When can muscovite Ar-40/Ar-39 dating constrain the timing of metamorphic exhumation?

Metamorphic cooling rates determined using muscovite Ar-40/Ar-39 ages underpin many tectonic models, but are based on several simplifying approximations. We demonstrate that a number of these approximations, especially those relating to the assignment of a single "closure temperature", might not hold for certain metamorphic scenarios, and particularly for metamorphism during rapid orogenic cycles. Using numerical diffusion models that include a recently reported significant pressure dependence of Ar diffusion in muscovite, we systematically interrogate the approximations associated with linking Ar-40/Ar-39 dates to pressure-temperature histories. The results of the simulations are presented in a simple graphic form allowing evaluation of the pressure-temperature (PT) regions in which Ar diffusion is efficient, and hence those in which the muscovite may yield true Ar-40/Ar-39 cooling ages. We suggest that a robust method for determining whether metamorphic muscovite Ar-40/Ar-39 ages relate to the timing of cooling involves: (1) the determination of the PT conditions and relative timing of muscovite crystallisation during the metamorphic cycle, (2) the collection of high precision and high spatial resolution Ar-40/Ar-39 data from muscovite grains and (3) the comparison between analytical data and numerical diffusion models which test different post-crystallisation pressure-temperature histories. (C) 2011 Elsevier B.V. All rights reserved.