HR: 15:25h
AN: MR33A-08    [Abstracts]
TI: Oxidation Kinetics and Textures for Natural and Synthetic MORB Glasses and Subliquidus Melts: RBS Analyses
AU: * Burgess, K
EM: katherine_burgess@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912-1846, United States
AU: Cooper, R F
EM: reid_cooper@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912-1846, United States
AU: Cherniak, D J
EM: chernd@rpi.edu
AF: Rensselaer Polytechnic Institute, Department of Earth & Environmental Sciences, Science Center 1W19, Troy, NY 12180, United States
AB: The dynamics of oxidation of basaltic melts and glasses involves the diffusive motion of network-modifying cations out of the material (i.e., to the free surface) where they react with atmospheric oxygen to produce oxide precipitates or thin films. The cation flux is countered by an inward flux of electron holes; thus the material, modified by the loss of cations, experiences, too, oxidation at an internal front [e.g., Cook & Cooper, 2000]. The texture so produced is a complex, non-equilibrium one. For glasses of similar polymerization to basalts, the internal oxidation front often involves the precipitation of nanometer-scale ferrites; in the oxidation dynamic, because of the persistent, metastable texture, there is no reason to believe that these ferrites should have compositions predicted by chemical equilibrium. The effect on understanding paleomagnetic field strength recorded in MORBs could be profound: this is the long-range goal of our experiments. We report here on oxidation textures produced for three glasses and melts produced from (i) a natural MORB and (ii) mixed oxides to produce MORB analogs \--- one free of all alkali cations. All materials were melted at ambient pressure at 1430°C in an atmosphere buffered at FMQ. Glasses so prepared had their glass-transition temperatures (T_g) characterized by thermal analysis. Oxidation experiments were performed on well- annealed glass specimens at temperatures T_g ± ~25°C as well as on crystallizing melts at 900°C; oxidation was done in dry air at ambient pressure. Ion (Rutherford) backscattering spectrometry (RBS) was used to characterize the near-surface texture (chemical distribution) of the oxidized specimens. The textures are found to be sensitive to oxidation temperature, initial melt polymerization, and the bulk chemistry (particularly as it affects the summed partial molar volumes of network-modifying oxides). A general result \--- applicable to all compositions \--- is that out-diffusion of ionic Fe and ionic Mg effects oxidation at higher temperatures, giving way to kinetic domination of ionic Ca at lower temperatures. Specific results will be presented. We are initiating transmission electron microscopy studies to characterize the compositions and distributions of internally-nucleated ferrites. Cook, G.B. and R.F. Cooper, Amer. Mineral. 85, 397-406 (2000).
DE: 1540 Rock and mineral magnetism
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3904 Defects
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting