AN EMPIRICAL-EVALUATION OF THE ARGON DIFFUSION GEOMETRY IN MUSCOVITE

The temperature-time significance of 40Ar/39Ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry, the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters. Laser 40Ar/39Ar studies of natural 40Ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion, commonly with an effective diffusion dimension equal to the crystal diameter. The present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural 40Ar diffusion gradient, parallel and perpendicular to 001, in a single euhedral muscovite phenocryst. The new data, combined with previous studies, clearly indicate loss of 40Ar via cylindrical volume diffusion and strongly suggest that loss of 40Ar across the 001 sheets (slab diffusion geometry) is minimal in undeformed crystals.