Thermochemistry of sulfide liquids. I. the system O-S-Fe at 1 bar

Abstract

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 liquids, despite their importance in igneous systems. This study presents results of a series of experiments designed to calibrate a thermodynamic model for sulfide liquids in the system O-S-Fe at one atmosphere pressure. Sulfide liquids were equilibrated under controlled oxygen and sulfur fugacities at temperatures between 1100 and 1350 ° C in equilibrium with a silica mineral and a silicate melt. Experiments were quenched in a high-speed double-roller “splat” quencher in order to assure that measured compositions were as close to equilibrium liquid values as possible. Sulfide liquids are not stable in equilibrium with a silica-saturated silicate melt at log10(fO2) > FMQ-1 at 1250 °C and log10(fS2)=−3. Iron content of the sulfide changes little with variations in oxygen and sulfur fugacity at a given temperature. Consequently, oxygen and sulfur contents are inversely correlated in these liquids. Sulfur is present entirely as sulfide. Iron appears to be present in both its ferric and ferrous states. Data from this study were combined with data compiled from the literature to calibrate 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-Fe binary. The observed inverse correlation between sulfur and oxygen is reflected in the predicted free-energy surface by a sharp energy valley running along a line of constant Fe content.