Plessite recording magnetic fields on planetesimals - Elias Mansbach - 2021 MagIC Workshop (1/20/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 25/01/2021

"Plessite as a Recorder of Magnetic Fields on Planetesimals" by Elias Mansbach, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology

Lightning poster talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
Plessite as a Recorder of Magnetic Fields on Planetesimals 

Authors: Elias N. Mansbach (1), Jay Shah (1), Clara Maurel (1), James F. J. Bryson (2), Benjamin P. Weiss (1)

(1) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (2) Department of Earth Sciences, University of Oxford

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 sufficiently 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 that are greater than 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) applied to the ungrouped iron meteorite Bacubirito, we analyzed the stepwise acquisition of an isothermal remanent magnetization to 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 images show that these grains exhibit no domain walls down to the resolution of the image, and our high-resolution backscattered and secondary electron images of the same etched grains show a lack of internal grain boundaries. This suggests that the Ni-rich grains are not aggregates of smaller grains. 

To complement the XPEEM observations and further understand the magnetization states of plessitic tetrataenite assemblages, we calculated the domain states of tetrataenite grains over a range of lengths and axial ratios using the micromagnetic modelling software MERRILL. Our models suggest that the Bacubirito tetrataenite grains analyzed in our XPEEM study could be in the single vortex domain state. Additionally, our calculations for the relaxation time of single vortex tetrataenite grains at room temperature suggest that if these grains are in fact in the single vortex state, they should be stable over the history of the solar system. 

The data and modeling results indicate that fine duplex plessite and black plessite have recoding properties that can make them useful for paleomagnetic analyses of a wide range of meteorites, enabling the investigation of past planetary magnetic fields. However, the process by which plessite acquires magnetization needs to be studied further.