Radiometric Dating: Potassium-Argon

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

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This is perhaps the closest approach to an ideal dating system. Potassium 40decays to Argon 40. Furthermore, since argon is an inert gas and doesn't combine naturally with other elements, it has no business being in a crystal lattice at all. It can only be there if it formed by radioactive decay. Well, there are a few complications:

Potassium 40 has a branched decay: only 1/9 of all K-40 atoms decay to Ar-40. The others decay to Ca-40, which is the only natural isotope of calcium. The correction is simple: multiply the number of argon atoms by 9 to find the total number of potassium atoms that decayed.

Second, the resulting argon atoms are merely mechanically trapped in the crystal lattice, not bonded in any way. That means even minor mechanical disturbance, like heating, can allow them to escape. So potassium-argon is the least stable commonly-used dating method.

Trapped Argon

Most of the things that upset K-Ar dating, like slow escape of Ar atoms, weathering, or heating, make ages too young by resetting the clock. Occasionally, ages can be too old, for example, historic lava flows can yield K-Ar ages of millions of years. These cases are due to trapped modern argon, either atmospheric argon or argon dissolved in ground water. The cure for this problem is pretty straightforward. Analyze small portions of the sample and plot a graph of argon versus potassium content. Most analyses should plot on a straight line, with argon proportional to potassium content. A line whose slope is proportional to the age of the sample is called an isochron. Any analyses with excess argon will plot off the isochron.

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