HR: 0800h
AN: GP41A-0864    [Abstracts]
TI: The Effects of Nanoscale Exsolution on Magnetic Properties: Insights From FORC Diagrams
AU: * McEnroe, S
EM: suzanne.mcenroe@ngu.no
AF: NGU, 7491, Trondheim, N-7491 Norway
AU: Hirt, A M
GP41A-0864 AF: Inst. of Geophysics , ETH, Zurich, CH 8093 Switzerland
AU: Harrison, R
GP41A-0864 AF: Dept. Earth Sci., Cambridge Univ., Cambridge, CB2 3EQ United Kingdom
AU: Robinson, P
GP41A-0864 AF: NGU, 7491, Trondheim, N-7491 Norway
AU: Brown, L
GP41A-0864 AF: Dept. Geosciences, Univ. Mass., Amherst, 01003 United States
AB: Samples of igneous and metamorphic rocks from the USA, Canada, Chile, Sweden, Norway and Australia contain oxides of either nearly `pure hematite' or from the hematite-ilmenite solid solution series. Samples with oxides from the hematite-ilmenite solid solution series all have nanoscale exsolution of hematite and ilmenite that formed as a chemical remanent magnetization (CRM) during cooling from igneous or metamorphic settings. The nearly pure hematite samples show relic exsolution of spinel and/or extensive twinning in the hematite. Hysteresis properties have been measured from 10 K to 300 K on most samples and to 900K on selected samples. All samples contain a hematite component either as exsolution lamellae, as the host material, or as a nearly pure oxide phase. Variations in room temperature coercivity ranged from 9mT to 440mT. With this large range in hysteresis behavior FORC measurements were made to help separate the possibilities of 1) co-existing phases, 2) exchange coupling or interactions and 3) contributions from the nanoscale exsolution. It is important to note that a saturated state may not have been reached at the start of the FORC measurements for the higher coercivity samples. However, FORC diagrams on samples with two or more oxides were very helpful in determining the contributions to the bulk coercivity of the different minerals. Numerous samples with exsolution showed distinct negative peak value of Hu. Separating out whether this response is due to the fact that these samples contain hematite, or due to interaction within the oxides is open to speculation. Most of the examined samples contained nanoscale exsolution as imaged with TEM, all had a negative Hu and the amount of displacement varied strongly. The amount of displacement cannot be correlated directly to the size, or spacing of the lamellae. Correlation of the distribution of Hc and Hu with TEM images and other rock magnetic properties will be discussed.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005