Komatiite-hosted Ni-Cu-PGE deposits in Finland: Their characterization, PGE content, and petrogenesis
The solubility of sulfur in silicate melt coexisting with iron sulfide melt has been determined as a function of temperature and pressure for three silicate compositions: Mt. Hood andesite, Grande Ronde basalt and Goose Island basalt (FeO, : 5.4, I l.l and l7 .UVo, respectively). Experimental temperatures vary from 1300-1460' C and pressures vary from 12.5-30 kbar. The oxygen fugacity in the experiments is near that of the C-CO-CO5O2 butrer (10-e bar at P, : 20 kbar) and the sulfur fugacity is buffered internally at a value above the iron-troilite equilibrium 0G, : I bar at Pt : 20 kbar). Sulfur solubility decreases with increasing pressure and increases with increasing temperature and FeOl content of the silicate melt; observed solubilities range from a low value of 0.05 wt.To at 30 kbar and 1420" C (andesite) to a high value of 0.26Vo at 12.5 kbar and 1420'C (Goose Island basalt). The FeO, content of the coexisting sulfide melt decreases with increasing pressure (from a maximum of 5.5% at 12.5 kbar and 1420" CGoose Island basalt), increases with FeO, content of the silicate melt and does not appear to be dependent on temperature. With increasing pressure, the miscibility gap between natural silicate melts and sulfide melts expands and the ratio of FeOiilicate meltTp"g,sulfide m lt increases. The sulfur solubility measurements establish a basis for evaluating the appearance of immiscible sulfide in erupted magmas, core segregation processes, and subsequent transition metal partitioning studies. Because of the strong negative pressure dependence of sulfur solubility in silicate melts, a magma may be sulfide-saturated in the source region but not on eruption until the late stages of crystallization.