Effects of fO (sub 2) , fS (sub 2) , temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid; implications for natural olivine-sulfide assemblages

Brenan, J.M.

Geochimica et Cosmochimica Acta 67(14): 2663-2681

2003


ISSN/ISBN: 0016-7037
DOI: 10.1016/s0016-7037(02)01416-3
Accession: 018811566

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Abstract
The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (K (sub D) = (X (sub NiS) /X (sub FeS) ) (sub liquid) / (X (sub NiSi12O2) /X (sub FeSi12O2) ) (sub olivine) ; X (sub i) = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO (sub 2) = 10 (super -8.1) to 10 (super -10) and fS (sub 2) = 10 (super -0.9) to 10 (super -1.7) ) over the temperature range 1200 to 1385 degrees C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO (sub 2) of -8.7+ or -0.1, and log fS (sub 2) of -0.9 to -1.7, K (sub D) is relatively insensitive to sulfur fugacity, but comparison with previous results shows that K (sub D) increases at very low sulfur fugacities. K (sub 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 (sub 2) of -8.1, lessens with decreasing fO (sub 2) , and K (sub D) becomes independent of melt Ni content at log fO (sub 2) < or = -9.5. The origin of this variation in K (sub D) with fO (sub 2) and fS (sub 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 (sub D) values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO (sub 2) , fS (sub 2) , and the nickel content of the sulfide liquid. Estimates of fO (sub 2) based on K (sub 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 (sub D) values recorded in some suites, such as Disko Island, probably reflect low fS (sub 2) during sulfide saturation, which is consistent with indications of low fO (sub 2) for those samples. It is concluded that the variation in K (sub D) values from natural samples reflects olivine-sulfide melt equilibrium at conditions within the T-fO (sub 2) -fS (sub 2) range of terrestrial mafic magmas.