Coarse-grained magnetites in biotite as a possible stable remanence-carrying phase in Vredefort granites

Norihiro Nakamura, Kensaku Okuno, Minoru Uehara, Tetsuya Ozawa, Lisa Tatsumi-Petrocholis, Michael Fuller

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6 Citations (Scopus)


The Archean granites of the Vredefort impact structure show a high intensity of natural remanent magnetization (NRM) and a random dispersion of directions of high-coercivity components on the centimeter scale. It has been suggested that this anomalous remanence is carried by rod-shaped single-domain (SD) magnetites along planar deformation features (PDFs) in shocked quartz produced as a consequence of the impact event. To determine the carriers of this NRM, we conducted surface magnetic field observations using scanning magneto-impedance (MI) magnetic microscopy during stepwise alternating field (AF) demagnetization over a 1-mm-thick slice of Vredefort granite. We found that the stable component after demagnetization gives rise to just three strong magnetic anomalies. Progressive thinning of the scanned section and micro-Raman spectroscopy revealed that the source of these magnetic anomalies, the highly coercive remanence-carrying mineral, is an assemblage of relatively coarse-grained (1-200 μm) magnetite in biotite, not single-domain magnetite embedded along PDF lamella.

Original languageEnglish
Pages (from-to)165-172
Number of pages8
JournalSpecial Paper of the Geological Society of America
Publication statusPublished - 2010


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