Mn. Ducea et al., Experimental determination of compositional dependence of hydrous silicatemelts on sulfate solubility, EUR J MINER, 11(1), 1999, pp. 33-43
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.