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

Isothermal annealing and continuous cooling experiments have been performed on synthetic orthopyroxene crystals with compositions En(81)Fs(19) and En( 49)Fs(51.) Their intracrystalline Fe2+,Mg-partitioning was determined by X- ray structure analysis. En(81)Fs(19) was equilibrated at 550 degreesC, 675 degreesC and 850 degreesC, En(49)Fs(51) at 550 degreesC, 650 degreesC, 675 degreesC, 800 degreesC, and 850 degreesC. The Fe2+,Mg distribution coeffici ents vary with temperature according to In(K-D) = 0.633(31)- 2625(31)/T[K] (En(81)Fs(19)) In(K-D) = 0.594(52) - 2581(51)/T[K] (En(49)Fs(51)), suggesting that in this compositional range In(K,) values do not significan tly depend on the Fs content. Rate constants were determined in ordering ru ns at 550 degreesC: k(dis) = 0.00165(37) [h(-1)] (En(81)Fs(19)) k(dis) = 0.0080(18) [h(-1)] (En(49)Fs(51)). The results closely agree with predictions from Arrhenius equations given b y Ganguly & Tazzoli (1994) and Kroll et al. (1997). Non-linear continuous cooling experiments were performed on both orthopyrox enes to test the ability of the Mueller rate equation (Mueller, 1967, 1969) to correctly predict the evolution of ordering. The crystals were cooled f rom 850 degreesC to 250 degreesC at an average rate of 13 degreesC/day. The frozen site occupancies could be fully reproduced by the calculated orderi ng paths when the Mueller equation was run with the temperature dependencie s of K-D given above and Arrhenius parameters for k(dis) (T), taken from Kr oll et al. (1997) and slightly adjusted within their error limits. Site refinements were also performed under the assumption that an error of +/-1% total pc had occurred in the microanalysis. This resulted in In(K-D) lines which almost coincide for En(49)Fs(51), but are clearly different for En(81)Fs(19). Consequently, the error of calculated cooling rates is negli gible for En(49)Fs(51), but becomes significant for En(81)Fs(19).