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Mineralogy and Petrology

, 97:251 | Cite as

Perthite microstructure in magmatic alkali feldspar with oscillatory zoning; Weinsberg Granite, Upper Austria

  • Rainer AbartEmail author
  • Elena Petrishcheva
  • Stefan Käßner
  • Ralf Milke
Original Paper

Abstract

Feldspar megacrysts from the Weinsberg granite (Moldanubian Zone) show oscillatory zoning with respect to the albite- and orthoclase components. All growth zones show perthitic exsolutions which take the form of bleb- and lens shaped albite-rich precipitates in an orthoclase-rich host. The average sizes and shapes of the precipitates show systematic variation with the integrated bulk compositions of the respective growth zones. The precipitates are abundant and relatively small in growth zones with intermediate bulk composition (Or50Ab41An09 - Or80Ab18An02), and they are less abundant and larger in more orthoclase-rich zones (Or88Ab11An01). Small precipitates have a relatively high aspect ratio, whereas the large precipitates in the potassium-rich zones are more spherical. The relation between microstructure and integrated bulk composition suggests that exsolution and subsequent growth and coarsening occurred by different mechanisms in the respective growth zones. Numerical modeling shows that rapid growth of precipitates over extended periods of time and attainment of relatively large final size is favored, if only few nuclei are formed in an oversaturated host. In contrast, precipitates can grow rapidly only over limited time intervals and remain relatively small, if abundant nuclei are present. During cooling of the oscillatorily zoned alkali-feldspar, exsolution started at relatively high temperatures in growth zones of intermediate integrated bulk composition as compared to exsolution in the more orthoclase-rich growth zones. Irrespective of whether exsolution occurred by spinodal decomposition or by nucleation at relatively high temperatures in the growth zones of intermediate integrated bulk composition, it produced abundant nuclei and resulted in relatively small precipitates. In contrast, comparatively few nuclei were formed in the orthoclase-rich growth zones resulting in large precipitates. The Na/K partitioning between precipitates and the host is independent of the integrated bulk composition of the respective growth zone reflecting re-equilibration during cooling down to relatively low temperatures (< 400°C). The shape of the precipitates probably has evolved from an initially lamellar or spindle-like geometry with high aspect ratio to more isometric, spheroidal shapes during precipitate growth and coarsening. Host/precipitate interfaces served as fluid pathways during late stage deuteric alteration.

Keywords

Growth Zone Spinodal Decomposition Oscillatory Zoning Precipitate Size Alkali Feldspar 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgements

We want to thank Friedrich Koller for providing the samples and Lukas Keller for his help on the scanning electron microscope. Funding by the Deutsche Forschungsgemeinschaft, project AB 741/3-1 is gratefully acknowledged. We are grateful to two anonymous reviewers for their valuable comments and advice.

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Rainer Abart
    • 1
    Email author
  • Elena Petrishcheva
    • 2
  • Stefan Käßner
    • 2
  • Ralf Milke
    • 2
  1. 1.Department of Lithosphere ResearchUniversität WienWienAustria
  2. 2.Institute of Geological SciencesFreie Universität BerlinBerlinGermany

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