Correlation between the age of orogeny and the cooling rate of metamorphic rocks (through ∼500–180 °C), as determined through various radioisotopic dating methods, suggests that ancient orogens cooled at rates as much as two orders of magnitude slower than those of active orogens. I propose that the correlation has arisen because ancient orogens have undergone long postorogenic periods, as long as hundreds of millions of years, during which the rocks remained effectively isothermal. Laboratory experiments suggest that argon diffusion in biotite may be sufficiently rapid at temperatures of ∼220 °C to produce the correlation by slow loss of argon during extended isothermal periods. One interesting realization is that nature has provided a long-term argon diffusion experiment, the results of which have been inverted in this paper to give an independent estimate of the diffusivity of argon in biotite under natural conditions. This estimate compares favorably with a projection of the results of laboratory hydrothermal experiments down 10 orders of magnitude. The analysis confirms the theory that there is a critical thermal window below accepted closure temperatures in which long residence will result in significant partial resetting of thermochronometers.