40Ar/39Ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations

Artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3:1, 1:1, and 1:3 and analyzed by the 40Ar/39Ar vacuum furnace step-heating method. The original micas are Late Jurassic and Late Cretaceous samples that yield flat age spectra, with plateau ages defined by 97–100% of the gas release and no evidence for excess argon. The age spectra from mica mixtures yield patterns that systematically decrease with decreasing Jurassic mica content and increasing Cretaceous component. However, the mixed muscovite spectra are relatively flat, consistent with the two original samples showing similar degassing patterns during vacuum furnace heating. In contrast, the mixed biotite spectra are highly discordant with an overall decrease in age during the step-heat analysis. This is consistent with the degassing patterns of the original mica samples, which indicate the Jurassic biotite (high Fe/Mg) outgases at lower temperatures and the Cretaceous biotite (low Fe/Mg) remains retentive until the higher temperature steps. The individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating. All but one of the mixed samples failed to yield ages reflecting the age of the original micas. These results imply that complex age spectra obtained for multiply-deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing (i.e. mixing of sample reservoirs) during heating. Comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas/retain argon in a predictable manner. These conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating.