Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmgp53a0956k&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #GP53A-0956
Other
1519 Magnetic Mineralogy And Petrology, 1521 Paleointensity, 1540 Rock And Mineral Magnetism, 6030 Magnetic Fields And Magnetism
Scientific paper
The magnetic record of individual chondrules has been examined on two low-metamorphic grade ordinary chondrites - Bjurböle (L4) and Avanhandava (H4). The chondrule magnetic conglomerate study was done by removing oriented chondrules from the meteorite. The magnetic directions of the chondrules in both meteorites seems to be randomly oriented within both meteorites. The paleofield method based on the REM ratio (NRM/SIRM; Natural Remanent Magnetization / Saturation Isothermal Remanent Magnetization) calibrated for the TRM (Thermal Remanent Magnetization; Kletetschka et al., 2004) magnetization process reveals approximate paleofields between 5 μT and 20 μT (REM ~ 0.002) for Avanhandava chondrules and between 12 μT and 45 μT (REM ˜ 0.0015-0.0048) for Bjurböle chondrules. However, the timing of the magnetizing process remains unclear. The random pattern of the individual chondrule magnetic directions suggest that the chondrules were not remagnetized by recent terrestrial weathering nor by the exposure to the artificial magnetic fields. However the effect of post aggregation metamorphism (reaching 400 - 600°C) and impact related shock demagnetization on the parent body may be responsible for partial or complete loss of the magnetic information carried by the lower coercivity grains. In the case of Avanhandava meteorite this can be observed in the REM ratio. The values of REM ratio calculated in low AF (alternating Field) demagnetizing fields are progressively lower than in the higher AF fields. The Bjurböle meteorite contains significant fraction of the tetrataenite mineral. The origin of tetrataenite is related to the slow ordering of the FeNi lattice at temperatures below ~ 350°C). Thus the origin of tetrataenite must be post-metamorphic ruling out the high temperature TRM scenario. The origin of the magnetization in the Bjurböle chondrules are most likely associated to the post-metamorphic low temperature processes (i.e. electric discharges or impact related shock remanent magnetization on the parent body). The random magnetic directions in this meteorite can be partly controlled by the strong magnetic anisotropy of tetrataenite. Moreover, the paleofield method based on REM ratio (Kletetschka et al., 2004) is calibrated for the TRM magnetizing scenario and not for low temperature acquisition processes and thus of limited use in the Bjurböle case. The results of the paleofield studies of the Avanhandava meteorite shows that the paleofield carried out by its chondrules can be significantly underestimated due to post-formation metamorphic and impact events. Those processes can in the same way influence the paleointensity results previously published for other chondritic meteorites. The interpretation of Bjurböle data may be even more difficult due to the presence of tetrataenite. Acknowledgements: Authors would like to thank to Peter J. Wasilewski from NASA/GSFC for constructive consultations and access to the laboratory facilities. References: Kletetschka, G., Acuna, M. H., Kohout, T, Wasilewski, P. J., and Connerney, J. E. P. (2004), An empirical scaling law for acquisition of thermoremanent magnetization, Earth Planet. Sci. Lett., 226, 521- 528.
Kletetschka Gunther
Kohout Tomas
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