HR: 0800h
AN: T11B-1255    [Abstracts]
TI: Dislocation Microstructures and Dissociation in Deformed Olivine Displaying the A-, B- and C-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: Inst. of Geophysics and Planetary, 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 Australia
AU: Karato, S
EM: hun-ichiro.karato@yale.edu
AF: Geology and Geophysics, Yale University, New Haven, CT 06520 United States
AB: Seismic anisotropy in the upper mantle is generally considered to result from olivine fabrics that result from dislocation creep. Jung and Karato, (2001) have shown that the fabrics developed in experimentally deformed olivine are dependent on H$_{2}$O fugacity and stress, suggesting that olivine fabric and the resulting seismic anisotropy can provide a means of probing H$_{2}$O content and stress in the upper mantle. Olivine fabrics, known as B-type and C-type, occur in olivine deformed under high H$_{2}$O fugacity and high stress (B-type) and low stress (C-type) whereas the more common A-type fabric occurs in olivine that is deformed under lower stress and H$_{2}$O fugacity. To better understand the development of B- and C-type fabrics in olivine, we have used TEM and HRTEM to investigate the deformation microstructures and dislocation core structures in experimentally deformed olivine displaying the A-, B-, and C-type fabrics. In a sample displaying the C-type fabric, tilt boundaries along (100) and (001), consisting of edge dislocations with b = [100] and b = [001], respectively, suggest dominance of the (001)[100] and (100)[001] slip systems. 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. In a B-type fabric sample, we find b = [100] and b = [001] dislocations with mixed character in (010), indicating that the (010)[001] and (010)[100] slip systems are active. Tilt boundaries along (001) with b = [001] suggest that the (100)[001] system is also active. HRTEM imaging of dislocations with b = [001] indicate that these dislocations are dissociated in two different ways. The first is the same described above for the C-type fabric whereas the second consists of two 1/2[001] partial dislocations. The A-type fabric sample contains many (100) tilt boundaries consisting of b = [100] edge dislocations, suggesting that the (010)[100] slip system is dominant. This sample also contains many b = [001] with predominantly screw character. HRTEM imaging of b = [001] dislocation cores was not possible, and b = [100] dislocations show no evidence of dissociation. The EBSD fabric analyses of Jung and Karato (2001) are consistent with the dominance of the (100)[001], (010)[001] and (010)[100] slip systems in the C-, B-, and A-type fabrics, respectively. The effect of high H$_{2}$O fugacity on the deformation mechanisms in olivine is to enhance slip along [001] on either the (100) or (010) planes for the C- and B-type fabrics, respectively. Dissociation of b = [001] edge dislocations in C- and B-type fabric samples is unlikely to enhance slip because the partial dislocations are out of the slip plane. However, the presence of hydrogen in olivine may reduce Peierls stresses for b = [001] in both C- and B-type fabrics.
DE: 8162 Rheology--mantle
DE: 5120 Plasticity, diffusion, and creep
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting