Radiometric dating,radioactive dating orradioisotope dating is a technique which is used todate materials such asrocks orcarbon, in which trace radioactiveimpurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurringradioactive isotope within the material to the abundance of itsdecay products, which form at a known constant rate of decay.[1] Radiometric dating of minerals and rocks was pioneered byErnest Rutherford (1906) andBertram Boltwood (1907).[2][3] Radiometric dating is now the principal source of information about theabsolute age of rocks and othergeological features, including the age offossilized life forms or theage of Earth itself, and can also be used to date a wide range of natural andman-made materials.
Together withstratigraphic principles, radiometric dating methods are used ingeochronology to establish thegeologic time scale.[4] Among the best-known techniques areradiocarbon dating,potassium–argon dating anduranium–lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages offossils and the deduced rates ofevolutionary change. Radiometric dating is also used to datearchaeological materials, including ancient artifacts.
Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
All ordinarymatter is made up of combinations ofchemical elements, each with its ownatomic number, indicating the number ofprotons in theatomic nucleus. Additionally, elements may exist in differentisotopes, with each isotope of an element differing in the number ofneutrons in the nucleus. A particular isotope of a particular element is called anuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergoradioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, includingalpha decay (emission ofalpha particles) andbeta decay (electron emission,positron emission, orelectron capture). Another possibility isspontaneous fission into two or more nuclides.[5]
While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decaysexponentially at a rate described by a parameter known as thehalf-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide ordecay product. In many cases, the daughter nuclide itself is radioactive, resulting in adecay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g.,tritium) to over 100 billion years (e.g.,samarium-147).[6]
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially constant.[7] This is known because decay constants measured by different techniques give consistent values within analytical errors and the ages of the same materials are consistent from one method to another. It is not affected by external factors such astemperature,pressure, chemical environment, or presence of amagnetic orelectric field.[8][9][10] The only exceptions are nuclides that decay by the process of electron capture, such asberyllium-7,strontium-85, andzirconium-89, whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.[citation needed] This predictability allows the relative abundances of related nuclides to be used as aclock to measure the time from the incorporation of the original nuclides into a material to the present.
The radioactive decay constant, the probability that an atom will decay per year, is the solid foundation of the common measurement of radioactivity. The accuracy and precision of the determination of an age (and a nuclide's half-life) depends on the accuracy and precision of the decay constant measurement.[11] The in-growth method is one way of measuring the decay constant of a system, which involves accumulating daughter nuclides. Unfortunately for nuclides with high decay constants (which are useful for dating very old samples), long periods of time (decades) are required to accumulate enough decay products in a single sample to accurately measure them. A faster method involves using particle counters to determine alpha, beta or gamma activity, and then dividing that by the number of radioactive nuclides. 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 thePb–Pb system.[citation needed]
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 ofalteration.[12] Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. 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 anisochron. This can reduce the problem ofcontamination. Inuranium–lead dating, theconcordia 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 theAmitsoq gneisses from western Greenland was determined to be 3.60 ± 0.05Ga (billion years ago) using uranium–lead dating and 3.56 ± 0.10 Ga (billion years ago) using lead–lead dating, results that are consistent with each other.[13]: 142–143
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 involvesisotope-ratio mass spectrometry.[14]
The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating cannot be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.[15]
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 throughdiffusion, resetting 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.[16][17] This temperature varies for every mineral and isotopic system, so a system can beclosed for one mineral butopen for another. Dating of different minerals and/or isotope systems (with differing closure temperatures) within the same rock can therefore enable the tracking of the thermal history of the rock in question with time, and thus the history of metamorphic events may become known in detail. These temperatures are experimentally determined in the lab byartificially resetting sample minerals using a high-temperature furnace. This field is known asthermochronology or thermochronometry.[citation needed]
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The mathematical expression that relates radioactive decay to geologic time is[16][18]
where
The equation is most conveniently expressed in terms of the measured quantityN(t) rather than the constant initial valueNo.[citation needed]
To calculate the age, it is assumed that the system isclosed (neither parent nor daughter isotopes have been lost from system),D0 either must be negligible or can be accurately estimated,λ is known to high precision, and one has accurate and precise measurements of D* andN(t).[citation needed]
