Abstract

New alkali feldspar/felsic magma trace-element partition coefficients (D-values) for Rb, Sr, Ba, Eu, Y, Zr, Nb, Ga, Zn, trivalent REE, Be, Cs, Hf, Pb, Th, and U for 30 samples of peralkalic quartz trachyte and rhyolite are presented. D-values of incompatible elements vary systematically with melt polymerization parameters, increasing with whole-rock silica, but decreasing in rocks with higher agpaitic indices [A.I. = mol (Na + K)/Al]. D-values for Sr and Ba (evaluated to be accurate) vary systematically with crystal chemistry, probably substituting for Na in the Ca-poor alkali feldspar phases. Apparent D-values for Sr and Ba from pre-Quaternary systems are fraught with contamination and analytical errors, respectively, and should be used with caution. DEu decreases exponentially with A.I., ranging from compatible in weakly peralkalic (A.I. < 1.1) rocks to strongly incompatible in very peralkalic (A.I. > 1.2) rocks. These regular variations strongly suggest that partition coefficients for these elements may be predicted accurately if whole-rock and crystal-chemical parameters are known.

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