Evidence for a low-O2 Archean atmosphere from nickel-rich chrome spinels in 3.24 Ga impact spherules, Barberton greenstone belt, South Africa

The composition of spinels in 3.24-billion-year-old Archean impact spherules in the S3 spherule bed in the Barberton greenstone belt, South Africa, provides important clues about the environments within which they formed, including the redox conditions of the impact plume and the oxygen levels in the early atmosphere. Despite pervasive diagenetic alteration of the impact spherules and nearly complete alteration of primary mineralogy, primary Ni-rich chrome spinel is preserved. The impact spinels are significantly more oxidized than detrital spinels of komatiitic origin that are also present in the spherule bed. The average Fe+ 3/FeT (atoms) value in the Ni-rich impact spinels is 0.43 whereas the average Fe+ 3/FeT of detrital spinels is 0.17. eT ratios of the impact spinels range from 0.26 to 0.69 (atoms) and suggest formation at oxygen fugacities below 10−4 bar based on comparisons with experimental results. Comparison of the S3 impact spinels with similar spinels from the K/T boundary layer, Eocene impact layer, and Late Pliocene impact layer also suggests much lower O2 levels in the 3.24 Ga atmosphere. Oxidation of spinels present in condensed melt droplets was affected by the oxygen fugacity of the atmospheric component mixed with the ejected plume, and the broad range in values may reflect fO2 heterogeneity both temporally and spatially within the impact-produced rock-vapor plume. Compositional and morphological variability of spinels in the spherules suggests temperature and cooling rate were heterogeneous through the plume.