BASALTIC GLASS WITH HIGH-TEMPERATURE EQUILIBRATED IMMISCIBLE SULFIDE BODIES WITH NATIVE IRON FROM DISKO, CENTRAL WEST GREENLAND

Immiscible sulphide bodies show eutectic quench textures in a basaltic glass rock (mg=66) from a native iron-bearing dyke chilled at T=1,200° C and P=250 bars. The sulphide bodies are composed of troilite (90-91%), iron (9-10%) and very scarce vanadium-rich chromite and approach a ternary cotectic in the Ni-poor part of the system Fe-Ni-S. Transition element partition between olivine (mg=83), silicate glass (mg=59) and sulphide blebs indicate that the phases were equilibrated at 1,200° C. Dvanadium(olivine/glass) is close to unity and reflect the reducing nature of the rock, for which estimates of fO2∼10-12 to -13 and fS2∼10-5 have been made. Dnickel, cobalt, copper (sulphide/glass)=4,300, 230 and 380 respectively, are much higher than reported experimentally determined D's onmonosulphide/basalt glass at the same temperature and show increasing positive deviation (ΔNi>ΔCo>ΔCu) with the increasingly siderophile character of the elements. KDnickel-iron (sulphide/olivine)=63 is much higher than an experimentally reported value (33) and comparison with published thermodynamic data on Ni-partition between olivine and iron metal suggests that the positive deviation is roughly proportional to the excess metal component in the sulphide melt. The occurrence of strongly Ni-depleted reduced basalts on Disko shows that fractionation of metal and sulphides was a common and geologically important process. © 1979 Springer-Verlag.