HR: 10:20h
AN: T32B-01 INVITED     [Abstracts]
TI: Hydrogen Mobility in Orthopyroxene in Comparison to Other Upper Mantle Minerals
AU: * Stalder, R
EM: rstalde@gwdg.de
AF: Department of Earth Sciences, G\"{o}ttingen University, Goldschmidtstr.1, G\"{o}ttingen, 37077 Germany
AB: The kinetics of hydrogen self-diffusion in orthopyroxene has been studied from 1 to 5000 bar for several chemically distinct synthetic samples. Samples were synthesized at 25 kbar and $1150\deg$C under H$_{2}$O and D$_{2}$O saturated conditions, respectively, and analysed by FTIR-spectroscopy and electron microprobe. For the kinetic studies, oriented wafers were produced and either exposed to H$_{2}$-atmosphere at $800\deg$C directly or equilibrated with H$_{2}$O or D$_{2}$O in a sealed Au-tube in an internally heated pressure vessel. After verification that H-D-exchange was fully reversible, diffusion coefficients were determined in both time dependent exchange experiments and from diffusion profiles. H-self-diffusion in pure enstatite turned out to be anisotropic, D[001] being approximately one order of magnitude faster than D[010]. Increasing pressure enhances H-mobility (-logD[010] = 12.9 (12.0) at 1 (2000) bar and $800\deg$C, respectively). Incorporation of Al and Cr in amounts comparable to natural mantle samples decreases H-mobility by one and two orders of magnitude, respectively. H-mobility in natural orthopyroxene from the Earth`s mantle is probably achieved by electron-hole diffusion coupled to a counter flux of protons, which is rate-limited by proton self-diffusion. Therefore, H-diffusivities determined in this study correspond to the same diffusion mechanism and are applicable to mantle processes. Compared to other nominally anhydrous mantle minerals, H-mobility in enstatite at $800\deg$C is slightly slower than in olivine and clinopyroxene [1-3], but may be faster at mantle temperatures if the higher activation energy is considered [4]. The data for Cr-free samples imply that H-diffusivities are fast enough to allow considerable H-exchange between a mantle xenolith consisting of mm-sized crystals and its host magma upon ascent. However, if it is assumed that Cr does not affect the activation energy for H-diffusion, Cr-rich orthopyroxenes may be protected from equilibration with the host magma. The anisotropic behavior of H-diffusion in orthopyroxene is very similar to that of clinopyroxene. Although pyroxenes show less pronounced anisotropic H-mobility than olivine, their generally higher water solubility may suggest that both may contribute to the electrical anisotropy of the asthenosphere inferred from magnetotelluric data [5]. [1] Mackwell SJ and Kohlstedt DL (1990) J Geoph Res 95, 5079-5088 [2] Carpenter Woods S, Mackwell S, Dyar D (2000) Amer Mineral 85, 480-487 [3] Hercule S and Ingrin J (1999) Amer Mineral 84, 1577-1587 [4] Stalder R and Skogby H (2003) Phys Chem Min 30, 12-19 [5] Bahr K and Duba A (2000) Earth Planet Sci Lett 178, 87-95
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3904 Defects
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting