Changes in thermomagnetic and X-ray diffraction properties of Asby dolerite with distance from the Lockne impact structure, Sweden

Abstract

The Lockne impact structure in Sweden formed due to a meteorite impact into the Proterozoic basement rocks in the Ordovician. The dykes of Asby dolerite are part of the target basement and are located outside the crater rim. We investigated the variation in thermomagnetic properties and X-ray diffraction (XRD) spectra of Asby dolerite with distance from the crater centre to find evidence of a potential low shock overprint. Ti-poor, (Ti)-magnetite with dominantly pseudo-single domain behaviour is the main magnetic carrier in the studied samples. Towards the centre of the crater, the Verwey transition temperature (T_v) and Curie temperature (T_c) decrease, whereas alteration indices increase systematically. Higher alteration indices and lower T_v near the crater centre may be owed to more intense fracturing of the silicate minerals which facilitated low-temperature oxidation of (Ti)-magnetite and alteration of mafic dykes by improving fluid circulation in a network of fractures in the silicate minerals. Higher deformation near the crater centre is supported by our XRD results, as the peak position (2θ value) of plagioclase (040), pyroxene $(\overline{3}11$) and magnetite (311) peaks decrease, while the width of the plagioclase (040) and pyroxene $(\overline{3}11$) peaks increase towards the centre. This study shows that changes in thermomagnetic properties and lattice parameters of magnetite are a powerful tool to characterise low shock pressure deformation in impacted target rocks.