Effects of fO 2, fS 2, temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid: implications for natural olivine–sulfide assemblages

Abstract

The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (K D = (X NiS/X FeS) liquid/(X NiSi12O2/X FeSi12O2) olivine; X i = mole fraction) has been measured at controlled oxygen and sulfur fugacities ( fO 2 = 10 −8.1 to 10 −10 and fS 2 = 10 −0.9 to 10 −1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO 2 of −8.7 ± 0.1, and log fS 2 of −0.9 to −1.7, K D is relatively insensitive to sulfur fugacity, but comparison with previous results shows that K D increases at very low sulfur fugacities. K D values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO 2 of −8.1, lessens with decreasing fO 2, and K D becomes independent of melt Ni content at log fO 2 ≤ −9.5. The origin of this variation in K D with fO 2 and fS 2 is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts. Application of these experimental results to natural samples shows that the relatively large dispersion that exists in K D values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO 2, fS 2, and the nickel content of the sulfide liquid. Estimates of fO 2 based on K D and sulfide melt composition in natural samples yields a range from fayalite–magnetite–quartz (FMQ)–1 to FMQ-2 or lower, which is in good agreement with previous values determined for oceanic basalts that use glass ferric/ferrous ratios. Anomalously high K D values recorded in some suites, such as Disko Island, probably reflect low fS 2 during sulfide saturation, which is consistent with indications of low fO 2 for those samples. It is concluded that the variation in K D values from natural samples reflects olivine–sulfide melt equilibrium at conditions within the T- fO 2- fS 2 range of terrestrial mafic magmas.