Variations in the Magnetic Properties of Meteoritic Cloudy Zone

Iron and stony-iron meteorites form the Widmanstätten pattern during slow cooling. Thispattern is composed of several microstructures whose length-scale, composition and magnetic propertiesare dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscaletetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic reviewof how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X-rayphotoemission electron microscopy is used to compare compositional and magnetization maps of thecloudy zone in the mesosiderites (slow cooling rates),the IAB iron meteorites and the pallasites(intermediate cooling rates), and the IVA ironmeteorites (fast cooling rates). The proportions of magneticphases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the firstobservations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slowcooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large-scale regionsof uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. Forthe most rapidly cooled IVA meteorites, the time available for Fe-Ni ordering is insufficient to allowtetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The mostreliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates(∼2–500◦C Myr−1) which produces islands that are “just right” in both size and degree of Fe-Ni order