Potassium-argon dating
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Potassium-argon or K-Ar dating is a geochronological method used in many geoscience disciplines. It is based on measuring the products of the radioactive decay of potassium (K), which is a common element found in materials such as micas, clay minerals, tephra and evaporites.
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[edit] Decay series
Potassium naturally occurs in 3 isotopes - 39K (93.2581%), 40K (0.0117%), 41K (6.7302%).
The radioactive isotope 40K decays to 40Ar and 40Ca with a half-life of 1.26x109 years. 40Ca is the most common form of Ca, however, so the increase in abundance due to K decay results in a negligible increase in total abundance making it less useful as a geochronometer. The 40Ar isotope is much less abundant however, and is therefore a more useful isotope.
[edit] Method
Because argon is a gas, it is able to escape from molten rock. However, after the rock solidified, the 40Ar produced by potassium decay will begin to accumulate in the crystal lattices. In order to determine the 40Ar content of a rock, it must be melted and the isotopic composition of the released gas measured via mass spectrometry. It is also necessary to separately measure the amount of 40K in the sample. This can be measured using flame photometry or atomic absorption spectroscopy. The ratio between the 40Ar and the 40K is related to the time elapsed since the rock was cool enough to trap the Ar. This is the key principle behind K-Ar dating.
A problem with K-Ar dating is that if there is heterogeneity in the sample, then the aliquots used for determining K concentrations and 40Ar abundances may have different K/Ar. This can lead to an inaccurate date. Argon-argon dating is a similar technique which measures the isotopic content of the same sample, so this problem can be avoided.
[edit] Applications
Due to the long half-life, the technique is most applicable for dating minerals and rocks more than 100,000 years old. Although it finds the most utility in geological applications, it plays an important role in archaeology. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits. It has also been indispensable in other early east African sites with a history of volcanic activity such as Hadar, Ethiopia.
The K-Ar method continues to have utility in dating clay mineral diagenesis (Aronson and Lee, 1986). Clay minerals are less than 2 microns thick and cannot easily be irradiated for Ar-Ar analysis because Ar recoils from the crystal lattice. Some research groups have overcome this by using vacuum encapsulation during irradiation.
[edit] References
- K-Ar N. Mex. Geochron. Lab
- Potassium-Argon Dating Univ. Cal.
- J. L. Aronson and Mingchou Lee "K/Ar systematics of bentonite and shale in a contact metamorphic zone, Cerrillos, New Mexico" Clays and Clay Minerals, Aug 1986; 34: 483 - 487.
