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Magnetic microscopy of meteorites: probing the magnetic state of the early solar system

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Microstructural and geochemical studies of meteoritic metal have been instrumental in shaping our current views of differentiated asteroids, providing constraints on their cooling rate, their size, the timing of their differentiation and their fractional crystallisation and impact histories. The characteristic Widmanstätten microstructure, familiar to anyone who has looked at a polished and etched section of an iron meteorite with the naked eye, hides a nanoscale complexity that is revealed only with high-resolution electron microscopy – a legacy of stranded diffusion profiles, metastability, martensitic transformations, chemical segregation and ordering during slow cooling over millions of years on the parent body. The presence of soft bcc iron has traditionally lead to the meteoritic metal being dismissed as a reliable carrier of paleomagnetic information. However, we have shown that, under favourable circumstances, paleomagnetic information can be recorded and retained on a local scale within a unique nanoscale intergrowth called the cloudy zone (CZ). High-resolution X-ray imaging methods enable the magnetic state of the CZ to be imaged and analysed quantitatively, opening up new avenues of research into the nanopaleomagnetism of a range of meteorites. Such studies are not only revealing new insight into the thermochemical properties of asteroids in the early solar system, but provide us with unique opportunities to learn about how magnetic fields are generated on planetary bodies in general, and the underlying physics of the dynamo generation process itself.

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