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Here, we reevaluate the origin of these enigmatic microtextures from a strictly nonbiological standpoint, using a case study on submarine glasses from the western North Atlantic Ocean DSDP A. Our findings have important implications for geomicrobiology, astrobiological exploration of Mars, and understanding of the long-term Fission-track dating relies on the decay of the art of nuclear waste glass.
Understanding and successfully identifying examples of preserved microbial life from extreme environments on planet Earth are pertinent to the astrobiological exploration of Mars, and this was highlighted during recent debates over Martian meteorite ALH e.
Knowledge about the geomorphology and geological setting of these environments at the macroscopic scale on Earth can help with landing site selection for Mars astrobiology missions [ 13 ]; however, even more imperative to the successful astrobiological exploration of Mars is the ability of scientists to distinguish with absolute certainty whether or not relict signs of life are present in a returned rock sample e.
Numerous lines of evidence will probably be necessary to indicate that a true biosignature is present in such a sample and, among others, may include geochemical and stable isotopic constraints [ 1415 ], the identification of biologically produced minerals Fission-track dating relies on the decay of the art 1617 ], detection of biomolecules [ 18 ], and paleontological arguments such as recognition of microscopic morphological biomarkers [ 319 ].
In fact, it is quite common at this scale of observation e. These three examples clearly demonstrate the value in seeking both Fission-track dating relies on the decay of the art and nonbiological explanations for the origin of putative microscopic morphological biomarkers in rocks, especially when found in extreme environments on Earth that may have similar counterparts at or below the surface of Mars.
All of these claims, however, need to be scrutinized, questioned, and tested by the scientific community, but, remarkably, this is something that has not yet taken place for this vast putative microbial ecosystem in volcanic glass on planet Earth. Throughout this year period i. Certainly, when investigating the origin of conspicuous microtextures in petrographic thin sections of volcanic rocks, a petrological i.
Historically, this was actually the case for several earlier studies on partially palagonitized basaltic glasses that identified the presence of microchannels or etch-pits in fresh basaltic glass immediately adjacent to the glass-palagonite interface e.
And although their significance was not initially addressed to much extent in the literature as highlighted by Zhou and Fyfe [ 44 ]some of these authors suggested in passing that such microscopic cavities are simply the result of dissolution processes associated with the incipient stage of palagonitization during aqueous alteration of the glass [ 40 ], such Fission-track dating relies on the decay of the art corrosion [ 41 ] or the formation of etch-pits [ 44 ], and saw that there is no need to invoke microbial activity in the formation of these tiny etch features.
It is quite amazing that in the face of this daunting body of scientific research purporting to have documented bona fide microscopic morphological biomarkers in volcanic glasses worldwide—complex microtextures supposedly resulting from microbial bioalteration, biocorrosion, or biogenic microboring of volcanic glass i.
A few studies have touched on the topic of examining possible nonbiological origins for microscopic etch-tunnels in submarine glasses e. In addition, morphologically similar i. Similarly, experimental chemical etching of alpha-recoil tracks Figure 4 caused by the radioactive decay of U and Th e.
Furthermore, radiation damage in synthetic borosilicate nuclear waste glasses for which basaltic glass is an ideal natural analog: Consequently, in the present study, we evaluate the origin of complex microscopic alteration textures commonly observed at the glass-palagonite interface in submarine basaltic glasses worldwide i. Figures 1 and 2from a strictly nonbiological standpoint. In particular, we consider the likely effects of the accumulation of radiation damage—that is, in the form of randomly distributed spontaneous fission tracks and alpha-recoil tracks caused by radioactive decay of U and Th—on process of natural abiotic corrosion i.
Rationale for this study comes from the well-known fact that spontaneous fission tracks and alpha-recoil tracks in silicate glasses are known to etch preferentially over undamaged regions of glass [, — ], commonly resulting in microscopic etch-pits e.
