HR: 1340h
AN: MR13C-1398    [Abstracts]
TI: Point defects in (Mg,Fe)O at high pressures: where does hydrogen dominate over ferric iron?
AU: * Otsuka, K
EM: kazuhiko.otsuka@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511, United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511, United States
AB: The point defects play an important role in transport processes of minerals including diffusion, electrical conduction and plastic deformation. Point defects caused by ferric iron and/or hydrogen (proton) are dominant defects in most of the iron-bearing minerals including olivine and (Mg,Fe)O. In many upper-mantle minerals such as olivine, the concentration of ferric iron is much smaller than that of hydrogen, and therefore the small amount of hydrogen changes their transport properties dramatically. However, the situation is very different for lower- mantle minerals such as (Mg,Fe)O. In this presentation, we will review the available experimental data on point defects in (Mg,Fe)O and discuss the relative importance of ferric iron and hydrogen at high pressures based on atomic models. The existing low-pressure data indicate that the maximum solubility of ferric iron in (Mg,Fe)O is on the order of 0.1 (atomic fraction in the total iron), which is much higher than that of hydrogen. However, experimental studies by Bolfan-Casanova et al (2002, 2006) indicate that the solubility of ferric iron decreases while that of hydrogen increases with pressure. This suggests that the dominant impurity to generate point defects in (Mg,Fe)O may change from ferric iron to hydrogen at high pressure. Therefore it is important to quantify the pressure dependence of the solubility of ferric iron and hydrogen. We have explored two models of ferric iron- related defects and found that the existing experimental data suggest that ferric iron may occur at two lattice sites: the tetrahedral site as interstitial atoms as well as the octahedral site. The pressure dependence of the solubility of hydrogen in (Mg,Fe)O are also estimated based on the experimental data and defect models. The cross-over of defect solubility likely occurs in the lower mantle, but the exact depth is poorly constrained because of large uncertainties in the hydrogen solubility and the mechanisms of hydrogen dissolution in (Mg,Fe)O. Possible implications of this cross-over on some physical properties of (Mg,Fe)O will be discussed.
DE: 3904 Defects
DE: 5139 Transport properties
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting