Fractionation of major elements between coexisting H 2O-saturated silicate melt and silicate-saturated aqueous fluids in aluminosilicate systems at 1–2 GPa

Abstract

From experimental data in the systems Na 2O-Al 2O 3-SiO 2-H 2O, K 2O-Al 2O 3-SiO 2-H 2O at 1100°C, and CaO-Al 2O 3-SiO 2-H 2O at 1200°C in the 1–2 GPa pressure range, the solution behavior of the individual oxides in coexisting H 2O-saturated silicate melts and silicate-saturated aqueous fluids appears to be incongruent. Recalculated on an anhydrous basis, in the CaO-Al 2O 3-SiO 2-H 2O system, CaO fluid/CaO melt < 1, whereas in the Na 2O-Al 2O 3-SiO 2-H 2O and K 2O-Al 2O 3-SiO 2-H 2O systems, K 2O fluid/K 2O melt and Na 2O fluid/Na 2O melt both are greater than 1. The aqueous fluids are depleted in alumina relative to silicate melt. In the Na 2O-Al 2O 3-SiO 2-H 2O, K 2O-Al 2O 3-SiO 2-H 2O, and CaO-Al 2O 3-SiO 2-H 2O systems, fluid/melt partition coefficients for the individual oxides range between ∼0.005 and 0.35 depending on oxide, bulk composition and pressure. The alkali partition coefficients are about an order of magnitude higher than that of CaO. Alumina and silica partition coefficient values in the CaO-Al 2O 3-SiO 2-H 2O system are 10–20% of the values for the same oxides in the Na 2O-Al 2O 3-SiO 2-H 2O and K 2O-Al 2O 3-SiO 2-H 2O systems. Positive correlations among individual partition coefficients and oxide concentrations in the aqueous fluids are consistent with complexing in the fluid that involves silicate polymers associated with alkalis and alkaline earths and aluminosilicate complexes where alkalis and alkaline earths may serve to charge-balance Al 3+, which is, perhaps, in tetrahedral coordination. Alkali aluminosilicate complexes in aqueous fluid appear more stable than Ca-aluminosilicate complexes.