Coupled with the observations that midocean ridge basaltic glasses worldwide are known to contain trace amounts of U and Th [ ] and are routinely dated by the fission track method [, ], this provides direct evidence that radiation damage could potentially play a very important role in the natural corrosion and dissolution of basaltic glass by seawater i.
Moreover, naturally etched fission tracks i. In the present study, we challenge these three points by demonstrating unequivocally that such complex corrosion microtextures at the glass-palagonite interface in submarine volcanic glasses e.
To accomplish these objectives, we have focussed this multidisciplinary petrographic rock-textural and theoretical modelling study on partially palagonitized, Our study combines petrographic observations, high resolution SEM imaging, considerations of the geological setting, determination of trace element concentrations U and Th in fresh basaltic glass by Inductively Coupled Plasma-Mass Spectrometry ICP-MSand theoretical modelling of radiation damage in basaltic glass caused by radioactive decay of U and Th i.
The natural breakdown and corrosion of basaltic glass have also long been considered to
Fission-track dating relies on the decay of the art an important natural analog process pertinent to understanding the long-term breakdown of high level nuclear waste glasses stored in geological repositories .
Detailed geochemical analyses i. According to their down-hole depths and relative positions in the volcanostratigraphic sequence at DSDP A Figure 5 ball three of the basaltic glass pillow margin samples in the present study occur within chemical i. Of particular interest to the present study aimed at understanding the likely role of self-incurred radiation damage on the development of complex corrosion microtextures in submarine basaltic glassfission track dating of basaltic glasses from DSDP A was actually carried out early on as well Fission-track dating relies on the decay of the art ].
Furthermore, U concentrations in basaltic glass samples DSDPA and DSDPA[34—37] stratigraphically higher up in the volcanic succession than the rocks from this study were determined to be The success of this early fission track dating study, coupled with the measurement of trace U in basaltic glasses at DSDP Hole A, provides important rationale for the present study Fission-track dating relies on the decay of the art the corrosion of radiation damage in DSDP A basaltic glasses—because it already proves for us that such radiation damage and U is actually there, implying that fission tracks might indeed play an important role in controlling the microscopic patterns of corrosion during preferential dissolution and palagonitization of basaltic glass by seawater.
The down-hole depths at which such putative tubular and granular bioalteration microtextures have been documented in these previous studies are indicated in Figure 5 b in redalong with the positions of basaltic glass pillow margins sampled in this study in black in which we document the occurrence of identical but clearly abiotic tubular and granular microtextures arising from preferential corrosion of randomly distributed fission tracks and alpha-recoil tracks in basaltic glass.
In order to carry out accurate theoretical modelling below in Section 5 of the accumulation of randomly distributed radiation damage i. Because the amount of material we had for each pillow margin sample in this study is quite small, and no suitable U-bearing minerals are present in these glasses such as zircon or baddeleyitewe were not able to carry out precise and accurate U—Pb isotopic dating of these rock samples e.
Accordingly, this fission track age was interpreted in that study as a estimate for the timing of pillow eruption, glass quenching, and formation of the M0 linear magnetic anomaly intersected by these drill holes: Figures 5 a and 5 dall of which formed during ancient ca.
Mid-Cretaceous seafloor spreading during early ocean opening of the central Atlantic Figure 5 c. However, additional radiometric dating of secondary analcites, celadonites, and smectites showing greater overall Rb—Sr enrichments that originate from volcanic basement rocks of nearby DSDP Hole A situated on the same M0 linear magnetic anomaly: However, given the occurrence of fresh basaltic glass in some pillow margin samples e.
Firstly, another radiometric dating study of DSDP A and D basalts was carried out early on [ ], which employed an 40 Ar— 39 Ar stepwise degassing method of dating that was performed on seven whole rock samples of crystalline rocks basalts recovered from these drill holes. Furthermore, the occurrence of another important nanofossil datum Corrolithion acutum even lower down in the sedimentary succession intersected by two of these three drill holes D and A was interpreted to indicate that earliest sedimentation above volcanic basement began in lower Aptian times i.
