Argon diffusion in white mica

Muscovite is dated via 40Ar/39Ar analysis and commonly yields age spectra with well-defined age gradients of somewhat questionable geological and thermochronological significance.  

Muscovite is known to record in-situ age gradients suggestive of volume diffusion with older cores and concentrically younger age domains, however measured data are not consistent with the thermal histories suggested by core and rim ages.

Typically, it has been held that structural instability within vacuum-furnace conditions precludes Ar loss by volume diffusion – a necessary condition for interpreting geologically accurate thermal histories based on laboratory degassing data. In-situ laser analysis and step heating of muscovite often yield results that are inconsistent with volume diffusion.

Modeled age spectra based on volume diffusion from a single size domain yields either match the shape of the age profile, or the terminal age, but not both. However, volume diffusion from multiple diffusion domains can explain the measured data provided that argon degasses via volume diffusion under laboratory conditions.

To address this controversy, I conduct diffusion experiments by in-vacuo heating of muscovite and subsequent thermal modeling to determine the extent to which 40Ar/39Ar age gradients generated in-vacuo are consistent with the experimentally induced thermal histories.

This work is conducted in collaboration with Matthew Heizler at the New Mexico Geochronology Research Laboratory, with implications for understanding the thermal evolution of continental crust throughout time and space.