Maniitsoq structure

The Maniitsoq structure is a proposed 3 billion-year-old (3 Ga) impact structure located in the Akia terrane of the North Atlantic Craton,^[1][2] centred about 55 km (34 mi) south-east of the town of Maniitsoq, Greenland, at 65°15′N 51°50′W / 65.250°N 51.833°W / 65.250; -51.833 (Maniitsoq). Its origin has been debated since it was first proposed as an impact structure in 2012.^[1] The Maniitsoq structure is not recognised as an impact structure by the Earth Impact Database.^[3]

The proposal was criticised for not meeting established criteria for recognising impact craters.^[2][4] Subsequent studies in the region have demonstrated that there is no evidence for an impact structure, and a number of observations directly contradict the earlier impact structure proposals.^[5][6][7][8]

In support of the proposal, a study published in 2023,^[9] used electron microscopy to examine zircon grains from seven sites, including the Maniitsoq structure. The study found distinctive shock-induced planar microstructures in the zircon grains from the four recognized impact structure, as well as in the Maniitsoq structure. These microstructures were not found in grains from the two non-impact tectonic deformation structures.

Garde et al.^[1] suggested the presence of a ~100 kilometres (62 mi) scale impact structure, formed by the impact of a large comet or meteorite, in the Maniitsoq region. They argued that consensus accepted diagnostic criteria for recognising impacts should be relaxed when searching for particularly large, ancient, and eroded impacts, and instead suggested the presence of an impact structure on the basis of the following observations:

1. the presence of an irregular aeromagnetic anomaly;
2. curved ~100 km scale deformation patterns;
3. intense fracturing;
4. sheets of crushed rock without the presence of faults;
5. a 35 by 50 square kilometres (14 sq mi × 19 sq mi) central domain of homogenised rocks (the Finnefjeld Orthogneiss Complex);^[10]
6. remelting of rocks around the central domain;
7. formation of breccias;
8. proposed evidence of direct K-feldspar melting;^[11]
9. planar elements within minerals;
10. presence of shear zones;
11. presence of ultramafic sills (the Maniitsoq Norite Belt);^[12]
12. proposed widespread hydrothermal alteration;
13. a coincidence of a zircon U-Pb ages at approximately 2975 million years ago (Ma). The impact was argued to post-date the end of deformation in the Maniitsoq region.^[1] The age was subsequently refined to 3000.9 ± 1.9 Ma based on mean age of five orthogneiss samples suggested to represent rocks melted and hydrothermally altered by the impact.^[13]

The proposal was criticised by Reimold et al.^[4] for devising new criteria for recognising an impact, because it failed to meet existing criteria. Furthermore, they argued that the structure was not circular, that there was no evidence for shock metamorphism, and no geochemical evidence for an impact.^[2] In particular, they demonstrated that Garde et al. had mistaken features commonly found in deformed and metamorphosed terranes, such as migmatites and inclusion trails in quartz, for shock features, such as microbreccias and planar deformation features.^[2][4]

Subsequent studies in the Maniitsoq region demonstrated that deformation in the region continued after the proposed impact age, with major metamorphic and deformation events at ~2.86–2.70 Ga ^[5] and ~2.55 Ga.^[14][15] Extensive deformation was noted both near the proposed impact centre ^[5] and in ultramafic rocks previously suggested to be post-tectonic.^[8][16] Kirkland et al.^[5] noted that it was difficult to reconcile the preservation of a circular impact structure and other proposed impact related features with the severe deformation that followed, and instead interpreted the 'impact' features as the result of multiple phases of high-grade metamorphism and partial melting.

