Measurement of the age of meteorites using the amount of radiogenic argon from the decay of potassium-40, or of helium from uranium and thorium, to indicate the time since the meteorite was last at a temperature at which gas leakage could occur. This is based on the principle that at high temperatures gases diffuse easily through silicate lattices, e.g. ⁴⁰A from ⁴⁰K diffuses out and is lost at temperatures above about 300 °C. If a meteorite has not suffered reheating its gas-retention age is the same as its formation age.