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.
ASJC Scopus subject areas
- Geochemistry and Petrology