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	Long-lived magnetism from solidification-driven convection on the pallasite parent body James F. J. Bryson , Claire I. O. Nichols , Julia Herrero-Albillos , Florian Kronast , Takeshi Kasama , Hossein Alimadadi , Gerrit Van Der Laan , Francis Nimmo , Richard J. Harrison Nature 517 (7535), 472 - 475 DOI: 10.1038/nature14114 PMID: 25612050 Show Abstract ▼ Abstract: Palaeomagnetic measurements of meteorites1, 2, 3, 4, 5 suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields6. Convection on these bodies is currently thought to have been thermally driven7, 8, implying that magnetic activity would have been short-lived9. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging10 of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core11, 12, 13 is consistent with our measured magnetic field characteristics and cooling rates14. Solidification-driven convection was probably common among small body cores15, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.	Jan 2015
	Nanopaleomagnetism of meteoritic Fe-Ni studied using X-ray photoemission electron microscopy James Bryson , Julia Herrero-Albillos , Florian Kronast , Massimo Ghidini , Simon A. T. Redfern , Gerrit Van Der Laan , Richard J. Harrison Earth And Planetary Science Letters 396, 125 - 133 DOI: 10.1016/j.epsl.2014.04.016 Open Access Show Abstract ▼ Abstract: X-ray photoemission electron microscopy (XPEEM) enables natural remanent magnetisation to be imaged with resolution across a field of view of 5–20 μm. The method is applied to structural features typical of the Widmanstätten microstructure (kamacite – tetrataenite rim – cloudy zone – plessite) in the Tazewell IIICD iron meteorite. Kamacite lamellae and the tetrataenite rim are multidomain, whereas plessite consists of laths of different phases displaying a range of stable magnetisation directions. The cloudy zone (CZ) displays a complex interlocking domain pattern resulting from nanoscale islands of tetrataenite with easy axes distributed along three possible crystallographic directions. Quantitative analysis of the coarse and intermediate CZ was achieved using a combination of image simulations and histogram profile matching. Remanence information was extracted from individual regions of interest wide, demonstrating for the first time the capability of XPEEM to perform quantitative paleomagnetic analysis at sub-micron length scales. The three tetrataenite easy axis orientations occur with equal probability in the coarse and intermediate CZ, suggesting that spinodal decomposition in these regions was not strongly influenced by internal interaction fields, and that they are suitable candidates for future paleomagnetic studies. The fine CZ shows a strong dominance of one easy axis. This effect is attributed to island–island exchange interactions that render the fine CZ unsuitable for paleomagnetic study. Variations in the relative strength (proportion of dominant easy axis) and direction (direction of dominant easy axis) of a paleomagnetic field can be resolved from different regions of the CZ using XPEEM, raising the prospect of obtaining a time-resolved measurement of the active dynamo period in meteorites originating from the upper unmelted regions of differentiated asteroids (e.g. chondrites, pallasites, mesosiderites).	Jun 2014

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