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

Figure 7

Diversity of abiotic corrosion microtextures in DSDP 418A basaltic glass linked with palagonitization (all thin section photomicrographs are taken in plane polarized light (uncrossed nicols); (a–e): sample DSDP-418A-75-3[120–123]; (f): sample DSDP-418A-72-4[13–15]). (a) Overview, highlighting fresh basaltic glass (FG), vesicles (v), plagioclase phenocrysts (plg), early fracturing (f₁) associated with incipient (ip) and ongoing palagonite (P), late fracturing (f₂) associated with white (K-Al-Si)-rich devitrified zones (D), regions where white devitrified zones have been enveloped (labelled “e”) by ongoing palagonitization, and the corrosion front (cf) associated with etched radiation damage. Inset BSE image: Paleoproterozoic ( Ma) zircon (Z) from the BD2 mafic dyke swarm, India, highlighting a similar corrosion front developed in the relatively U- and Th-rich (radiation damaged) zircon core during recent tropical weathering (Appendix  A in French [194]; Figures  1 and 2 in French [195]). (b) Close-up from (a) highlighting fresh basaltic glass, vesicles, white (K-Al-Si)-rich devitrified zones exhibiting weak evidence of axiolitic internal microtextures, palagonite, and alpha-recoil track etch-tunnels (ARTETs). (c) Close-up from (b) highlighting the characteristic tortuosity of ARTETs in fresh basaltic glass and places where necking (N) has pinched off certain portions of these nanotunnels. This image is a photomosaic of numerous image fragments taken at 15 different focal depths throughout the entire depth of the ~30 μm thick petrographic thin section (a single image from this same area—see (b)—is shown in Figure 1(f), in which the alpha-recoil tracks appear to dive in and out of focus). Two inset SEM (secondary electron) images highlight the tiny size of these ARTETs, where they intersect the surface of the polished thin section (~100–200 nm wide), that is, about the same size as an alpha-recoil track (or ART; see ~120 nm pink dots in Figures 7(c) and 9(d)). (d) Region where palagonite fingers (PF) have now “overprinted” previously existing ARTETs. (e) Four regions where incipient ARTETs have been affected by prolonged overetching (i.e., etch-tunnel widening), possibly in relation to pressure solution, resulting in a diversity of etch-tunnel sizes and shapes, including elongate wide tunnels (EWT), string-of-pearls texture (SOP), irregular bulbous cavities (IBC), and boudinaged tunnels (BT). Note: the zone of incipient ARTETs (at right in the left image) is a photomosaic of five focal depths, and the two small images at far right are of the same region but different focal depths. (f) Two photomicrographs highlighting development of “granular palagonite ART alteration” (GP), as well as additional examples of palagonite fingers (PF) and a single (ARTET).

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