Impact controversies: Impact recognition criteria and related issues

We were surprised to see the cover of the August 2013 issue of Meteoritics & Planetary Science (MAPS) adorned by a field image of a Greenlandic migmatite. The caption refers to the Maniitsoq structure in Greenland and explains that the lithology shown on the cover "is interpreted as due to a crustal-scale hydrothermal convection cell in a now deeply exhumed Archean impact structure" (Scherstén and Garde 2013). We found this statement surprising, as the Maniitsoq structure (Garde et al. 2012, 2013) has not been widely accepted as an impact structure, because the evidence reported in these papers does not comply with established recognition criteria for impact (see, e.g., French and Koeberl [2010] and references therein; Reimold et al. 2013). Garde and colleagues assert that, when established criteria "do not work," new ones can be devised. To present such a statement as that related to the MAPS cover—highlighting this structure, in our view gives unnecessary credit to a (as we insist on calling) proposed but unconfirmed impact structure. The article by Scherstén and Garde (2013), to which this cover refers, contains high-quality U-Pb data for zircon, which are interpreted as hydrothermal re-equilibration of the isotope systems as the result of an impact event. Reimold et al. (2013) recommended a detailed investigation of Maniitsoq zircon grains to investigate the possible presence of impact evidence in the form of planar fractures, shock-induced granular texture, or twinning in zircon. The numerous zircon images shown in Scherstén and Garde (2013), however, fail to illustrate any textural evidence of shock deformation. While we explicitly appreciate the high-quality U-Pb ages, we must insist that there is still no tangible evidence for impact at Maniitsoq. Nor does the circumstantial evidence preclude alternatives, such as magmatic/tectonic explanations, for the observations and data reported so far from Maniitsoq. Let us assume that Maniitsoq could be a deeply eroded impact structure with a diameter of some 150 km. This would place Maniitsoq within the range of "large, old, eroded impact structures," such as Sudbury (Canada) or Vredefort (South Africa). In these cases, and especially in that of the long controversial, deeply eroded Vredefort Structure, the ultimate proof of impact origin came from the discovery of shatter cones and planar deformation features (PDFs) in quartz, as well as the presence of a meteoritic component in impact melt rocks, all of which are accepted in the impact community as unambiguous proof of impact origin. The often annealed and/or decorated planar deformation features in Vredefort quartz were shown by Leroux et al. (1994) through detailed TEM analysis to represent bona fide shock deformation in the form of basal Brazil twins and decorated, higher order PDFs. Only with this proof in hand—further supported by unambiguous evidence for shock deformation in zircon (planar fractures and granular zircon texture)—were various authors able to later interpret breccia bodies, structural observations, and morphometric data within an impact context. Planar is the key term in planar deformation features, so that the description of "commonly curved, coarsened, and partially annealed" features as "planar features" (p. 1474) by Scherstén and Garde (2013) is not consistent with the published definitions. Their features are not imperfectly preserved, but have never been planar. Hence, we consider that they cannot provide evidence for a shock event. They also state "Proven but imperfectly preserved PDFs in known impact structures are only rarely reported in the literature," which represents another obscure finding—as "imperfectly preserved" (in their sense referring to curved) features only rarely would be recognized as impact-diagnostic PDFs. As further emphasized below, "decorated PDFs" are still planar, even if their traces in thin section only represent straight fluid inclusion trails. Maniitsoq is just one of several recent instances where the impact origin of geological structures was proposed based on evidence unrelated to the commonly accepted criteria for impact. These recognized criteria include the presence of evidence of shock metamorphism (such as shatter cones—Figs. 1a and 1b, or planar deformation features [PDFs—see Fig. 2] in various minerals), and remnants or chemical traces of extraterrestrial projectiles (e.g., see the recent reviews by French and Koeberl 2010 and Koeberl 2014). In the Maniitsoq case, Garde and coworkers developed their own criteria for shock metamorphism by interpreting nonplanar (i.e., irregular) fluid inclusion trails as equivalent to proper PDFs. As impact workers, and some of us as associate editors of MAPS as well, we are concerned that this disregard for accepted procedure may gain traction. In his discussion of the "impact bandwagon," Reimold (2007) drew attention to a number of problems that were plaguing the impact cratering community already at that time: (1) inclusions in the Earth Impact Database (EID, currently curated by the University of New Brunswick, Canada) that did not seem to be justified on the basis of geological contexts and/or lack of bona fide impact evidence; (2) the less than careful use of the terms "confirmed, probable, or possible" impact structures in the literature; (3) a number of reports of alleged impact structures without proper documentation of acceptable evidence; and (4) the confusion about the usage of the unqualified term "planar feature" as shock evidence and the sometimes unwarranted reporting of PDFs where no such definite shock metamorphic evidence had been documented. Questionable cases discussed by Reimold (2007) included the unconfirmed