potassium–argon dating A dating technique for certain rocks that depends on the decay of the radioisotope potassium–40 to argon–40, a process with a half-life of about 1.27 × 10¹⁰ years. It assumes that all the argon–40 formed in the potassium-bearing mineral accumulates within it and that all the argon present is formed by the decay of potassium–40. The mass of argon–40 and potassium–40 in the sample is estimated and the sample is then dated from the equation: ⁴⁰Ar = 0.1102 ⁴⁰K(e^λt – 1),

where λ is the decay constant and t is the time in years since the mineral cooled to about 300°C, when the ⁴⁰Ar became trapped in the crystal lattice. The method is effective for micas, feldspar, and some other minerals.

potassium—argon dating (K—Ar method) Geologic dating technique based on the radioactive decay of potassium (⁴⁰K) to argon (⁴⁰Ar). This potassium isotope has a half-life (see DECAY CONSTANT) of 1.3 billion (10⁹) years, making this a valuable dating method. The minimum age limit for this dating method is about 250 000 years.

potassium–argon dating(K–Ar method) A dating technique based on the radioactive decay of potassium (⁴⁰K) to argon (⁴⁰Ar). This potassium isotope has a half-life of 1.3 billion (10⁹) years, and the minimum age limit for this dating method is about 250 000 years.