Impact Structures: How post-impact thermal conditions influence magnetic behavior

Large scale meteorite impact events are some of the most catastrophic and instantaneous geological processes in nature. These events release high amounts of energy, and generate pressures which will vaporize, melt and metamorphose the target rock. Large impact events create impact structures with very characteristic features, one of which being intense magnetic anomalies. These anomalies are caused by a contrast of the total magnetization in the shocked target, newly formed impactites (that is, rocks formed by, or related to the impact event) and the regional background magnetization. The total magnetization (Mtot) is the sum of natural remanent magnetization (NRM) and induced magnetization (κ*B, κ is magnetic susceptibility and B is Earth’s magnetic field). Shock deformation causes a sharp loss of up to 90% of NRM and κ in the magnetite-bearing target. The demagnetization of magnetite is caused by the shock-induced physical deformation, such as brittle grain fracturing and fragmentation, and ductile crystal lattice defects. Physical defects also enhance magnetic domain wall-pinning, which decreases the apparent domain state of magnetite, from originally multi-domain (MD) towards pseudo-single domain (PSD), or even single domain (SD) states. ... mehrThe change in apparent domain state is concomitant with an increase in magnetic coercivity, and further decreases κ. However, a recent study found that post-shock thermal annealing of shocked magnetite causes a “healing” of some of the reversible ductile lattice defects, which reduces domain wall-pinning and allows for the restoration and recovery of some magnetic properties, including an increase of κ.
Natural post-impact magnetic recovery through temperature has not been studied to date, and is particularly relevant for large impact craters such as the Chicxulub (Mexico). In this study we compared the large Chicxulub (diameter ca. 200 km) with the smaller Nördlinger Ries structure (Germany, diameter ca. 25 km). In both impact structures, high temperature impactites (>900°C) and post-impact hydrothermalism may lead to optimal conditions for natural annealing to take place. The effect that this phenomenon has on the characteristic magnetic signature of the craters remains unclear until now.
To address this knowledge gap, magnetite samples from the Chicxulub and Nördlinger Ries craters were investigated through rock-magnetic, petrographic and mineral chemical methods. Two generations of magnetite common to both craters were observed: (1) a pure, stoichiometric shocked magnetite with large (~100 μm) grains in the basement; and (2) a newly formed, generally low-cation substituted (Ti-) magnetite, with smaller (~10-50 μm) grains and no visible shock deformation, in the impactites.
Naturally shocked MD magnetite was found to show an apparent domain state decrease towards PSD, similar to magnetite shocked experimentally in laboratory conditions. Temperatures above 540°C create an irreversible increase in κ, and restore some apparent MD state contributions. The threshold for annealing and magnetic recovery is estimated to be ~540°C. If the magnetite is oxidized and hematite is present in the sample, it transforms back to magnetite if experimentally heated above 560°C in an argon atmosphere. The transformation creates very small magnetite grains with mottled textures and SD to superparamagnetic (SP) domain states, in a process that masks annealing if both occur concomitantly. This is an underappreciated phenomenon, that may lead to NRM remagnetization and overprint in nature, under certain conditions.
In Chicxulub, the shocked magnetite of the uplifted basement shows shock-reduced NRM and κ values, leading to a low total magnetization in the peak-ring. We attribute the negative anomaly in the peak-ring to this lack of magnetization. The hydrothermal system did not significantly overprint the magnetic signal, and its temperatures (~450°C) were not high enough to anneal the magnetite naturally, and thus it did not affect significantly the magnetic anomaly in the peak-ring. Natural annealing takes place in contact with the impact melt, where basement magnetite shows increased κ, and a transformation of pre-impact oxidation-derived hematite into newly formed SD-SP magnetite. The impact melt shows strong NRM and κ values, but in the peak-ring it constitutes only a thin layer, which also does not significantly contribute to the anomaly.
In Nördlinger Ries, the basement magnetite is similarly demagnetized, and natural annealing occurs locally due to the prevalence of melt bearing suevite dykes in the basement, with high emplacement temperatures (>900°C). The hydrothermal system in Nördlinger Ries had shorter duration and lower temperatures than in Chicxulub (max. 300°C), and also does not significantly affect the magnetic anomalies of the crater. The impact melt shows weak magnetization, with κ values comparable to the shocked and demagnetized basement. On the other hand, impact breccia (suevite) in Nördlinger Ries show high κ and strong reverse polarity magnetization, which contributes to the negative magnetic anomalies.