Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history |
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| Authors: | J Gattacceca C Suavet P Rochette B P Weiss M Winklhofer M Uehara Jon M Friedrich |
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| Institution: | 1. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, , Cambridge, Massachusetts, 02139 USA;2. CNRS, Aix‐Marseille Université, , 13545 Aix en Provence, France;3. Department of Earth and Environmental Sciences, Ludwig‐Maximilians‐University Munich, , D‐80333 Munich, Germany;4. Department of Chemistry, Fordham University, , Bronx, New York, 10458 USA;5. Department of Earth and Planetary Sciences, American Museum of Natural History, , New York City, New York, 10024 USA |
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| Abstract: | Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study. |
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