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gas-retention age 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.