Argon-Argon Dating: The Simple Version

  Sciency Thoughts has a post up on recent high-precision ³⁹Ar/⁴⁰Ar dating of the Toba supervolcano in Indonesia.  Unfortunately he seems a bit confused about the technique.

Argon has three naturally occurring isotopes: ³⁶Ar, ³⁸Ar, and ⁴⁰Ar. Potassium also has three isotopes, ³⁹K, ⁴⁰K, and ⁴¹K.  One of these isotopes, ⁴⁰K, is radioactive, with a half life of about 1248 million years, and one of its stable decay products is ⁴⁰Ar.

 In the universe, and in Jupiter and the Sun locally, ³⁶Ar is the most abundant argon isotope, followed by ³⁸Ar.  In the cosmic scheme of things, ⁴⁰Ar is so rare that we don’t even know what its overall abundance is.

However, Earth is a rocky planet.  It was not able to hold onto much gas during its formation, so there is very little ³⁶Ar and ³⁸Ar here.  Earth has lots of potassium though, so almost all the Ar in the atmosphere is ⁴⁰Ar, which is the decay product of ⁴⁰K.

In a potassium-bearing mineral, the ⁴⁰K decays into ⁴⁰Ar, so you can measure the ratio of these two isotopes to figure out how old the mineral is.

The problem is that it is technically very difficult to measure a potassium argon ration accurately, because one is a reactive solid, and the other is an inert gas.  They require different sorts of ion sources, different mass spectrometers, and there are all sorts of chemical effects that complicate the measurement.

If you want an accurate ratio, it is much easier to measure isotopes of the same element.

So for ³⁹Ar-⁴⁰Ar dating, what happens is that the mineral of interest is put into a nuclear reactor and bombarded by neutrons.  Some of the ³⁹K (the most abundant stable potassium isotope) absorbs a neutron, ejects a proton, and transmutes into radioactive ³⁹Ar.  ³⁹Ar has a half-life of a few hundred years, and is virtually non-existent in nature.   So as long as you know your nuclear ³⁹K to ³⁹Ar conversion ration well, this method allows you to use the ³⁹Ar as a proxy for ³⁹K.  The handy thing is that because it is argon, not potassium, it behaves chemically just like the other naturally occurring argon isotopes, so you can measure it in a gas source mass spectrometer much more accurately than you can measure the chemically different ³⁹K and ⁴⁰Ar.

The initial ³⁹K-⁴⁰K ratio doesn’t very much in nature, and is taken as constant (I think- I’ve never actually done Ar-Ar). But the take-home point is that ³⁹Ar-⁴⁰Ar dating is not its own decay system.  It is the ⁴⁰K-⁴⁰Ar decay system, but using a nuclear reactor to change some of the potassium into an unstable argon isotope to make the nuts and bolts of measuring it easier.