Radiometric dating - Wikipedia
The technique of comparing the abundance ratio of a radioactive isotope to to determine the age of a material is called radioactive dating. Once living things die, they no longer can exchange carbon with the environment. Radioactive dating is a method of dating rocks and minerals using Some do not change with time and form stable isotopes (i.e. those that form to 40Ar ( argon) and 40Ca (calcium) was worked out between and Radiometric dating or radioactive dating is a technique used to date materials such as rocks or Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
The oldest crystals on Earth that were formed on Earth are zircon crystals, and are approximately 4. Asteroids in the solar system have been clocked at 4. We assume that the Earth is probably as old as the asteroids, because we believe the solar system to have formed from a collapsing nebula, and that the Earth, being geologically active, has simply destroyed any older zircon crystals that would be its true age, but we can't really be certain.
The building blocks that the Earth is made of, the asteroids are 4. Based on astronomical models of how stars work, we also believe the Sun to be about 4. Radiometric dating is a widely accepted technique that measures the rate of decay of naturally occurring elements that have been incorporated into rocks and fossils.
Every element is defined by the particular number of protons, neutrons, and electrons that make up it's atoms. Sometimes, the number of neutrons within the atom is off. These atoms, with an odd number of neutrons, are called isotopes. Because they do not have the ideal number of neutrons, the isotopes are unstable and over time they will convert into more stable atoms. Scientists can measure the ratio of the parent isotopes compared to the converted isotopes.
The rate of isotope decay is very consistent, and is not effected by environmental changes like heat, temperature, and pressure. This makes radiometric dating quite reliable. However, there are some factors that must be accounted for. For example, sometimes it is possible for a small amount of new "parent" isotopes to be incorporated into the object, skewing the ratio.
This is understood and can be corrected for. Carbon is the most commonly used isotope for dating organic material plants, animals. Plants and animals continually take in carbon during their lifespan. When they die, they no longer acquire carbon and so we can measure the decay of the isotope to determine when the plant or animal died. Because carbon decays relatively rapidly compared to other isotopes, it can only be used to date things that are less than 60, years old.
Anything older would have so little carbon left that you couldn't accurately measure it. However, the rapid decay allows precise dating - accuracy within just a couple decades. When dating older objects, namely rocks, it is necessary to use other isotopes that take a much longer time to decay. The most common isotopes used are uranium and uranium there are multiple isotopes of uranium. We often test them under controlled conditions to learn when and why they fail so we will not use them incorrectly.
We have even discredited entire techniques. For example, after extensive testing over many years, it was concluded that uranium-helium dating is highly unreliable because the small helium atom diffuses easily out of minerals over geologic time. As a result, this method is not used except in rare and highly specialized applications. These methods provide valuable and valid age data in most instances, although there is a small percentage of cases in which even these generally reliable methods yield incorrect results.
Such failures may be due to laboratory errors mistakes happenunrecognized geologic factors nature sometimes fools usor misapplication of the techniques no one is perfect. Not only that, they have to show the flaws in those dating studies that provide independent corroborative evidence that radiometric methods work. This is a tall order and the creationists have made no progress so far. It is rare for a study involving radiometric dating to contain a single determination of age.
Usually determinations of age are repeated to avoid laboratory errors, are obtained on more than one rock unit or more than one mineral from a rock unit in order to provide a cross-check, or are evaluated using other geologic information that can be used to test and corroborate the radiometric ages. Scientists who use radiometric dating typically use every means at their disposal to check, recheck, and verify their results, and the more important the results the more they are apt to be checked and rechecked by others.
As a result, it is nearly impossible to be completely fooled by a good set of radiometric age data collected as part of a well-designed experiment. The purpose of this paper is to describe briefly a few typical radiometric dating studies, out of hundreds of possible examples documented in the scientific literature, in which the ages are validated by other available information. I have selected four examples from recent literature, mostly studies involving my work and that of a few close colleagues because it was easy to do so.
I could have selected many more examples but then this would have turned into a book rather than the intended short paper.
The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks.
The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time. Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer the Crow Creek Member within a thick rock formation known as the Pierre Shale.
The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus.
The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section Figure 1. The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds.
There are three important things to note about these results. First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent.
Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds. Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should.
Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure The Ages of Meteorites Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system. There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula.
Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules.
Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4. Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results.
The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories.
This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors.
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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. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry.
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. Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. 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. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to 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.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No.
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. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
Modern dating methods[ edit ] 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.
Radioactive dating - Australian Museum
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.
Explainer: what is radiocarbon dating and how does it work?
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 dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe. 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.
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. 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. Samarium—neodymium dating method[ edit ] Main article: Samarium—neodymium dating 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.