Probably one of the best methods for estimating Fission-track dating relies on the decay of the art age of eruption of pillow lavas at DSDP A that is currently available right now until precise and accurate U—Pb isotopic ages become available for these rocks is to consider the geological context of these lavas within the broader scale regional age patterns of development of the oceanic crust and associated linear magnetic anomalies in the North Atlantic Ocean. Therefore, before drilling was carried by the DV Glomar Challengerearly reconnaissance geophysical survey work was carried out by the USNS Lynchin order to provide the necessary scientific means for drill site targeting [ ].
Consequently, the age of eruption of DSDP A pillow lavas and quenching of associated pillow rim basaltic glasses in this study coincides with the age of oceanic crust formation associated with the final stages of development of the M0 magnetic anomaly. Linear magnetic anomalies recorded in the oceanic crust associated with the M0 Chron have now been correlated globally across the oceanic portions of several major tectonic plates—and beneath several different oceans , notably including the North American, Eurasian, and African plates beneath the North Atlantic Ocean note: Therefore, in the present study on DSDP A basaltic glasses, we also consider this time interval of Here, it is important to note that the M0 Chron and associated seafloor spreading linear magnetic anomalies are quite crucial to understand many aspects of Earth evolution and plate tectonic theory, especially regarding the origin and opening of the Atlantic Ocean.
In addition, the base of the globally correlated M0 magnetic anomaly, which is currently estimated at To study and characterize alteration microtextures preserved in basaltic glass pillow margin samples in this study, we prepared polished petrographic thin sections and studied them Sections 3.
Accordingly, the basaltic glass in these samples ranges locally from fresh unaltered glass e. The development of such orange-brown palagonite during aqueous alteration of basaltic glass is a common feature in both submarine [ 44 ] and terrestrial e.
Figure 8 cand are composed primarily of microcrystalline K-feldspar i. Locally, these white K-Al-Si -rich devitrified zones exhibit a weakly fibrous internal microtexture, visible with transmitted light microscopy Figure 8 b and define an overall axiolitic structure i.
Similarly, the white devitrified zones that form halos around plagioclase phenocrysts also exhibit a fibrous internal microtexture, but in this case the fibers appear to radiate around the phenocrysts, maintaining perpendicularity to the plagioclase contact Figures 8 d and 8 e. Allaby [ ], which states that such axiolitic fibers are only visible by petrographic microscope.
The white K-Al-Si -rich devitrified zones described here are similar in size, shape, geological context, and chemical composition to light coloured K-rich zones documented in a previous study of DSDP A basaltic glasses [ ], Fission-track dating relies on the decay of the art they were also found to occur in basaltic glass along some fractures and as rims around some plagioclase phenocrysts and interpreted as poorly crystalline, secondary K-feldspar.
Although the glass-devitrification interface is for the most part quite sharp and curvilinear e. Aside from being the possible end product of the solid-state transformation of basaltic glass into poorly crystalline materials Fission-track dating relies on the decay of the art. High temperature devitrification of submarine glasses i.
Furthermore, the composition of these white K-Al-Si -rich devitrified zones is consistent with poorly crystalline K-feldspar Figure 8 j and not plagioclase or clinopyroxene, and they do not form masses of coalescing spheroidal bodies of radiate fibers that are typical of high temperature devitrification textures e. However, some minor occurrences of varioles do exist locally in some of the basaltic glass pillow margin samples studied e.
Figure 9 m and are interpreted as primary high temperature igneous quench features—similar in nature to the dark-coloured varioles described by Fisk and McLoughlin [ ].
Such slow advancement of the glass-palagonite interface i. Subsequent to the development of these narrow incipient i.
