HR: 0830h
AN: GP11B-0261 INVITED     [PDF]
TI: Orientations of Exsolved Magnetite Inclusions in Clinopyroxene and Plagioclase Determined With Electron Backscatter Diffraction (EBSD)
AU: * Feinberg, J M
EM: feinberg@eps.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Wenk, H
EM: wenk@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Scott, G R
EM: gscott@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Dept. of Earth and Planetary Science, University of California, 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
AB: Crystallographically oriented magnetite inclusions occur as subsolidus exsolution features in slowly cooled mafic rocks and are of interest to paleomagnetism because of their highly stable magnetic remanence. Many inclusions in both clinopyroxene and plagioclase exist as elongate laths with generalized dimensions of 1 $\mu$m x 2 $\mu$m x 50 $\mu$m. Of initial interest is the formation temperature and orientation of these elongate inclusions with respect to their silicate hosts. In this study, the electron backscatter diffraction (EBSD) technique is used to determine orientation relationships across exsolution boundaries for magnetite inclusions $>$ 0.5 $\mu$m in diameter in host crystals of both clinopyroxene and plagioclase. Magnetite inclusions in clinopyroxenes from the Early Cretaceous Messum Complex of Namibia occur as two arrays growing within (010) of clinopyroxene and elongated subparallel to either the [100] or [001] axes. Inclusions subparallel to [100]$_{cpx}$ have [-110]$_{mag}$ // [010]$_{cpx}$, (-1-11)$_{mag}$ // (-101)$_{cpx}$, and [112]$_{mag}$ // [101]$_{cpx}$. Inclusions subparallel to [001]$_{cpx}$ have [-110]$_{mag}$ // [010]$_{cpx}$, (111)$_{mag}$ // (100)$_{cpx}$, and [-1-12]$_{mag}$ // [001]$_{cpx}$. Both arrays of inclusions are oriented such that planes of roughly close-packed oxygen atoms in both phases, \{111\} in magnetite and (-101) and (100) in clinopyroxene, are aligned. These EBSD-derived orientation relationships agree well with previous TEM and X-ray diffraction studies on similar materials, and are consistent with a high-temperature exsolution origin for the magnetite and coexistent amphibole. In contrast to clinopyroxene, the orientation relationships between plagioclase and its exsolved magnetite inclusions have so far been unexplored. Such inclusions are responsible for stable magnetic remanence in both layered intrusions and oceanic gabbros. Exsolved magnetite inclusions in plagioclase crystals from anorthosites in the Early Jurassic Freetown Complex of Sierra Leone have been investigated. Preliminary results from EBSD analyses of plagioclase (An$_{66}$) enable us to discern at least two arrays of magnetite inclusions. In the first array, (100)$_{mag}$ // (101)$_{pl}$ and (111)$_{mag}$ // (011)$_{pl}$. In the second array, (311)$_{mag}$ // (101)$_{pl}$ and (111)$_{mag}$ // (111)$_{pl}$. A unit cell with c = 14.203 $\AA$ was used to index the plagioclase diffraction patterns.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting