Blocking temperature is the temperature below which isotopes in a mineral are no longer free to move, at which point the radiometric dating clock starts.¹ It is specific to a particular material and isotopic system.²

These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as blocking temperature and represents the temperature below which the mineral is a closed system to isotopes. ²Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the blocking temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to blocking temperature.

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