Ongoing palagonitization of basaltic glass by infiltrating seawater i. This corrosion front is locally characterized by a pronounced etch-tunnel zone Figures 1 b1 d1 f7and 9 that extends out in front of the palagonite zone into fresh basaltic glass for distances of up to a few hundred microns e. Therefore as a first step, in the next few Sections we provide a range of detailed observations of complex corrosion microtextures that occur at the glass-palagonite interface in these studied "Fission-track dating relies on the decay of the art" of basaltic glass pillow margins from DSDP A.
Palagonitization is interpreted to have started early—soon after quenching in the Early Cretaceousin association with f 1 fracturing as outlined in Section 3. Similar alteration microtextures have also been observed in the interior of some zircon grains caused by preferential corrosion of high U and Th radiation damaged regions during weathering e. Palagonite at the glass-palagonite interface locally appears to be superficially much darker in colour i.
These etch-tunnels extend outwards into the fresh glass for up to several hundred microns past the palagonite zone Figure 7 btracing out intricate 3-dimensional curvilinear pathways through the glass Figure 7 c that exhibit a high degree of tortuosity i. The similarity in diameter between hypothetical alpha-recoil tracks and the observed etch-tunnels i. For that reason also see Sections 3.
Subsequent to their formation at the glass-palagonite interface, late secondary modification of some of these alpha-recoil track etch-tunnels has also locally taken place. Furthermore, prolonged overetching perhaps caused by pressure solution etch-tunnelling—this idea is explored further in Section 5.
In many places along the glass-palagonite interface, alpha-recoil track etch-tunnels are abundant e. For instance, along the f 1 fracture highlighted in Figures 9 e9 iand 9 m — 9 oa prominent alpha-recoil track etch-tunnel zone occurs at the glass-palagonite interface on one side i. We interpret this pattern of alteration as evidence that the infiltration of seawater into the glass during palagonitization takes place not only by preferential etch-tunnelling through alpha-recoil track damaged sites in the glass i.
Key evidence to support this claim comes from the observation that the areal density and distribution of palagonite granules imaged by SEM e.
Therefore, we propose that granular palagonite alteration microtextures found in submarine volcanic glasses worldwide e. Consequently, we suggest that the recently proposed microbial trace fossil i. Therefore, in order to circumvent this problem in the present study, we also prepared hand-crushed i. The sub-millimeter-sized glassy pillow margin fragments i.
We surveyed a large number of fresh fracture surfaces of various basaltic glass chip samples from DSDPA[—] and located partially palagonitized basaltic glass and associated etch-tunnels on several different grains, and a high resolution SEM overview of a representative region of the glass-palagonite interface is shown in Figure 11 a.
The etch-tunnel zone occurs between the palagonite zone and fresh basaltic glass Figures 11 a — 11 c and consists of fresh basaltic glass that is riddled with porosity i.
Porosity in the etch-tunnel zone occurs in the form of a complex 3-dimensional network of anastamosing nanoscopic tunnels i. In cross section i. The typically peanut-shaped larger tunnels i. SEM images of four cross sections through the larger variety of etch-tunnel Figures 13 a — 13 d may all be interpreted to represent variably oriented planar sections through a single type of peanut-shaped void space i.
The smaller etch-tunnels i. On the basis of morphology, size, geological context, and textural relationships with surrounding glass, this filamentous material is interpreted to be authigenic imogolite Figures 12 e12 g12 hand 14 h ; also see Figure 17 c and 17 f —introduced in a later Section—and .
The authigenicity of these imogolite filaments and platy smectite occurring within small versus large etch-tunnels, respectively i. This distinctive microtexture occurring at the glass-water interface i. This adds further support for our conclusion in Section 3. As such, we interpret the etch-tunnel zone at the glass-palagonite interface Figures 7911 — 1315 a15 band 15 d ; also see Figures 1 b1 d1 f3 e4 c —left to have formed by preferential dissolution and etch-tunnelling by seawater through radiation damaged regions of basaltic glass i.
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