Experimental determination of compositional dependence of hydrous silicatemelts on sulfate solubility

We measured the solubility of SO3 in quartz-normative and nepheline-normati ve compositions in the system Na2O-Al2O3-SiO2-H2O at temperatures from 800- 825 degrees C and pressures between 1.5 - 1.7 kbar. We added 1.5, 3, 10 or 15 wt% CaSO4 to two H2O-saturated synthetic compositions near the eutectic s in quartz-albite-H2O and nepheline-albite-H2O. The low sulfate solubiliti es measured in the quartz-normative quenched glasses (0.06-0.12% SO3) are s imilar to previous determinations on quartz-normative melts from natural ro ck compositions. The sulfate contents of quenched glasses in the nepheline- normative system are up to 6 times higher (0.17-0.36 %SO3). Precipitation o f anhydrite in the quartz-normative melt requires addition of about 6 wt% C aSO4 at 800 degrees C, and 10 wt% CaSO4 at 820 degrees C. In contrast, <1.5 wt% CaSO4 added to the nepheline-normative liquid is sufficient to cause p recipitation of hauyne or an immiscible sulfate-rich liquid. We conclude th at the sulfate solubility in hydrous magmas is significantly influenced by silica activity (melt alkalinity) and SiO2 concentration in melts.