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

Figure 18

Schematic illustrations depicting the abiotic, stepwise development of complex petrographic microtextures in DSDP 418A basaltic glass (with emphasis on the corrosion of radiation damage). The final illustration (at lower right) highlights the key dissolution/palagonitization microtextures in DSDP 418A basaltic glass that are readily observable by transmitted light microscopy and SEM. Steps in microtextural/petrographic development: (1) nucleation and growth of plagioclase and clinopyroxene phenocrysts in the parent magma; (2) formation of basaltic glass, vesicles, and varioles, upon pillow eruption and quenching of glassy pillow margins; (3) early (f₁) fracturing of glass, allowing the infiltration of seawater into pillow margin interiors; (4) formation of inicipient (initial) palagonite—that is, before the onset of radiation damage—resulting in a sharp glass-palagonite interface; (5) the accumulation of significant numbers of alpha-recoil tracks (ARTs) in basaltic glass (originating from radioactive decay of U and Th) begins soon after pillow eruption (~23,500 ARTs/cm² after only the first 10,000 years); (6) deep burial of the pillow lavas being studied to depths of ~408–461 m beneath the overlying volcanic pile causes (f₂) fracturing and development of white (K-Al-Si)-rich devitrified zones, both along f₂ fractures and as halos surrounding plagioclase phenocrysts; (7) early-formed ARTs in the vicinity of f₁ fractures undergo preferential palagonitization (7a) and/or preferential dissolution/etch-tunnelling (7b); (8) ARTs continue to accumulate inside fresh basaltic glass; (9) eventually, fission tracks begin to accumulate in significant numbers within the glass, albeit in a much more sluggish fashion than ARTs (i.e., only ~11 fission tracks/cm² after the first 1,000,000 years); (10) over the passage of many tens of millions of years, basaltic glass becomes “riddled” with radiation damage. Meanwhile, the local oceanic crust ages and subsides under a deepening ocean (Figure 16), causing large incremental increases in hydrostatic pressure. The combination of these two processes leads to more advanced corrosion (dissolution and palagonitization) of basaltic glass, including development of a complex etch-tunnel network at the glass-palagonite interface defined by immense numbers of ARTETs (step 10a) interconnected with comparatively sparse FTETs (step 10b). In addition, multitudes of additional ARTs undergo “selective palagonitization” that collectively results in the development of a “granular palagonite ART alteration microtexture” (step 10c). (11-12) More ARTs and fission tracks would continue to form with the passage of time (step 11), ultimately resulting in an increase in complexity of both “granular palagonite ART alteration microtextures” (step 12a) and the ARTET-FTET network (step 12b). Eventually, some previously existing ARTETs/FTETs would become completely overprinted by the advancing palagonite zone, resulting in the formation of “palagonite fingers” that extend outwards into glass (step 12c; Figures 7(d) and 7(f)). Incremental increases in hydrostatic pressure could lead to “pressure solution” etch-tunnelling (step 12d), resulting in more peculiar etch-tunnel structures such as string-of-pearls (SOP), elongate wide tunnels (EWT), and overetched ARTETs (Figures 7(e) and 17). (13) Locally, the permeability of f₁ fractures could be reduced to zero by infilling of fractures with clays/palagonite, terminating the corrosion process. (14) Even after the termination of etch-tunnelling and alteration of the glass by seawater, additional ARTs (and fission tracks) would continue to form within fresh glass. ART: alpha-recoil track; ARTETs: alpha-recoil track etch-tunnels; D: devitrified zone; f₁: early fractures (associated with incipient/initial and ongoing palagonitization); f₂: late fractures (associated with devitrification); FTET: fission track etch-tunnel; FG: fresh glass; FV: flare-out void (formed by dissolution of multiple nearby ARTs); GP: granular palagonite; ip: initial palagonite; L: loop; P: palagonite; PF: palagonite fingers; SOP, EWT, and so forth: string-of-pearls texture, elongate wide tunnels, and other peculiar etch-tunnel varieties caused by prolonged overetching of alpha-recoil tracks and/or pressure solution etch-tunnelling; see Figures 7(e) and 17.