Solubility of sulfur in an ultramafic melt and the relevance of the system Fe-S-O

Abstract

An ultramafic silicate melt (SiO 2 = 45.6; Al 2 O 3 = 8.75; FeO = 12.43; MgO = 22.96; CaO = 7.73; NaO (sub 1/2) = 0.50 wt%) has been equilibrated at 1,450 degrees C, and a series of sulfur fugacities (ranging from 10 (super -4) to 10 (super -1.4) atm) at two different oxygen fugacities (10 (super -10.4) and 10 (super -9.2) atm) and the sulfur content determined for each of the experimental charges. These data, coupled with the results of previous investigations, indicate that the solution of sulfur can be represented by the reaction:FeO (sub (in melt)) + 1/2 S 2 FeS (sub (in melt)) + 1/2 O 2 At the lower fO 2 , an immiscible sulfide liquid first segregated from the melt at a sulfur fugacity between 10 (super -2.4) and 10 (super -2) atm, at which stage the melt contained between 0.16 and 0.27 weight percent S. It was not possible to saturate the melt at the higher fO 2 even with the fS 2 = 10 (super -1.4) atm.An immiscible sulfide-rich liquid will segregate from a silicate melt when the activities of all components in the liquid equal those in the melt. Using data from the metallurgical literature and from Naldrett (1969), a portion of the system Fe-S-O has been contoured in terms of a FeO , fO 2 , and fS 2 at 1,450 degrees and 1,200 degrees C. If the a FeO in the silicate melt is known, and if it is assumed that the sulfide-rich liquid that eventually segregates from it consists essentially of Fe, S, and O, it is possible to predict from the contour diagrams the sulfur fugacity for any given oxygen fugacity at which the a FeO in the sulfide-rich liquid will equal that in the silicate melt and hence at which segregation will occur. Knowing (from experimental study of the reaction defined above) the relationship between dissolved sulfur content and sulfur fugacity in a silicate melt for any given temperature and oxygen fugacity, it is thus possible to predict the sulfur concentration in the melt when segregation occurs. Predictions for our melt at an fO 2 = 10 (super -9.2) atm suggest that this should occur at an fS 2 between 10 (super -1) and 10 (super -0.7) and with a sulfur concentration of between 0.33 and 0.47 weight percent S.