Pyrite oxidation and sulfate-oxygen isotopes - what are the missing pieces?

Oxygen isotopes have been used to gain insight into pyrite oxidation mechanisms for some 25 years. Two distinct approaches are currently being used to interpret field data. We show the discrepancies between these approaches and recent models of pyrite oxidation, identifying the areas of uncertainty that need to be addressed. The first approach uses fractionation factors derived from experimental work to express sulfate-oxygen in terms of the percentage of water-oxygen and atmospheric-oxygen. Recent models of pyrite oxidation suggest that only water-oxygen interacts with the S-moiety on the pyrite surface, regardless of whether Fe3+ or O2 is the oxidant. In this case, atmospheric-oxygen can only be incorporated into the final sulfate during the oxidation of dissolved sulfoxyanion intermediates. Published pathways of sulfoxyanion oxidation suggest a maximum of 34% atmospheric-oxygen in the final sulfate. In contrast, field studies using this first approach suggest that >50% atmospheric-oxygen can be incorporated. The second approach presents two end-member scenarios and is more consistent with recent pyrite oxidation models: (i) 100% water-oxygen from the direct release of sulfate from the pyrite surface, resulting in little or no fractionation; (ii) release of intermediate sulfoxyanions, oxidation to sulphite, sulphite-water oxygen isotopic exchange followed by oxidation to sulfate. This approach predicts a maximum of 25% atmospheric-oxygen in the final sulfate but laboratory studies have shown the incorporation of up to 45% atmospheric-oxygen. We present recommendations for the further experimental work needed to refine our understanding of oxygen isotopes in pyrite oxidation.