Hematitized greenstones in Pilbara, Western Australia: evidence for an oxygenated surface environment before 2.76 Ga

Abstract

We report the discovery of abundant hematite in Archaean greenstones in a deep drill core from the Pilbara Craton, Western Australia. The greenstones are mostly composed of basaltic rocks that primarily erupted on the seafloor about 3.46 Gyr ago, and were exposed before 2.77 Gyr ago and subsequently covered by a continental flood basalt. We have obtained a Re-Os age of 2,763 +/- 15 Ma on pyrite veinlets that clearly cross cut (younger than) the hematitized greenstones. Geologic history of the Pilbara district and geochemical data on the hematitized greenstones (including an oxygen isotope value of ca. 0 permil for hematite separates) suggest the hematite was formed by reactions with O₂-rich surficial water and thus an oxic atmosphere developed before 2.76 Gyr ago.