Petrology and geochronology of "muscovite age standard" B4M

Heri, Alexandra; Robyr, Martin; Villa, Igor Maria (2014). Petrology and geochronology of "muscovite age standard" B4M. In: Advances in 40Ar/39Ar Dating: from Archaeology to Planetary Sciences. Geological society of London: special publications: Vol. 378 (pp. 69-78). London: Geological Society of London 10.1144/SP378.2

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ArAr-281_Heri_suppub.csv - Supplemental Material
Electron microprobe results on two grain mounts of the unprocessed B4M separate as distributed and on a whole-rock thin section of Brione gneiss from the teaching collection of the Universität Bern are provided as a .csv file

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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 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 characterizes 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 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.

Item Type:

Book Section (Book Chapter)


08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Heri, Alexandra, Robyr, Martin, Villa, Igor Maria


500 Science > 550 Earth sciences & geology




Geological society of London: special publications


Geological Society of London




Sarah Antenen

Date Deposited:

02 Nov 2013 12:00

Last Modified:

05 Dec 2022 14:26

Publisher DOI:





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