Khebira, Gilf Kebir, Arkenu, Suavjärvi, Bedout, Ramghar, and Luna structures, as well as a putative impact structure in Antarctica. In addition, the lack of impact evidence for a crater strewn field in the Chiemgau district of southern Germany was discussed, and already at that time, an alleged impact event at the Younger Dryas was controversial. Reimold (2007) observed that "misleading, incomplete, or incorrect information can find its way quickly into mainstream science (and even faster into the public domain via the internet)" and that it "may be rapidly absorbed into valuable databases." To date, the situation has not improved: The Chiemgau impact scenario is still pursued in nonspecialist discussions and in poorly reviewed or nonreviewed media. The hype about a Younger Dryas impact catastrophe continues to split the scientific community, despite absence of any proper evidence supporting an impact hypothesis (see, e.g., Pinter et al. 2011; Boslough et al. 2012, 2013). New questionable reports of geological structures alleged to be of impact origin have been published—mainly in online journals that do not appear to follow specialist peer review and adherence to best editorial practice, but unfortunately also in some reputable journals. Examples are the alleged Zirouki structure in Iran (Daneshwar and Bagherzadeh 2013), the Bajada del Diablo crater-strewn field in Argentina (Acevedo et al. 2009, 2012), and the repeated assertion of twin impact structures (lakes Isli and Tislit) in Morocco (e.g., Ibhi et al. 2013; Nachit et al. 2013). The latter was refuted by Chaabout et al. (2013). A very large impact structure was proposed by Glikson et al. (2013) in the Warburton Basin region of central Australia (interestingly duplicating much earlier suggestions by Duane and Reimold 1990) without (in our view) providing convincing evidence. Anomalous microstructures in quartz grains were recently reported from Mt. Oikeyama (Japan) by Sakamoto et al. (2010), on a MAPS cover page. Acevedo et al. (2009, 2012) alleged the presence of an impact-crater strewn field in a volcanic province of Argentina—without presenting petrographic or geochemical evidence, but by comparison with the equally unproven Chiemgau claims. Most recently, another report of poor content and editing in an online journal by Velázquez et al. (2013) introduced the Colônia structure (Brazil) into the Earth Impact Database (last checked on 14 January 2014), with evidence that can be reduced to a single image (their fig. 6A). This image does not allow the determination of whether it shows PDFs in a feldspar grain or exsolution/twin lamellae of nonshock credibility. Lajeunesse et al. (2013) reported on a new, possible impact structure termed Corossol, located close to Sept-Îles, some 250 km southeast of the Manicouagan impact structure. They described detailed geophysical data sets and find a general similarity of Corossol with complex impact structures. They also refer to a single 4 cm sized fragment of a breccia with many particles of glass as well as a single quartz grain with possible PDFs (which remain unconfirmed). This breccia fragment was not part of a coherent breccia layer in the structure, but is said to represent loose material from the central uplift—in their words "assumed to be locally derived." Unfortunately, no one knows whether the material is locally derived and what its relation to a (possibly buried) structure is. As the age of the proposed Corossol structure is rather unconstrained between "Mid Ordovician and pre-Quaternary regional sea-level lowstands" (Lajeunesse et al. 2013, p. 2542), it cannot be excluded that this fragment was deposited by, e.g., glacial activity from a distal impact structure, of which there are many in Quebec. We applaud the authors for defining the Corossol discovery as a "possible impact structure," but want to caution against accepting this breccia fragment and its attributes as confirming evidence of an impact origin of this proposed structure, at this time. Another somewhat unusual impact-related hypothesis was recently proposed by Tohver et al. (2013), involving the moderately sized (40 km diameter) Araguainha impact structure: the impact event was suggested to have liberated sufficient methane from the Paraná-Karoo basin sediments through seismic shaking to account for the tremendous negative carbon isotope anomalies found worldwide at marine P-Tr boundary sections. This hypothesis challenges previous thinking that a Chicxulub-sized impact event (180 km crater diameter) would be required for global mass extinction. This innovative, although entirely unproven, idea has since been prolifically quoted in the public media, which seems to be the fate of such spectacular hypotheses. What many of the cases of questionable impact structures have in common is reporting of supposed presence of PDFs, mainly in quartz, as evidence for impact. Frequently, this can be revealed to be based on obvious misperceptions of what does and what does not constitute true PDFs. Not only have many of these reports been published as primary erroneous literature, but these reports have entered secondary literature as well, causing further misperceptions, especially by newcomers and less-informed beginners in the impact cratering study field. An example is the work on Chiemgau published in an unrefereed (as stated on the webpage of the publishing society) conference proceedings volume, but then cited elsewhere, e.g., in the case of the alleged Bajada del Diablo crater strewn field in Argentina (Acevedo et al. 2009, 2012). Too many erroneous reports have been published in peer-reviewed journals, despite the obvious lack of firm evidence presented by the authors. This is problematic and illustrates lack of basic knowledge