Further zircon U-Pb dating also contradicts an impact model. The ages of rocks interpreted as impact melts within the impact structure ^[13] are indistinguishable from the ages of the unaffected rocks from outside the impact structure.^[6] This requires that the impact coincidentally occurred at the same time as major (non impact-related) crustal formation in the region, which Gardiner et al. consider unlikely.^[6] Furthermore, Gardiner et al. note the presence of a second homogeneous body of orthogneiss further east within the Akia terrane, the Taserssuaq Orthogneiss Complex, which formed at 2982 Ma and contains homogeneous gneisses and magnetic anomalies that are very similar to the Finnefjeld Orthogneiss Complex, interpreted to be the centre of the impact structure. This orthogneiss complex is too young to have formed in response to the proposed impact, and demonstrates that similar orthogneiss complexes and magnetic anomalies can be generated without an impact event.^[6] Dating of metamorphic zircon and rocks formed during high temperature metamorphism at ~3 Ga, indicate that the metamorphic event lasted for >40 million years, which is too long to have been caused by a single impact.^[7][8] Instead, the metamorphism and deformation is better explained by endogenic (terrestrial) processes, such as stagnant lid processes ^[7] or an ultra-hot orogenic event.^[8] Finally, new dating of the ultramafic intrusions of the Maniitsoq Norite Belt shows that these formed at 3013 Ma, and are therefore too old to have been generated by the impact event.^{[6][8][16][17]}

Further evidence against an impact origin comes from analyses of oxygen isotopes within the ultramafic intrusions of the Maniitsoq Norite Belt,^[8] which show no evidence of the widespread hydrothermal alteration asserted to have been caused by the impact.^[13] This is supported by geochemical and petrographical observations from the same rocks, which show that most rocks were largely dry, with only limited local hydrothermal alteration occurring adjacent to intrusions of much younger granitic rocks.^[8]

Due to the reasons outlined above, the Maniitsoq Structure is widely believed not to have formed due to a giant impact,^[3] and is instead interpreted to reflect terrestrial tectonic processes.^{[2][4][5][6][7][8]}

In 2023, a team led by Adam Garde reported findings^[9] that supports the theory that an impact was the cause of the Maniitsoq structure. Their study identified features in zircon grains from confirmed impact structures that distinguish them from grains found in non-impact tectonic deformation seismites. The particular features they found in the confirmed impact structures were also found in the Maniitsoq structure and were not found in the non-impact structures.

In an impact structure, the shock wave generated by the impact causes the crystal lattice of quartz to break, forming planes of glass within the crystal. These are known as shock lamellae. Shock lamellae are well known in quartz, where they are a typical indication of impact craters. Deep inside the Earth's crust, however, quartz is soft and the shock lamellae are therefore difficult to recognize or are completely destroyed by subsequent heat (annealing) and deformation. Zircon is a much more resistant mineral that is better suited to preserving possible shock structures. These features are still recognizable in zircon grains from older structures where post-cratering deformation, heat and recrystallisation have occurred.^{[9](Introduction; Discussion: Interpretation of CPs as PDFs)}

The study found that shocked zircons contain two principally different types of planar microstructures, only one of which is diagnostic of impact. The team calls the first type of microstructure contiguous planar microstructures (CPs). These consist of closely spaced and contiguous planar structures. These planes form in sets that are essentially parallel to each other and spaced about 1 µm apart. Adam Garde adds: "They can be altered by water penetrating from the surrounding Earth's crust, causing nanometer-sized bubbles to form along the original shock lamellae. Under an electron microscope, these bubbles look like tiny beads on a string."^{[18][9](A revised observation-based nomenclature)}

The second type of microstructures they call planar fractures (PFs). These are open, widely and irregularly spaced fissures spaced around 5–10 µm that are caused by intense seismic shaking. Where both CPs and PFs are present, the PFs are always younger than the CPs and re-use CP orientations, commonly in a stepwise pattern. Because PFs are found in zircons from non-impact seismites, they are not impact-diagnostic. Various other studies have often lumped CPs and PFs together under the term planar deformation features (PDFs).^{[9](A revised observation-based nomenclature; Conclusions:Planar fractures (PF))}

The team used electron microscopy to examine exterior and interior planar microstructures and twins in numerous zircon grains from four large impact structures and from two non-impact seismites. They then studied zircon grains from the Maniitsoq structure.

The impact structures studied were the young, upper-crustal, well-preserved impact structures of Manicouagan in Canada and Rochechouart in France, and the much older, geologically reworked Sudbury Basin in Ontario, Canada, and the more deeply exhumed Vredefort impact structure in South Africa.

The non-impact seismites studied were the Caledonian deformation zone in SW Norway (also known as the Svarthumlevatnet metagabbro), where non-impact exterior planar microstructures in zircons had been previously found, and the Alpine seismite in northern Italy at Premosello along the Insubric line in the Ivrea–Verbano zone, where non-impact interior planar fractures had been previously found.

