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Understanding the role of impact cratering in Earth's evolution through state-of-the-art geochronology

Periodic Reporting for period 1 - Crater Chron (Understanding the role of impact cratering in Earth's evolution through state-of-the-art geochronology)

Reporting period: 2018-07-01 to 2020-06-30

Impact cratering – the collision of asteroids or comets with planetary bodies such as the Moon and Earth – is a fundamental geological process that has played a primary role in the evolution of the inner Solar System. However, a major impediment to fully understanding the role of impacts in Earth’s history has been the poor geochronological constraints on most of Earth’s impact structures. Of the approximately 200 known impact structures on Earth – that range from less than 1 km to over 300 km in diameter – only about 15% are accurately and precisely dated. Simply put, we don’t know when most of Earth’s impact structures formed.
The overall objectives of this project were to (i) take advantage of recent advances in our understanding of how rocks and minerals record impact ages and the analytical techniques used to determine such ages to refine existing, and develop new, techniques for dating impact structures; (ii) apply these newly developed protocols to dating impact structures whose precise formation age may have broad scientific significance, such as impact structures that have been proposed as having contributed to mass extinctions.
Impacts hold a rare position in science in that they capture the imagination of the general public while also being scientifically important. The project, and our broad understanding of the role of impacts in Earth’s history, is important to society given the proposed role that impacts have played in, for example, the development of life on Earth (both its origin and its fate in mass extinctions), the onset of tectonic processes, and the formation of economic mineral deposits.
The project focused on dating impact structures with a mineral that is almost ubiquitous in the Earth’s crust – zircon. Zircon can form when molten rock cools and solidifies, and it can thus be used to date this process (see image on left in attached figure). Zircon can also be affected by a hypervelocity impact and this project explored how disturbed zircon grains can record the age of the impact event (see image on right in attached figure). A standard protocol for dating impact structures with state-of-the-art analysis of so-called “shocked zircon” was developed at the host institution. This was applied to an impact structure that had previous been dating using another technique (Ar/Ar analysis), offering one of the first insights into how the different techniques complement each other and record the cooling of an impact structure over hundreds of thousands of years after the initial impact event. After demonstrating the utility of the method in dating impact structures, it was applied to other impact structures in collaboration with researchers around the world. Some of these ages have already been published in peer-reviewed journals with several more publications expected.
The first strand of the project was to reproduce and complement an existing age for an impact structure with an age determined using state-of-the-art techniques in geochronology of zircon. This was done for the Lappajärvi impact structure, Finland, which has a diameter of approximately 23 km. Sub-millimeter-scale zircon grains were separated from rocks from the impact structure. The grains were imaged and characterized using state-of-the-art scanning electron microscopy (SEM) techniques and were then dated using uranium–lead radiometric dating. The new age obtained for this impact structure was actually slightly older than the previously accepted age of the structure and offered insights into the cooling of impact structures and the accurate dating of impact events. The results were published in Geochimica et Cosmochimica Acta.
After demonstrating the utility of the newly established protocol, the next strand saw application of the method to another impact structure in Finland, the Sääksjärvi impact structure. The work led to a new age for the impact structure, which was tens to hundreds of millions of years older than previous estimates for the age of the structure. This resolved issues with the previously suggested younger ages and demonstrated that dating small impact structures may help constrain the timing of other geological processes in the Earth’s crust. The results were published in the Journal of the Geological Society, London.
In addition to the mineral zircon, the project also involved work on the mineral apatite. Apatite and related minerals are much more common than zircon in rocks from the Moon and Mars and so understanding how apatite responds to the impact events is important for dating impacts and other events on these planetary bodies. Using the same imaging and characterization techniques earlier applied to zircon, it was shown that a hypervelocity impact can induce recrystallization and chemical changes in apatite. These findings were published in Geology.
Exploitation of results: The routines for dating impact structures with zircon that were developed in the course of the project are now being applied to impact structures around the world in collaboration with researchers globally. The researcher has also been working with a group of researchers who focus on the evolution of the Moon and Mars, and the insights gained in this project have also benefitted the interpretation of data from lunar and Martian samples. Furthermore, that the routines developed during the project have been published a peer-reviewed articles means that other researchers can now apply, and build on, this methodology in other laboratories, further expanding the impact of the project.
Dissemination of results: At the time of final report writing, results from the project had been published in four peer-reviewed publications. It is envisaged that at least four further publications will come from the project. The results were also disseminated to the scientific community at conferences, including the Annual Meeting of the Meteoritical Society (Moscow, Russia; 2018), the Lunar and Planetary Science Conference (Houston, Texas, USA; 2019) and the Large Meteorite Impacts Conference (Brasilia, Brazil; 2019). Invited lectures were given at a range of institutions. Results were disseminated to the general public through outreach events including the “Borrow a researcher” event run by the host institution in association with European Researchers’ Night.
The project has successfully pushed the state-of-the-art forward by testing and demonstrating an efficient methodology for dating impact structures on Earth with modern analytical techniques. This is now being applied to a range of impact structures with published results already advancing our understanding of the cooling of impact structures and the preservation of impact ages, as well as having wider implications for regional geology. A number of further publications are in preparation and will, for example, shed light on the timing of the largest impact events over the past several hundred million years and how these relate to mass extinctions.
The primary direct impacts of the project will be to have (i) introduced a simplified, and to some extent standardized, protocol for dating impacts that will have implications long after the project, (ii) demonstrate the utility of impact ages to other aspects of geology (that is, understanding and dating regional geological events), and (iii) shed light on the relationship between mass extinctions and the largest known impact structures on Earth.
The zircon grain on the right records the age of the impact event that caused it to recrystallize.