Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa |
| |
Authors: | Marthinus Cloete Rodger J Hart Herbert K Schmid Martyn Drury Chris M Demanet K Vijaya Sankar |
| |
Affiliation: | (1) Council for Geoscience, Private Bag X112, Pretoria, RSA, ZA;(2) Schonland Research Centre, Wits University, P.O. Box 3, Johannesburg 2050, ZA;(3) Division of Materials Science and Technology, CSIR, P.O. Box 395, Pretoria 0001, ZA;(4) Department of Geology, Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508TA Utrecht, Netherlands, NL;(5) Department of Physics, University of Transkei, Private Bag X1, Umtata 5100, South Africa, ZA |
| |
Abstract: | Submicroscopic 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 occur in several distinct modes. In one sample magnetite occurs along 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> 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 microfractures (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 microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat 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 μm 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.
Received: 16 November 1998 / Accepted: 17 May 1999 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|