Why is potassium 40 useful in dating igneous rocks
Radioactive dating of rock samples determines the age of rocks from the time it was formed.
A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample.
If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.
Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.
As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.
He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes that any argon-40 that he measures in his rock sample must have been produced by the radioactive decay of potassium-40 since the time the rock solidified.
He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running.For example, if the age is higher than he expected he will say that his rock contains ‘excess argon’ or ‘parentless argon’.By this he means that argon gas in his rock has come from the melting of some older rocks deep underground and contaminated his sample with a higher concentration of argon-40, which is why its age is too old.However, we cannot rely on all the argon being lost, and if it is not then when we apply K-Ar dating this will give us an essentially arbitrary date somewhere between the formation of the rock and the metamorphosis event.For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.When a rock undergoes metamorphism, some or all of its argon can be outgassed.