Potassium-argon dating is one of a series of methods for obtaining an estimate of the relative time of crystallization of igneous rocks. This technique is useful over a large range of apparent ages from about 1 million or less to several billion years. It is based upon the relative abundance of the radioactive isotope of potassium, 40K, an isotope with a half life of 1.2 billion years. This isotope is present in all potassium in a ratio of approximately one atom to 10,000 atoms of common (non-radioactive) 39K. When an atom of 40K decays, 88.8% of the time it gives rise to an atom of 40Ca. Since 40Ca is common calcium, it is not usually of use in radiometric dating. However, 11.2% of the 40K decays into 40Ar, which, as an inert gas, is generally considered to be lost to the system during the crystallization of magma. Subsequently, argon resulting from the breakdown of 40K, accumulates in the rock within the crystal lattice and is trapped. The amount of potassium in the rock is determined, and the rate of argon production is estimated from this. Using this rate value, a radiometric age can be assigned to crystallization, based upon the amount of 40Ar contained in the rock. Like all radiometric age determinations, Potassium Argon dating has its share of assumptions. Some of these involve higher levels of asumption than others.
1: The half life for decay of 40K has not changed. This is perhaps the least assailable assumption.
2: The branching ratio has not appreciably changed.
3: The starting level of potassium, and particularly radiogenic potassium can be determined with certainty.
4: All preexisting 40Ar was removed from the rock prior to cooling.
5: Any nonradiogenic argon contained in the rocks has the composition of atmospheric argon.
6: No argon has been lost since the rock cooled. Beyond these are the problems of choosing a sample that has even the potential of giving precise information that is relevant for dating, and carefully assessing that the above criteria have been correctly evaluated in each sample.
2010 Arthur V. Chadwick, Ph.D.