Nature's diffusion experiment: The cooling-rate cooling-age correlation

Correlation between the age of orogeny and the cooling rate of metamorphic rocks (through similar to 500-180 degrees C), as determined through various radioisotopic dating methods, suggests that ancient orogens cooled at rate s as much as two orders of magnitude slower than those of active orogens, I propose that the correlation has arisen because ancient orogens have under gone long postorogenic periods, as long as hundreds of millions of years, d uring which the rocks remained effectively isothermal. Laboratory experimen ts suggest that argon diffusion in biotite may be sufficiently rapid at tem peratures of similar to 220 degrees C to produce the correlation by slow lo ss of argon during extended isothermal periods. One interesting realization is that nature has provided a long-term argon diffusion experiment, the re sults of which have been inverted in this paper to give an independent esti mate 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 res etting of thermochronometers.