Abstract

The partitioning of Sr, Ba and Rb between alkali feldspar and melt has been determined at 0.1 GPa water vapour pressure, mainly on one Na-rich series and one K-rich series within the system nepheline–kalsilite–quartz. Experiments were also carried out with small amounts of the anorthite molecule or peralkaline components (Na,K metasilicates). The compositions of the alkali feldspars and coexisting quenched glasses were determined by electron microprobe analysis. Except for some peralkaline compositions, the crystal/liquid partition coefficients for Sr and Ba are always >1; the crystal/liquid partition coefficient for Rb is always <1. For sodic alkali feldspars DSr > DBa and for potassic feldspars DSr < DBa. Partition coefficients for Sr and Ba increase: (1) with decreasing temperature; (2) with increasing Or content of feldspar; (3) with increasing silica-undersaturation of the melt; (4) with decreasing peralkalinity. The variation in the value of DRb is less clear, but it is higher for K-rich feldspars. Multiple linear regression equations are fitted to correlate ln(D) with independent compositional and physical variables. Where rock/groundmass major-element data are available for felsic natural rocks, equations are recommended for obtaining reliable alkali feldspar partition coefficients for modelling fractional crystallization processes. The structural properties of silicate melts and crystal chemical relations are used to rationalize trends in partition coefficients.

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