Critical testing of potential cellular structures within microtubes in 145 Ma volcanic glass from the Argo Abyssal Plain

Microtubes within 145 Ma volcanic glass from the Argo Abyssal Plain possess intriguing internal microtextures that under light microscopy resemble biological septa and ovoid microbial cells. These microtextures have previously been used as part of a suite of evidence to support the biogenicity of such microtubes, and similar textures are beginning to be used in attempts to taxonomically classify microtubes from both the modern and ancient oceanic crust within an ichnofossil (trace fossil) hierarchy. Here we use high spatial resolution correlative microscopy to characterize the morphology and chemistry of the Argo microtubes in order to critically assess the origin of these microtextures and increase our understanding of the potential formation mechanisms of microtubes in volcanic glass. Electron microscopy shows that the microtubes contain abundant elongated void spaces and when these are reconstructed in three dimensions they closely replicate the morphology and distribution of the previously described ‘septa’. No organic material is associated with the void spaces and so we reinterpret the ‘septa’ as cracks within the clay mineral phase that infills the microtubes, probably formed during sample collection and/or preparation. One ovoid body also appears to correlate with void space but further data are required to substantiate such an origin. We caution that the study of micro-textures within volcanic glass-hosted microtubes by optical microscopy alone may be misleading, hence each individual occurrence should be subject to detailed micro- to nano-scale in situ morphological and chemical investigation before being used as a potential biosignature. Several microtubes do contain elevated levels of carbon, typically found within amorphous carbonate minerals that, along with nontronite clay, have precipitated within the microtubes. One microtube contains organic carbon; this is heterogeneously distributed, occurs away from void spaces and is spatially associated with elevated levels of titanium. This organic carbon could originate from in situ biological activity but it could also have been introduced by circulating seawater. Titanium adsorbed onto this organic material may provide a titanium source for the commonly observed titanite mineralization found in ancient volcanic glass-hosted microtubes within greenstone belts and ophiolites that have experienced low grade metamorphism. Elemental enrichments and depletions in three chemically distinct regions (glass, alteration rim, and tube interior) provide further insights into microtube formation mechanisms. Alteration rims have sharply defined edges, are about 0.1 μm wide independent of microtube diameter, and are primarily composed of Si, Al, O ± Ti. The tube interiors are depleted in Si and Al, and most other elements (Ca, Mg, Na, Mn) relative to fresh glass but K and Fe may be enriched. There is no evidence for depletion of elements in the glass immediately exterior to the alteration rim. This favours a mechanism whereby microtubes grow by increasing in length, rather than increasing in diameter. In this model protons are the major agent of glass alteration and the supply of protons and the kinetics of the formation of the Si-Al alteration rim control the diameter of the microtubes.