Experimental determination of compositional dependence of hydrous silicate melts on sulfate solubility

We measured the solubility of SO₃ in quartz-normative and nepheline-normative compositions in the system Na₂O-Al₂O₃-SiO₂-H₂O at temperatures from 800-825°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 eutectics in quartz-albite-H₂O and nepheline-albite-H₂O. The low sulfate solubilities measured in the quartz-normative quenched glasses (0.06-0.12% SO₃) are similar to previous determinations on quartz-normative melts from natural rock compositions. The sulfate contents of quenched glasses in the nepheline-normative system are up to 6 times higher (0.17-0.36 %SO₃). Precipitation of anhydrite in the quartz-normative melt requires addition of about 6 wt% CaSO₄ at 800°C, and 10 wt% CaSO₄ at 820°C. In contrast, <1.5 wt% CaSO₄ added to the nepheline-normative liquid is sufficient to cause precipitation of hauyne or an immiscible sulfate-rich liquid. We conclude that the sulfate solubility in hydrous magmas is significantly influenced by silica activity (melt alkalinity) and SiO₂ concentration in melts.