Mineral records of atmospheric-oceanic environments and biological processes in archean to paleoproterozoic banded iron formations

Abstract

Banded iron formations (BIF) are Precambrian sedimentary rocks characterized by alternating iron- and silica-rich bands. The minerals and their structures in BIF are ultrafine and have been overprinted by diagenetic to metamorphic processes. Current studies on BIFs are majorly based on observations at low-resolutions. This study uses high-resolution electron microscopes with the combination of geochemical and spectroscopic techniques to investigate the mineralogical, microstructural, crystallographic and geochemical features of six BIFs aged from >3760 Ma to ca. 2200 Ma. The geneses and material source of the BIFs, atmospheric-oceanic environments and biological processes during the BIF deposition are explored.

The six BIFs have similar mineralogy, but the abundances and structures of the minerals differ from one BIF to another. Euhedral magnetite with homogeneous internal structure and chemical composition contains substantial structural Si but little crustal-sourced elements. It has not suffered multi-redox alterations or interferences from external materials, thus, could reveal the depositional environment for BIF. Irregularly shaped massive magnetite is commonly brecciated and contains appreciable Al. It is likely recrystallized in later stage hydrothermal events with detrital inputs. Partly or completely oxidized octahedral magnetite has been repeatedly reduced and oxidized. Cautions should be taken while using these two types of magnetite to interpret the depositional environments for BIF. Submicrometer-size euhedral hematite crystals in the chert of Fe-Si transitional bands and 3-5 nm hematite nanocrystals in iron-rich bands are primary hematite inherited ferric iron from precursor minerals. This indicates photosynthetic Fe(II)-oxidation, by either direct or indirect biotic mechanisms, did exist in Neoarchean. The BIFs formed before the Great Oxidation Event (GOE) contain extremely low As, but the post-GOE Lüliang BIF contains abundant. For all BIFs, arsenic was adsorbed as As(V) by ferric iron precursors. These results suggest the Lüliang BIF was deposited in an environment with relatively more intensive oxidative weathering. This is consistent with the XANES results on Mn: Mn exists as oxidic Mn(III) in iron oxides from the Lüliang BIF, but as Mn(II) in the other BIFs. The Ge/Si ratios of magnetite are within the range of hydrothermal inputs, while the Ge/Si ratios of chert are close to that of modern marine. In all cases, the Ge/Si ratios increase with the increase of Fe, indicating Si and Fe have different sources: Si is mainly from seawater whereas Fe is from hydrothermal fluxes. The micro- to nano-sized flower-like phosphates from three Neoarchean to Paleoproterozoic BIFs are highly similar to biogenic phosphates from Mesoproterozoic to Pliocene sedimentary records. It reveals that microbialmediated phosphatization already occurred in Neoarchean. This also implies biogenic phosphate with specific mineralogical features could be preserved for billions of years, and could be used as a potential biosignature for tracing life on Mars.

With the combined high-resolution electron microscopic, Mössbauer spectroscopic and Synchrotron-radiation-based X-ray spectroscopic investigations, ultrafine structures of the minerals and the short-range environments of some elements in iron oxides from the studied BIFs are characterized. This study suggests that the genetic and evolutionary histories of minerals in BIF may provide clues on the ecophysiology of early marine environments.