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Isothermal annealing and continuous cooling experiments on synthetic orthopyroxenes

¹ Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloß, D-53115 Bonn, Germany. – e-mail: schlenz{at}uni-bonn.de
² Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
³ Department of Earth, Ecological and Environmental Sciences, The University of Toledo, 2801 W. Bancroft Street, Toledo, Ohio 43606, U.S.A.

Isothermal annealing and continuous cooling experiments have been performed on synthetic orthopyroxene crystals with compositions En₈₁Fs₁₉ and En₄₉Fs₅₁. Their intracrystalline Fe²⁺, Mg-partitioning was determined by X-ray structure analysis. En₈₁Fs₁₉ was equilibrated at 550°C, 675°C and 850°C, En₄₉Fs₅₁ at 550°C, 650°C, 675°C, 800°C, and 850°C. The Fe²⁺, Mg distribution coefficients vary with temperature according to

suggesting that in this compositional range In(K_D) values do not significantly depend on the Fs content. Rate constants were determined in ordering runs at 550°C:

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°C to 250°C at an average rate of 13°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_D given above and Arrhenius parameters for k_dis, 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 ±1% total Fe had occurred in the microanalysis. This resulted in In (K_D) lines which almost coincide for En₄₉Fs₅₁, but are clearly different for En₈₁Fs₁₉. Consequently, the error of calculated cooling rates is negligible for En₄₉Fs₅₁, but becomes significant for En₈₁Fs₁₉.

Key-words: orthopyroxene, cooling rate, isothermal annealing, Fe²⁺, Mg distribution, Mueller rate equation.

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