PYRRHOTITE SOLUBILITY IN HYDROUS ALBITE MELTS (SULFUR)

The solubility of pyrrhotite in hydrous NaAlSi(,3)O(,8) melt has been determined as a function of temperature, pressure, mole fraction of water in the melt (X(,H(,2)O)('m)), and pyrrhotite composition at an oxygen fugacity approximating the fayalite-magnetite-quartz buffer. Temperature ranged from 900(DEGREES)C to 1000(DEGREES)C, pressure ranged from 1 to 6 kbar, X(,H(,2)o)('m) ranged from 0.25 to 0.62, and the iron content of pyrrhotite ranged from 48.75 to 46.00 At%. Experiments were run using internally heated pressure vessels fitted with hydrogen membranes. Glass run products were analyzed using an electron microprobe; pyrrhotite run products were analyzed using an X-ray diffraction technique.;The sulfur content of the melt was most strongly dependent on and inversely proportional to the iron content of the melt. This inverse proportionality suggests that pyrrhotite dissociates into iron-bearing and sulfur bearing species on dissolution in hydrous aluminosilicate melts.;With increasing activity of FeO, the iron content of pyrrhotite-saturated melts increased from 0.02 to 2.1 wt% FeO. Pressure, temperature, and X(,H(,2)O)('m) did not significantly affect the iron content of the melt. The sulfur content of pyrrhotite-saturated melt, which ranged from less than 0.02 to 0.4 wt% S, increased with increasing activity of H(,2)S and pressure, and decreased with increasing iron content of the melt. Temperature and X(,H(,2)O)('m) had no significant effect on the sulfur content of the melts.;In natural systems, magnetite-saturated magmas should require very little sulfur ((TURN)0.01 wt% S) to saturate the system with respect to pyrrhotite. Thus, in the presence of magnetite, many magmas probably become saturated with pyrrhotite fairly early in their crystallization history. Magmas that do not contain magnetite may contain appreciable concentrations of dissolved sulfur. Even though sulfur contents of melts saturated with magnetite may be low, there is still more than enough sulfur in these magmas for them to act as the principal source of sulfur for porphyry copper deposits.;When a vapor phase was present, the activities of volatile species and iron oxides could be calculated from the pyrrhotite composition, total pressure, temperature, and hydrogen partial pressure. At vapor-undersaturated conditions, these activities were determined from these same parameters plus X(,H(,2)O)('m).