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Possible effects of39Ar recoil in40Ar—39Ar dating. So if all has gone well, and if there were no problems with argon loss or excess argon, then the age of the sample would be given by the following formula: But what is J? J is a factor which depends on the nature of the neutron bombardment. J is not calculated on theoretical grounds, but is found experimentally; alongside the sample we're interested in, we irradiate and then heat a sample of known age a standard. Measuring the 39Ar and 40Ar emitted from the standard, and knowing the time t that it was formed, we can put these figures into the equation above and solve it for J.
So now we know J, and we have measured the R-value of the sample we're actually interested in dating, so we can use these data to solve the equation for t, giving us the age we're looking for. You will note that this means that we have to be able to date some rocks accurately using some method other than Ar-Ar, so that we can find a standard to use for the determination of J; fortunately we can do this, and geologists have put a lot of effort into identifying rocks which can be accurately dated and used as standards.
Advantages of the Ar-Ar method[ edit ] So far, all we seem to have done is taken the K-Ar dating method and made it much more complicated for no apparent reason. However, there are advantages to this more complex method. In the first place, recall that one of the potential problems with the K-Ar method is that it requires two different samples, one to measure the potassium and the other to measure the argon; if the two samples had different chemical compositions when they first formed then this will introduce an error.
However, in Ar-Ar dating the two isotopes of argon are both measured from the same sample, and so at least one potential source of error is eliminated. The other important advantage of Ar-Ar dating is the extra data gained from step heating: What's the point of this? Well, different minerals within the rock will give up their argon at different temperatures, so each step will give us a ratio of 40Ar to 39Ar which we can use in the equation to calculate a date.
Now, recall that we said that when the rock was first formed, the 39K and 40K from which these are derived must have appeared in the same ratio in each mineral, because both isotopes of potassium have the same chemical properties. This means that if the rock cooled rapidly enough that all the minerals in it have the same date, and if there has been no argon loss, and if there is no excess argon added to the system, then the dates we calculate at each step of the heating will be the same date.
If we don't get the same date at each step, then we may be able to work out what's going on. For example, if the date increases at each step, then we are quite possibly looking at a slow-cooling igneous rock in which different minerals crystallized out of the magma at different times, a possibility we can investigate further.
Potassium-Argon Dating Methods
Ar–Ar and K–Ar Dating
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