Comments and corrections to the Letter to the Editor, <i>Meteoritics &amp; Planetary Science</i>, May 2014: “Impact controversies: Impact recognition criteria and related issues,” and discussion of shock mineral melting at Maniitsoq and Vredefort

The Maniitsoq structure in West Greenland features prominently in a recent letter to the editor of MAPS (Reimold et al. 2014). As authors of the paper that prompted this letter (Scherstén and Garde 2013) and the first paper on the Maniitsoq structure (Garde et al. 2012, 2013) we would like to present a few clarifications and comments to those parts of the letter that deal with the Maniitsoq structure. A major part of the letter is devoted to a discussion of shock lamellae in quartz, and the reader is given the impression that poorly preserved, subplanar arrays of fluid inclusions have incorrectly been designated as PDFs and used as evidence for impacting at Maniitsoq. This is not correct. On the contrary, Garde et al. (2012) did not designate the microstructures as PDFs and concluded that their orientations were compatible with being PDFs but that they were not diagnostic. The strongest part of their microstructural data was orientation measurements using indexing that strictly followed the guidelines by Ferrière et al. (2009), and including a correlation test that was performed on suggestion by W.U. Reimold (see Garde et al. 2013 for details). It remains uncertain to which extent the authors of the letter now acknowledge that postshock deformation of quartz crystals will necessarily affect the planarity of their PDFs (see e.g., examples from Charlevoix in Trepmann and Spray 2005; or from Sudbury and Vredefort in Garde et al. 2012; an issue that was dealt with extensively in Garde et al. 2012, 2013). We do not understand how the illustrations in the letter of non-PDFs might contribute to this discussion. The authors of the letter note that impacting at Vredefort was ultimately proven by identification of PDFs in quartz and go on to argue that if an impact structure at Maniitsoq with a diameter of 100–150 km is assumed, it would be comparable to Sudbury and Vredefort and therefore should contain similar, rather well-preserved PDFs. As is clear from previous discussion (Garde et al. 2013; Reimold et al. 2013), such direct comparison is incongruent. The remains of an impact structure at Maniitsoq with a present diameter of 100–150 km would have a crustal setting substantially different from those of Sudbury and Vredefort. Whereas the presently exposed targets of the latter structures were cold, upper crustal rocks eroded to a maximum of around 10 km below the impacted surface, the presently exposed host environment for the Maniitsoq event at 3.0 Ga constituted preheated rocks in the lower continental crust with ambient upper amphibolite to granulite facies P–T conditions. This is clearly seen from the many published age determinations of the Mesoarchaean igneous, metamorphic, and hydrothermal events at Maniitsoq; a convenient overview of these data has now been published in table 1 of Garde et al. (2014). Well-preserved PDFs cannot be expected to be found in deeply exhumed, lower crustal Maniitsoq-like settings where quartz is highly ductile during and after impacting. This is not an “ad hoc” argument or an attempt to circumvent accepted practice, but merely a necessary consequence of the ambient physical conditions of the target rocks. This is supported by detailed observations on the Finnefjeld domain that forms the central part of the Maniitsoq structure (Garde et al. 2014; published after the letter but considered highly appropriate for the ongoing discussion). The annihilation of orogenic structures and mineral textures in the central part of the Maniitsoq structure is characterized by mixed rheological behavior characterized by intense brittle fracturing of plagioclase, direct melting of K-feldspar (shock melting, using the terminology of the letter), and concomitant ductile deformation of quartz (Garde et al. 2014). This type of intense, brittle, pure-strain deformation of plagioclase, ductile deformation of quartz, and direct mineral melting in an approximately 35 by 50 km large area in the deep crust is unknown from endogenic orogenic events. It was ascribed to intense deep-crustal seismic reverberation, which was in turn interpreted as induced by impacting (Garde et al. 2014). If an impact indeed occurred at Maniitsoq, it is obvious that the chances of finding well-preserved PDFs in quartz would be small, at least in the central part of the structure, and obvious that PDFs in quartz do not constitute an appropriate identification tool in this context. The collection of samples in the eastern part of the Maniitsoq structure for the zircon study by Scherstén and Garde (2013) was devised to obtain a range of rock types that was as diverse as possible, following previous advice (Reimold et al. 2014). We have published all of the analytical data from all of the initially selected samples. We were ourselves surprised to find such pervasive evidence in the zircon of the hydrothermal event. The authors of the letter write that no evidence has so far been produced from Maniitsoq that precludes alternative magmatic/tectonic explanations. If the study of possible quartz PDFs is inconclusive, which other types of compelling evidence for impacting might form and become preserved in the lower crust due to giant impacts? We consider that shock melting of rock-forming minerals probably constitutes the most suitable, generally accepted and potentially widely applicable identification tool that is currently available for structures like Maniitsoq, and, as it happens, work by the authors of the letter and their coworkers during the last decade has contributed significant insight into this phenomenon. Beginning with K-feldspar, Gibson et al. (2002, fig. 2d) described highly cuspate, shock-melted K-feldspar in a breccia vein from the granofels in the core of the Vredefort dome. They used the descriptive term “poikilitic K-feldspar,” but might also have described the K-feldspar as cuspate. It is implicit in this interpretation that the K-feldspar shock melt was re-formed as contiguous, millimeter-sized grains rather than glass (or devitrified glass) as in shallow and geologically young impact structures (e.g., Machado et al. 2009), or as fine-grained aggregates as elsewhere in the central Vredefort structure (Gibson et al. 2002). Millimeter-sized, highly irregular and cuspate K-feldspar showing the same characteristic microstructure was described by Garde et