Sulfur, as sulfide, is most readily dissolved in silicate liquids approaching magmas in composition by displacing oxygen solely bonded to ferrous iron. Sulfide released by lowering the activity of FeO forms an immiscible liquid phase at magmatic temperatures. Activity of FeO can be controlled by such methods as changing the SiOz content or the oxidation state of the liquid. Iron silicate liquid (Ix) coexists with iron sulfide liquid (1±) throughout a large volume of the experimental system. Solubility of FeS in iron silicate liquid is maximum when in equilibrium with iron and decreases with increasing fugacity of oxygen ([o2). Excess FeS forms an immiscible liquid which has its lowest temperature of coexistence with silicate liquid when in equilibrium with fayalite (fa), tridymite (td), magnetite (rot) and vapor (v). The composition of liquids, temperature and phases present at some important invariant points are listed below: FeS FeO Fe 2O 4 SiO 2 (wt. %) 16 57 1 26 (IQ± 1,140 ± 5°C iron, fa, td, h, 12, v 83 15 0.5 1.5 (12)j 10 40 21 29 (1Q'± 1,095 + 5°C mt, fa, td, lx, h, v 57 26 15 2 (h)3 63 20 16 1 955 4-5°C mr, fa, td, po, h, v 62 22 15 1 945 + 5°C ws, mt, fa, po, 12, v (ws = wustite, po-pyrrhotite) When an iron silicate liquid in a low oxidation state and saturated with FeS is oxidized to a higher state, a significant quantity of FeS is released as immiscible sulfide liquid. Partial melting in the system under a fo±. exerted by the quartz-fayalite- magnetite assemblage yields sulfide liquid 140 ° below the appearance of silicate liquid. In the mantle, sulfides (their melting points and solubilities in silicate minerals are increased little by pressure) would be melting before silicate liquid formed. A comparison of the sulfur content of the mantle (0.03 wt.%) to basalts (0.03 wt.%) and gabbros (0.09 wt.%), indicates that sulfide liquid could coexist with silicate magma until at least one-third of the mantle is melted, and that basalts are probably always saturated with sulfide. Ore concentrations of sulfide could be deposited in the crust by intrusion of mixed sulfide and silicate liquids, or by rapid oxidation of intruding magma. © 1969 Society of Economic Geologists, Inc.
