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Isothermal annealing and continuous cooling experiments on synthetic orthopyroxenes; temperature and time evolution of the Fe, Mg distribution

Hartmut Schlenz, Herbert Kroll and Michael W. Phillips
Isothermal annealing and continuous cooling experiments on synthetic orthopyroxenes; temperature and time evolution of the Fe, Mg distribution
European Journal of Mineralogy (August 2001) 13 (4): 715-726

Abstract

Isothermal annealing and continuous cooling experiments have been performed on synthetic orthopyroxene crystals with compositions En (sub 81) Fs (sub 19) and En (sub 49) Fs (sub 51) . Their intracrystalline Fe (super 2+) , Mg-partitioning was determined by X-ray structure analysis. En (sub 81) Fs (sub 19) was equilibrated at 550 degrees C, 675 degrees C and 850 degrees C, En (sub 49) Fs (sub 51) at 550 degrees C, 650 degrees C, 675 degrees C, 800 degrees C, and 850 degrees C. The Fe (super 2+) , Mg distribution coefficients vary with temperature according to ln(K (sub D) ) = 0.633(31) - 2625(31)/T[K] (En (sub 81) Fs (sub 19) ); ln(K (sub D) ) = 0.594(52) - 2581(51)/T[K] (En (sub 49) Fs (sub 51) ), suggesting that in this compositional range ln(K (sub D) ) values do not significantly depend on the Fs content. Rate constants were determined in ordering runs at 550 degrees C: k (sub dis) = 0.00165(37) [h (super -1) ] (En (sub 81) Fs (sub 19) ) k (sub dis) = 0.0080(18) [h (super -1) ] (En (sub 49) Fs (sub 51) ). The results closely agree with predictions from Arrhenius equations given by Ganguly & Tazzoli (1994) and Kroll et al. (1997). Non-linear continuous cooling experiments were performed on both orthopyroxenes to test the ability of the Mueller rate equation (Mueller, 1967, 1969) to correctly predict the evolution of ordering. The crystals were cooled from 850 degrees C to 250 degrees C at an average rate of 13 degrees C/day. The frozen site occupancies could be fully reproduced by the calculated ordering paths when the Mueller equation was run with the temperature dependencies of K (sub D) given above and Arrhenius parameters for k (sub dis) (T), taken from Kroll et al. (1997) and slightly adjusted within their error limits. Site refinements were also performed under the assumption that an error of + or -1% total Fe had occurred in the microanalysis. This resulted in ln(K (sub D) ) lines which almost coincide for En (sub 49) Fs (sub 51) , but are clearly different for En (sub 81) Fs (sub 19) . Consequently, the error of calculated cooling rates is negligible for En (sub 49) Fs (sub 51) , but becomes significant for En (sub 81) Fs (sub 19) .


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 13
Serial Issue: 4
Title: Isothermal annealing and continuous cooling experiments on synthetic orthopyroxenes; temperature and time evolution of the Fe, Mg distribution
Affiliation: Universitaet Bonn, Mineralogisch-Petrologisches Institut, Bonn, Federal Republic of Germany
Pages: 715-726
Published: 200108
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 35
Accession Number: 2001-065609
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Westfaelische Wilhelms-Universitaet Muenster, DEU, Federal Republic of GermanyUniversity of Toledo, USA, United States
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute.
Update Code: 200120
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