HR: 1340h
AN: GP13A-0040    [Abstracts]
TI: Temperature Dependence of Magnetic Susceptibility as a Potential indicator of fine-scale oxyexsolution in igneous rocks
AU: * Smirnov, A V
EM: aleksey.smirnov@yale.edu
AF: Yale University Department of Geology and Geophysics, P.O. Box 208109, New Haven, CT 06511 United States
AU: Bauch, D G
EM: db007k@mail.rochester.edu
AF: University of Rochester Department of Earth and Environmental Sciences, Hutchison Hall 227, Rochester, NY 14627 United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester Department of Earth and Environmental Sciences, Hutchison Hall 227, Rochester, NY 14627 United States
AB: Rock magnetic characterization is an essential prerequisite for paleomagnetic research. David Dunlop's contributions in rock magnetism have provided a better understanding of the fundamental physical processes related to natural remanent magnetizations, greatly facilitating the application of such magnetizations to geological and geophysical questions. Here we report an apparent conflict between scanning electron microscopy (SEM) and magnetic analyses of mafic dikes cutting the ~3.2 Ga Kaap Valley Pluton (South Africa). At face value, the SEM data suggest that the magnetic remanence carrier is homogeneous titanomagnetite of intermediate composition (x ~ 0.4-0.5). Such a carrier should have Curie temperatures of ~350-400 degrees C. In contrast, the temperature dependence of magnetic susceptibility (k) reveals the presence of a magnetic phase with a Curie temperature close to 585 C, consistent with pure magnetite. A characteristic peak observed at -153 C, associated with the Verwey transition, further indicates high magnetite stoichiometry. Irreversibility of k(T) curves was observed upon heating and cooling in argon. On heating, the samples show a gradual increase of k with a pronounced Hopkinson peak, followed by a sharp decrease to the Curie temperature. However, on cooling, the Hopkinson peak disappears and the k(T) values trace a curve lower than the heating curve, with a broad maximum at 350-450 C. We interpret the observed k(T) behavior as reflecting the presence of very fine magnetite-ilmenite intergrowths produced by oxyexsolution. We speculate that the k(T) irreversibility may be caused by partial homogenization of the intergrowths upon heating. We suggest that the intergrowths were not identified using conventional SEM analysis because of their very small (<50 nm) size. This interpretation is supported by analyses using a high-resolution field-emission gun scanning electron microscope. Because of the characteristic changes with heating, we further propose that k(T) data may be a sensitive indicator of fine-scale oxyexsolution in some igneous rocks.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005