Lightning-induced remanent magnetization-the Vredefort impact structure, South Africa

Abstract

Earlier studies at the large Vredefort impact structure since 1960 have shown that values of natural remanent magnetizations (NRMs) and, hence, Koenigsberger's Q values (ratio of remanent over induced magnetization), for different rock lithologies are elevated compared to the values for similar rock types around theworld. Three origins for the highQvalues have been suggested, namely shock by meteorite impact, enhanced plasma field and lightning strikes.We have studied whether laboratory lightning experiments can produce enhanced NRMs in the Vredefort target rocks. For comparison, we also included rocks from the Johannesburg dome, which is not a meteorite impact site. The results revealed increased NRMs, susceptibility and Q values of the rocks from both Vredefort and Johannesburg domes. Rock magnetic measurements and scanning electron microscope analyses of lightning pulsed and unpulsed samples showed that the lightning included changes in magnetic properties of the rocks.We suggest that in some samples lightning have changed magnetic mineralogy by oxidizingmagnetite to maghemite. Indication of this oxidation came from the low-temperature variation of the remanent magnetization where we observed several hallmarks of maghemitization in samples treated by lightning strikes. Further indications of mineralogical changes include increased Curie points above the magnetite's Curie point (580° C) and appearance of pronounced lower temperature (200-400° C) phases in susceptibility versus temperature curves. These changes are interpreted to indicate partially oxidized magnetite (maghemitization) coupled with grain fragmentations and by thisway grain size reduction. High-temperature hysteresis and REM (= NRM/saturation isothermal remanent magnetization) studies support these conclusions. Our resultswere analogous with the ones for lodestones and protolodestones where partially oxidized magnetite is thought to make magnetization more intense. © The Authors 2013 Published by Oxford University Press on behalf of The Royal Astronomical Society.