Recovery of muscovite age gradients by 40Ar/39Ar vacuum furnace step-heating analysis

Abstract

40Ar/39Ar furnace step heating analysis of 3 size fractions of Crazy Basin muscovite was performed using ‘conventional’ (12 steps) and ‘detailed’ (29 steps) heating schedules. The intracrystalline age gradients previously documented by laser spot ablation analysis were reproduced in the step-heating age spectra. Total gas and plateau ages from ‘conventional’ results correlate positively with grain size indicating that muscovite preserves natural dimensions during sample processing and the crystal size influences argon retention during geologic cooling. Age spectra from ‘detailed’ analyses exhibit incrementally increasing ages over the low-temperature portion of the gas release (first ~10%) followed by more gently increasing ages over the remainder of the gas release. The ages in the ‘detailed’ spectra span the range previously documented by laser spot ablation analysis including the core ages reported by Hames and Bowring (1994) indicating the age zonation in the mica crystals was not homogenized during furnace heating. Arrhenius diagrams constructed from the incremental 39Ar release during the ‘detailed’ experiments yield consistent results with those recently reported by Harrison et al. (2009). Interpreting the initial Arrhenius array as subdomains yields domain distribution plots (log r/ro vs. % 39Ar released) exhibiting strong correlation with the shape of the associated age spectra. This correlation supports the assertion that the same process controlling diffusion during geologic cooling is active during vacuum furnace degassing in the laboratory. Therefore, muscovite step-heating data, may afford recovery of thermal histories.