Wednesday, October 23, 2013
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
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.