Radiometric dating
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale.
By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.
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.
It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field.
[11] The in-growth method is one way of measuring the decay constant of a system, which involves accumulating daughter nuclides.
A faster method involves using particle counters to determine alpha, beta or gamma activity, and then dividing that by the number of radioactive nuclides.
[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.
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.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
[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 its closure temperature.
[citation needed] Radiometric dating has been carried out since 1905 when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.
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.
[25] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead.
[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.
[citation needed] While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured.
Carbon-14, 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 releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent; in contrast, the amount of carbon-14 was increased by above-ground nuclear bomb tests that were conducted into the early 1960s.
Also, an increase in the solar wind or the Earth's magnetic 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 the spontaneous 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 with slow neutrons.
Older materials can be dated using zircon, apatite, titanite, epidote and garnet which have a variable amount of uranium content.
[38] Large amounts of otherwise rare 36Cl (half-life ~300ky) were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958.
Over time, ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such as quartz and potassium feldspar.
The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried.
[41] Other methods include:[citation needed] Absolute radiometric dating requires a measurable fraction of parent nucleus to remain in the sample rock.
By measuring the decay products of extinct radionuclides with a mass spectrometer and using isochronplots, it is possible to determine relative ages of different events in the early history of the solar system.
When a consistent 129Xe/128Xe 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 from 127I to 128Xe.
[citation needed] Another example of short-lived extinct radionuclide dating is the 26Al – 26Mg chronometer, which can be used to estimate the relative ages of chondrules.
The dating is simply a question of finding the deviation from the natural abundance of 26Mg (the product of 26Al decay) in comparison with the ratio of the stable isotopes 27Al/24Mg.
