The measured stable O18 isotopic equilibrium temperature of muscovite and biotite is 300 deg C in a metapelitic mylonite. The Ar-Ar age of dynamically recrystallized muscovite is c.11-12 Ma. Can we consider the 300 deg C temperature of muscovite as its closure temperature for Ar-Ar? What are the constraints here? Which option(s) can we further take to deduce the closure temperature of this recrystallized muscovite?
To me, it is never easy to interpret a K-Ar or a Ar/Ar age as a cooling age therefore using a closure temperature in mylonitic rocks. This procedure has to be prefered in unstrained rocks and rocks that did not experiences pervasive fluids percolations (not easy to proove). The first test you can do is to try to calculate, using the equation of Dodson (1973), the effect of the grain-size reduction. This can explain (alone) younger ages. Besides, if the muscovite indeed recrystalized below ca. 400°C, then this crystal did crystalize below the closure temperature and must be considered as a crystallization age. Accordingly, if you could, even crudely, constraint the conditions or crystallisation (thermobarometric estimates, fluid inclusions, RSCM data...), then the age may reflect the time ellapsed since crystallization at the conditions you found. So franckly speacking, forget about the closure temperature concept if you work of higly strained rocks!
From my experience, white mica closure temperature depends on grain size. In hydrothermal muscovites (sericites) it is most likely not above 300 degrees, perhaps lower.
I think,
Alteration may be effected on muscovite (and muscovite exchange to sericites). Ar is nobel gas, and it can be decreased by alteration that is why cooling age and temperature can be lower than early stage.
Kalin T. McDannell, 1) The age spectrum is a plateau like and 2) my samples are step-heated.
I know this is an old post but I thought I might add some literature! See below.
Article Diffusion of 40Ar in muscovite
Article 40Ar/39Ar dating of white micas from an Alpine high-pressure...
Article On the loss of 40Ar* from muscovite during polymetamorphism
Article Thermochronology of economic mineral deposits; dating the st...
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