A comparative analysis of potential biosignatures in basalt glass by FIB-TEM

Bulk analyses of fresh and altered subseafloor basalts have elucidated the large-scale geochemical fluxes that result from water–rock interactions, which ultimately affect both seawater and mantle compositions, but these analyses do not provide insights into the physical and chemical processes occurring on the scale of microorganisms. Micron-sized tubules that may represent microbial boring into basalt glass are ubiquitous throughout the seafloor and can potentially be preserved over geological time. These putative biosignatures give clues into the history of life on Earth, and may be significant for the search for life on other planets; however, their formation and mineralization are still poorly understood. We have used a combination of focused ion beam (FIB) milling and transmission electron microscopy (TEM) to analyze the textural and geochemical characteristics of tubular alteration features from four basalt glass samples representing different sample locations, collection depths, and ages, including one sample from the Cretaceous Troodos ophiolite.High-resolution TEM imaging consistently identified platy layers of partially crystalline material infilling the tubules. The lattice spacings and diffraction patterns showed that this material comprises a mix of two-layer and three-layer silicates. Closer inspection of the contact point between the tubules and the surrounding basalt glass commonly revealed a thin leached rim around the tubules. Electron energy loss spectroscopy mapping demonstrated that these rims are depleted in everything but Si and O, which implies that they are formed by a leaching process, likely the incongruent dissolution of the glass during formation of the tubule. The infilling material is depleted in Ca and enriched in K, and in most cases also enriched in Fe, which is consistent with their identification as Fe-bearing phyllosilicates. These observations are consistent across the samples, which implies that the processes that form and mineralize tubular glass alteration features are similar throughout the oceanic subsurface and through time.

► FIB-TEM reveals important clues about micron-scale processes.
► A thin leached rim marks the contact between tubules and fresh glass.
► Mineralization of the tubules is primarily by iron-bearing phyllosilicates.
► Tubular alteration is a common process throughout the subseafloor and through time.