Striped domains of coarse-grained magnetite observed by X-ray photoemission electron microscopy as a source of the high remanence of granites in the Vredefort dome

Hiroto Kubo, Norihiro Nakamura, Masato Kotsugi, Takuo Ohkochi, Kentaro Terada, Kohei Fukuda

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

The characteristics of a coarse-grained high-remanence magnetite obtained from shocked Vredefort granite were investigatedby X-ray magnetic circular dichroism(XMCD) analysis and X-ray absorption spectroscopy (XAS). The study utilized a spectroscopic photoelectron low-energy electron emission microscope (SPELEEM) and was conducted in the SPring-8 large-synchrotron radiation facility. It is generally believed that the strong and stable bulk remanence of Vredefort granites is due to the presence of minerals that have been strongly magnetized by either an impact-generated magnetic field or terrestrial lightning strikes. Although coarse-grained magnetite is traditionally characterized by weak coercivity and remanence, the specimen used in the present study exhibited high coercivity and an intense remanent magnetization. The presence of hematite lamellae observed on the partially oxidized magnetite specimen indicated an array of striped domains, intensifying a remanence and coercivity. We also conducted XAS and XMCD analyses on a natural lodestone permanent magnet produced by lightning strikes; while maghemite was found to be present, no magnetic domain structures were observed. Considering that the nucleation of hematite lamellae on magnetite/maghemite grains is due to oxidation, we attribute the intense remanent magnetization and magnetic hardening of Vredefort granites to post-impact hydrothermal activity.

Original languageEnglish
Article number31
JournalFrontiers in Earth Science
Volume3
DOIs
Publication statusPublished - 2015 Jun 17

Keywords

  • Corase-grained magnetite
  • Hematite lamellae
  • Lightning
  • Lodestone
  • Post-impact hydorothermal activity
  • Striped domain
  • XMCD

ASJC Scopus subject areas

  • Earth and Planetary Sciences(all)

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