Argon–argon dating
A refinement of potassium–argon dating that measures argon isotopes after neutron irradiation, widely used to date heating and impact events.
Potassium-40 decays partly to argon-40, with a half-life of about 1.25 billion years. In argon–argon (⁴⁰Ar/³⁹Ar) dating, the sample is irradiated with fast neutrons in a nuclear reactor alongside a standard of known age, converting some ³⁹K into ³⁹Ar. Potassium content can then be measured as argon, so both parent and daughter are determined in a single isotopic analysis, avoiding separate chemical measurements.
The sample is usually heated in steps, or with a laser. Each step yields an apparent age; together they form an age spectrum. A flat plateau across much of the gas suggests an undisturbed system, while staircase patterns reveal partial argon loss. Because argon is a gas that escapes readily when rocks are heated, the method mostly records the last major thermal event rather than original crystallisation. For meteorites that often means an impact.
This makes it the workhorse for shock histories. Many shocked L chondrites give argon–argon ages near 470 million years, matching the fossil L chondrites in Ordovician limestone in Sweden and pointing to a catastrophic break-up of the L chondrite parent body. Many eucrites and lunar impact melts record resetting between about 3.4 and 4.1 billion years ago. Corrections for cosmogenic and trapped argon are essential.
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