In situ sulfur isotope analysis of pyrite coexisting with microfossils in the 3.4 Ga Strelley Pool Formation, Western Australia

Abstract

The activities of early life and its environment remain uncertain. The Strelley Pool Formation (SPF) in the Pilbara Craton, Western Australia is one of the oldest sedimentary rocks containing microfossils, making it a suitable sample for elucidating early life activities and its environment. In this study, small spherical pyrite with a concentric growth structure with about 20µm in diameter was discovered coexisting with microfossils. Using a microscope, field emission scanning electron microscope (FE-SEM), and secondary ion mass spectrometry (NanoSIMS), we performed morphology, microstructure, and micron-scale carbon and sulfur isotope analysis on this spherical pyrite. These results showed that the spherical pyrite formed in the early diagenetic stage of metamorphism of the organic-rich chert layer, at least before the solidification of the sediments. Furthermore, the negative sulfur isotope value and its variation indicated the contribution of biological activity to the pyrite formation.