Distribution of chloride between aqueous fluids and felsic melts at 2 kbar and 800 degrees C

In the five model systems, albite-quartz-HCl-H (sub 2) O, albite-quartz- NaCl-H (sub 2) O,orthoclase-quartz-HCl-H (sub 2) O, orthoclase-quartz-KCl-H (sub 2) O andalbite-orthoclase-quartz-NaCl-KCl-H (sub 2) O, chloride always partitions strongly into the fluid. At the same Cl concentration in the fluid, less Cl is dissolved in the melt in the K-bearing systems than in the Na-bearing systems. While the partition coefficient is constant in the two HCl-bearing systems, K (super roman melt/fluid) (sub D ) strongly decreases with Cl concentration in the other three systems. This effect can be attributed entirely to variation in the activity coefficients of NaCl and KCl in the fluid. Measurements of the Cl partition coefficient between fluids and silicate melts thus provide a simple means for obtaining activity coefficients in fluids at supercritical conditions. Since Cl always strongly partitions into the fluid, any exsolution of water vapour from the melt will tend to strip a magma of virtually its entire Cl content; therefore, only magmas that remain water-undersaturated until shortly before eruption are expected to contribute large amounts of Cl to the atmosphere. The ratio of H (sub 2) O to Cl in the melt also controls the enrichment of trace elements in an evolving fluid; if the ratio is low, Cl can be highly enriched in residual melts. When water saturation is finally reached in such a case, the fluid phase in equilibrium with a melt containing 0.1-0.3 wt.% Cl can contain approximately 50 wt.% of dissolved alkali chloride; such highly saline solutions are extremely efficient in extracting trace elements out of magmas.