40Ar/39Ar geochronology and the diffusion of 39Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo
Step-heating experiments in vacuo are routine when conducting 40Ar/39Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program (eArgon) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40Ar/39Ar geochronology may allow direct dating of white mica mineral growth during metamorphism.