Experimental partitioning of Sr and Ba in Kiglapait feldspars

Abstract

New electron microprobe analyses of Sr and Ba in feldspars and coexisting experimental glasses show very strong correlations of the partition coefficients with the ternary Or content of the feldspar. The correlations therefore scale inversely with temperature, as also found in the literature. The Sr content of feldspars is maximized in mesoperthite and decreases with evolution of feldspar to the ternary minimum. The relationship of DSr/L to decreasing XAn is initially flat at a value of 1.8 but curves sharply upward past 6.5 after the arrival of apatite at An38. A similar concave-up curve at much higher values of D is found in the literature. The value of D (nine points) scales linearly with ternary Or; R2 = 0.99. Experimental feldspars are consistently richer in Sr than natural ones at comparable An. They jump from 200 ppm higher to 400 ppm higher at A p+. Experimental glass compositions are correspondingly low in Sr, so the material balance is conserved. Inversion of natural feldspars to liquid compositions using the experimental partition coefficients fails because the experimental crystals contain enhanced values of Sr from the apatite and feldspar components of the melted rock compositions. The partitioning of Ba (five points; range 0.4 to 6.1) also scales with ternary Or; R2 > 0.99; and there is no systematic difference between the natural and experimental concentrations of Ba in the feldspars. The inversion of D to liquid compositions may be safe for Ba but must be approached with caution for Sr.