about PDFs by the authors and/or failure of the peer-review process in that some editors and reviewers are simply not familiar with some basic aspects of shock metamorphism. Therefore, we believe that it is timely to demonstrate how to distinguish between "true" and "false" impact evidence. In particular, the phenomenon PDF must be clarified to avoid such misinterpretations in the future. This editorial provides a short and readily comprehensible discussion of PDFs in quartz, accompanied by some characteristic photomicrographs to illustrate what does and does not constitute PDFs in quartz (compare Figs. 2 and 3). As a result of shock compression during an impact event, quartz (and other silicate minerals) develops irregular fractures and planar microdeformation features (for a concise explanation of the process, refer to Langenhorst and Deutsch 2012). Irregular fractures in quartz (and other minerals) can never be considered diagnostic shock effects, as such fractures can originate from various geological deformation processes and can even, to some extent, be induced during sampling or preparation of thin sections. Such nonplanar microdeformation features also occur in impact-affected rocks, e.g., at Vredefort, but irregular fractures or their healed counterparts (irregular fluid inclusion trails) in quartz grains still do not at all support an impact origin (which takes us back to the wrong chain of arguments used in the Maniitsoq case). Planar microstructures are categorized into planar fractures (PFs) and PDFs (e.g., Stöffler and Langenhorst 1994; French and Koeberl 2010 and references therein). The orientations of both PFs and PDFs are crystallographically controlled, i.e., they are oriented exclusively parallel to specific crystallographic planes. They can be characterized using transmission electron microscopy (TEM) (e.g., Goltrant et al. 1991) or with optical microscopy using the spindle stage (e.g., Bohor et al. 1987; Langenhorst and Deutsch 1994) or the universal stage (e.g., Ferrière et al. 2009). The fundamental characteristic of "planar deformation features" is that they are planar (besides being parallel and narrowly spaced). This seems still to be unclear for a large number of people who present microstructures as being planar that cannot even be called "sub-planar," because they are either of irregular geometry or of wavy appearance. If a shocked quartz grain is, however, strained before or after impact deformation, PDFs may give the appearance of being somewhat "curved"; this can be clearly established by careful observation in cross polarized light, whereby oscillatory extinction will illustrate low-angle kinkbands in such a grain). Even though these PDF sets seem to be "curved," from a crystallographic point of view they are planar (and parallel to each other) and only appear "curved" because they follow the c-axis orientation that changes through the grain. Thus, we argue that, in the past, in a number of cases impact structures have been accepted as falling into the category confirmed on the basis of microdeformation features that do not de facto represent PDFs. However, it is equally possible that in some instances PDFs have not been recognized as such, as they have been oriented subparallel or slightly oblique to the thin section plane and rather than having the distinct sharp and planar appearance of proper PDFs, have resembled broad and at best subplanar fluid inclusion trails. An investigation of microdeformation in quartz in arenite samples from a recently identified (possible) impact structure in Brazil (Santa Marta—Uchôa et al. 2013) has illustrated such a case, as seen in the two photomicrographs of Fig. 2e and f that contrast the same features in plane polarized light with normal incident light (2e) and slight tilting of the thin section plane to the incident light beam (2f). Figure 2f shows the true nature of these features as sharp, characteristic PDFs. Note that we refer to Santa Marta as a possible impact structure, at this time, as full documentation of the evidence has not been published yet, only in an abstract. Planar fractures are by definition planar, thin, parallel-walled fractures that are generally spaced more than 15–20 μm apart (compare Figs. 2a and 2b and see Stöffler and Langenhorst 1994; Montanari and Koeberl 2000). Formed at low pressures, PFs are not regarded as unambiguous evidence of shock metamorphism, as they may also occur in quartz grains that were subject to nonimpact stress. In contrast to PFs, PDFs are not open fractures. PDFs are typically composed of narrow, individual planes of amorphous material that are less than 5 μm thick, comprising straight, parallel sets of features that are spaced 2–10 μm apart (e.g., Stöffler and Langenhorst 1994). In samples of low-shock degree (approximately 8–15 GPa), PDFs generally occur as one set per host grain; at higher shock pressures, two or multiple sets become prominent, until at shock pressures in excess of 25–30 GPa grains become partially or completely converted to diaplectic glass (partial to complete isotropization; Langenhorst and Deutsch 1994; Huffman and Reimold 1996). PDFs can be either "decorated" or "non-decorated" (Figs. 2d and 2e). The decoration is formed mainly by fluid inclusions developed by exsolution of water upon recrystallization of originally amorphous PDFs, but occasionally can also include solid inclusions derived from shock dissociation of rock-forming minerals. Decorations are generally <2 μm in size; their presence greatly facilitates detection of PDFs at the scale of the optical microscope. Detailed TEM analysis has shown that PDFs can be composed of amorphous material or represent twin planes or high-density dislocation planes (e.g., Gratz et al. 1988, 1992). Leroux et