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Diffusion of (40)Ar in muscovite RID H-1802-2011

Author:
Harrison, T. Mark  Celerier, Julien  Aikman, Amos B.  Hermann, Joerg  Heizler, Matthew T.  


Journal:
GEOCHIMICA ET COSMOCHIMICA ACTA


Issue Date:
2009


Abstract(summary):

Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced (40)Ar* loss that is revealed in (40)Ar/(39)Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate (40)Ar* loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 degrees C. Diffusion data generated in this manner yield an activation energy (F) of 63 +/- 7 kcal/mol and frequency factor (D(o)) of 2.3 (+70)(2.2) cm(2)/s. Experiments at 20 kbar yield diffusivities lower by about all order of magnitude and correspond to an activation Volume of similar to 14 cm(3)/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (T(c)) of 425 degrees C for a 100 mu m radius grain cooling at 10 degrees C/Ma at 10 kbar (T(c)-405 degrees C at 5 kbar. Age and log (r/r(o)) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous (40)Ar/(39)Ar thermochronology to low grade, regionally metamorphosed terranes. (C) 2008 Elsevier Ltd. All rights reserved.


Page:
1039---1051


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