are given below for further reading, both for textbooks, non-classroom books, and web resources. Resources On the Web :. Virtual Dating-a very helpful educational course on half-lives and radioactive decay was put together by Gary Novak at California State University in Los Angeles.
This site has several interactive web "workbooks" to help the reader understand various concepts involved with radiometricdating. Reasons to Believe-a Christian ministry supporting the old-Earth viewpoint. Hugh Ross, the founder and head of the ministry, holds a PhD in Astronomy. The ministry supports an accurate interpretation of the Bible while also supportive of science as a tool to study God's creation.
Most of the members hold an old-Earth view, though membership is open to anyone supporting their positional statement. This website has numerous resources on theology and Bible-science issues. There is a wealth of information, including presentations on the interpretation of Genesis chaptersa resource list of apologetics ministries, etc. Reviewed by Rev. John W. Origins-this site is devoted mainly to evidences for intelligent design in nature. Talk Origins-an archive dedicated to creation-evolution issues.
It includes separate resource sections on the reliability of radiometric dating, introductory articles, advanced articles, radiocarbon dating, etc. C Dating-The radiocarbon laboratories at Oxford England and Waikato New Zealand Universities jointly operate this website which gives very comprehensive information on radiocarbon dating.
Portions of it were written specifically for use by K students, so it is easy to understand. The site contains explanations on measurements, applications, calibration, publications, and other areas. Cornell University Geology Lecture Notes-A large number of files of geology lecture notes are available on the web.
These are university-level lecture notes describing radiometric dating and related topics. Radiometric dating textbooks: The following books are popular college-level Geology texts that deal in depth with various dating techniques.
Geologic Time is very easy to read and has been around for quite some time. The text by Dalrymple is meant to be relatively easy to read, but is also very comprehensive. The Faure and Dickin texts are regular textbooks for Geology, including more mathematics and more details. Dickin, Alan P. Cambridge University Press, pp. Dalrymple, G. Brent The Age of the Earth. Stanford University Press, pp. MacMillan Pub. Faure, Gunter Principles of Isotope Geology2nd edition.
Wiley, New York, pp. Eicher, Don L. Atheneum Books, New York, 92 pp. This is a book designed for easy reading on the general subject of dating. This short book covers topics from archeology to tree ring dating to radiocarbon dating of the dead sea scrolls, to dating of meteorites and moon rocks. The book is out of print, but slightly used copies can be obtained from online dealers like Amazon. Wagner, G? Springer-Verlag, New York, pp. This book is a quite comprehensive reference on all methods for determining dates less than about a million years old.
Strahler, Arthur N.
Prometheus Books, Buffalo, pp. This book is a very thorough and comprehensive refutation of young-Earth ideas, written by a non-Christian. The only negative ct is that at one point Strahler throws in a bit of his own theology-his arguments against the need for a God. This book is long and in small print; it covers a wealth of information.
For ice core studies, the Journal of Geophysical Research, volumestarting with page 26, has 47 papers on two deep ice cores drilled in central Greenland. Books on scripture, theology, and science :. He addresses typical objections brought up by young-Earth adherents, including the death of animals before Adam and Eve's sin, entropy or decay before the fall, the six days of creation, and the flood.
This is a very readable theological book about Genesis. Sailhamer has served on the translation committees for two versions of the book of Genesis. Hugh Ross has a PhD in Astronomy. In this book Dr. Ross defends modern science and an old age for the universe, and refutes common young-Earth arguments. He firmly believes in the inerrancy of the Bible. Schroeder, Paramount, CA, pp. A persuasive book written for the Christian layman.
Stoner uses arguments both from the theological and the scientific side. He talks somewhat philosophically about whether God deceives us with the Genesis account if the Earth is really old. Stoner also tries to discuss the meaning of the Genesis 1 text. Van Till Howard J. This book talks about the misuse of science by both hard-line atheists and by young-Earth creationists. A good deal of the book is devoted to refuting young-Earth arguments, including a substantial section on the Grand Canyon geology.
Its authors are well-known Christians in Geology and Physics. Wiester, John The Genesis Connection. John Wiester has taught Geology at Westmont and Biola University, and is active in the American Scientific Affiliation, an organization of scientists who are Christians. This book discusses many scientific discoveries relating to the age of the Earth and how these fit into the context of Genesis 1.
