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Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes.
Due to the relatively heavy atomic weight of potassium, inificant fractionation of the different potassium isotopes occurs. However, the 40 K isotope is radioactive and therefore will be reduced in quantity over time. But, for the purposes of the KAr dating system, the relative abundance of 40 K is so small and its half-life is so long that its ratios with the other Potassium isotopes are considered constant.
Argon, a noble gas, constitutes approximately 0. 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 ificantly 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 Potassium 40 dating accuracy 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. These are:. 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. 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 Potassium 40 dating accuracy, 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. Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control.
For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain. The from a laser probe can be plotted in several graphical ways, including a map of a grain showing lateral argon distribution. Total fusion is performed using a laser and are commonly plotted on probability distribution diagrams or ideograms.
For the J to be determined, a standard of known age must be irradiated with the samples of unknown age. Traditionally, this primary standard has been a hornblende from the McClure Mountains, Colorado a. Some of these include other isotopic dating techniques e.
This uncertainty from 1 the branched decay scheme of Potassium 40 dating accuracy K and 2 the long half-life of 40 K 1. Because the J value is extrapolated from a standard to an unknown, the accuracy and precision on that J value is critical. J value uncertainty can be minimized by constraining the geometry of the standard relative to the unknown, both vertically and horizontally.
The NMGRL does this by irradiating samples in machined aluminum disks where standards and unknowns alternate every other position. J error can also be reduced by analyzing more flux monitor aliquots per standard location. This is caused by the net loss of 39 Ar K from the sample by recoil the kinetic energy imparted on a 39 Ar K atom by the emission of a proton during the n,p reaction.
Recoil is likely in every potassium-bearing sample, but only becomes a ificant problem with very fine grained minerals e. For multi-phase samples such as basaltic wholerocks, 39 Ar K redistribution may be more of a problem than net 39 Ar K loss. In this case, 39 Ar may recoil out of a low-temperature, high-potassium mineral e. K-feldspar into a high-temperature, low potassium mineral e. Such a phenomenon would great affect the shape of the age spectrum.Potassium 40 dating accuracy
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