Abstract
The characteristics of a coarse-grained high-remanence magnetite obtained from shocked Vredefort granite were investigated by 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 high-temperature oxidation, we attribute the intense remanent magnetization and magnetic hardening of Vredefort granites to post-impact hydrothermal activity.
Highlights
The Vredefort dome is known as the largest and oldest (2023 ± 4 Ma) terrestrial impact structure
Our results showed that coarse-grained magnetite in Vredefort granites contained an array of hematite lamellae, resulting from a partial oxidation
We found that our magnetite contained very dense hematite lamellae that run horizontally and vertically, and this suggests the occurrence of secondary high-temperature metamorphism
Summary
The Vredefort dome is known as the largest and oldest (2023 ± 4 Ma) terrestrial impact structure. Previous researchers have hypothesized that the origin of these unique magnetic remanence properties is associated with very intense impact-generated small-wavelength magnetic fields that produce strong remanence, and randomize the directions of the remanence (e.g., Hart et al, 1995; Cloete et al, 1999; Carporzen et al, 2005) This hypothesis has been rejected based on the observation of terrestrial lightning remagnetization in studies that utilized 10m borehole paleomagnetic measurements (Carporzen et al, 2012) and artificial lightning experiments (Salminen et al, 2013). Such fluids enhance the remanence of primary low-coercivity coarse-grained magnetite by high-temperature oxidation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
