Radiometric dating - Wikipedia
Lead–lead dating is a method for dating geological samples, normally based on ' whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium–lead stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb. Uranium–uranium dating is a radiometric dating technique which compares two isotopes of on the way to the stable isotope Pb. Other dating techniques using this decay series include uranium–thorium dating and uranium–lead dating . Uranium-lead is one of the oldest and most refined of the radiometric dating Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it.
Together, these samples define an isochron, whose slope gives the age of meteorites as 4. Patterson also analyzed terrestrial sediment collected from the ocean floor, which was believed to be representative of the Bulk Earth composition.
Because the isotope composition of this sample plotted on the meteorite isochron, it suggested that earth had the same age and origin as meteorites, therefore solving the age of the Earth and giving rise to the name 'geochron'. Lead isotope isochron diagram used by C. Patterson to determine the age of the Earth in Animation shows progressive growth over million years Myr of the lead isotope ratios for two stony meteorites Nuevo Laredo and Forest City from initial lead isotope ratios matching those of the Canyon Diablo iron meteorite.
Precise Pb—Pb dating of meteorites[ edit ] Pb—Pb isochrons for the oldest known material in the solar system. Hence precise dating of these objects is important to constrain the early evolution of the solar system and the age of the earth. The U—Pb dating method can yield the most precise ages for early solar-system objects due to the optimal half-life of U. However, the absence of zircon or other uranium-rich minerals in chondrites, and the presence of initial non-radiogenic Pb common Pbrules out direct use of the U-Pb concordia method.
In the distant past, around the time that the solar system formed, there were more kinds of unstable high-weight isotopes available, and the four chains were longer with isotopes that have since decayed away. Today we have manufactured extinct isotopes, which again take their former places: In particular, we have through the large-scale production of neptunium successfully resurrected the hitherto extinct fourth chain.
The four most common modes of radioactive decay are: Of these decay processes, only alpha decay changes the atomic mass number A of the nucleus, and always decreases it by four. Because of this, almost any decay will result in a nucleus whose atomic mass number has the same residue mod 4, dividing all nuclides into four chains.
The members of any possible decay chain must be drawn entirely from one of these classes. All four chains also produce helium-4 alpha particles are helium-4 nuclei. Three main decay chains or families are observed in nature, commonly called the thorium series, the radium or uranium series, and the actinium series, representing three of these four classes, and ending in three different, stable isotopes of lead.
The long-lived starting isotopes of these three isotopes, respectively thoriumuraniumand uraniumhave existed since the formation of the earth, ignoring the artificial isotopes and their decays since the s. 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.
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. 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.
- Decay chain
- Uranium-lead dating
- Uranium–uranium 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.
Uranium–uranium dating - Wikipedia
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.
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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.