Trace element partitioning in plagioclase feldspar

The location of the eutectic and the limits of mutual crystalline solubility were determined by reversed experiments at I1.3 kbar in the anhydrous system anorthite-sanidine. These results differ greatly from those of earlier work by Ai and Green (1989) and indicate that the anhydrous eutectic lies at 19 t 2 wto/o An and 1280 + l0'C. This eutectic composition lies much closer to the HrO-saturated eutectic than indicated by the previous data. Therefore, the amount of An depletion during crystallization of two feldspars from a hydrous melt along the 2 feldspar + L surface in the system An-Or-Ab-H'O is signiflcantly less than previously thought. Maximum mutual crystalline solubilities obtained are 9 wto/o An in sanidine and I I wt0/o Or in anorthite. Comparisons of these solubilities with various solution models for ternary feldspars permit assessment of these models and indicate that only the ternary feldspar models of Nekvasil and Burnham (1987) and Elkins and Grove (1990) yield calculated liquidus relations in good agreement with the data. INrnonucrroN The anhydrous system anorthite-sanidine [CaAlrSirOr(An)l-[I(AlSirOr(Or)] is of major importance in evaluating the crystallization history of high-temperature silicic magmas such as trachytes and syenites. Within this system lies the high-temperature termination of the 2 feldspar + L curve (at the eutectic), which separates the plagioclase * L and the alkali feldspar + L fields in the ternary feldspar system and indicates the compositions of melts in equilibrium with two feldspars. Knowledge of the melt composition at the eutectic is required for development and evaluation of thermodynamic mixing models for Ca- and K-rich silicic melts. Additionally, the feldspar compositions at the anhydrous eutectic provide high-temperature limits of the solubility of Or in plagioclase and An in sanidine. These are needed to constrain mixing models of ternary feldspar solid solutions at high tempefatures. Phase relations in the system An-Or have been studied by several workers. Yoder et al. (1957) experimentally determined the liquidus relations and solidus temperature at 5 kbar under HrO-saturated conditions. Their results showed the system to be characterized by a eutectic and limited solid solution in both crystalline phases. Schairer and Bowen (1947) studied this system at I atm. However, because of the incongruent melting of sanidine at low pressures, the eutectic between sanidine and anonhite is obscured by a field of leucite + L. Ai and Green (1989) experimentally investigated phase relations in the anhydrous system at l0 kbar. Their results indicated that the basic characteristics of the HrO-saturated system are preserved at high temperatures. They located the eutectic at 30 wto/o An and l2l5 + l5 'C and obtained maxlmum

Sanidine

Alkali feldspar

Anorthite

Anhydrous

Liquidus

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Trace element partitioning between plagioclase and melt in the system diopside-albite-anorthite

Jon Blundy

Anorthite

Diopside