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Potassium argon and argon argon dating

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Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon. 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, 40Ar is the decay product of 40K and therefore will increase in quantity over time.

The quantity of 40Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere. The decay scheme is electron capture and positron decay.

Argon–argon dating, Wikipedia

The material in question is a closed system. In the case of a volcanic mineral, this means rapid cooling. Likewise, potassium has not been gained or lost. The decay constants of 40K are accurately known.

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 40Ar and a parent isotope.

Because the relative abundances of the potassium isotopes are known, the 39ArK produced from 39K 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 39ArK produced in any given irradiation will be dependant on the amount of 39K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39K.

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 39Ar production from 39K, 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.

Potassium argon and argon argon dating, potassium-argon dating

For example, to determine the amount of reactor produced 40Ar from 40K, potassium-rich glass is irradiated with the samples. The desirable production of 38Ar 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.

Radioactive Dating


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