Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites.In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).
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.method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.Ar dating is a major method that researchers have used to understand the structural evolution of the Maria Fold and Thrust Belt.
Argon-argon dating works because potassium-40 decays to argon-40 with a known decay constant. This led to the formerly-popular potassium-argon dating method.Common phases to be used for argon-argon dating are white micas, biotite, varieties of potassium feldspar (especially sanidine because it is potassium-rich), and varieties of amphibole. This can be used to solve equation 2 for the sample.Second, the sample is irradiated along with a standard of a known age. A major advantage of the argon-argon method is that the sample can be heated incrementally.While this assumption holds true in the vast majority of cases, excess argon can occasionally be trapped in the mineral when it crystallizes, causing the K-Ar age to be a few hundred thousand to a few million years older than the actual cooling age.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.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].