Unmixing Multiple Metamorphic Muscovite Age Populations With Powder X-Ray Diffraction And Ar-40/ Ar-39 Analysis

A combination of modal estimates from powder X- diffraction (XRD) experiments and arson isotopic data shows that muscovite Ar-40/Ar-39 total gas age correlates with muscovite composition near the retrograde Bald Mountain shear zone (BMSZ) in Claremont, New Hampshire, and that the shear zone was active at similar to 245 Ma. Petrologic study demonstrates that chemical disequilibrium is preserved in muscovite grains in these samples. The recognition of this preservation is critical to the interpretation of the Ar-40/Ar-39 step-healing experiments, which never produce age plateaus and yield spectra with steps that range in age by similar to 20 Ma. Petrographic, compositional, and crystallographic data all indicate that the age spectra reflect dissolution of metastable Na-rich muscovite and precipitation of stable Na-poor muscovite associated with deformation in the BMSZ.Comparison of whole rock and muscovite concentrate XRD patterns from individual samples demonstrates that the mineral separation process can fractionate these muscovite populations. Therefore, four muscovite concentrates of varying magnetic susceptibility were prepared from a single hand sample, analyzed by XRD, and dated. These four splits define a mixing line that resolves end-member ages of 244.5 +/- 4.2 Ma and 302.5 +/- 12.5 Ma (1 sigma). Although the ages are imprecise, the petrologically supported conclusion that these schists preserve two discrete ages is distinct from an interpretation that the spectra reflect cooling through closure at similar to 270 Ma, as might be concluded in the absence of petrologic characterization. The XRD results also demonstrate that, even well above anchizone conditions, petrologic information relevant to Ar-40/Ar-39 dating is observable in subtle variations in the crystallography of muscovite grains.