Potassium-argon dating

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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Decay series

Potassium (K) exists 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.

Method

Because argon is a gas, it is able to escape from molten rock. However, when the rock solidifies, the decayed ⁴⁰Ar 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.

Applications

Due to the long half-life, the technique is most applicable for dating minerals and rocks >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.

References

• K-Ar N. Mex. Geochron. Lab
• Potassium-Argon Dating Univ. Cal.