10.6084/m9.figshare.3453248.v1 Alexandra R. Heri Alexandra R. Heri Martin Robyr Martin Robyr Igor M. Villa Igor M. Villa Petrology and geochronology of ‘muscovite age standard’ B4M Geological Society of London 2016 Rb relict phengite peak metamorphism step ages greenschist facies muscovite mica populations ratio fraction 20 µ m Sr reaction textures petrological equilibrium grain size fractions electron microprobe analyses micrometre scale Relict 4M mineral Ar Geology 2016-06-21 11:15:18 Dataset https://geolsoc.figshare.com/articles/dataset/Petrology_and_geochronology_of_muscovite_age_standard_B4M/3453248 <p>Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized 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 of 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 <sup>87</sup>Sr/<sup>86</sup>Sr 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 characterizes <sup>39</sup>Ar/<sup>40</sup>Ar 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 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 micrometre scale. </p>