Plessite as a Recorder of Past Magnetic Fields on Planetesimals

Abstract:

Paleomagnetic studies depend on the identification of ferromagnetic grains that can accurately record ancient magnetic fields and retain these records until the present day. In the past decade, paleomagnetic studies of iron meteorites have focused on cloudy zones, nanoscale intergrowths of 20-500 nm tetrataenite (𝛾”-FeNi) islands embedded in a paramagnetic Ni-poor matrix, which have been shown to be reliable recorders. However, cloudy zones do not exist in all metal-bearing meteorites. Another microstructure that can contain such small tetrataenite grains is plessite. Plessite is a nano- to microscale intergrowth of ferromagnetic kamacite with taenite and/or tetrataenite. It is ubiquitous among metal-bearing meteorite groups and can manifest a diversity of microstructures determined by the thermal history and composition of the host meteorite. One type of plessite, known as black plessite, consists of 20 – 100 nm tetrataenite blades in a kamacite matrix while another variety, duplex plessite, has coarser grains >200 nm. Due to its grain size, the tetrataenite in black plessite and finer duplex plessite should have high microcoervicities, making them resistant to subsequent remagnetization. Thus, the tetrataenite in black plessite and fine duplex plessite may enable analysis of a wider range of meteorites using paleomagnetic methods. Using X-ray photoemission electron microscopy (XPEEM) imaging of the ungrouped iron meteorite Bacubirito, we analyzed the stepwise acquisition of isothermal remanent magnetization in duplex plessite in applied fields from 0 to 324 mT. We found that 200 – 900 nm size Ni-rich grains have effective microcoercivities between 250 mT and 324 mT. The XPEEM maps show that these grains have a uniform magnetization state down to the resolution of the image (see figure). Our high-resolution backscattered and secondary electron images of the same etched plessite grains show a lack of internal grain boundaries. In addition, micromagnetic modeling of tetrataenite suggests that the grains analyzed are transitional in size between the single and multi-domain states. These data and calculations collectively indicate that these grains, and by implication the smaller grains in black plessite, may be capable of recording ancient magnetic fields and be stable over the history of the solar system.