On the loss of 40Ar* from muscovite during polymetamorphism

Recent laser studies have documented 40Ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years. A fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or, alternatively, loss during chemical reactions and deformation. New data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40Ar via lattice diffusion during overprinting events than has been inferred previously. Kyanite-muscovite assemblages in Proterozoic schist along the eastern flank of the Green Mountain massif, central Vermont Appalachians, formed during Proterozoic metamorphism at ca. 1.1 Ga and were remetamorphosed in the greenschist facies during both Taconian (ca. 465 Ma) and Acadian (ca. 390 Ma) events. The overprinting events resulted in deformation of earlier kyanite and muscovite, growth of chemically and isotopically distinct rims on earlier muscovite, and growth of neoblastic muscovite and chloritoid, with each event at inferred minimum temperatures of ca. 425 degrees C. Laser spot analyses of the Grenville muscovite range from 988 +/- II Ma in the centers of porphyroclasts to 397 +/- 7 Ma along rims. Ages younger than 750 Ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals. Parts of early-formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40Ar; laser spot analyses along the outer 100 mu m of an undeformed growth face yield ages close to the timing of Proterozoic metamorphism and cooling. The present study suggests episodic heating events are ineffective for producing measurable, diffusive argon loss along the edges of large muscovite crystals (ca. 1 mm in diameter), particularly in comparison with deformation and reaction mechanisms, and are unlikely to result in complete 40Ar/39Ar age resetting of such crystals. The results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work. Copyright (C) 1997 Elsevier Science Ltd.