The potassium-argon age of some meteorites is as old as 4,,, years, and volcanic rocks as young as 20, years old have been measured by this method. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval.
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Learn More in these related Britannica articles: This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially. In rubidium-strontium dating, micas exclude strontium when they form but accept much rubidium.
In uranium-lead U-Pb dating of zircon, the zircon is found to exclude initial lead…. The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method. Since radiogenic argon was first detected in by the American geophysicist….
Potassium—argon dating has made it possible to establish that the earliest remains of man and his artifacts in East Africa go back at least 2,, years, and probably further. Potassium-argon dating , for instance, can provide the age of a specimen by clocking the rate at which radioactive isotopes of these elements have decayed.
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When radiometric methods cannot be applied, investigators may still ascribe a relative age to a fossil by relating it to the…. More About Potassium-argon dating 5 references found in Britannica articles Assorted References major reference In dating: Analysis of separated minerals In dating: Potassium—argon methods age determination of tektites In tektite: Chemistry and petrography of tektites archaeology In archaeology: Argon can mobilized into or out of a rock or mineral through alteration and thermal processes. Like Potassium, Argon cannot be significantly fractionated in nature.
However, 40 Ar is the decay product of 40 K and therefore will increase in quantity over time.
The quantity of 40 Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere. This ratio is The decay scheme is electron capture and positron decay.
Certain assumptions must be satisfied before the age of a rock or mineral can be calculated with the Potassium-Argon dating technique. Argon loss and excess argon are two common problems that may cause erroneous ages to be determined. Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma.
Both techniques rely on the measurement of a daughter isotope 40 Ar and a parent isotope.
Because the relative abundances of the potassium isotopes are known, the 39 Ar K produced from 39 K by a fast neutron reaction can be used as a proxy for potassium. Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results. The amount of 39 Ar K produced in any given irradiation will be dependant on the amount of 39 K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39 K. However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age.
The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation:. In addition to 39 Ar production from 39 K, several other 'interference' reactions occur during irradiation of the samples.
Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample. These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses.
For example, to determine the amount of reactor produced 40 Ar from 40 K, potassium-rich glass is irradiated with the samples. The desirable production of 38 Ar from 37Cl allows us to determine how much chlorine is present in our samples. Multiple argon extractions can be performed on a sample in several ways. Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron.
Mechanical crushing is also a technique capable of releasing argon from a single sample in multiple steps.