Lightning remagnetization of the Vredefort impact crater: No evidence for impact‐generated magnetic fields

Abstract

[1] The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact-generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact-generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single-domain magnetite found along shock-generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact-generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact-generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos.

Preservation and detectability of shock‐induced magnetization

“…While many attempts have been made to identify SRM in natural samples [ Robertson , ; Halls , ; Jackson and Van der Voo , ; Fuller and Cisowski , ; Iseri et al ., ; Schmidt and Williams , ; Pesonen et al ., ; Carporzen and Gilder , ; Elbra et al ., ; Kontny et al ., ; Louzada et al ., ; Raiskila et al ., ; Carporzen et al ., ], reports of confirmed SRM in studied impact craters and extraterrestrial samples are rare to nonexistent. A key question is why SRM has not yet been conclusively identified.…”

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

“…By 10-m borehole paleomagnetic measurements, Carporzen et al (2012) showed that terrestrial lightning strike was the cause of the unusual magnetic rock properties of Vredefort granites. They also found that the natural remanence directions and their 0.55-m magnetic anomaly vectors mostly lie along a great vertical circle and are consistent with the circular field expected for a current flowing horizontally in the northeastern direction.…”

“…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 not only produce strong remanence, but also 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 10-m borehole paleomagnetic measurements (Carporzen et al, 2012) and artificial lightning experiments (Salminen et al, 2013). Salminen et al (2013) found that lightning-induced enhancement of remanence is due to the secondary production of partially oxidized magnetite, implying the presence of postimpact alteration.…”

Low temperature magnetic properties of the Late Archean Boolgeeda iron formation (Hamersley Group, Western Australia): environmental implications

“…Thus, TCDP-1 samples do not support the interpretation of a shock related origin of the two magnetite populations. Alternatively, lightninginduced partial melting and recrystallization of magnetite has been advocated as a potential mechanism for the unusual rock magnetic properties recorded in the Vredefort rocks (Carporzen et al, 2012). As the Boolgeeda section was formed below the storm-wave base and shows no sign of subaerial exposure, it is difficult to envision a 2.4 Ga lightning strike as a possible cause of our peculiar rock magnetite property.…”