Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr51b0969g&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR51B-0969
Physics
1540 Rock And Mineral Magnetism, 3944 Shock Wave Experiments
Scientific paper
Hypervelocity impacts play a key role in the magnetic records of many extraterrestrial bodies (asteroids, Mars, the Moon...). The understanding of magnetization changes induced by hypervelocity impacts is crucial for the interpretation of planetary magnetic anomalies and of the paleomagnetic signal of meteorites. Shocks may also permanently modify the intrinsic magnetic properties of rocks. In this work, we investigate the effects of shocks on both the intrinsic (hysteresis, susceptibility, magnetic anisotropy) and extrinsic (remanent magnetization) magnetic properties of magmatic and metamorphic terrestrial rocks, meteorites (ordinary chondrites and Martian meteorites) and synthetic samples. In order to span a large spectrum of shock duration bracketing the natural case (10-3 to 1 s), we use different experimental techniques: pulsed laser (~10-9-10-8s, up to 50 GPa), high-order explosives (~10-6s, up to 25 GPa) and measurement under pressure in a non-magnetic pressure cell (~102-103 s, up to 1.5 GPa). In order to calibrate experimentally the pressure profile in the impacted materials, we applied shocks induced by laser on rock slices of different thickness (0.5-4mm) to evaluate material response under strong and short pressure loading. Rear free surface velocity has been measured with VISAR (Velocimeter Interferometry for Any Reflector) technique. Velocity profiles are reproduced by simulation using hydrodynamic code including elasto-plastic behaviour of material. The agreement between calculations and experiment is rather good allowing an estimation of the pressure seen by the impacted rocks. The shock experiments demonstrate that above ~10 GPa, the intrinsic properties of all investigated magnetic minerals (magnetite, titanomagnetite, pyrrhotite) are permanently modified: coercivity increases, susceptibility decreases and in some cases a magnetic foliation is created perpendicular to the direction of shock. Therefore, the magnetic properties of shocked meteorites (e.g. Martian meteorites) may not be representative of the magnetism of their parent body. The effects on remanent magnetization are already noticeable at ~1 GPa. The efficiency of the shock remagnetization is strongly dependent on the magnetic mineralogy. For pressure of a few GPa, most magnetic minerals present in meteorites (taenite, kamacite, titanomagnetite, magnetite, pyrrhotite) are already largely demagnetized and/or remagnetized. Therefore, in the vast majority of cases, the paleomagnetic signal of meteorites cannot be original. Records of the possible original magnetic fields may only be looked for in very weakly shocked meteorites. Nevertheless the paleomagnetic signal of shocked meteorites can provide clues on the magnetic fields present during the impacts.
Berthe L.
Bezaeva N.
Boustie Michel
Gattacceca Jérôme
Lamali A.
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