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

Figure 15

Comparison of theoretical modelling of radiation damage in DSDP 418A basaltic glass with the observed distribution of natural etch-tunnels at the glass-palagonite interface. (a) Representative SEM (secondary electron) image of the etch-tunnel zone (close-up from Figure 11(a)). (b) Porosity map (constructed from (a)) highlighting the distribution of fission track etch-tunnels (FTETs) and alpha-recoil track etch-tunnels (ARTETs) observed at the glass-palagonite interface. (c) Theoretical areal distribution of alpha-recoil tracks (ARTs) in DSDP 418A basaltic glass intersecting a hypothetical flat fracture plane through the glass (calculated using (1)). In this close-up theoretical plot (c), the individual model ARTs are plotted as randomly distributed 120 nm diameter spheres, resulting in pink circles of varying size (up to 120 nm), depending on their depth relative to the plane of the page (i.e., flat fracture surface). (d) Close-up porosity map from (b), highlighting a representative region of the etch-tunnel zone that is predominated by ARTETs. Note the similarity in size and areal distribution of the natural ARTETs (d) with the theoretically modelled ARTs (c), indicating a causal relationship. Many of the ARTETs in (d) exhibit elongate anastamosing shapes, the result of seawater “etching out” several nearby alpha-recoil tracks in cumulative succession (see “step 10a” in the lower left panel of Figure 18). (e) Etch-pit map, showing the observed distribution of experimentally etched ARTs and one etched-out fission track, on the cleavage surface of a mica crystal (adapted from Figure  1(b) of Huang and Walker [130], no scale bar available). (f–h) Theoretical models of the present-day distribution of randomly distributed ARTs (pink) and fission tracks (green) intersecting a hypothetical flat fracture plane through DSDP 418A basaltic glass, as determined using (1) and (2). For clarity, ARTs are not shown in (g) and (h) because of their exceptionally high numbers. Note how in (f), one single large fission track is surrounded by a multitude of smaller ARTs, similar in nature to the bimodal size versus population distributions of experimentally etched ARTs and fission tracks in mica (e) and naturally formed ARTETs and FTETs in DSDP 418A basaltic glass (b), a key argument for the “radiation damage” origin of the ARTETs and FTETs in DSDP 418A basaltic glass (see Sections 3.4.2, 5.2, and 6.1 for discussion). Colours in (b) and (d): blue = ARTETs and FTETs; white = fresh basaltic glass; orange = palagonite. Colours in (c), (f), (g), and (h): white = fresh basaltic glass not affected by radiation damage; pink = model ARTs; green = model fission tracks (randomly oriented lines in (f) and (g); dots in (h)). Colours in (e): white = fresh cleavage surface of mica; blue = ART etch-pits and one fission track etch-pit.