Shock metamorphism and hydrothermal alteration of mafic impact ejecta from the Lockne impact structure, Sweden

Abstract

The local geology at Kloxåsen is characterised by ejecta deposits from the 458 Ma Lockne marine impact. The Kloxåsen ejecta are located on a Caledonian parautochthonous unit, approximately 7 km from the centre of the 7.5-km-wide Lockne crater structure. The ejecta were deposited on the seafloor and were covered with seawater immediately after the impact event. Of special interest is a mafic impact breccia within the ejecta, which before the impact was Åsby dolerite that belongs to the Jämtland suite of the 1.25 Ga Central Scandinavian Dolerite Group. The mafic impact breccia occurs mainly as a coherent thin domain within a larger block of granitic breccia, which we interpret as a result of the in situ brecciation of a dolerite sill within granitic bedrock. Shock pressure in the doleritic breccia was low, in the order of 0.4 GPa, constrained by the presence of mechanically twinned clinopyroxene. Low shock pressure and brecciation corresponds well to the spall zone of an impact crater, where ejecta originate from. Whereas spalled ejecta can also show signs of having been exposed to high shock pressures, including shocked quartz, evidence for this was not found in the Kloxåsen ejecta. The breccia has been hydrothermally altered, but the ejecta are too far removed from the crater to have been affected by hydrothermal circulation in relation to Lockne’s impact event. Fluid inclusion analyses suggest that most of the alteration happened later, during the Caledonian orogeny. Geochemical analyses reflect observed mineral alterations well, such as serpentinisation of olivine.

Keywords:

• Mafic impact ejecta
• Lockne impact structure
• shock metamorphism
• hydrothermal alteration

Acknowledgements

Most of this work was conducted as A.S.’s Bachelor thesis, which was mainly funded with a grant by the Paneth Meteorite Trust of the Royal Astronomical Society in London and the Swedish National Space Board under Contract number 59/80. This work is also funded by the Swedish National Space Board under Contract number 83/10, Swedish Research Council under grant number 2012-4364 and the Danish National Research Foundation under grant number DNRF53. Gabrielle Stockmann, Irene Melero Asensio, Jens Ormö, Reinhard Greiling and Joakim Mansfeld made the field work particularly enjoyable. Peter Chung was very helpful during SEM analysis in Glasgow. John Gilleece in Glasgow, Ali Firoozan in Gothenburg and Kjell Helge in Hunnebostrand made thin sections and fluid inclusion wafers. We are thankful for helpful suggestions by two reviewers, which helped to improve the manuscript significantly.