Diffusion-Controlled Reaction rim Growth Studied in Thin Films Down to the Nano Scale

HR: 09:00h
AN: T41B-04 [PDF]
TI: Diffusion-Controlled Reaction rim Growth Studied in Thin Films Down to the Nano Scale
AU: * Milke, R
EM: bergner-milke@t-online.de
AF: Mineralogisches Institut der Universit„t Basel, Bernoullistr. 30, Basel, CH-4056 Switzerland
AU: * Milke, R
EM: bergner-milke@t-online.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14478 Germany
AU: Dohmen, R
EM: ralf.dohmen@ruhr-uni-bochum.de
AF: Institut fr Geologie, Mineralogie und Geophysik, Ruhr-Universit„t Bochum, Bochum, D-44780 Germany
AU: Wiedenbeck, M
EM: michawi@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14478 Germany
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14478 Germany
AU: Abart, R
EM: rainer.abart@unibas.ch
AF: Mineralogisches Institut der Universit„t Basel, Bernoullistr. 30, Basel, CH-4056 Switzerland
AU: Becker, H
EM: becker@ep3.ruhr-uni-bochum.de
AF: Institut fr Physik mit Ionenstrahlen, Ruhr-Universit„t Bochum, Bochum, D-44780 Germany
AB: Grain boundaries play a crucial role for diffusion in polycrystalline materials. Diffusion through polycrystalline rims of enstatite-rich pyroxene was studied in pulsed-laser deposited thin films [1] by the rim growth method. The starting samples consisted of layers of isotopically doped ($^{18}$O, $^{29}$Si) olivine fo$_{90}$fa$_{10}$ (c. 500 nm thick) and pyroxene en$_{90}$fs$_{10}$ (c. 100 nm thick) on a polished quartz surface. Annealing experiments were performed at temperatures between 1000 and 1200 $\deg$C at fO$_{2}$ of 10$^{-10}$ bar. Layer thickness and composition were measured by Rutherford Back-Scattering (RBS) and TEM using Focused Ion Beam preparation methods. O and Si isotope profiles were measured by SIMS depth scanning. The miniaturization of the layer assembly allows one to study growth at lower temperatures than previously possible and avoids large extrapolations to natural conditions. During the experiments the enstatite layers thicken by rates for $\Delta$X$^{2}$ of 700 to 50000 nm$^{2}$/h at the chosen conditions. An activation energy of 398ñ30 kJ/mol was derived. The rim growth rates are slightly slower than expected from an extrapolation of high temperature data (1350-1450 $\deg$C) [2] but yield the identical activation energy, suggesting that rim growth over the entire temperature range is controlled by the same diffusion mechanism and that this is valid down to the nano scale. At 1000 $\deg$C enstatite rim growth in our dry experiments is slower by about 4 orders of magnitude than in high pressure experiments at 7 [3] resp. 10 kbar [4] in the presence of small amounts of H$_{2}$O. The isotope concentration profiles reveal that Si acts as a slow diffusing component compared to O. Effective diffusion coefficients D$_{eff}$ for Si were derived from the SIMS profiles using a moving boundary diffusion model. D$_{eff}$$^{Si}$ in the dry polycrystalline enstatite rims is 2-3 orders of magnitude smaller than in polycrystalline olivine under similar conditions [5]. At 1000 $\deg$C D$_{eff}$$^{Si}$ is 4 orders of magnitude smaller than in enstatite rims during hydrous high pressure experiments [6]. Ref.: [1] Dohmen et al. (2002) Eur J Miner 14: 1155-1168; [2] Fisler et al. (1997) Phys Chem Minerals 24: 264-273; [3] Yund (1997) Contrib Miner Petrol 126: 224-236; [4] Milke et al. (2001) Contrib Miner Petrol 142: 15-26; [5] Farver \& Yund (2000) Geophys Res Letters 27, 2337-2340; [6] Abart et al (2002) J Conf Abs 7, 4
DE: 3630 Experimental mineralogy and petrology
DE: 3947 Surfaces and interfaces
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: 2003 Fall Meeting