They finally studied planar microstructures in zircons from several parts of the Maniitsoq structure and compared them with those from the other six sites.

They used the following methods of scanning electron microscopy:^{[9](Samples And Analytical Methods)}

• backscattered electrons detection (BSE)
• Electron backscatter diffraction (EBSD)
• transmission electron microscopy (TEM)
• cathodoluminescence (CL)

Their observations of planar microstructures in zircons from the Manicouagan, Rochechouart, Vredefort and Sudbury impact structures can be summarised as follows. Exterior, closely spaced, contiguous planar microstructures are readily distinguished from open, texturally younger planar fractures. The first type of planar microstructures forms up to several sets with different orientations, and a spacing of ~1 µm; their actual thickness is below the scanning electron microscopy resolution that they could acquire. They documented that closely spaced interior planar microstructures in Manicouagan and Rochechouart zircons directly correspond to the external ones in terms of positions, spacing and orientations. In BSE images, they are less persistent than their exterior counterparts, decorated with abundant tiny pores mostly much smaller than 100 nm across, and appear to have been partly annealed. In some grains from Rochechouart and Sudbury, the exterior planar microstructures are smoothed by thin overgrowths and thereby gradually changed into small and locally intermittent grooves, erasing the distinction between the two types of planar microstructures recognized in other grains from the same impact structures. These observations collectively show that post-impact annealing of planar microstructures in shocked zircon is a common phenomenon.^{[9](Summary of microstructures in zircons from confirmed impact structures)}

They reexamined a seismite lent by H. Austrheim and F. Corfu from their 2009 study,^[19] and examined 186 newly extracted zircon grains from the deformation zone. In one grain, they observed a small area of exterior, closely spaced subplanar and curviplanar fractures, but without any of the distinctive characteristics of the tight, contiguous and strictly planar microstructures spaced at ~1 µm that occur in shocked zircons.^{[9](Summary of microstructures in zircons from Caledonian and Alpine seismites)}

They examined 83 zircon grains and found rare, subplanar to planar interior structures in zircons from this locality with variable and irregular misorientation. Exterior planar fractures in some grains have orientations that appear to match interior misorientations, but they are rare and most of them do not transect entire grains.^{[9](Summary of microstructures in zircons from Caledonian and Alpine seismites)}

They examined ~2600 zircon grains in 24 samples from the inner part of the Maniitsoq structure. Samples from several different parts of the Maniitsoq structure contained numerous zircons with planar microstructures comprising both exterior, closely spaced, contiguous planar microstructures and open planar fractures, and a few remnants of closely spaced interior contiguous planar microstructures. The planar microstructures closely resembled those in the confirmed impact structures studied. As also observed at Rochechouart, Vredefort and Sudbury, many of the planar microstructures of the Maniitsoq zircons appeared to have been variably affected by annealing. In particular, grain surfaces may have been smoothed. Also, the interiors of some grains were very homogeneous without preservation of growth zonation or planar fractures. The interiors of a few other grains contained one or two sets of narrow, contiguous lamellae a few micrometers wide with deformational boundaries parallel to the exterior contiguous planar microstructures.^{[9](Summary of microstructures in zircons from the Maniitsoq structure)}

In summary, the contiguous planar microstructures in the Maniitsoq zircons closely resembled those found in shock-metamorphosed zircons from major confirmed impact structures, and they were very different from the only known examples of rare, individual subplanar to planar fractures in non-impact zircons, where contiguous planar microstructures were completely absent.^{[9](Planar microstructures in Maniitsoq zircons)}

The team concluded that shock-induced microstructures in zircon are prone to heat annealing, recrystallisation and exterior overgrowth. Interior contiguous planar microstructures (CPs) appear to be the first impact-induced planar microstructures to disappear, whereas exterior CPs are probably the most robust of the diagnostic shock-induced microstructures in zircon found in this study. The different degrees of preservation of the shock-induced microstructures in the Manicouagan, Rochechouart, Yarrabubba, Vredefort and Sudbury impact structures suggest that annealing is primarily governed by depth of burial and geological reworking rather than by age alone.^{[9](Conclusions: Annealing)}

• List of possible impact structures on Earth