Deposition of 1.88-billion-year-old Iron Formations as a Consequence of Rapid Crustal Growth

Abstract

The reappearance of major iron formations at 1.88 billion years ago (after the rise in atmospheric oxygen) is puzzling because their deposition requires anoxic and iron-rich sea water, but is here explained as a consequence of major mantle activity and rapid crustal growth at that time. Ocean-floor iron formations are deposited when the oceans are depleted in oxygen, so the rise of atmospheric oxygen by around 2.32 billion years ago is thought to have been responsible for their decline. Their sudden reappearance 500 million years later, in an oxygenated atmospheric environment, remains a mystery. Most of these 1.88-billion-year-old iron formations are found in North America, but new geochronology data reported here show that major iron formations were also deposited in Australia at about the same time, suggesting that they reflect the chemistry and redox state of the global ocean. The authors propose that enhanced volcanic and hydrothermal activity released vast volumes of iron, changing ocean chemistry and depositing major iron formations. The decline of igneous activity led to the disappearance of iron formations. Iron formations are chemical sedimentary rocks comprising layers of iron-rich and silica-rich minerals whose deposition requires anoxic and iron-rich (ferruginous) sea water. Their demise after the rise in atmospheric oxygen by 2.32 billion years (Gyr) ago1 has been attributed to the removal of dissolved iron through progressive oxidation2 or sulphidation3,4 of the deep ocean. Therefore, a sudden return of voluminous iron formations nearly 500 million years later poses an apparent conundrum3,5. Most late Palaeoproterozoic iron formations are about 1.88 Gyr old6,7,8 and occur in the Superior region of North America5,9,10. Major iron formations are also preserved in Australia, but these were apparently deposited11 after the transition to a sulphidic ocean at 1.84 Gyr ago that should have terminated iron formation deposition4, implying that they reflect local marine conditions5,12. Here we date zircons in tuff layers to show that iron formations in the Frere Formation of Western Australia are about 1.88 Gyr old, indicating that the deposition of iron formations from two disparate cratons was coeval and probably reflects global ocean chemistry. The sudden reappearance of major iron formations at 1.88 Gyr ago—contemporaneous with peaks in global mafic–ultramafic magmatism13,14, juvenile continental and oceanic crust formation15,16, mantle depletion17,18 and volcanogenic massive sulphide formation5,19—suggests deposition of iron formations as a consequence of major mantle activity and rapid crustal growth5,10,15,20. Our findings support the idea that enhanced submarine volcanism and hydrothermal activity linked to a peak in mantle melting released large volumes of ferrous iron and other reductants that overwhelmed the sulphate and oxygen reservoirs of the ocean, decoupling atmospheric and seawater redox states, and causing the return of widespread ferruginous conditions. Iron formations formed on clastic-starved coastal shelves where dissolved iron upwelled and mixed with oxygenated surface water. The disappearance of iron formations after this event may reflect waning mafic–ultramafic magmatism and a diminished flux of hydrothermal iron relative to seawater oxidants.