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>