HR: 09:45h
AN: GP31A-07    [PDF]
TI: Stable Remanence from Crystallographically Oriented Magnetite Inclusions in Clinopyroxene
AU: * Scott, G R
EM: gscott@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Feinberg, J M
EM: feinberg@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AB: The unusually stable remanent magnetization found in many gabbroic rocks actually comes from clinopyroxene crystals. While not magnetic itself, clinopyroxene acts as a crystallographic host to two arrays of magnetite inclusions, formed during slow cooling by exsolution. In the cone-sheet samples studied (Messum, Namibia), this exsolution takes place at $850\deg$$\pm$$50\deg$C. The typical size of a magnetite inclusion in each array is 1$\mu$m x 2$\mu$m x 50$\mu$m. Even with such extreme shape anisotropy, these inclusions are too large to expect single domain behavior. However, alternating field and thermal demagnetization, hysteresis, and FORC experiments all show stable remanence behavior, significantly in excess of that predicted for single domain magnetite (e.g. mean destructive field $>$ 80 mT). When viewed with a magnetic force microscope (MFM), the magnetite inclusions from both arrays display a regular (and fixed) internal segmentation of domains. This segmentation was the result of a second stage of exsolution, in which the oxide has segregated into two phases: one a nearly pure magnetite, and the other a Ti-rich ulv\"{o}spinel. This internal oxide exsolution was crystallographically coherent (both minerals have almost identical inverse spinel crystal structure) and has generated regularly spaced septa of ulv\"{o}spinel along $\{$100$\}$ of magnetite. Three-dimensionally, the ulv\"{o}spinel has isolated the magnetite into rectangular boxes of approximately 50nm x 100nm x 200nm. The long dimension of each nanobox is nearly perpendicular to the magnetite inclusion length. Each box is a single domain, usually of opposite polarity to its adjacent neighbors. The domain walls are fixed in position with nonmagnetic spinel dividers. Multidomain features such as Bloch (or Ne\'{e}l) walls have not been observed. Thus each magnetite inclusion is an aggregate of $\sim$10$^{5}$ closely packed magnetite nanoboxes, separated by continuous walls of ulv\"{o}spinel. The factors that control the orientation and dimensions of the magnetite boxes and ulv\"{o}spinel septa, appear to be related to the crystallographic parameters and cooling history of the clinopyroxene host. This host-dictated form of the magnetite boxes also controls the anisotropy of remanence and hysteresis, which is dipolar (not linear or double dipolar). The origin of the anomalously high coercivity appears to be the strong magnetic exchange coupling between adjacent, single domain magnetite nanoboxes. In summary, the clinopyroxene has provided an architectural framework upon which a second (internal oxide) exsolution built a coherent set of single-domain magnetites with fixed domain walls. It is this nanostructured assembly that has unusually stable remanence.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 5112 Microstructure
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting