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As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.
(However, see the section below on the limitations of the method.) This suggests an obvious method of dating igneous rocks.
The isotopes the KAr system relies on are Potassium (K) and Argon (Ar).
Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals.
Potassium can be mobilized into or out of a rock or mineral through alteration processes.
Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs.
Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released.
Potassium is a common element found in many materials, such as micas, clay, tephra, and evaporites.
K has a half-life of 1.248 billion years, which makes it eminently suitable for dating rocks.
However, the Argon, a noble gas, constitutes approximately 0.1-5% of the Earth's present day atmosphere.
Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon.