Shock-induced anisotropy of magnetic susceptibility: impact experiment on basaltic andesite

Itoyuki Nishioka,Minoru Funaki,Toshimori Sekine

doi:10.1186/bf03352060

Abstract

Abstract Changes in the anisotropy of the low-field magnetic susceptibility (AMS) of basaltic andesite were induced by decaying stress waves and subsequently quantified. An initial shock pressure of 5 GPa was generated in a block of the target rock through impacting with a cylindrical projectile. Following the impact, the maximum or minimum principal susceptibility axes of the target were reoriented toward the shock direction at low (0.5–3 GPa) or high (>3 GPa) estimated shock pressures, respectively. Subtraction of the initial AMS demonstrated a parallelism between the induced susceptibility axes and the shock direction. These results suggest a potential application of AMS as an indicator of the propagation directions of stress waves generated in rocks at terrestrial impact structures.

Highlights

The effects of high pressures on rock magnetic properties are of interest in terms of gaining an understanding of the magnetism of rocks subjected to strong stress waves, such as lunar rocks and meteorites and terrestrial planets and satellites
To constrain the mechanism of these changes, we evaluated their stabilities against tumbling alternating field demagnetization (AFD) and changes in hysteresis parameters
The anisotropy of the low-field magnetic susceptibility (AMS) ellipsoids show a gradual change in shape with increasing pressure, with the shifts in the data points being more pronounced close to the impact point (i01–i05): specimen i01 records a high P value. These results indicate that the observed changes in the shape, degree, and orientation of the AMS ellipsoids occur at a lower pressure than that leading to a reduction in Km

Summary

Introduction

The effects of high pressures on rock magnetic properties are of interest in terms of gaining an understanding of the magnetism of rocks subjected to strong stress waves, such as lunar rocks and meteorites (paleomagnetic records) and terrestrial planets and satellites (crustal magnetization). The results of laboratory shock experiments reveal that the primary effects of impact are demagnetization or remagnetization, and magnetic hardening (Gattacceca et al, 2007 and references therein). Gattacceca et al (2007) demonstrated that explosive-driven shocks of about 10 GPa on basalt and microdiorite acted to change the anisotropy of their low-field magnetic susceptibility (AMS). An important implication of the results of the abovementioned shock experiments is that the minimum principal susceptibility axes are reoriented toward the shock directions, indicating that AMS may be a proxy to the propagation direction of ancient stress waves in rocks. We have conducted a laboratory impact experiment aimed at studying the shock-induced changes in AMS parameters in detail. To constrain the mechanism of these changes, we evaluated their stabilities against tumbling alternating field demagnetization (AFD) and changes in hysteresis parameters

Methods

Findings

Conclusion