THERMOCHEMISTRY OF SULFIDE LIQUIDS .1. THE SYSTEM O-S-FE AT 1 BAR

Recrystallized globules representing former immiscible sulfide liquids are found in a variety of igneous environments. Relatively little is known about the physical properties and thermochemistry of sulfide liq uids, despite their importance in igneous systems. This study presents results of a series of experiments designed to calibrate a thermodyna mic model for sulfide liquids in the system O-S-Fe at one atmosphere p ressure. Sulfide liquids were equilibrated under controlled oxygen and sulfur fugacities at temperatures between 1100 and 1350 degrees C in equilibrium with a silica mineral and a silicate melt. Experiments wer e quenched in a high-speed double-roller ''splat'' quencher in order t o assure that measured compositions were as close to equilibrium liqui d values as possible. Sulfide liquids are not stable in equilibrium wi th a silica-saturated silicate melt at log(10)( f(O2)) > FMQ-1 at 1250 degrees C and log(10)( f(S2))= -3. Iron content of the sulfide change s little with variations in oxygen and sulfur fugacity at a given temp erature. Consequently, oxygen and sulfur contents are inversely correl ated in these liquids. Sulfur is present entirely as sulfide. Iron app ears to be present in both its ferric and ferrous states. Data from th is study were combined with data compiled from the literature to calib rate an asymmetric regular solution thermodynamic mixing model for O-S -Fe liquids. This model reproduces miscibility gaps and data from this study quite well, but exhibits minor but systematic errors at the O-F e binary. The observed inverse correlation between sulfur and oxygen i s reflected in the predicted free-energy surface by a sharp energy val ley running along a line of constant Fe content.