Radiometric dating - Wikipedia
All rocks and minerals contain tiny amounts of these radioactive elements. Radioactive decay occurs at a constant rate, specific to each radioactive isotope. A commonly used radiometric dating technique relies on the breakdown of. Isotopic dating of rocks, or the minerals in them, is based on the fact that we know One of the isotope pairs widely used in geology is the decay of 40K to 40Ar. Radiometric dating or radioactive dating is a technique used to date materials such as rocks or . Dating of different minerals and/or isotope systems (with differing closure temperatures) within the same rock can therefore enable the tracking of.
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. 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 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.
Radioactive dating - Australian Museum
Potassium—argon dating This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1.
Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating 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.
BBC - GCSE Bitesize: Radioactive dating
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.
Uranium—thorium dating method[ edit ] Main article: Uranium—thorium dating 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 method[ edit ] Main article: Carbon is a radioactive isotope of carbon, with a half-life of 5, years,   which is very short compared with the above isotopes and 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 CO2. 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.
- Carbon dating
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- Radioactive dating
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.
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. Fission track dating method[ edit ] Main article: 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.
But it decays very slowly, taking years for half of a sample of carbon to be converted back to nitrogen Samples of wood, charcoal or cloth were originally living vegetable matter. We assume that while living, plants and trees absorb a constant ratio of C and C because the model says that the process of cosmic ray bombardment continues essentially at a constant rate.
Since animals are a part of the food chain which includes plants, they also receive a constant ratio of C and C, but in the form of carbohydrates, proteins and fats. The amount of C in any sample of carbon containing material can be found by measuring the level of radioactive decay, and comparing that with the decay rate observed in a carbon sample exposed to the continual mixing at the surface of the earth of C and C produced in the upper atmosphere.
Using the ratio of C to total carbon, one can determine the age of the sample. There is evidence gathered from tree rings that the ratio of C C has not remained constant but has varied significantly.
Tree ring studies on trees of great ages, such as bristlecone pines and sequoias, provide data to establish a base line ratio of 14C: Libby won the Nobel Prize for his invention of this technique.
A recent celebrated use of radiocarbon dating involved the Shroud of Turin. Some people claimed that the Shroud had been used to wrap the body of the prophet of Christianity after his crucifixion though no one disputed that its history was not known before the 12th century, when it had become the property of the cathedral at Turin, Italy. It was not an official Relic of the Church, but its reputation over the centuries had grown and it probably was responsible for many pilgrimages to the cathedral among the faithful.
Early proposals to use radiocarbon dating to determine its age were rejected because such a sizeable amount of material would have to be used to carry out the determination perhaps as much as 10 cm2 for each sample, and at least 3 samples must be taken to assure reproducibility. The fear was that if its age could be traced to the beginning of the first millennium, then it might well be named a Church Relic -- but one that had to be mutilated to gain that stature.
Meanwhile, back at the lab, techniques continued to improve, until reliable radiocarbon dating could finally be done with considerably smaller samples in the case of the Shroud, just a few short strands were needed for each sample. Such small sample sizes were judged by Church authorities not to constitute mutilation and the analysis went forward.
Samples were taken from the Shroud and sent to several laboratories along with other samples of fabrics of known ages. The laboratories were not told which was which. The reported values showed close agreement between the Shroud samples and none suggested an age of the fabric having been harvested from plants before the 12th century A. The committee which had taken on the task of judging the validity of the analysis was sufficiently satisfied to convince local Church authorities to retire the claim that it is a Holy Shroud.
Potassium-argon method There is another often used dating technique for samples considerably older than 60, years.