Petrology and geochronology of the “muscovite standard” B4M

Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterised by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction < 0.2 µm) and a muscovitic component (predominant in the fraction > 20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses on the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe, and F. Rb/K ratios vary by one order of magnitude. Rb-Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87Sr/86Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size - age relationship also characterises 39Ar-40Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635 ± 20 °C. A suitable mineral standard requires that especially its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the µm scale.