Characteristics of First Order Shock Induced Magnetic Transitions in Iron and Discrimination from TRM

Abstract-The magnetometer experiment (MAG) onboard the Near-Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2.1 x 10-6 Am 2 kg-l for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S-class asteroids (Gaspra,~2.4 x 10-2 Am 2 kg-I; Braille, 2.8 x 10-2 Am 2 kg-I) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments.The ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid-meteorite match based on magnetic property measurements. Natural remanent magnetization (NRM) intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the H, L, and LL groups, all well above the 2.1 x 10-6 Am 2 kg-I level for 433 Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent magnetization)) range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between NRM and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more ofthe NRM. Consequently, published values of the NRM should be considered suspect unless careful evaluation ofthe noise sources is done. NASA Goddard SFC studies of per unit mass intensities in large (>10 000 g) and small (down to <1 g) samples from the same meteorite demonstrate magnetic intensity decreases as size increases. This would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples. This would then argue for some level of demagnetization of large objects such as an asteroid. The possibility that 433 Eros is an LL chondrite cannot be discounted.

Section: Discussionmentioning

confidence: 99%

Section: Ratio Of Natural Remanent To Saturation Remanent Magnetizatimentioning

confidence: 99%

Section: R Mmentioning

confidence: 99%

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443 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

Wasilewski

Acuña

Kletetschka

2002

“…The Cu(Fe) alloys, which contain very fine Fe particles (<I00 nm), have a REM range of (13 to 43) x for TRM acquired in the geomagnetic field. These same Cu(Fe) alloys subject to a 5 GPa shock in a field comparable to the geomagnetic field show a restricted but reduced REM range of (0.6 to 8) x I k 3 (Wasilewski, 1977a). The Mount Fuji tree mold sample set is particularly instructive in that the tree mold contains samples with titanomagnetite and Fe that have acquired TRM at the same instant of time in the same geomagnetic field intensity.…”

Section: Rem Data and Magnetic Remanencementioning

confidence: 99%

Aspects of the validation of magnetic remanence in meteorites

Wasilewski

Dickinson

2000

Self Cite

Abstract-Meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution. Consequently, the validation of these records is important in meteorite research. The first step in the validation process should be the REM value. The REM value is the ratio of natural remanence (NRM) to saturation remanent magnetization imparted by a 1 T magnetic field (SIRM). The REM values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from Allende (C3V-SI), Bjurbole (L4-S1), and Chainpur (LL3-SI) meteorites.The REM values computed from published NRM and SIRM data identify many orders of magnitude range in the REM values including REM values >I00 x 10-3. These data suggest a dependence for the NRM intensity on the curatorial location from which the sample was obtained. Any earth rock acquiring thermoremanent magnetization (TRM) in the geomagnetic field has a restricted range in REM mostly between 5 and 50 x 10-3, the exception being the mineral hematite in the multidomain size range. The only terrestrial samples with REM much greater than 100 x 10-3 are those struck by lightning.The REM value provides a physical basis for recognition between valid records and those that "might be contaminated." The isothermal remanence acquisition (RA) curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed "REM" (RM/SIRM) value. In the case of the Bjurbole and Chainpur chondrules, with REM values > 100 x 10-3, the RA curve indicates that unrealistically large contamination magnetic fields would be required to give REM values greater than 100 x l e 3 . This would suggest contamination other than by a hand magnet that is normally available to an experimenter. This would require an explanation that would involve large magnetic fields during chondrule formation, or some extraordinary remanence acquisition mechanism that remains to be described.Magnetic contamination experiments, using -80 and -40 mT magnets, demonstrate that the "REM" values and extent of modification of the magnetic vector record are mineralogy dependent, and this is mostly related to the amount and characteristics of the mineral tetrataenite. The complexity of the meteorite records suggest validation of the record as a first step. The REM value is the first physical statement that can be made in this validation

“…Kletetschka et al (2004) showed that pressures of about 1 GPa are sufficient to partially demagnetize all of magnetite, hematite, and titanohematite. Wasilewski (1976Wasilewski ( , 1977 and Dickinson and Wasilewski (2000) reported remanent magnetization acquired by shock to 5 GPa in magnetic and nonmagnetic environments using samples containing precipitated grains of anti-ferromagnetic iron (nonmagnetic fcc-phase) in a copper field. The iron grains are transformed to the ferromagnetic bcc-phase by the shock.…”

Section: Introductionmentioning

confidence: 99%

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

Funaki

Syono

2008

Abstract-Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 µT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma-induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub-samples, the SRM intensities of sub-samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma-induced magnetization of impactites should be reconsidered.