Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars

Abstract

Magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters Hellas and Argyre than over surrounding regions. The reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact, in the absence of a significant ambient magnetic field. Lower than average magnetic field intensities are also observed above the Vredefort meteorite crater in South Africa, yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on Earth. We find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented, with vector directions changing over centimetre length scales. Moreover, the magnetite grains contributing to the magnetic remanence crystallized during impact, which directly relates the randomization and intensification to the impact event. The strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater. Seen from high altitudes, as for martian craters, the magnetic field appears much lower than that of neighbouring terranes, implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet's internally generated magnetic field at the time of impact.

Electronic and magnetic structures of the postperovskite-type Fe 2 O 3 and implications for planetary magnetic records and deep interiors

“…For example, magnetic fields can be generated during a shock event, providing a possible explanation for high magnetic remanence found in young lunar glasses (14). In addition, a recent study suggested that randomization of paleomagnetic records by impactgenerated fields may be responsible for the weak magnetic field intensity observed at the impact basins of Mars (15). As shown for pyrrhotite (16), impacts can demagnetize minerals, providing an alternative interpretation for the weak fields at the impact basins.…”

Acquisition of shock remanent magnetization for demagnetized samples in a weak magnetic field (7 μT) by shock pressures 5–20 GPa without plasma‐induced magnetization

“…It may be reasonable due to much slower projectile velocity (only 0.8 km/s) in this study compared with much faster velocity (5.2 to 6.0 km/s) as Crawford and Schultz (1991) claimed the PIRM. Carporzen et al (2005) found strongly magnetized and randomly oriented directions changing over centimeter length scales at the Vredefort meteorite crater in South Africa. They attributed the unusual magnetization to PIRM.…”

The Origin of Mercury’s Internal Magnetic Field

“…The largest volcanic edifices (Tharsis, Elysium, Olympus) as well as the largest impact craters (Hellas, Argyre, Isidis) are devoid of significant magnetic field at satellite altitude, which is generally attributed to local demagnetization of an otherwise magnetized crust. Thermal demagnetization due to volcanoes (Langlais et al 2004) and impacts as well as impact excavations are likely explanations (Artemieva et al 2005;Carporzen et al 2005).…”