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Potassium argon dating

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Potassium argon (40K-40Ar) dating is a form of radiometric dating widely used because of the range of dates for which it is useful. The technique can be used for dates ranging from earth's beginning, 4550 mya (4.55bn in US terminology) to about 100,000 years ago. The half-life of potassium40 is roughly 1.25 billion years, so four half-lives are about five billion years, or older than the earth. Other radiometric techniques, like carbon14, are useful for much more recent dates.

Potassium 40 decays into argon 40 through a process known as electron capture. In electron capture, an electron from the innermost electron shell "falls" into the nucleus, causing a proton to convert into a neutron. In the case of potassium-argon decay, this loss of a proton causes the atom to change from a reactive alkali metal to a non-reactive noble gas, which is an important characteristic. Because argon is an inert gas, if it is not physically trapped in a rock, it will diffuse into the atmosphere. In molten rock almost all of the argon will be released into the atmosphere; so in volcanic material, when the rock cools and hardens, the argon begins to accumulate in the crystals, effectively starting the clock.

Volcanic material is the ideal subject of potassium-argon dating because it is a closed system - there is none of the daughter element present when the material is formed to contaminate measurements. There is a possibility of contamination from atmospheric argon, but that can be adjusted for. Another factor that can skew measurements is reheating of the rock, which can partially reset the clock by releasing some of the stored argon into the atmosphere. This is not a significant problem for igneous rocks, but it affects the usefulness of the techniques for other forms of rock that have more complex heating histories. For these other rocks argon-argon (40Ar - 39Ar) dating can be more useful.

Because the method is primarily useful for volcanic materials, and fossils more commonly lie in sedimentary layers, the material that is dated is usually from volcanic strata (commonly volcanic ash layers) above and below that of the fossil. The fossil is given an age limit between the dated volcanic layers.

The first Hominid fossil dated using this method was an Australopithecus boisei (initially called Zinjanthropus boisei) specimen discovered by Mary Leakey in the Olduvai Gorge in 1959. Volcanic ash layers are relatively common along the Great Rift Valley of east Africa.

Argon can easily escape by diffusion which will result in too low ages. Since helium escapes more easily than argon, any diffusion loss can be excluded if the Potassium-Argon age and the Uranium-Thorium-Helium age of a sample are concordant. A more sophisticated technique which is capable of excluding diffusion loss is 39Ar-40Ar dating, known as the argon-argon method.

Sources

  1. Dalrymple, G. Brent. 1991. The Age of the Earth. Stanford University Press.
  2. O'Neil, Dennis. "The Record of Time: Chronometric Techniques - Part 2". [1]
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