See all related overviews in Oxford Reference ». An absolute dating technique similar to radiocarbon dating but applicable to much older deposits. It is used to determine the age of volcanic rock strata containing or sealing archaeological objects rather than to date the artefacts themselves. In volcanic rocks any argon present will have escaped when the rock was last molten but will start to accumulate again when it solidifies.
Thus by carefully measuring the amount of 40 K and 40 Ar present in a sample it is possible to work out how long ago it was that the rock solidified. Subjects: Science and technology. View all reference entries ». This method is used in many fields to determine the age of a sample. The process in rocks is based on the decay of radioactive potassium to radioactive argon Some parts also decay to calcium The ratio of these components in a sample or rock is the measure of its age.
The potassium argon method calculates the ages of several objects like meteorites, volcanic rocks, different types of minerals , etc. Some meteorites have dated back to crore years, and some volcanic rocks are aged just 20, years. The potassium argon dating process follows a particular formula to determine the age of a rock or a sample.
The decay profile of radioactive potassium determines the age and origin of radioactive argon. Radioactive potassium also decays to radioactive calcium. The radioactive form of potassium and argon are potassium and argon The ratio of radioactive potassium, radioactive argon, and radioactive calcium is measured. These dating methods have been under constant improvement for more than 50 years.
The learning curve has been long and is far from over today. With each increment in quality, more subtle sources of error have been found and taken into account. Good materials and skilled hands can yield ages that are certain to within 1 percent, even in rocks only 10, years old, in which quantities of 40 Ar are vanishingly small. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance.
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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.
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