Potassium 40 argon 40 dating
But consider what happens if the argon came from deep within the Earth, where it was formed by Ar ratio as is found in the atmosphere, and the formula that corrects for atmospheric carbon will not correct for this.Finally, we must consider the possibility of argon loss.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somewhere between the formation of the rock and the metamorphosis event.For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
Because argon is inert, it cannot be chemically incorporated in the minerals when they are formed, but it can be physically trapped in the rocks either during or after formation. If the source of this argon is atmospheric contamination, then we can correct for this.
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.
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.