By looking at something very small, a group of researchers have found the evidence for something very big. To others, it may seem strange that it has been difficult for geologists to find huge old meteorite craters, but it is nevertheless the case. A myriad of large and small meteor impacts were crucial to the formation of the early Earth, but the Earth is an active and dynamic planet, and therefore it is something of a treasure hunt for today’s geologists to locate and, not least, verify old, worn-out meteor craters. But perhaps we have also been looking for the wrong signs in the wrong places.
In a new study, which has just been published in Journal of Petrology , Adam Garde, emeritus researcher at GEUS, together with an international research group consisting of Leif Johansson, Nynke Keulen, Anja Schreiber and Richard Wirth, present some exciting results, which confirm a theory that the 3-billion-year-old Maniitsoq structure in West Greenland is the remains of a meteorite crater – and the results can make a big difference in the geoscientific method presently used to identify old crater structures deep in the Earth’s crust.
The origin and nature of the Maniitsoq structure in West Greenland, which is around three billion years old, has been the subject of scientific debate for years. Today, the almost round structure extends over 100 kilometers in diameter, but the original crater was probably about 500 kilometers across. Adam Garde from the Geological Survey of Denmark (GEUS) believes the crater to be the remains of a meteorite impact based on mineral analysis and very extensive cataclasis at an originally very deep part of the Earth's crust. This would make it the largest and oldest known impact crater on Earth. In contrast, other researchers posit that it could also be a tectonic structure, i.e. created by movement and earthquakes along the tectonic plates that make up the Earth's crust.
”Look at the outside of the crystals first”
In 2017, Adam Garde participated in a geological conference in Iceland. Here, he met a colleague who gave him a tip that would prove to be extremely useful for his further work with the zircons from the Maniitsoq structure. "Look first at the outside of the crystals, not the inside. The shock structures are best preserved on the outside,” said Balz Kamber, Queensland University of Technology (QUT). And so, that is what Adam Garde and his colleagues did.
The researchers have examined more than 3400 zircon crystals from four large crater structures, which are confirmed internationally as meteorite impacts. For comparison, they also examined zircons from two intense ancient earthquake zones in Norway and Italy without meteorite impact, and not least zircons from the Maniitsoq structure in Greenland.
“We realised something very important. We recognized two completely different kinds of microstructures on top of each other in the zircons we examined. The primary structure consists of very closely spaced, coherent microplanes. These are so-called shock lamellae, and they are formed by the shock wave at the time of the impact itself. Much later, the shock lamellae can be affected by influx of water inside the crust of the Earth. This water ends up as nanometre-scale bubbles along the original shock lamellae. In an electron microscope, the altered original shock lamellae now look like tiny pearls on a string,” explains Adam Garde.
Illustration: Click the illustration to open a bigger version . Shock lamellae: The extreme compression and relaxation of the shock wave induce instantaneous melting and freezing along certain crystal planes. (Adam garde, GEUS).
The second kind of planar structures in shocked zircons are open fissures or joints, which are formed by intense seismic shaking caused by the shock wave. These kinds of open fissures can also form, although rarely, in intense earthquake zones, and therefore cannot be used to identify a meteorite crater.
Shock lamellae are particularly well known in quartz, where they are the most common tool for reliably identifying a meteorite crater. However, deep down in the crust, quartz is soft, and the shock lamellae can therefore easily be altered, deformed and difficult to recognise, or they are completely gone due to subsequent alteration and further deformation. Zircon is a much more robust mineral that can better preserve their shock structures and be used to verify an extraterrestrial shock that reached deep into the Earth's crust.
Illustration: Click the illustration to open a bigger version . Shock effects in the mineral zircon – a micro-diagnostic tool. (Adam garde, GEUS).
The group has also examined many zircons in the vicinity of the crater, and as expected without finding any shock structures in them. However, this local check-up is not included in the new article.
Slow but meticulous research
It has taken several years and required many samples and high-tech equipment to be able to present the new results. Most of the investigations were performed with the scanning electron microscope at the Earth Science department, Lund University in Sweden, whereas the most detailed investigations at nanometre scale have been carried out by Richard Wirth and Anja Schreiber using transmission electron microscopy at the German GeoForschungsZentrum in Potsdam, thanks to funding from the EU research programme EXCITE. Here, ultra-thin slices of the zircons have been taken out and instead of light rays, electrons have been sent through them, which has made it possible for the researchers be able to see the tiny details in the shock lamellae.
“This is slow and thorough research. The results have been a long time in the making, partly because we have also looked seriously into alternative interpretations, but I expect that our investigations will leave their mark for a long time to come,” says Adam Garde.
Adam Garde refers to two things: one, Greenland can again, based on geoscience, argue that the world's largest and oldest known meteor crater structure is located in West Greenland – a place that Adam Garde predicts will be of significant geological interest when it comes to understanding the Earth’s early development and the formation of the Earth's crust. And two, the geosciences have acquired a refined and safe tool for the identification of very old and deeply eroded craters.