K ar dating method
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.The severity of this problem decreases as the accuracy of our instruments increases.Still, as a general rule, the proportional error in K-Ar dating will be greatest in the youngest rocks.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.(However, see the section below on the limitations of the method.) This suggests an obvious method of dating igneous rocks.If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago.
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.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somewhere between the formation of the rock and the metamorphosis event.For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.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.The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.