Potassium–argon dating
Potassium–argon dating measures argon-40 produced by the decay of potassium-40 to find when a rock last cooled.
Potassium–argon dating relies on potassium-40, which decays partly to argon-40 with a half-life of about 1.25 billion years. Argon is a gas, so when a rock is molten or very hot, argon escapes. Once the rock cools, argon produced by decay is trapped, and the clock starts.
The method measures the time since a rock last cooled below the temperature at which argon is retained. In meteorites, that event is often an impact rather than the original formation, because collisions heat rocks enough to drive out argon. Many L chondrites have argon ages near 470 million years, recording a major collision in the asteroid belt that also sent a shower of meteorites to Earth.
The modern refinement is argon–argon dating, which converts some potassium to argon in a nuclear reactor so that only argon isotopes need to be measured. It gives step-by-step heating results that reveal disturbance more clearly than the older method.
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