Basaltic glass with high-temperature equilibrated immiscible sulphide 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. D _{vanadium(olivine/glass)} is close to unity and reflect the reducing nature of the rock, for which estimates of f _O210^{–12 to –13} and f _S210^–5 have been made. D _{nickel, cobalt, copper (sulphide/glass)}=4,300, 230 and 380 respectively, are much higher than reported experimentally determined D's on_{monosulphide/basalt glass} at the same temperature and show increasing positive deviation (Ni>Co>Cu) with the increasingly siderophile character of the elements. K _{Dnickel-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.