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Micromagnetic Coercivity Distributions and Interactions in Chondrules With Implications for Paleointensities of the Early Solar System
Yin, Q - University of California, Davis, Department of Geology, Davis, CA 95616 United States
Acton, G - University of California, Davis, Department of Geology, Davis, CA 95616 United States
Verosub, K L - University of California, Davis, Department of Geology, Davis, CA 95616 United States
Jovane, L - University of California, Davis, Department of Geology, Davis, CA 95616 United States and Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, 00143 Italy
Roth, A - University of California, Davis, Department of Geology, Davis, CA 95616 United States
Jacobsen, B - University of California, Davis, Department of Geology, Davis, CA 95616 United States
Ebel, D S - American Museum of Natural History, Department of Earth and Planetary Sciences, New York, NY 10024 United States
Chondrules in chondritic meteorites record the earliest stages of formation of the Solar System, potentially providing information about the magnitude of early magnetic fields and early physical and chemical conditions. Using first-order reversal curves (FORCs), we map the coercivity distributions and interactions of 32 chondrules from the Allende, Karoonda, and Bjurbole meteorites. Distinctly different distributions and interactions exist for the three meteorites. The coercivity distributions are log-normal shaped, with Bjurbole distributions being bimodal or trimodal. Allende FORC distributions have coercivities that extend out to about 250-350 mT, with little or no interaction above 10 mT. Karoonda FORC distributions are triangular shaped with high interactions at low coercivity and progressively lower interactions out to the peak coercivity of about 130 mT. In Bjurbole chondrules, a high coercivity mode (400-700 mT) arising from tetrataenite interacts strongly with one or more lower coercivity modes in a manner unlike that seen in terrestrial rocks. Such strong interactions have the potential to bias paleointensity estimates. Moreover, because a significant portion of the coercivity distributions for most of the chondrules is <10 mT, low-coercivity magnetic overprints are common. Therefore, paleointensities based on the REM method, which rely on ratios of the natural remanent magnetization (NRM) to the saturation isothermal remanent magnetization (IRM) without magnetic cleaning, will probably be biased. The paleointensity bias is found to be about an order of magnitude for most chondrules with low-coercivity overprints. Paleointensity estimates based on a method we call REMc, which uses NRM/IRM ratios after magnetic cleaning, avoid this overprinting bias. Allende chondrules, which are the most pristine and possibly record the paleofield of the early Solar System, have a mean REMc paleointensity of 10.4 µT. Karoonda and Bjurbole chondrules, which have experienced some thermal alteration, have REMc paleointensities of 4.6 and 3.2 µT, respectively.
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