Micrometer-scale (4–8 μm) Rb Sr age mapping was applied to millimeter-sized muscovite from Cambro-Ordovician metapegmatites of the western Bohemian Massif (SE Germany, Erbendorf–Vohenstrauß Zone), overprinted by Devonian amphibolite-facies metamorphism (~380 Ma, >600 °C). These relict magmatic muscovite crystals (ms1) show a wide range of deformation states, from undeformed magmatic “books” to lozenge-shaped, plastically deformed porphyroclasts. Neocrystallized muscovite (ms2) forms along the grain boundaries of white mica fish and within micro-shear zones that are only a few hundred micrometers in width. The ms2 mica uniformly records the ~380 Ma ages displaying sharp age gradients at the interface towards pristine ms1 muscovite, with one case recording a ~ 100 Ma range over <20 μm. Contrary, both deformed and undeformed ms1 mica displays domains in which the Rb Sr system is only partially reset spanning the full range from 380 to 480 Ma, which the older ages found towards the crystal interior. This partial age reset results from 87 Sr loss and/or Rb gain along cleavage-parallel bands that extend several millimeters into the grain interior. These bands display sigmoidal concentration profiles, from which apparent (bulk effective) diffusion coefficients could be extracted. In deformed muscovite, apparent 87 Sr diffusion coefficients span two orders of magnitude (7.9 × 10 −18 to 9.6 × 10 −20 cm 2 /s; mean = 1.4 × 10 −18 cm 2 /s), likely reflecting intracrystalline differences of strain accommodation. Closure temperature estimates using this mean apparent diffusion coefficient and the Dodson equation yield ~530 °C for 87 Sr in muscovite, ~220–280 °C higher than in biotite using published Sr diffusion data. Notably, diffusive age resetting is observed along both muscovite–plagioclase and muscovite–quartz grain boundaries. Given that quartz cannot serve as an appreciable reservoir or sink for Rb or Sr, these observations imply that fluids acted as the primary medium for isotopic exchange. The outward diffusion of 87 Sr is consistently faster (by up to an order of magnitude) than inward diffusion of Rb. In deformed muscovite, the mean apparent Rb diffusivity (7.5 × 10 −19 cm 2 /s) is markedly slower than in undeformed crystals, with strong crystallographic control: 3.5 × 10 −21 cm 2 /s perpendicular and 2.7× 10 −22 cm 2 /s parallel to the c-axis. Our results demonstrate that the most efficient mechanism for Rb Sr isotope redistribution in muscovite is neo- or recrystallization. However, age resetting via thermally activated diffusion also plays a significant role, particularly in deformed crystals at such elevated temperatures. These findings highlight the importance of deformation and crystallographic orientation for the resetting of the Rb Sr system and the preservation of associated geochronological information.
Kutzschbach et al. (Sun,) studied this question.