Commentary: Is the Neoproterozoic oxygen burst a supercontinent legacy?

The oxygen content of the atmosphere is thought to have increased in two steps: the Paleoproterozoic Great Oxidation Event and the Neoproterozoic Oxidation Event. The younger event is still a matter of debate (Och and Shields-Zhou, 2012). Macouin et al. (2015) suggest that it may have been triggered by volcanic degassing from unusually oxidized magmas occurring in a subduction ring surrounding the Rodinia supercontinent. Evidence for subduction-related oxidized magmas is supported by the study of 780 Ma biotite and pink granites of high K-calc-alkaline affinities and associated gabbros from Socotra Island (Denèle et al., 2012). Magnetite and subordinate hematite are revealed by magnetic properties, reflected-light microscopy, and scanning electron microscopy. Macouin et al. (2015) suggest that hematite is a primary phase witnessing a very high oxygen fugacity at magmatic conditions. In igneous rocks, Ti-rich titanohematite (ilmenite) is a primary phase, whereas stoichiometric hematite represents a common secondary mineral due to post-magmatic alteration processes. Indeed, hematite was uniquely found to be a liquidus phase in an experimental peralkaline residua Na 2 O-Al 2 O 3-Fe 2 O 3-SiO 2 system (Edgar, 1974), but the investigated system had no ferrous (Fe 2+) iron, hence it has no terrestrial equivalent in common tectonic settings. The microphotographs of iron oxides presented as evidences of primary hematite by Macouin et al. (2015) in their Figures 6, 7 show subhedral magnetite crystals with typical subsolidus exsolution features such as fine trellis-lamellae of ilmenite and minor hematite parallel to (111) planes, or larger (sandwich) ilmenite lamellae in only one (111) plane. The petrographic nature (biotite granite, pink granite, or gabbro) of the relevant samples is not provided. Macouin et al. (2015) infer that some primary magnetite grains were fractured and subsequently filled by ilmenite (see caption of their Figure 7). We argue that sandwich ilmenite lamellae in Fe-Ti oxide grains do not arise from a reaction with biotite, unlike the small ilmenite needles at the contact between magnetite and biotite (Figure 7d of Macouin et al., 2015). Indeed, primary igneous magnetite is generally titanomagnetite containing a percentage of ulvöspinel (Fe 2 TiO 4) in solution. During slow cooling, subsolidus oxidative exsolution takes place from ∼650 • C to 450 • C yielding ilmenite + magnetite (Figure 1A). Temperature and oxidation conditions control the exsolved phase proportions, hence the width of ilmenite lamellae. This is the common interpretation of Fe-Ti oxide intergrowths supported by experimental evidence (Buddington and …