Closure temperature

Closure temperature is the temperature below which a mineral or material system ceases to exchange a particular element or isotope with its surroundings, effectively closing the system and starting a radiometric clock. Above this temperature, diffusion is rapid enough that daughter products escape as fast as they form; below it, the mineral retains its isotopic signature. The closure temperature is not a fixed material constant — it depends on cooling rate, grain size, and diffusion geometry. A slowly cooled rock will have a higher effective closure temperature than the same mineral rapidly quenched. This dependence makes closure temperature a systems parameter, not a simple physical constant, and it is one of the reasons radiometric dates are interpreted as cooling ages rather than crystallization ages.

The concept of closure temperature exposes a common misunderstanding: we do not date when a rock formed. We date when it stopped forgetting.