Generally, there are four main concepts that students struggle with when thinking about radioactive decay:. Radioactivity and radioactive decay are spontaneous processes. Students often struggle with this concept; therefore, it should be stressed that it is impossible to know exactly when each of the radioactive elements in a rock will decay. Statistical probablity is the only thing we can know exactly. Often students get bogged down in the fact that they don't "understand" how and why radioactive elements decay and miss the whole point of this exercise. If they can begin to comprehend that it is random and spontaneous, they end up feeling less nervous about the whole thing. Radioactive decay involves the spontaneous transformation of one element into another.
Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals, such as evaporites, may contain elements suitable for radioisotopic dating. Igneous pyroclastic layers and lava flows within a sedimentary sequence can be used to date the sequence.
Cross-cutting igneous rocks and sills can be used to bracket the ages of affected, older sedimentary rocks. There are several ways radioactive atoms decay. We will consider three of them here- alpha decay, beta decay, and electron capture. Alpha decay is when an alpha particle, which consists of two protons and two neutrons, is emitted from the nucleus of an atom. This also happens to be the nucleus of a helium atom; helium gas may get trapped in the crystal lattice of a mineral in which alpha decay has taken place.
When an atom loses two protons from its nucleus, lowering its atomic number, it is transformed into an element that is two atomic numbers lower on the Periodic Table of the Elements. The loss of four particles, in this case, two neutrons and two protons, also lowers the mass of the atom by four. For example, alpha decay takes place in the unstable isotope U, which has an atomic number of 92 92 protons and a mass number of total of all protons and neutrons. When U spontaneously emits an alpha particle, it becomes thorium Th.
The radioactive decay product of an element is called its daughter isotope and the original element is called the parent isotope. In this case, U is the parent isotope and Th is the daughter isotope.
The half-life of U is 4. This isotope of uranium, U, can be used for absolute dating the oldest materials found on Earth, and even meteorites and materials from the earliest events in our solar system. Beta Decay is when a neutron in its nucleus splits into an electron and a proton.
The electron is emitted from the nucleus as a beta ray. For example, Th is unstable and undergoes beta decay to form protactinium Pawhich also undergoes beta decay to form uranium U.
Radioactive decay Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms emit particles. This releases energy (in the form of radiation) and often transforms one element into another.
Notice these are all isotopes of different elements but they have the same atomic mass of The decay process of radioactive elements like uranium keeps producing radioactive parents and daughters until a stable, or non-radioactive, daughter is formed. Such a series is called a decay chain. The decay chain of the radioactive parent isotope U progresses through a series of alpha red arrows on the adjacent figure and beta decays blue arrows until it forms the stable daughter isotope, lead Pb.
Electron capture is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron. This produces one of two different effects: 1 an electron jumps in to fill the missing spot of the departed electron and emits an X-ray, or 2 in what is called the Auger process, another electron is released and changes the atom into an ion. The atomic number is reduced by one and the mass number remains the same.
An example of an element that decays by electron capture is potassium 40 K. Radioactive 40 K makes up a tiny percentage 0. Below is a table of some of the more commonly-used radioactive dating isotopes and their half-lives. Some common isotopes used for radioisotopic dating [ 7 ; 8 ]. For a given sample of rock, how is the dating procedure carried out? The parent and daughter isotopes are separated out of the mineral using chemical extraction.
In the case of uranium, U and U isotopes are separated out together, as are the Pb and Pb with an instrument called a mass spectrometer [ 9 ]. Here is a simple example of age calculation using the daughter-to-parent ratio of isotopes.
This can be further calculated for a series of half-lives as shown in the table. The table does not show more than 10 half-lives because, after about 10 half-lives, the amount of remaining parent is so small it becomes too difficult to accurately measure via chemical analysis. Modern applications of this method have achieved remarkable accuracies of plus or minus two million years in 2. The existence of these two clocks in the same sample gives a cross-check between the two.
The ratio of parent to a daughter in terms of half-life. Another radioisotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone.
Radiocarbon datingalso called carbon dating, uses the unstable isotope carbon 14 C and the stable isotope carbon 12 C. Carbon is constantly being created in the atmosphere by the interaction of cosmic particles with atmospheric nitrogen 14 N [ 11 ]. Cosmic particles such as neutrons strike the nitrogen nucleus, kicking out a proton but leaving the neutron in the nucleus. The collision reduces the atomic number by one, changing it from seven to six, changing the nitrogen into carbon with the same mass number of The 14 C quickly bonds with oxygen O in the atmosphere to form carbon dioxide 14 CO 2 that mixes with another atmospheric carbon dioxide 12 CO 2 while this mix of gases is incorporated into living matter.
However, when it dies, the radiocarbon clock starts ticking as the 14 C decays back to 14 N by beta decay, which has a half-life of 5, years. The radiocarbon dating technique is thus useful for 57, years or so, about 10 half-lives back. Radiocarbon dating relies on daughter-to-parent ratios derived from a known quantity of parent 14 C. Early applications of carbon dating assumed the production and concentration of 14 C in the atmosphere remained fairly constant for the last 50, years.
However, it is now known that the amount of parent 14 C levels in the atmosphere. Comparisons of carbon ages with tree-ring data and other data for known events have allowed reliable calibration of the radiocarbon dating method.
Geologic History 5 Radioactive Dating
Taking into account carbon baseline levels must be calibrated against other reliable dating methods, carbon dating has been shown to be a reliable method for dating archaeological specimens and very recent geologic events.
