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

Abstract

We measured the solubility of SO_3 in quartz-normative and nepheline-normative compositions in the system Na_2O-Al_2O_3-SiO_2-H_2O at temperatures from 800-825 °C and pressures between 1.5-1.7 kbar. We added 1.5, 3, 10 or 15 wt% CaSO_4 to two H_2O-saturated synthetic compositions near the eutectics in quartzalbite-H_2O and nepheline-albite-H_2O. The low sulfate solubilities measured in the quartz-normative quenched glasses (0.06-0.12% SO_3) 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 %SO_3). Precipitation of anhydrite in the quartz-normative melt requires addition of about 6 wt% CaSO_4 at 800°C, and 10 wt% CaSO_4 at 820°C. In contrast, < 1.5 wt% CaSO_4 added to the nepheline-normative liquid is sufficient to cause precipitation of haüyne or an immiscible sulfate-rich liquid. We conclude that the sulfate solubility in hydrous magmas is significantly influenced by silica activity (melt alkalinity) and SiO_2 concentration in melts.

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