EXPERIMENTAL EVIDENCE FOR DIFFUSIVE AR TRANSPORT DURING IN-VACUO HEATING: IMPLICATIONS FOR 40AR/39AR THERMOCHRONOLOGY OF MUSCOVITE

Muscovite dated via ⁴⁰Ar/³⁹Ar analysis commonly yields age spectra with age gradients of questionable geological and thermochronological significance. It has been held that structural instability under vacuum-furnace conditions precludes Ar loss by volume diffusion. Here we show that variably sized muscovite crystal aggregates and single crystals partially degassed in-vacuo prior to irradiation yield age spectra and Arrhenius parameters that are consistent with argon loss via volume diffusion. Sized fractions of muscovite known to have flat age spectra were heated in-vacuo to induce partial Ar loss prior to irradiation. Subsequent step-heating yielded age gradients that vary systematically with both grain size and temperature of pre-irradiation heating. Fine grained aliquots are unanimously more degassed than coarse grained aliquots, as are samples subjected to higher pre-irradiation heating. All samples yield age gradients that can be interpreted within a multiple diffusion domain (MDD) framework. Single grains (~100-200 µm) also yield age spectra consistent with argon loss controlled by physical size. Further, physically cutting these crystals to reduce their grain size after irradiation disrupts the age spectrum adding support that the concentration profile of ⁴⁰Ar* is developed within the physical crystal rather than mixing of neo-crystalline particles grown during pre-irradiation heating. MDD-derived model age spectra based on the known pre-irradiation thermal history closely match the measured age spectra. Assuming the experimentally determined kinetic parameters D₀ = 2.3 cm2/s and E = 63 kcal/mol for muscovite, the diffusion lengths predicted by the MDD model distribution are smaller than the observed physical grain sizes. However, varying the Do to account for pressure dependence and the activation energy within its published uncertainty yields model diffusion length scales comparable with those of physical grains. These data provide an intriguing counterpoint to arguments against the validity of diffusion-based thermal history information from muscovite, and are part of a growing body of evidence that suggests the MDD method can, in some circumstances, successfully be applied to extract geologically meaningful thermal history information from muscovite.