HR: 14:10h
AN: T42D-03    [PDF]
TI: Dislocation Microstructures in Deformed Olivine Displaying the C-type and B-type Fabrics
AU: * Sharp, T G
EM: tom.sharp@asu.edu
AF: Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Jung, H
EM: hjung@citrus.ucr.edu
AF: Earth Sciences, University of California, Riverside, CA 92521 United States
AU: Fitz Gerald, J
EM: John.FitzGerald@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Austria
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520 United States
AB: Because seismic anisotropy in the upper mantle is commonly a result of lattice preferred orientations (LPOs) of minerals as a result of mantle flow, seismic anisotropy is used to probe fabrics and therefore flow directions in the mantle. Jung and Karato, (2001) have shown that the fabrics developed in experimentally deformed olivine are dependent on H2O fugacity and stress, suggesting that olivine fabric in the upper mantle can provide a means of probing H2O content and stress. Olivine fabrics, known as B-type and C-type, occur in olivine deformed under high H2O fugacity and high stress. In order to extrapolate these experimental deformation fabrics to the mantle, one must understand how H2O changes the mechanisms of plastic deformation in olivine. Experimentally deformed olivine samples with C-type and B-type fabrics have been examined by TEM to determine dislocation microstructures, core structures and active slip systems. In the C-type sample, we examined a relatively large olivine grain with its b axis normal to the TEM foil and the a and b axes approximately 45$\deg$ from the slip direction. Tilt boundaries along (100) and (001), consisting of edge dislocations with b = [100] and b = [001], respectively, indicate that the dominant slip systems are (001)[100] and (100)[001]. HRTEM imaging of the b = [100] and b = [001] edge dislocations along [010] shows no dissociation for b = [100], whereas the b = [001] dislocations are dissociated into 1/2[-101] and 1/2 [101] partial dislocations separated by 3 nm along the (-101) plane. The C-type fabric, which occurs in wet olivine deformed at low stress, results from the domination of the (100)[001] slip system over the (010)[100] slip system that produces the A-type fabric. Based on the abundance of b = [001] edge dislocations, we infer that the presence of hydrogen in olivine reduces Peierls stresses for glide of b = [001] screw dislocations in (100). The more complicated microstructures in the B-type sample will be presented at the meeting.
DE: 3900 MINERAL PHYSICS
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2003 Fall Meeting