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below itsclosure temperature. This is well established for most isotopic systems.[17][20] However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Anisochron plot is used to solve the age equation graphically and calculate the age of the sample and the original composition.[citation needed]
Radiometric dating has been carried out since 1905 when it wasinvented byErnest Rutherford as a method by which one might determine theage of the Earth. In the century since then the techniques have been greatly improved and expanded.[19] Dating can now be performed on samples as small as a nanogram using amass spectrometer. The mass spectrometer was invented in the 1940s and began to be used in radiometric dating in the 1950s. It operates by generating a beam ofionized 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.[citation needed]
Uranium–lead radiometric dating involves using uranium-235 or uranium-238 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.[22][23] An error margin of 2–5% has been achieved on youngerMesozoic rocks.[24]
Uranium–lead dating is often performed on themineralzircon (ZrSiO4), though it can be used on other materials, such asbaddeleyite andmonazite (see:monazite geochronology).[25] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes forzirconium, but strongly reject lead. 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.In situ micro-beam analysis can be achieved via laserICP-MS orSIMS techniques.[26]
One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost. 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.[citation needed]
This involves thealpha decay of147Sm to143Nd with ahalf-life of 1.06 x 1011 years. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable.[27]
This involveselectron capture orpositron decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, so this method is applicable to the oldest rocks. Radioactive potassium-40 is common inmicas,feldspars, andhornblendes, though the closure temperature is fairly low in these materials, about 350 °C (mica) to 500 °C (hornblende).[citation needed]
This is based on the beta decay ofrubidium-87 tostrontium-87, with a half-life of 50 billion years. This scheme is used to date oldigneous andmetamorphic rocks, and has also been used to datelunar 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.[28]
A relatively short-range dating technique is based on the decay of uranium-234 into thorium-230, a substance with a half-life of about 80,000 years. It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 32,760 years.[citation needed]
Whileuranium is water-soluble,thorium andprotactinium are not, and so they are selectively precipitated into ocean-floorsediments, from which their ratios are measured. The scheme has a range of several hundred thousand years. A related method isionium–thorium dating, which measures the ratio ofionium (thorium-230) to thorium-232 inocean sediment.[citation needed]
Radiocarbon dating is also simply called carbon-14 dating. Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years[30][31] (which is very short compared with the above isotopes), and decays into nitrogen.[32] In other radiometric dating methods, the heavy parent isotopes were produced bynucleosynthesis in supernovas, meaning that any parent isotope with a short half-life should be extinct by now. Carbon-14, though, is continuously created through collisions of neutrons generated bycosmic rays with nitrogen in theupper atmosphere and thus remains at a near-constant level on Earth. The carbon-14 ends up as a trace component in atmosphericcarbon dioxide (CO2).[33]
A carbon-based life form acquires carbon during its lifetime. Plants acquire it throughphotosynthesis, and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon-14, and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon-14 an ideal dating method to date the age of bones or the remains of an organism. The carbon-14 dating limit lies around 58,000 to 62,000 years.[34]
The rate of creation of carbon-14 appears to be roughly constant, as cross-checks of carbon-14 dating with other dating methods show it gives consistent results. However, local eruptions ofvolcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon-14 and give inaccurate dates. The releases of carbon dioxide into thebiosphere as a consequence ofindustrialization have also depressed the proportion of carbon-14 by a few percent; in contrast, the amount of carbon-14 was increased by above-groundnuclear bomb tests that were conducted into the early 1960s. Also, an increase in thesolar wind or the Earth'smagnetic field above the current value would depress the amount of carbon-14 created in the atmosphere.[35]
This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by thespontaneous fission of uranium-238 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 withslow neutrons. This causes induced fission of235U, as opposed to the spontaneous fission of238U. The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and theneutron flux.[36]
This scheme has application over a wide range of geologic dates. For dates up to a few million yearsmicas,tektites (glass fragments from volcanic eruptions), and meteorites are best used. Older materials can be dated usingzircon,apatite,titanite,epidote andgarnet which have a variable amount of uranium content.[37] Because the fission tracks are healed by temperatures over about 200 °C the technique has limitations as well as benefits. The technique has potential applications for detailing the thermal history of a deposit.[38]
Large amounts of otherwise rare36Cl (half-life ~300ky) were produced by irradiation of seawater during atmospheric detonations ofnuclear weapons between 1952 and 1958. The residence time of36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water,36Cl is also useful for dating waters less than 50 years before the present.36Cl has seen use in other areas of the geological sciences, including dating ice[39] and sediments.