al. (1994) demonstrated that PDFs parallel to the basal plane in quartz constitute Brazil twins. These very specific microstructures are quite different from tectonic deformation features, such as Böhm lamellae and other wavy and nonparallel features that can be seen in strained quartz grains (examples are shown in Fig. 3; in French and Koeberl 2010 and by Langenhorst and Deutsch 2012). In some cases, such nonimpact microdeformation structures that may be mistaken for PDFs by nonexperts are not limited to a single grain, but continue into neighboring grains, which is never observed in the case of PDFs or PFs—unless in the specific case where a host quartz grain has been transformed by metamorphic overprint into a number of subgrains after the shock deformation event. These may show individual extinction behavior, but PDFs may seem to be continuous across subgrain boundaries. The optical microscope may not be sufficient to identify the true nature of apparently planar microstructures; however, in all the cases of alleged impact craters mentioned here, the misidentified microstructures can be easily discriminated from bona fide shock deformation on the basis of the published photomicrographs. In the past, transmission electron microscopy has been called upon to clarify such ambiguous cases. Recently, and due to the diligent work of innovative researchers (e.g., Hamers and Drury 2012; Hamers 2013), more widely available scanning electron microscopy techniques, supported by cathodoluminescence and electron back-scatter diffraction analysis, have substantially enlarged the tool-kit for the proper identification of shock features—but still, the final assessment requires some experience. Finally, a word about shatter cones: The examples of shatter cones shown in Figs. 1a and 1b illustrate the nature of these features, which have served to confirm a large number of impact structures listed in the Earth Impact Database. Emanating and diverging from a millimeter to centimeter wide apex/apical area, striations slope downward with more or less curvature. Complete cones are relatively rare, but between 360° cone structures and nearly planar-though-still-striated fracture surfaces a wide range of striated features have been described (e.g., Wieland et al. 2006 and references therein). Sizes of shatter cones vary between a few millimeters, from apex to base, to 12 m. Frequently, the so-called horsetailing phenomenon has been observed—a distinct overlapping of V-shaped bundles of striations. However, not everything that appears striated represents shatter cones, as discussed in detail by Lugli et al. (2005). On a number of occasions, ventifacts, crystallization textures (Fig. 1c), diagenetic cone-in-cone structures, or percussion marks at fossil waterfalls have been mistaken for shatter cones. In conclusion, we call for caution by authors, reviewers, and editors when preparing and judging possibly controversial manuscripts. We are not promoting the view that new ideas or hypotheses should not be would to we that authors of such use proper and not about one possible is to arguments and recognized knowledge and criteria should be in of so far unproven and for impact. should the basin in be of impact as recently by (2013), without any of impact evidence for such a should irregular fluid inclusion trails that have been as being of impact origin be to the of a diagnostic for impact in the case of We simply that reviewers and editors are careful about how they and In other us to clearly from especially from one with a that any of a new possible impact structure should be in a way that the arguments in of this hypothesis established evidence for impact metamorphism and the to find confirming evidence to "impact The is If at an unproven structure the of evidence for impact origin some authors have that some evidence is because impact structure is is that suggestions for of evidence must be to in the cases of confirmed impact i.e., it is to demonstrate that such evidence is characteristic of impact. A number of impact structures still remain to be on but the of discovery of a new impact structure cannot scientific The in impact is that the presence of unambiguous shock-induced and Koeberl that this and/or shock metamorphism, that the structure from which this was in an impact event. of shock features, however, does not a a negative further On the other one should not that or unusual observations are impact-related just because they occur in an unusual geological The impact community is that there may be still of impact that to be further characterized or especially in the low-shock and some have to for that in (e.g., et al. To date, no evidence has been presented that features proposed by and as and known from a large number of impact be related to any other deformation process but impact. To this that lack proper scientific impact origin and appear in the mainstream literature are at a when publishing in poorly reviewed and online journals is more and more The recent inclusion of the (as Colônia structure into the Earth Impact Database was such cases unnecessary confusion about the criteria used in this discussed by Reimold the recent (2013) and Planetary in a discussion was about how to of the Earth Impact Database. This is as more and more and are the for impact structures and their an was by the of the Earth Impact Database to an to investigate of to the An was at an the of the should for inclusion into the Earth Impact Database. Such a would not preclude or of but would that only confirmed impact structures would appear in the to the of the for way would be a We are to the of Meteoritics & Planetary Science who publishing this The reviewers and are for valuable and

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