Biogenic nitrogen and carbon in Fe‐Mn‐oxyhydroxides from an Archean chert, Marble Bar, Western Australia

Abstract

To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460‐Myr‐old Fe‐rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe‐ and Si‐rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe‐Mn‐oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe‐Mn‐oxyhydroxides, but a minor fraction is found in K‐feldspars and Ba‐mica dispersed in the silica matrix. The N and C isotopic composition of Fe‐oxides suggests the presence of a unique biogenic source with δ15NAIR values from +6.0 ± 0.5‰ to 7.3 ± 1.1‰ and a δ13CPDB value of −19.9 ± 0.1‰. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion‐controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe‐isotopic composition of the Marble Bar chert (δ56Fe = −0.38 ± 0.02‰) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact with oxygenated waters. To explain the N and C isotopic composition of Marble Bar chert, we propose either (1) a later addition of N and C at the end of Archean when oxygen started to rise or (2) an earlier development of localized oxygenated environments, where biogeochemical cycles similar to modern ones could have developed.