Widespread chemically oscillating reactions and the phosphatization of hematite filaments and tubes in the oldest BIF from the Nuvvuagittuq Supracrustal Belt

Accurately distinguishing between biotic and abiotic microstructures is crucial for understanding the evolution of early life and the search for extraterrestrial life. The oldest putative fossils reported occur in the form of hematite filaments and tubes in the jasper-carbonate BIF from the Nuvvuagittuq Supracrustal Belt (NSB), Québec, possibly as old as 4.3 Ga. Although these twisted and branched hematite filaments and tubes are very similar to the Fe-oxyhydroxide filaments produced by Fe-oxidizing bacteria in modern hydrothermal deposits, they are still being questioned because morphologically and compositionally similar abiotic filamentous biomorphs can be produced in “chemical gardens”. Additionally, the origin of ubiquitous circularly concentric rosettes that occur with the filaments and tubes remains unclear. Systematic mineralogical and morphological characterization of these microstructures using a variety of correlated in-situ micro-analytical techniques such as polarizing microscopy, Raman spectroscopy, SEM-EDS, and XPS now yield a new understanding of these ancient microscopic objects.

Firstly, new observations of hematite filaments and tubes preserved in apatite crystals indicate phosphatization as another taphonomic mode of preservation. These apatites with filaments that are several hundred micrometers in size, and usually distributed in discontinuous bands between the silicon-rich and iron-rich microbands. The diameter of these hematite filaments and tubes is 4 to 8 μm, while their lengths are 10 to 200 μm. They are thinner than those previously reported preserved in quartz and their diameter is closer to that of modern iron-oxidizing bacteria. As for co-occurring hematite tubes, their interior is usually filled with apatite. The walls of tubes are often straight, and even crossing crystal boundaries between apatite and microcrystalline quartz. Furthermore, new Raman spectra show the occasional presence of organic matter in these filaments preserved in apatite, independently supporting a biological origin.

Secondly, rosettes widely present in the quartz have circularly concentric layers, radially geometric crystals of acicular hematite, and circular double or triple twins. These microstructures are akin to patterns seen in botryoidal minerals and likely produced by abiotic chemically oscillating reactions (COR). In addition, the walls of the tubes preserved in quartz are also sometimes wavy, curved, or botryoidal-like, along with concentric layers, which is comparable to botryoidal coatings on modern hollow filaments of ferrihydrite in deep-sea hydrothermal ecosystems, indicating the interaction between iron-containing minerals and decaying organic matter from biomass during diagenesis.

The latest observations suggest that in the early Earth's submarine hydrothermal environments rich in phosphate and organic acids, the widespread phosphatisation enables the oldest life preserved in the apatite in the form of hematite filaments and tubes. The new observations also emphasize the potential role of abiotic COR in the formation of rosettes, as well as the modifications of the surface features of microfossils during diagenesis. These biological and abiotic “biosignatures” provide a valuable reference to search for life signals in extraterrestrial environments such as Mars and icy moons.