A 3D nanoscale approach to nebular paleomagnetism in the Semarkona LL3.0 ordinary chondrite

Friday, 19 December 2014
Joshua Franz Einsle1, Roger R Fu2, Benjamin P Weiss3, Takeshi Kasama4, Paul A Midgley1 and Richard J Harrison1, (1)University of Cambridge, Cambridge, United Kingdom, (2)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (3)MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States, (4)Technical University of Denmark, Kongens Lyngby, Denmark
Solar nebular models suggest that magnetic fields are central to the redistribution of mass and angular momentum in the protoplanetary disk. Using individual chondrules with patches of dusty olivine the strength of these magnetic fields can be measured due to presence of nanoscale Fe inclusions. Since chondrules formed by rapid heating and cooling in the early solar nebula, individual chondrules have the potential to record the magnetic field that was present during their formation, and retain this signal for several billion years. Recently the first robust paleointensity measurement of nebular fields was compleated by measureing dusty olivine grains from the Semarkona LL3.0 ordinary chondrite meteorite in a SQUID microscope. (Fu et al. this meeting) Extracting quantitative information from the paleomagnetic meaurements requires a full understanding of the underlying physical mechanisms producing the measured magnetic signal.

Here we characterise the magnetic behaviour of the same dusty olivine chondrules, using a variety of electron microscopy techniques. Electron holography and Lorentz imaging confirm the dominance of single vortex (SV) states in the majority of the remanence carriers. In-field measurements demonstrate the high stability of this SV state, making them suitable carriers of paleomagnetic information. We present a 3D volume reconstruction of the dusty olivine using Focussed-Ion-Beam (FIB) slice-and-view tomography. Combining the selective milling properties of FIB with the high spatial resolution of the Scanning Electron Microscope we are able to capture images as we make successive slices through a selected region of the sample. For this initial study we present a collection of 400 images taken every 10 nm as we slice through an 10 µm x 10 µm x 4 µm volume of the dusty olivine patch within a single chondrule. Each image possesses resolution around 10 nm allowing us to resolve particles in both the single domain and single vortex size ranges. Once assembled the full data provies quantitative statistics on particle-size distribution, shapes and interparticle spacing. The information is then used to model the macroscopic paleomagnetic properties. This work further extends the central role of electron microscopy in determining the underlying physics of the remanence acquisition process.