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During the latter half of this century anthropological surveys in East Africa have Berger, G.W., and York, D. Precision of the 40Ar/39Ar dating technique. methods are typically used involving longer-lived radionuclides. Most frequently . Method has advantage of parallel counting of 39Ar and 40Ar atoms in mass. The 40Ar/39Ar step-heating method can be applied successfully to any sample which is suitable for K-Ar dating. G.B. Dalrymple, Lanphere ann-estetyka.info technique of K/Ar dating: a comparison with the Current Anthropology, 6 () , pp.
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. 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 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 results 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 results 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.
Argon Geochronology Methods
This uncertainty results from 1 the branched decay scheme of 40K and 2 the long half-life of 40K 1. J Factor 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 39ArK from the sample by recoil the kinetic energy imparted on a 39ArK atom by the emission of a proton during the n,p reaction.
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