al. (2012, fig. 5) from a fluidized microbreccia in the Maniitsoq structure. Further examples of similar K-feldspar grains were presented by Garde et al. (2014). The main difference between such K-feldspar occurrences at Vredefort and Maniitsoq is that they are confined to breccia veins within 5 km from the center of the former structure, but are widespread in the latter and have been found up to at least 50 km from its center. In order to verify the branching, cuspate, and poikilitic, millimeter-sized K-feldspar grains at Vredefort and Maniitsoq as shock-metamorphic, distinction from cuspate K-feldspar of endogenic origin is critical. Endogenic melt pseudomorphs of various minerals with characteristic, bifurcating, and locally branching habits and cuspate tips have been described from contact metamorphosed rocks adjacent to mafic intrusions, and cuspate K-feldspar, plagioclase, and quartz anatectic melt pseudomorphs occur in high-grade, migmatitic, and restitic gneisses (Holness and Sawyer 2008). The development of these endogenic melt pseudomorphs is controlled by a balance between supersaturation and nucleation in anatectic melt pockets and requires diffusion over at least the grain size of the melt pseudomorphs themselves. They are therefore restricted to tiny patches and short microveins, which are typically located at triple junctions of their host minerals and rarely exceed 50 μm in thickness (Holness and Sawyer 2008), in contrast to the large and pervasive K-feldspar grains with their numerous inclusions, branches, and cusps described from Vredefort and especially Maniitsoq. Now turning to biotite, quenched shock melts of this mineral were recently described by Ogilvie et al. (2011) in a shock-experimental study of cold and mildly preheated pelitic rocks (25 and 400 °C). On p. 1575 it was stated that “Vesiculated biotite shock melts are markedly mobile and appear to be injected to distances up to 100 μm from sites of generation into fractures and fissures in adjacent, more shock-resistant phases.” Scherstén and Garde (2013, fig. 2) demonstrated similar textures in a sample collected just outside the outer margin of the Finnefjeld domain in the Maniitsoq structure, and showed that shock-melted biotite was injected into fractured plagioclase. However, the biotite shock melt at Maniitsoq was crystallized again as biotite and not quenched as in the experiments. The higher postshock confining pressure at Maniitsoq would have prevented escape of the hydrous component of the biotite melt, and combined with the high ambient postshock temperature this would have allowed renewed crystallization of biotite. At Vredefort, former biotite shock melts have mainly crystallized as fine-grained anhydrous mineral phases (Gibson and Reimold 2008; fig. 19e), but also 1–2 mm large, poikilitic grains of re-formed biotite have been observed in the above mentioned breccia vein (Gibson et al. 2002), as observed at Maniitsoq (Scherstén and Garde 2013). Moving briefly on to plagioclase, Gibson (2002, pp. 63–64) described partial shock melting of sodic plagioclase adjacent to K-feldspar in the central part of the Vredefort dome, leaving an unmelted, more calcic core. Garde et al. (2012) showed a comparable example of partial melting of plagioclase adjacent to K-feldspar, where cataclastic fragments of intermediate plagioclase are set in a matrix of former albitic melt. However, a full understanding of the behavior of plagioclase during shock melting obviously requires further study. We consider it unfortunate that none of these complementary observations of K-feldspar, biotite, and plagioclase and their microstructures at Vredefort, Maniitsoq, and from experimental shock melting were addressed in the letter. The Maniitsoq structure also contains many major, regional features which do not individually constitute compelling evidence of impacting based on any of the generally accepted criteria, but which are exceedingly difficult to explain by endogenic processes especially when considered collectively (Garde et al. 2012). These include the Finnefjeld domain with its complete mechanical destruction of all endogenic lithological features (Garde et al. 2014); the 75 km long, curvilinear, Ni-mineralized Norite belt in the eastern part of the center which is derived from extensive melting of depleted mantle and admixing of felsic continental crust (Kokfelt et al. 2013); the anomalous, boninitic composition of rift-related Palaeoproterozoic dykes in the outcrop area of the Maniitsoq structure (Hall and Hughes 1987); and not least the geographically very extensive, high-temperature hydrothermal alteration which is particularly thorough in the eastern part of the structure. It was no coincidence that Scherstén and Garde (2013) chose a field photograph of one of these, seemingly enigmatic, intensely brecciated and hydrothermally altered orthogneiss-amphibolite outcrops for the front cover of MAPS Vol. 48. No. 8. The letter by Reimold et al. (2014) highlights the need for new insight into the testimony of the shock wave in deep-seated rocks after a giant impact, and there is a wide gap between the information that can be achieved by experiments and what happens under the real-time P–T conditions in the lower crust during and after giant impacts. The observations at Maniitsoq have so far just scratched the surface of this interesting issue. New experimental work might improve our insight into issues like re-formation of shock-melted minerals at high post-shock temperature and pressure, as well as the fate of PDFs in quartz during oscillatory deviatoric stress at high ambient temperature and pressure. The latter might be addressed by subjecting naturally shocked granitic rocks to repeated deformation at high strain rates under conditions where quartz, but not plagioclase, would deform in a crystal-plastic manner. Ongoing and future research targets at Maniitsoq itself include further investigations of shock-melting phenomena, oscillatory deformation, detailed study of hydrothermal alteration and fluid-assisted melting, and geochemical and isotopic modeling of the genesis of the Norite belt and boninitic dykes. We encourage interested scientists to visit the rocks at Maniitsoq themselves. We thank associate editor M. Poelchau for constructive comments and suggestions aiming at improving the documentation and research outlook and making the text more accessible to the general MAPS readership. We also thank MAPS editor-in-chief T. Jull for editorial handling. Dr. Michael Poelchau

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.