High precision radioisotopic dating of terrestrial impact craters

Overview

Establishing the timing of impact crater formation is essential to exploring the relationship between bolide impact and biological evolution, constraining the tempo of planetary surface evolution and understanding the formation of large ore deposits. Unfortunately, precise and accurate impact geochronology can be challenging. Many of the rock products of impact – referred to here as impactites – contain relict, pre-impact phases that may have had their isotopic systematics completely reset during the impact event, only partially reset, or not reset at all. Such inconsistencies can lead to uncertainties about how best to interpret the age significance of specific isotopic dates. Similar issues arise when impactite isotopic systematics are disturbed by subsequent thermal events, such as those related to alteration, plate tectonics, or – especially on airless bodies like the Moon – additional impact events.

This project will date a series of key impact structures from around the world that may, or may not, be related to key events in the extinction and emergence of life on Earth. For example, the Acraman impact crater in Australia is a key target and has proved difficult to geochronologically constrain. Biostratigraphic and chemostratigraphic studies of Australian late Neoproterozoic (Ediacarian) fossil plankton (acritarch) successions reveal a striking relationship between a radical palynofloral change, a short-lived negative excursion in the carbon isotope composition of kerogen, and a debris layer from the Acraman bolide impact event. Establishing a high precision age for such a crater would allow for interrogation of cause-and-effect.

The project aims to use a variety of radioisotopic dating techniques (Ar/Ar, U-Pb and Rb-Sr) to directly determine the age of past impact events. The project builds on recent work the research group has conducted on the Chicxulub and Boltysh impact craters that have temporal associations with the K-Pg mass extinction and eruption of Deccan Traps.

Methodology

Fieldwork to sample materials for high-precision geochronology from various exposed impact sites around the world, including Australia. The Scottish Universities Environmental Research Centre offers a full range of geochronological techniques to support the project in addition to sample characterisation using petrographical and electron microscopes. The student will be immersed in the high precision geochronological unit and have carte blanche access to the U-Pb, Rb-Sr and Ar/Ar dating suites. Time will be spent at St Andrews to geochemically characterise zircon. During the project the student will spend 3 months working directly with Thermo Scientific (CASE Partners) in their Bremen factory to push the limits of noble gas and thermal ionisation mass spectrometry.

The dating techniques to be applied have different thermal sensitivities so in addition to directly dating the impact craters of interest, they will allow for reconstruction of the thermal history of the Earth’s crust. As required, the geochronology can be supplemented by isotope geochemistry (e.g., oxygen isotopes) to further understand the context and geological history of the impact craters.

Links to key papers describing the applications of techniques to impact craters can be found below.

Project Timeline

Year 1

– Literature review

– Hands on experience learning isotope dating techniques

– Fieldwork to sample impact craters

– Characterisation of collected materials for geochronology using the Selfrag selective fragmentation device, petrography including scanning electron microscopy and element mapping. If required, electron microprobes will be used to characterise the chemistry of the materials.

Year 2

– High precision geochronology, analysis of material characterised in year 1. This will be an extremely intensive year analytically with the student working across 3 isotope dating techniques.

– Attendance at national conference.

Year 3

– Complete dating of materials.

– Supplement dating of materials with required isotope geochemistry.

– Data interpretation and development of publications.

– Attendance at international conference.

– Start to draft PhD thesis.

Year 3.5

– Completion of PhD thesis and submission.

Training
& Skills

Training in geological mapping and field assessment of impact craters and impact materials across a range of scales. The student will be training in high precision geochronology with an expectation to establish themselves as an emerging expert in multi-chronometer dating of impact craters. The student will be trained in a range of mass spectrometer techniques: noble gas mass spectrometry, thermal ionisation mass spectrometry and plasma mass spectrometry. This will include experience of clean laboratory chemistry and data handling and interpretation. The student will use a range of programming languages for data interrogation and receive generic skills training (e.g., writing and presenting with impact) at the University of Glasgow and as a part of the IAPETUS DTP.

References & further reading

Pickersgill, A. E. , Mark, D. F. , Lee, M. R. , Kelley, S. P. and Jolley, D. W. (2021) The Boltysh impact structure: An early Danian impact event during recovery from the K-Pg mass extinction. Science Advances, 7(25), eabe6530. (doi: 10.1126/sciadv.abe6530)

Pickersgill, A. , Mark, D., Lee, M. and Osinski, G. (2020) 40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada. Geochimica et Cosmochimica Acta, 274, pp. 317-332. (doi: 10.1016/j.gca.2020.01.025)

Rasmussen, C., Stockli, D. F., Ross, C. H., Pickersgill, A. , Gulick, S. P., Schmieder, M., Christeson, G. L., Wittmann, A., Kring, D. A. and Morgan, J. V. (2019) U-Pb memory behavior in Chicxulub’s peak ring — applying U-Pb depth profiling to shocked zircon. Chemical Geology, 525, pp. 356-367. (doi: 10.1016/j.chemgeo.2019.07.029)

Mark, D.F. , Lindgren, P. and Fallick, A.E. (2014) A high-precision 40Ar/39Ar age for hydrated impact glass from the Dellen impact, Sweden. Geological Society, London, Special Publications, 378(1), pp. 349-366. (doi: 10.1144/SP378.22)

Further Information

To discuss the project further contact: Darren.Mark@glasgow.ac.uk.

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