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The role of internal deformation in resetting argon ages of micas has been investigated by measuring 40Ar/39Ar ratios of biotite and muscovite, before and after experimentally deforming them. Neither mica crushed cataclastically at room temperature on-line with a mass spectrometer showed any measurable change in 40Ar/39Ar age. Muscovite crystals either sheared along the K-interlayer and/or kinked at 400 C and 100-200 MPa confining pressure, exhibit small (0.7-1.0%) reductions in age and marked increases in bulk diffusion coefficients, as determined from argon release during the initial stages of step-heating between 550 and 810 C. We conclude that the relatively young ages of fine-grained mica populations in naturally deformed mylonites result primarily from grain size refinement and reductions in length scale for volume diffusion and/or by syntectonic neocrystallization. Internal deformation involving dislocation slip and kinking may contribute to some argon loss by pipe diffusion, but reductions in closure temperature associated with multipath diffusion are small. 2001 Springer-Verlag.
Contributions to Mineralogy and Petrology
author list (cited authors)
Dunlap, W., & Kronenberg, A.