ABSTRACT
Potassium-Argon dating is a method of radiometricdating based on spontaneous decay of the unstable 40K isotope. The decay of 40K is relatively slow, with a half life of ca. 1.25 Myr, and the isotope occurs in only one out of 8,600 atoms of potassium. Most 40K decay events proceed to 40Ca by emission of a beta particle, and only 10.5 percent involve electron capture and gamma emission with decay into the 40Ar isotope instead. Nevertheless, because of the abundance of potassium in granitic rock, it is calculated that the dual decay of this isotope is responsible for approximately one-third of the geothermal heat flow, nearly all of the background radioactive flux, and more than 99 percent of the argon in the atmosphere (ca. 1 percent of the total by weight). Radiogenic 40Ca cannot be distinguished from the natural 40Ca isotope of calcium, but all 40Ar is radiogenic. This means that the amount of 40Ar trapped within a mineral is a function of the age of the crystal and the amount of original 40K (as a fixed percentage of total K, noted above). Applying the constant for the number of 40K decay events per year ( 5.543 10-10 yr-1) and a correction for any background, or atmospheric, 40Ar that may have been incorporated at the beginning, year-ages can be calculated directly from the ratio of the parent and daughter isotopes.
