A TEM and EELS study of carbon in a melt fragment from the Gardnos impact structure

1Paula Lindgren,2Lydia Hallis,3Fredrik S. Hage,2Martin R. Lee,4John Parnell,1Anders Plan,5Alistair Doye,5Ian MacLaren
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13381]
1Department of Geology, Lund University, S€olvegatan 12, 223 62 Lund, Sweden
2School of Geographical & Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
3SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK
4School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK
5School of Physics & Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
Published by arrangement with John Wiley & Sons

A carbon-rich melt fragment from the Gardnos impact structure has been studied for a better understanding of the preservation and structural form(s) of carbon that have been processed by impact melting. The carbon was analyzed in situ in its original petrographic context within the melt fragment, using high-resolution techniques including focused ion beam-transmission electron microscopy and electron energy loss spectroscopy. Results show that the carbon is largely uniform and has a nanocrystalline grain size. The Gardnos carbon has a graphitic structure but with a large c/a ratio indicating disorder. The disorder could be a result of rapid heating to high temperatures during impact, followed by rapid cooling, with not enough time to crystallize into highly ordered graphite. However, temperature distribution during impact is extremely heterogenous, and the disordered Gardnos carbon could also represent material that avoided extreme temperatures, and thus, it was preserved. Understanding the structure of carbon during terrestrial impacts is important to help determine if the history of carbon within extraterrestrial samples is impact related. Furthermore, the degree of preservation of carbon during impact is key for locating and detecting organic compounds in extraterrestrial samples. This example from Gardnos, together with previous studies, shows that not all carbon is lost to oxidation during impact but that impact melting can encapsulate and preserve carbon where it is available.

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