Argon loss during deformation of micas: constraints from laboratory deformation experiments

The role of internal deformation in resetting argon ages of micas has been investigated by measuring Ar-40/Ar-39 ratios of biotite and muscovite, befo re and after experimentally deforming them. Neither mica crushed cataclasti cally at room temperature on-line with a mass spectrometer showed any measu rable change in Ar-40/Ar-39 age. Muscovite crystals either sheared along th e K-interlayer and/or kinked at 400 degreesC and 100-200 MPa confining pres sure, exhibit small (0.7-1.0%) reductions in age and marked increases in bu lk diffusion coefficients, as determined from argon release during the init ial stages of step-heating between 550 and 810 degreesC. We conclude that t he relatively young ages of fine-grained mica populations in naturally defo rmed mylonites result primarily from grain size refinement and reductions i n length scale for volume diffusion and/or by syntectonic neocrystallizatio n. Internal deformation involving dislocation slip and kinking may contribu te to some argon loss by pipe diffusion, but reductions in closure temperat ure associated with multipath diffusion are small.