Abstract EANA2024-97

Microscopic filaments and tubes composed of iron (oxyhdry)oxides and clay minerals are very common across a variety of rock types on Earth. They have been interpreted as the oldest known fossils [1], the oldest known fungus-like organisms [2] (hence possibly the oldest known eukaryotes [2]), as fossils from the ancient deep biosphere [3], and as targets in the search for life in extraterrestrial samples [1,3,4]. Such objects are often found in void spaces in basaltic rocks [2-4], commonly entombed by silica. We can readily anticipate that similar objects may soon be found on Mars, an iron- and clay-rich, dominantly basaltic planet where aqueous fluids have deposited abundant silica.

Some of these objects are certainly fossil organisms [3]. However, others are of dubious and contested biogenicity, often despite impressive morphological resemblance to known forms of life such as bacteria and fungi [5,6]. Here, we present new results from the study of filamentous microstructures in agates: natural casts formed within cavities in rock (typically basalt) by the precipitation of microcrystalline silica. Agates are extremely widespread on Earth and very commonly contain either or both: (a) filaments and tubes like those mentioned above; (b) other, very diverse and morphologically complex structures (e.g., plumes, spirals, dendrites) made of the same minerals but plainly abiogenic, which promise to shed light on self-organization processes active under the conditions of agate formation.

This presentation will focus on a new form of biomorphic filamentous structure from agates hosted by Palaeozoic and Cenozoic lavas from Scotland and Mexico respectively. These structures are composed of long, impressively sinuous, branching filaments about 0.5–1 micron in typical diameter, arranged radially around opaque cores (rather like spider legs around a spider). Second and third-order Y-branching is common; side-branching and even something like anastomosis can also be observed. The filaments are composed primarily of the iron minerals goethite and hematite but are also slightly enriched in carbon (as well as copper and sulfur). Nevertheless, these filaments are not fossil microbes but complex crystals: scanning electron microscopy reveals that their cross-sections are distinctly polygonal. This also suggests that they are not chemical gardens. We will suggest alternative explanations, some of which shed light on general problems of agate genesis and are now being tested experimentally in the lab (preliminary results of these experiments will be shown).  We conclude that biomorphic microstructures may occur in basaltic rocks on Mars [7,8].

 

References

[1] Dodd M.S., et al. (2017) Nature 543, 60-64. [2] Bengtson, S., et al. (2017). Nature E&E 1, 0141. [3] McMahon, S., & Ivarsson, M. (2019). BioEssays 41, 1900052. [4] Hofmann B.A. et al. (2008) Astrobiology, 8, 87-117. [5] McMahon, S., et al. (2021). Geobiology 19, 473-488. [6] McMahon, S., & Cosmidis, J. (2022). J. Geol. Soc. 179, jgs2021-050. [7] Sainz‐Díaz, C.I., et al. (2021). GRL 48, e2021GL092883. [8] Escamilla-Roa, E., et al. (2022). Astrobiology 22, 863-879.