Variation in Magnetic Fabrics at Low Shock Pressure Due to Experimental Impact Cratering

Abstract

Magnetic fabrics provide important clues for understanding impact cratering processes. However, only a few magnetic fabric studies for experimentally shocked material have been reported so far. In the framework of MEMIN (Multidisciplinary Experimental and Modeling Impact Research Network), we conducted two impact experiments on blocks of Maggia gneiss with the foliation oriented perpendicular (A38) and parallel (A37) to the target surface. Maggia gneiss has plenty of biotite bands forming a strong rock foliation. The bulk magnetic susceptibility varies from 0.376 × 10−3 to 1.298 × 10−3 SI in unshocked and from 0.443 × 10−3 to 3.940 × 10−3 SI in shocked gneiss. The thermomagnetic curves reveal a Verwey transition at −147 °C and a Curie temperature between 576 and 579 °C in unshocked and shocked samples, indicating nearly pure magnetite, which carries the magnetic fabrics. In A37 and A38 kinking is prominent from the point source down to a depth of 2 and 4.2 dp (projectile diameter) or 1 and 2.1 cm, respectively. Kinking, folding, and fracturing changed the position of magnetite grains with respect to each other to reorient the magnetic fabrics. Reorientation of magnetic fabrics is conspicuous down to 20 dp (10 cm) in A38, where no other impact‐related deformation is visible. The reorientation of magnetic fabrics may, therefore, aid in identifying impact processes at very low pressures, starting at 0.1 GPa, when other common indicators are absent.

Curie temperature of weakly shocked target basalts at the Lonar impact crater, India

“…On the other hand, present observations agree with the trends in cratering experiments. Here experimental shock pressure (< 3 GPa) increases the bulk magnetic susceptibility of rocks with small magnetite grains, < 10 µm (Agarwal et al 2019). Although the mechanism behind this increase is not known, their results agree well with the present observations.…”

“…Until now, the shock demagnetization of magnetic minerals such as magnetite, Ti-magnetite, hematite, Ti-hematite, and pyrrhotite (see Louzada et al 2011 and the references therein) was the only effect of low shock pressures (< 3 GPa), which was considered of importance at a regional scale. However, now, increase in magnetic susceptibility of Ti-magnetite, at low shock pressures, is clear from present investigations Table 1 Table compiles the on naturally shocked target basalts at Lonar impact structure, and from experiments (Agarwal et al 2019).…”

“…As most of the target rocks in the crater subsurface are generally weakly shocked, < 3 GPa (Pierazzo and Melosh 2000;Kenkmann et al 2014), such an increase in magnetic susceptibility will affect the induced magnetization, thus, changing the magnetic anomalies at these impact structures. While there are no other studies on the T C of weakly shocked rocks, there is only one study on magnetic susceptibility, which shows its increase with shock pressures below 3 GPa, without commenting on the mechanism (e.g., Agarwal et al 2019). Changes in the crystal lattice, such as straining of (311) peak, decrease in apparent crystallite size, and grain fragmentation may be among the possible reasons for one of the observed trends in T C and magnetic susceptibility.…”

Impact Experiment on Gneiss: The Effects of Foliation on Cratering Process

“…In agreement to the reorientation of the magnetic fabrics, the Vertical Profile of A38 shows intense straining, which is lower on either side, in the Left and Right Oblique Profiles of A38. Agarwal et al (2019) show that in A37 (horizontal foliation) the reorientation of the magnetic fabrics is less below the crater, but more at the sides. A similar trend is seen in the present study.…”

Rock magnetism of ejected basaltic boulders from Lonar crater, India: Implications for the existence of a short‐lived impact‐generated weak magnetic field

“…Hence, based on these experiments it can be concluded that the shock pressure generated due to impact has affected the rock magnetic properties of the target basalts presently exposed around Lonar crater rim and surrounding areas (Arif et al 2012b). Agarwal et al (2019), in their recent experiments on gneissic target, show that the reorientation of magnetic fabrics is possible at shock pressure as low as 0.1 GPa. This experiment is perhaps not analogous to Lonar impact because like the gneissic target, Lonar target basalt does not have a strong preimpact magnetic fabric that could influence the orientation of AMS axes due to impact.…”