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

Abstract

Sub microscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occurs in several distinct modes. III one sample magnetite occurs along relict planar deformation features (PDF) 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 disorientated such that they share a sub parallel [101] direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in micro fractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as micro fractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heal may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 mu m with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles.