Determining cooling rates from mica 40Ar/39Ar thermochronology data: Effect of cooling path shape

Abstract

Tectonic models are commonly underpinned by metamorphic cooling rates derived from diffusive-loss thermochronology data. Such cooling ages are usually linked to temperature via Dodson's closure temperature (TC) formulation, which specifies a 1/time-shaped cooling path (Contributions to Mineralogy and Petrology, 1973, 40, 259). Geologists, however, commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite-difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data.