Luminescence dating methods are not radiometric dating methods in that they do not rely on abundances of isotopes to calculate age. Instead, they are a consequence ofbackground radiation on certain minerals. Over time,ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such asquartz andpotassium feldspar. The radiation causes charge to remain within the grains in structurally unstable "electron traps". Exposure to sunlight or heat releases these charges, effectively "bleaching" the sample and resetting the clock to zero. The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried. Stimulating these mineral grains using either light (optically stimulated luminescence or infrared stimulated luminescence dating) or heat (thermoluminescence dating) causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial and specific properties of the mineral.[40]
These methods can be used to date the age of a sediment layer, as layers deposited on top would prevent the grains from being "bleached" and reset by sunlight. Pottery shards can be dated to the last time they experienced significant heat, generally when they were fired in a kiln.[41]
Other methods include:[citation needed]
Absolute radiometric dating requires a measurable fraction of parent nucleus to remain in the sample rock. For rocks dating back to the beginning of theSolar System, this requires extremely long-lived parent isotopes, making measurement of such rocks' exact ages imprecise. To be able to distinguish the relative ages of rocks from such old material, and to get a better time resolution than that available from long-lived isotopes, short-lived isotopes that are no longer present in the rock can be used.[43]
At the beginning of the solar system, there were several relatively short-lived radionuclides like26Al,60Fe,53Mn, and129I present within the solar nebula. These radionuclides—possibly produced by the explosion of a supernova—areextinct today, but their decay products can be detected in very old material, such as that which constitutesmeteorites. By measuring the decay products of extinct radionuclides with amass spectrometer and using isochronplots, it is possible to determine relative ages of different events in the early history of the solar system. Dating methods based on extinct radionuclides can also be calibrated with the U–Pb method to give absolute ages. Thus both the approximate age and a high time resolution can be obtained. Generally a shorter half-life leads to a higher time resolution at the expense of timescale.[citation needed]
129
I beta-decays to129
Xe with a half-life of16.14±0.12 million years.[44] The iodine-xenon chronometer[45] is an isochron technique. Samples are exposed to neutrons in a nuclear reactor. This converts the only stable isotope of iodine (127
I) into128
Xe via neutron capture followed by beta decay (of128
I). After irradiation, samples are heated in a series of steps and the xenonisotopic signature of the gas evolved in each step is analysed. When a consistent129
Xe/128
Xe ratio is observed across several consecutive temperature steps, it can be interpreted as corresponding to a time at which the sample stopped losing xenon.[citation needed]
Samples of a meteorite called Shallowater are usually included in the irradiation to monitor the conversion efficiency from127
I to128
Xe. The difference between the measured129
Xe/128
Xe ratios of the sample and Shallowater then corresponds to the different ratios of129
I/127
I when they each stopped losing xenon. This in turn corresponds to a difference in age of closure in the early solar system.[citation needed]
Another example of short-lived extinct radionuclide dating is the26
Al –26
Mg chronometer, which can be used to estimate the relative ages ofchondrules.26
Al decays to26
Mg with ahalf-life of 720 000 years. The dating is simply a question of finding the deviation from thenatural abundance of26
Mg (the product of26
Al decay) in comparison with the ratio of the stable isotopes27
Al/24
Mg.[46]
The excess of26
Mg (often designated26
Mg*) is found by comparing the26
Mg/27
Mg ratio to that of other Solar System materials.[47]
The26
Al –26
Mg chronometer gives an estimate of the time period for formation of primitive meteorites of only a few million years (1.4 million years for Chondrule formation).[48]
In a July 2022 paper in the journalApplied Geochemistry, the authors proposed that the terms "parent isotope" and "daughter isotope" be avoided in favor of the more descriptive "precursor isotope" and "product isotope", analogous to "precursor ion" and "product ion" inmass spectrometry.[49]
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