HR: 08:15h
AN: GP21C-02 INVITED     [Abstracts]
TI: Lamellar Magnetism: A New Magnetic Substructure?
AU: * McEnroe, S A
EM: suzanne.mcenroe@ngu.no
AF: Geological Survey of Norway, NGU, Trondheim, N-7491 Norway
AU: Robinson, P
AF: Geological Survey of Norway, NGU, Trondheim, N-7491 Norway
AU: Harrison, R J
EM: rjh40@esc.cam.ac.uk
AF: Dept. Earth Sciences, Cambridge Univ., Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Langenhorst, F
EM: Falko.Langenhorst@Uni-Bayreuth.DE
AF: Bayerisches Geoinstitut, Univ. Bayreuth, Bayreuth, D-95440 Germany
AB: Nearly 1 billion year old samples from Proterozoic terranes in Norway, Sweden and USA, contain finely exsolved members of the hematite ilmenite (Fe2O3 FeTiO3) series. Samples have strong and extremely stable remanent magnetization, suggesting an explanation for some magnetic anomalies in the deep Earth and on planetary bodies that no longer produce a magnetic field. Due to the high thermal stability and coercivity of these samples, understanding the nature of the magnetization may have commercial applications. Common to all samples are grains of ilmenite or hematite with multiple generations of exsolution lamellae. Observations of exsolution have been made at high resolution in transmission electron microscopy and, using images produced by electron energy loss spectroscopy showing exsolution ranging down to (1 to 2 nm) about that of one six-layer unit cell of a rhombohedral oxide. Images also show that the interfaces of the finest lamellae are coherent and have considerable lattice strain, a feature that may enhance coercivity and unblocking temperatures. Atomic simulations of the nanoscale exsolutions has led us to propose a new ferrimagnetic substructure created by ferrous ferric `contact layers' that reduce charge imbalance along lamellar contacts between antiferromagnetic hematite and paramagnetic ilmenite. Under perfect magnetic in phase-conditions, we estimate that such a lamellar magnetic material could have a saturation magnetization up to 150 kA/m, 70 times stronger than pure hematite, while retaining the high coercivity and thermal properties of single-domain hematite. Current research is focused on rock-magnetic experiments at low and high temperatures, observational and analytical TEM, effects of pressure and compositional variations on the Fe2O3 FeTiO3 phase diagram, with emphasis on the magnetic phases, crystal-chemical reconstructions, and additional atomic simulations of lamellar interfaces.
DE: 6225 Mars
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 1517 Magnetic anomaly modeling
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting