Experimental determination of composition dependence of hydrous silicate melts on sulfate solubility
Experimental determination of composition dependence of hydrous silicate melts on sulfate solubility
European Journal of Mineralogy (February 1999) 11 (1): 33-43
The solubility of SO (sub 3) in quartz-normative and nepheline-normative compositions in the system Na (sub 2) O-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O was measured at 800-825 degrees C and 1.5-1.7 kbar, 1.5, 3, 10 or 15 wt.% CaSO (sub 4) being added to two H (sub 2) O-saturated synthetic compositions near the eutectics in quartz-albite-H (sub 2) O and nepheline-albite-H (sub 2) O. The low sulphate solubilities measured in the quartz-normative quenched glasses (0.06-0.12% SO (sub 3) ) are similar to previous results on quartz-normative melts from natural rock compositions, but the sulphate contents of quenched glasses in the nepheline-normative system are < or = six times higher (0.17-0.36% SO (sub 3) ). Precipitation of anhydrite in the quartz-normative melt requires the addition of approximately 6 wt.% CaSO (sub 4) at 800 degrees C, and 10 wt.% CaSO (sub 4) at 820 degrees C. In contrast, < 1.5 wt.% CaSO (sub 4) added to the nepheline-normative liquid is sufficient to cause precipitation of hauyne or an immiscible sulphate-rich liquid. It is concluded that sulphate solubility in hydrous magmas is significantly affected by silica activity (melt alkalinity) and SiO (sub 2) concentration in melts.
