STAIRCASE AR-40 AR-39 SPECTRA OF FINE-GRAINED-WHITE MICA - TIMING ANDDURATION OF DEFORMATION AND EMPIRICAL CONSTRAINTS ON ARGON DIFFUSION/

Staircase Ar-40/Ar-39 spectra of 2 to 6 pm white micas in ductile carb onate mylonites (Morcles-Doldenhorn nappe, Switzerland) record ages be tween 32 and 13 Ma. These ages are within independent geochronologic a nd biostratigraphic time Limits for the beginning and ending of deform ation of this fold-thrust nappe. We interpret these data to record pro tracted white mica neocrystallization and growth during low-temperatur e ductile deformation, such that the differences between the maximum a nd minimum ages in the Ar-40/Ar-39 spectra record the time span over w hich mica formed and grew. We have modeled the argon concentrations in these micas, taking into account the thermal history of the nappe, mi ca growth, and diffusional loss of argon. Our results indicate that av ailable experimental data underestimate the activation energy for argo n diffusion in muscovite, and that the retentivity of argon in white m ica is greater than generally thought.