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Book Chapter

Petrology and geochronology of ‘muscovite age standard’ B4M

By
Alexandra R. Heri
Alexandra R. Heri
1
Institut für Geologie, Universität Bern, 3012 Bern, Switzerland
2
Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong
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Martin Robyr
Martin Robyr
1
Institut für Geologie, Universität Bern, 3012 Bern, Switzerland
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Igor M. Villa
Igor M. Villa
1
Institut für Geologie, Universität Bern, 3012 Bern, Switzerland
3
Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano Bicocca, 20126 Milano, Italy
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Published:
January 01, 2014

Abstract

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.

Supplementary 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 available at: http://www.geolsoc.org.uk/SUP18590.

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Contents

Geological Society, London, Special Publications

Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences

F. Jourdan
F. Jourdan
Curtin University, Australia
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D. F. Mark
D. F. Mark
Scottish Universities Environmental Research Centre, UK
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C. Verati
C. Verati
University of Nice, France
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Geological Society of London
Volume
378
ISBN electronic:
9781862396623
Publication date:
January 01, 2014

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