K ar dating example
The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.
But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.
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.
Young rocks have low levels of Ar, so as much as several kilograms may be needed.
Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.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somewhere between the formation of the rock and the metamorphosis event.For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.The potassium-argon dating method has been used to measure a wide variety of ages.The potassium-argon age of some meteorites is as old as 4,500,000,000 years, and volcanic rocks as young as 20,000 years old have been measured by this method.