Chemistry Reference
In-Depth Information
layers provide a minimum and maximum age for the activity of humans.
The method, which is particularly useful for dating mineral, rock, or ash
rich in potassium, can be applied to mineral matter as old as the earth (4.5
billion years ago) and as recent as under 100,000 years ago (Walter 1997;
Guillot and Cornette 1986).
Methodology
The determination of the potassium-argon age of rock involves the fol-
lowing experimental stages:
1.
Sampling the rock
2.
Isolating the potassium-40 and argon-40 in the sample from the rest
of the mineral matter
3.
Determining the relative amounts of these elements in the sample
4.
Calculating the potassium-argon date of the mineral or rock
Experience shows that samples weighing 0.2-20 g are usually sufficient for
dating by this method. The determination of potassium-40 is, at present,
FIGURE 11
The decay of potassium-40.
Potassium-40 may decay by one of
two different radioactive decay modes: either by the emission of beta particles
or by electron capture. The emission of beta particles is, however, its main mode
of decay: almost 90% of radioactive potassium-40 decays by this mode to
calcium-40; the other nearly 10% decays by electron capture, to excited argon-
40. The diagram illustrates the two different decay modes. Only the decay by
electron capture to excited argon-40, which is followed by the emission of
gamma rays to argon-40, is the decay path used in the potassium-argon method
of dating.
