Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by mea ns of micro-analytical techniques with high spatial resolution such as elec tron diffraction, orientation contrast imagery and magnetic force microscop y. The opaque particles have been identified as nano- to micro-sized magnet ite that occur in several distinct modes. III one sample magnetite occurs a long relict planar deformation features (PDFs) in quartz generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel [101] dire ction. The magnetite groups of all measured quartz grains thus appear to ha ve a crystallographic preferred orientation in space. In a second sample, o rientations of magnetite particles have been measured in microfractures (no n-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientatio ns of magnetite particles are diverse, with only a minor portion having a p referred orientation. Scanning electron microscopy shows that magnetite alo ng the relict PDFs is invariably associated with other microcrystalline pha ses such as quartz, K-feldspar and biotite. Petrographic observations sugge st that these microcrystalline phases crystallized from locally formed micr o-melts that intruded zones of weakness such as microfractures and PDFs sho rtly after the shock event. The extremely narrow widths of the PDFs suggest that heal may have dissipated rapidly resulting in melts crystallizing rel atively close to where they were generated. Magnetic force microscopy confi rms the presence of magnetic particles along PDFs. The smallest particles, <5 mu m with high aspect ratios 15:1 usually exhibit intense, uniform magne tic signals characteristic of single-domain magnetite. Consistent offsets b etween attractive and repulsive magnetic signals of individual single-domai n particles suggest consistent directions of magnetization for a large prop ortion of particles.