Abstract
X-ray photoemission electron microscopy (XPEEM) enables natural remanent magnetisation to be imaged with ∼30 nm 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 ∼400 nm 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).
Highlights
Meteorites are our primary source of information regarding the magnetic fields generated billions of years ago by planetary bodies (Weiss and Elkins-Tanton, 2013)
We have demonstrated the potential of X-ray photoemission electron microscopy (XPEEM) to perform paleomagnetic measurements on sub-micrometer length scales in meteoritic metal
The method was used to show that the cloudy zone (CZ) in the Tazewell IIICD iron meteorite contains equal proportions of the possible easy axes among the coarser and intermediate sized islands
Summary
Meteorites are our primary source of information regarding the magnetic fields generated billions of years ago by planetary bodies (Weiss and Elkins-Tanton, 2013). J.F.J. Bryson et al / Earth and Planetary Science Letters 396 (2014) 125–133 core solidification and dynamo field decay, and thereby contribute significantly to our understanding of planetary evolution. Bryson et al / Earth and Planetary Science Letters 396 (2014) 125–133 core solidification and dynamo field decay, and thereby contribute significantly to our understanding of planetary evolution It has recently been shown (Bryson et al, 2014) that time-resolved records of asteroid dynamos can potentially be encoded within spinodal nanostructures unique to meteoritic metal, termed the cloudy zone (CZ). Extracting this signal, requires paleomagnetic measurements to be made on the micrometer to sub-micrometer length scales. In this study we demonstrate a new approach based on synchrotron X-ray magnetic imaging that, combined with the unique properties of the CZ, allows quantitative analysis of nanoscale remanence and the potential to study the temporal evolution of asteroid dynamo fields
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