Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.211c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 211
Other
Scientific paper
The remanent magnetism and other magnetic properties of three howardites (Kapoeta, Petersburg, and Le Teilleul) and one eucrite (Sioux County) have been investigated. The dominant carrier of magnetic properties in all four meteorites is low-nickel kamacite, present to the extent of <1% in the eucrite, Kapoeta and Le Teilleul and probably 3-5% in Petersburg. Recent studies of the natural remanent magnetization (NRM) of ordinary chondrites (Morden and Collinson 1992) and the Estherville mesosiderite (Collinson 1991) showed that their NRM is randomly directed within a sample, which has important implications for their evolution. Studies were therefore undertaken on HEDs to investigate, among other properties, the homogeneity or otherwise of their NRM and assess the implications of the nature of the NRM for their formation and evolution. The initial NRM in Kapeota, Sioux County, and Le Teilleul is widely scattered in direction in mutually oriented subsamples, with a mean intensity of ~10^-4-10^-5 Am^2.kg^1. Petersburg has a much stronger initial NRM of ~10^-3 Am^2.kg^-1. However, after an initial alternating field demagnetizing step of 2.5 milliteslas, the intensity in subsamples decayed to about 10^-5 Am^2.kg^-1 and directions of NRM became scattered. This suggests the presence of a contaminatory, soft magnetization in the Petersburg sample. The subsamples used in this work are typically of 1-2 g mass. With the exception of Le Teilleul, alternating field demagnetization of the NRM shows highly irregular behaviour, with no clear evidence of secondary magnetization. This behavior appears to be a consequence of the inhomogeneity of NRM directions within the samples, and it is unlikely that any secondary magnetization is distinguishable using this technique. Thermal demagnetization confirms the presence of a very stable primary magnetization in the subsamples. Le Teilleul shows more conventional behaviour on demagnetization, with a smooth decay of intensity with increasing demagnetizing field and, in one subsample, the removal of a secondary NRM component of NRM. A second property of interest is anisotropy of magnetic susceptibility. The anisotropy ellipsoid defines a rock fabric, and uniformity of susceptibility ellipsoid orientation among mutually oriented subsamples indicates a uniform fabric throughout the meteorite material. Nonuniformly oriented subsample ellipsoids indicate a local fabric associated with each subsample. All the samples possess a generally uniform foliated fabric. The degree of anisotropy (the ratio of maximum to minimum susceptibility) is typically in the range 1.1-1.2. The randomly directed NRM in Kapoeta, Petersburg, and Sioux County shows that in these samples the observed NRM arises from the magnetization acquired by matrix, lithic clasts, and mineral fragments before they accumulated into the final meteoritic material. The most likely origin of this magnetization is a thermoremanent magnetization (TRM) acquired in an ambient magnetic field during cooling after heating in the original parent body. The magnetic field could have been an enhanced interplanetary field or one generated within the parent body. The latter source is currently favored. There is also evidence from the nature of the NRM that there was no heating above ~750 degrees C, or very severe shock, after final accumulation of the meteorites. This would result in uniform remagnetization or demagnetization of the meteoritic material, according to the presence or absence of an ambient magnetic field. The uniform fabric indicated by the susceptibility anisotropy observations was acquired either during or after the final accumulation of the meteorites. Its probable origin is in the distortion of magnetic particles towards a disc shape by shock compression. The results so far obtained for the NRM of Le Teilleul are more difficult to interpret. The inhomogeneous NRM in the other meteorites is believed to be the main cause of their anomalous demagnetization behavior, yet the Le Teilleul subsamples demagnetize conventionally in intensity and direction. A possible explanation is that the NRM is approximately uniform within each of the measured subsamples, but scattered in direction between subsamples. The most likely origin of the primary NRM is again a TRM in an ambient magnetic field on the parent body. References: Collinson, D.W. (1991) Meteoritics 26. 1-10. Morden, S.J. and Collinson, D.W. (1992) Earth Plan. Sci. Lett. 109, 185-204.
Collinson D. W.
Morden S. J.
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