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

Metamorphic cooling rates determined using muscovite 40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar cooling ages. We suggest that a robust method for determining whether metamorphic muscovite 40Ar/39Ar 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 40Ar/39Ar data from muscovite grains and (3) the comparison between analytical data and numerical diffusion models which test different post-crystallisation pressure–temperature histories.