Young, Davis A. He argues for an old Earth and refutes many of the common young-Earth claims including their objections to radiometric dating. The following people are sincerely thanked for their contributions to the first edition: Drs.
Davis Young Calvin CollegeDr. Guillermo Gonzalez U. New Mexico. Kenneth Van Dellen. I thank my wife Gwen, and children, Carson and Isaac, for supporting me in this work, and I thank God for giving us the intelligence to understand little bits and pieces of His amazing creation.
More about the author : Dr. Wiens received a bachelor's degree in Physics from Wheaton College and a PhD from the University of Minnesota, doing research on meteorites and moon rocks. He spent two years at Scripps Institution of Oceanography La Jolla, CA where he studied isotopes of helium, neon, argon, and nitrogen in terrestrial rocks. He worked seven years in the Geological and Planetary Sciences Division at Caltech, where he continued the study of meteorites and worked for NASA on the feasibility of a space mission to return solar wind samples to Earth for study.
Wiens wrote the first edition of this paper while in Pasadena. In he joined the Space and Atmospheric Sciences group at Los Alamos National Laboratory, where he has been in charge of building and flying the payload for the solar-wind mission, as well as developing new instruments for other space missions. He has published over twenty scientific research papers and has also published articles in Christian magazines.
Wiens became a Christian at a young age, and has been a member of Mennonite Brethren, General Conference Baptist, and Conservative Congregational, and Vineyard denominations.
He does not see a conflict between science in its ideal form the study of God's handiwork and the Bible, or between miracles on the one hand, and an old Earth on the other. Alpha decay Radioactive decay in which the atom's nucleus emits an alpha particle. An alpha particle consists of two neutrons and two protons-the same as a helium atom nucleus. In alpha decay, the daughter is four atomic mass units lighter than the parent. Alpha decay is most common in heavy elements. Atom The smallest unit that materials can be divided into.
An atom is about ten billionths of an inch in diameter and consists of a nucleus of nucleons protons and neutrons surrounded by electrons.
Beta decay Radioactive decay in which the atom's nucleus emits or captures an electron or positron. The daughter ends up with the same mass as the parent, but ends up with one more neutron and one less proton, or vice versa.
Because of the different number of protons, the daughter is a different element with different chemical properties than the parent. However, it is challenging and expensive to accurately determine the number of radioactive nuclides. Alternatively, decay constants can be determined by comparing isotope data for rocks of known age.
This method requires at least one of the isotope systems to be very precisely calibrated, such as the Pb-Pb system. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.
The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.
Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium-lead datingthe concordia diagram is used which also decreases the problem of nuclide loss.
Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement except as described below under "Dating with short-lived extinct radionuclides"the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established.
On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
The closure temperature or blocking temperature represents the temperature below which the mineral is a closed system for the studied isotopes. If a material that selectively rejects the daughter nuclide is heated above this temperature, any daughter nuclides that have been accumulated over time will be lost through diffusionresetting the isotopic "clock" to zero.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. This field is known as thermochronology or thermochronometry.
The mathematical expression that relates radioactive decay to geologic time is  . The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value N o. The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. An isochron plot is used to solve the age equation graphically and calculate the age of the sample and the original composition. Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s.
It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.
Uranium-lead radiometric dating involves using uranium or uranium to date a substance's absolute age. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. Uranium-lead dating is often performed on the mineral zircon ZrSiO 4though it can be used on other materials, such as baddeleyiteas well as monazite see: monazite geochronology. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert.
Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
One of its great advantages is that any sample provides two clocks, one based on uranium's decay to lead with a half-life of about million years, and one based on uranium's decay to lead with a half-life of about 4.
Radiometric dating steps
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1.
This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. Application of in situ analysis Laser-Ablation ICP-MS within single mineral grains in faults have shown that the Rb-Sr method can be used to decipher episodes of fault movement.
A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years. It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured.
The scheme has a range of several hundred thousand years. A related method is ionium-thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment. Radiocarbon dating is also simply called carbon dating. Carbon is a radioactive isotope of carbon, with a half-life of 5, years   which is very short compared with the above isotopesand decays into nitrogen.
Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth. The carbon ends up as a trace component in atmospheric carbon dioxide CO 2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesisand animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years.
The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. The rate of creation of carbon appears to be roughly constant, as cross-checks of carbon dating with other dating methods show it gives consistent results. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates.
The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s.
Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere. This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities.
The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons.