Abstract
The partition of Ni between olivine, crystallised from basalt liquids, and iron-nickel monosulfide has been determined experimentally at 1160 and 1050 °C using alumina crucibles in sealed silica glass tubes. The compositional ranges investigated are 83 to 88 mol. % Fo and 5 to 50 mol. % NiS. The average distribution coefficient for the exchange reaction FeS +NiSi0.5O2⇌NiS+FeSi0.5O2 is remarkably similar to the literature value of 33.2 for 900 °C. It is concluded that within the temperature and pressure range of magmatic olivine crystallisation the exchange reaction is effectively independent of temperature, pressure, olivine composition and composition and physical state of the sulfide. The NiO contents of early magmatic olivine are appreciably greater than the calculated NiO contents for equilibration with any hypothetical iron-rich immiscible sulfide liquid, suggesting that an immiscible sulfide liquid may not be a normal product of upper mantle magmatic processes. Similarly, it appears unlikely that sulfide liquid immiscibility had any role in the genesis of nickel sulfide ores. Olivine from upper mantle and crustal rock associations has characteristic NiO/MgO distributions. Both distributions appear to be the products of extensive fractionation by crystal-liquid processes although the crustal olivine distribution may be complicated by the added factor of heterogeneity of mantle magma source.
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