HR: 16:45h
AN: GP24A-04    [Abstracts]
TI: Effects of Temperature on Ilmenite-Hematite: Microstructure and Magnetic Properties in the Ecstall Pluton, British Columbia
AU: * Brownlee, S J
EM: sbrownlee@berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science 307 McCone Hall #4767, Berkeley, CA 94720-4767, United States
AU: Feinberg, J
EM: jfei05@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Harrison, R
EM: rjh40@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Kasama, T
EM: tk305@cam.ac.uk
AF: Electron Microscopy Group, Department of Materials Science and Metallurgy University of Cambridge Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
AU: Scott, G
EM: gscott@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AU: Renne, P R
EM: prenne@bgc.org
AF: University of California, Berkeley, Department of Earth and Planetary Science 307 McCone Hall #4767, Berkeley, CA 94720-4767, United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AB: The ~91 Ma Ecstall pluton exhibits paleomagnetic directions and NRM intensities that vary spatially in a systematic way (Butler et al. 2002, Brownlee et al. 2006). This spatial variation may be related to reheating from the adjacent ~58 Ma Quottoon pluton of the Coast Mountains batholith (CMB) (Hollister et al. 2004, Brownlee et al. 2006). In order to determine the mineralogic origins of the magnetic variations we studied single grains of ilmenite-hematite from samples along a transect on the South side of the Skeena River using hysteresis, FORC, and remanence v. temperature experiments in concert with petrographic, SEM, and TEM observations. Light microscope and SEM observations indicate 3 basic types of grains; (1) hematite host with ilmenite lamellae (found >13 km from the CMB); (2) hematite with large, irregular ilmenite host regions (found ≤ 13 km from the CMB); and (3) hematite host with blitz texture formed by many large rutile needles in 6 orientations (found <8 km from the CMB). Single crystals distant from the CMB have coercivities from 250-350 mT, and FORC coercivity distributions that are curved and have a negative H_a offset, which we attribute to exchange coupling between hematite lamellae. Single crystals closer to the CMB appear to have two magnetic phases. Hysteresis loops are wasp-waisted, and FORC coercivities have two distinct peak distributions; one at 300-350 mT with a negative H_a offset, and another at 10-15 mT with no apparent offset. Remanence v. temperature, and TEM analyses indicate magnetite as the second magnetic phase. Samples far from the CMB show no evidence of this second magnetic phase indicating that magnetite growth may be related to reheating. Significant grain-to-grain variability in the ratio of magnetite to hematite within a sample makes FORC analysis useful in distinguishing the hysteresis properties of the two magnetitc phases. Magnetite coercivity distributions change only in peak intensity, while for hematite there are trends in the shape of the coercivity distribution, maximum coercivity, and Ha offset that may be correlated with distance from the CMB.
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting