Discovery of Naturally Etched Fission Tracks and Alpha-Recoil Tracks in Submarine Glasses: Reevaluation of a Putative Biosignature for Earth and Mars

Figure 4

Background on alpha-recoil tracks and examples of alpha-recoil track etching. (a) Schematic illustration of alpha-recoil track development during complete ²³⁸U-series radioactive decay (note: alpha-recoil tracks also originate from ²³⁵U- and ²³²Th-series decay—not shown). Beta decays are indicated by small arrows pointing down (enclosed in boxes/circles), while alpha decay parent-daughter pairs are denoted by identical bracket styles. (b) Etched alpha-recoil tracks in micas, revealed by experimental chemical etching of fresh cleavage surfaces. The image at left is of muscovite (etched for 2 hours at 20°C using 48% HF) and was obtained using phase contrast microscopy [130] (no scale bar is available for this image). The other two images (center and right) are of phlogopite (etched using 40% HF and then coated with a thin layer of carbon and imaged via an optical microscope equipped with an incident light device combined with Nomarski-Differential Interference Contrast [133]). Note the increase in average size of alpha-recoil track etch-pits with time, which is evident when comparing the 60 s and 300 s images. (c) Naturally occurring alpha-recoil track etch-tunnels (ARTETs)—identified at the glass-palagonite interface in the glassy margins of midocean ridge pillow basalts (this study). Left image: sample DSDP-418A-75-3[120–123] (see Figure 11 for petrographic context). Right image: sample DSDP-418A-68-3[40–43].