Effect of oxygen fugacity on sulfur isotope compositions and magnetite concentrations in the Kirkpatrick Basalt, Mount Falla, Queen Alexandra Range, Antarctica

Abstract

The δ 34S-values of total sulfur in the Jurassic tholeiite flows on Mt. Falla in Antarctica range from −1.45 to +11.73‰. The concentrations of sulfur range from 80 to 480 ppm, which is typical of subaerial lava flows that lose varying proportions of sulfur by out-gassing of SO 2. The concentrations of magnetite range from less than 1% to more than 4% and appear to correlate inversely with the total Fe content of the flows. However, the five flows which are anomalously enriched in 34S also have elevated magnetite concentrations. We suggest that the elevated magnetite concentrations and the 34S enrichment were both caused by high oxygen fugacities ( f O 2) in the melt. The magnetite concentrations are affected by the fugacity of oxygen through equilibrium in the FMQ buffer whereas the enrichment of the flows in 34S resulted from outgassing of SO 2 at f O 2 greater than ∼ 10 −8 atm. The dependence of δ 34S and the magnetite concentrations of the flows on f O 2 is supported by the stratigraphic variation of these parameters and by their direct linear correlation.