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” (Scherst en 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 Scherst en 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 Scherst en 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 Scherst en 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 Meteoritics & Planetary Science 49, Nr 5, 723–731 (2014) doi: 10.1111/maps.12284