The work of Hutton and other scientists gained attention after the Renaissance see Chapter 1spurring exploration into the idea of an ancient Earth. In the late 19 th century William Thompson, a. Lord Kelvin, applied his knowledge of physics to develop the assumption that the Earth started as a hot molten sphere. He estimated the Earth is 98 million years old, but because of uncertainties in his calculations stated the age as a range of between 20 and million years [ 12 ; 13 ].
This animation also shown below illustrates how Kelvin calculated this range and why his numbers were so far off, which has to do with unequal heat transfer within the Earth. In the s, Clair Patterson - thought he could determine the age of the Earth using radioactive isotopes from meteorites, which he considered to be early solar system remnants that were present at the time Earth was forming.
Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer. The current estimate for the age of the Earth is 4.
It is remarkable that Patterson, who was still a graduate student at the University of Chicago, came up with a result that has been little altered in over 60 years, even as technology has improved dating methods. Radioactive isotopes of elements that are common in mineral crystals are useful for radioisotopic dating.
Zircon is resistant to weathering which makes it useful for dating geological events in ancient rocks.
During metamorphic events, zircon crystals may form multiple crystal layers, with each layer recording the isotopic age of an event, thus tracing the progress of the several metamorphic events [ 16 ]. Geologists have used zircon grains to do some amazing studies that illustrate how scientific conclusions can change with technological advancements.
Zircon crystals from Western Australia that formed when the crust first differentiated from the mantle 4. The zircon grains were incorporated into metasedimentary host rocks, sedimentary rocks showing signs of having undergone partial metamorphism.
The host rocks were not very old but the embedded zircon grains were created 4. From other properties of the zircon crystals, researchers concluded that not only were continental rocks exposed above sea level but also that conditions on the early Earth were cool enough for liquid water to exist on the surface.
The presence of liquid water allowed the processes of weathering and erosion to take place [ 17 ]. Researchers at UCLA studied 4. Igneous rocks best suited for radioisotopic dating because their primary minerals provide dates of crystallization from magma. Detrital sedimentary rocks are less useful because they are made of minerals derived from multiple parent sources with potentially many dates. However, scientists can use igneous events to date sedimentary sequences.
For example, if sedimentary strata are between a lava flow and volcanic ash bed with radioisotopic dates of 54 million years and 50 million years, then geologists know the sedimentary strata and its fossils formed between 54 and 50 million years ago.
Radioactive 40 K makes up a tiny percentage (%) of naturally occurring potassium, most of which not radioactive. 40 K decays to argon (40 Ar) with a half-life of billion years, so it is very useful for dating geological events. Below is a table of some of the more commonly-used radioactive dating isotopes and their half-lives. Radiometric dating is the method of obtaining a rock's age by measuring the relative abundance of radioactive and radiogenic isotopes. Using demonstrations of half-life such as a coin toss for large classes or M&M demonstration for smaller classes can help . The mathematical expression that relates radioactive decay to geologic time is called the age equation and is: Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures are complex. The numbers of parent and daughter isotopes in each specimen are determined by various kinds of analytical methods.
Another example would be a 65 million-year-old volcanic dike that cut across sedimentary strata. This provides an upper limit age on the sedimentary strata, so these strata would be older than 65 million years. Primary sedimentary minerals containing radioactive isotopes like 40 K has provided dates for important geologic events. Luminescence aka Thermoluminescence : Radioisotopic dating is not the only way scientists determine numeric ages.
Luminescence dating measures the time elapsed since some silicate minerals, such as coarse-sediments of silicate minerals, were last exposed to light or heat at the surface of Earth.
All buried sediments are exposed to radiation from normal background radiation from the decay process described above. Some of these electrons get trapped in the crystal lattice of silicate minerals like quartz. When exposed at the surface, ultraviolet radiation and heat from the Sun release these electrons, but when the minerals are buried just a few inches below the surface, the electrons get trapped again.
Samples of coarse sediments collected just a few feet below the surface are analyzed by stimulating them with light in a lab. This stimulation releases the trapped electrons as a photon of light which is called luminescence.
The amount luminescence released indicates how long the sediment has been buried. Additionally, elements may exist in different isotopeswith each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide.
Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture.
Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is umulticoingames.comedictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-lifeusually 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 or decay product.
In many cases, the daughter nuclide itself is radioactive, resulting in a decay chaineventually 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. For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially constant.
It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field. 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.
This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Nature has conveniently provided us with radioactive nuclides that have half-lives which range from considerably longer than the age of the universeto less than a zeptosecond. This allows one to measure a very wide range of ages. Isotopes with very long half-lives are called "stable isotopes," and isotopes with very short half-lives are known as "extinct isotopes.
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.
Unfortunately for nuclides with high decay constants which are useful for dating very old sampleslong 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.
Radioactive decay geologic dating
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.
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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.
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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. 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.
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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.
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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.
Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events. Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time. In addition, they have had to develop special techniques with which to dissolve these highly refractory .
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. This causes induced fission of U, as opposed to the spontaneous fission of U. 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 the neutron flux. This scheme has application over a wide range of geologic dates.
For dates up to a few million years micastektites glass fragments from volcanic eruptionsand meteorites are best used. Older materials can be dated using zirconapatitetitaniteepidote and garnet which have a variable amount of uranium content. The technique has potential applications for detailing the thermal history of a deposit. The residence time of 36 Cl in the atmosphere is about 1 week. Thus, as an event marker of s water in soil and ground water, 36 Cl is also useful for dating waters less than 50 years before the present.
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 of background radiation on certain minerals. Over time, ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such as quartz and potassium 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.