Closure temperature
The closure temperature is the temperature below which a mineral stops losing the daughter isotopes of a radioactive clock.
When a mineral is hot, atoms move through it easily and daughter isotopes produced by radioactive decay can escape. As it cools, diffusion slows until the daughter isotopes are effectively locked in. The temperature at which this happens is the closure temperature. A radiometric age therefore records when a mineral cooled below its closure temperature, not necessarily when it first formed.
Different clocks and minerals close at different temperatures. Argon in feldspar closes at a few hundred degrees Celsius, while lead in zircon or uranium-rich phosphates closes at much higher temperatures. Combining several clocks in the same meteorite gives a cooling curve, showing how temperature dropped over time.
This approach has been used for ordinary chondrites to reconstruct the structure of their parent asteroids. If more heated type 6 material cooled more slowly than type 4 material, the parent body was probably layered like an onion, with the most heated rocks at the centre.
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