The distribution of Na, K, Rb, Sr, Al, Ge, Cu, W, Mo, La, and Ce between granitic melts and coexisting aqueous fluids

Abstract

The distribution of Na, K, Rb, Sr, Al, Ge, Cu, W, Mo, La, and Ce between H 2O, NaCl, NaCl + KCl, HCl, NaF, Na 2CO 3, or Na 2CO 3 + K 2CO 3 aqueous fluids and granitic melts was determined at 750–800°C and 1–4 kbar. The distribution coefficients D Na, D K, D Rb, and D Cu (D i = C iv /C im , where C iv and C im are the concentrations of element i in the aqueous fluid and the melt, respectively) increase linearly with the (Na, K)Cl concentration in the fluid, indicating the presence of (Na, K, Rb, or Cu)Cl complexes. D Sr shows a quadratic relation with the chloride concentration, suggesting a SrCl 2 complex in these fluids. With the (Na, K)Cl-bearing aqueous fluids, Na, K, and Rb, and especially Cu strongly partition toward the fluid. D K and D Rb are about half of D Na at comparable Cl concentrations. In contrast, Al, Ge, Mo, W, La, and Ce strongly partition toward the melt. NaF has little effect on the partitioning of these elements, except for Al, W, and Mo. D Al increases with increasing NaF content. At low NaF concentrations, W and Mo are enriched in the aqueous fluid, but at higher NaF contents they partition toward the silicate melt. With (Na,K) 2CO 3, all elements except Mo and Cu partition strongly towards the silicate melt, although Ge is slightly more soluble in the carbonated aqueous fluid. The quenched glasses are highly peralkaline in the experiments with (Na,K) 2CO 3, slightly peralkaline with NaF, slightly peraluminous with pure H 2O or (Na,K)Cl, and highly peraluminous with HCl. D Al and D Ge increase slightly in peralkaline melts. In the experiments with highly peraluminous melts, the distribution coefficient for all the elements, except Al, Ge, and W, is ≫1. With an increase of the (Na + K)/Al ratio to 0.3, the distribution coefficients become <1, except for Cu and Mo. Raising pressure to 4 kbars does not significantly affect partitioning of these elements except for D Ge and D Mo, which show an increase.