Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis. This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium.

Is K-Ar dating accurate?

Potassium-argon dating is accurate from 4.3 billion years (the age of the Earth) to about 100,000 years before the present. At 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40, pushing the limits of present detection devices.