Fe–Ni exchange between olivine and sulphide liquid: implications for oxygen barometry in sulphide-saturated magmas

Abstract

In order to better understand the behaviour of nickel in magmatic processes, we have measured the apparent equilibrium constant ( K D ) for the exchange of Fe and Ni between coexisting olivine and sulphide liquid at controlled oxygen and sulphur fugacities ( f O 2 = 10 −8–10 −10 and f S 2 = 10 −2–10 −4) over the temperature range 1200 to 1400°C and with 5 to 50 wt.% nickel in the sulphide liquid. Measured values of K D are independent of temperature and sulphur fugacity, but increase linearly with the nickel content of the sulphide liquid, and follow a power-law increase with oxygen fugacity; behaviour that is consistent with previous measurements of K D under controlled conditions of f O 2 and f S 2 . The variation of K D with melt nickel content and f O 2 is most likely the result of nonideal mixing in the sulphide liquid, which results in a decrease in γ NiS/γ FeS with melt metal/sulphur ratio. As a consequence of the systematic dependence of K D on f O 2 , a new oxygen barometer is proposed for estimating oxygen fugacity in igneous rocks that were cosaturated in olivine and sulphide liquid. Application of the experimental results to natural samples shows that the relatively large variations that exist in K D values from different olivine + sulphide-saturated rock suites can be interpreted as arising from variations in f O 2 and/or the nickel content of the sulphide liquid. Oxygen fugacities calculated for oceanic basalt samples using the proposed Fe–Ni exchange oxybarometer are found to be relatively high (10 −8.5–10 −10.4) which is in accord with the range of values determined using glass ferric/ferrous ratios. Moreover, the very low f O 2 (∼10 −14) calculated for the mafic dike from Disko Island is consistent with the presence of native iron in these samples and is in quantitative agreement with indicators of f O 2 based on chromite- and olivine-melt partitioning of vanadium. Consideration of the f O 2 exhibited by olivine + sulphide-saturated intrusive suites reveals a range from relatively oxidized (i.e., similar to oceanic basalts) to values at or below that of Disko Island. Field, petrographic, and geochemical evidence suggests that the production of the low f O 2 suite of samples is probably the result of reduction accompanying the assimilation of carbonaceous country rock.