Abstract
The fractionation trend of trace elements with large ionic radii capable of substituting for Ca in plagioclase is calculated for the model system diopside-albite-anorthite. After determining the necessary parameters to reproduce Bowen's (1915) phase diagram of the system, the following assumptions were made on the system. 1. (1) Plagioclase is an ideal solution and diopside has a fixed composition. 2. (2) The thermodynamics of the melt can be described in terms of three “molecules”, diopside, albite and anorthite. Trace elements can substitute Ca and Na in the plagioclase “molecule” of the melt, but not for Ca of the diopside “molecule”. 3. (3) The exchange partition coefficient of a trace element between plagioclase and melt is a constant, independent of temperature and composition of the system. This model leads to the composition and temperature dependence of the “Nernst type” partition coefficient and gives rise to the appearance of maximum trace-element concentration in the melt at an intermediate stage of fractional crystallization. These considerations on the model system allow the qualitative interpretation that during magmatic differentiation Sr concentrates at an intermediate stage of fractionation, while Ba concentrates at the latest stage.
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