Of the naturally occurring isotopes of potassium, 40K is radioactive and decays into 40Ar at a precisely known rate, so that the ratio of 40K to 40Ar in minerals is always proportional to the time elapsed since the mineral formed [ 40K is a potassium atom with an atomic mass of 40 units; 40Ar is an argon atom with an atomic mass of 40 units].This relationship is useful to geochronologists, because quite a few minerals in the Earth’s crust contain measurable quantities of potassium (e.g. In theory, therefore, we can estimate the age of the mineral simply by measuring the relative abundances of each isotope.The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present.Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral.
Secondly, K-Ar dating assumes that very little or no argon or potassium was lost from the mineral since it formed. it does not bond to any other elements), it can readily escape from minerals if they are exposed to significant amounts of heat for a prolonged period of time.
The versatility of the Ar method permits determining the timing of processes and events such as igneous intrusions and extrusions, ore mineralization and hydrothermal fluid circulation, metamorphic cooling and exhumation, mineral formation and recrystallization, and shallow crustal faulting.
Scientists are dependent on the geochronologist for data and interpretations to determine these parameters.
Though we know that K-Ar dating works and is generally quite accurate, however, the method does have several limitations.
First of all, the dating technique assumes that upon cooling, potassium-bearing minerals contain a very tiny amount of argon (an amount equal to that in the atmosphere).