HR: 1340h
AN: DI33A-1119    [Abstracts]
TI: Micro-XANES Determination Fe Speciation in Natural Basalts at Mantle-Relevant fO2
AU: * Fischer, R
EM: rebecca-fischer@northwestern.edu
AF: Northwestern Univ., 633 Clark St., Evanston, IL 60208, United States
AU: * Fischer, R
EM: rebecca-fischer@northwestern.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, United States
AU: Cottrell, E
EM: cottrelle@si.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, United States
AU: Lanzirotti, A
EM: lanzirotti@bnl.gov
AF: National Synchrotron Light Source, Brookhaven Natl. Lab., Upton, NY 11973, United States
AU: Kelley, K A
EM: kelley@gso.uri.edu
AF: Grad. Sch. of Oceanography, Univ. of Rhode Island, Narragansett, RI 02882, United States
AB: We demonstrate that the oxidation state of iron (Fe³⁺/ΣFe) can be determined with a precision of ±0.02 (10% relative) on natural basalt glasses at mantle-relevant fO₂ using Fe K-edge X-ray absorption near edge structure (XANES) spectroscopy. This is equivalent to ±0.25 log unit resolution relative to the QFM buffer. Precise determination of the oxidation state over this narrow range (Fe³⁺/ΣFe=0.06-0.30) and at low fO₂ (down to QFM-2) relies on appropriate standards, high spectral resolution, and highly reproducible methods for extracting the pre-edge centroid position. We equilibrated natural tholeiite powder in a CO/CO₂ gas mixing furnace at 1350°C from QFM-3 to QFM+2 to create six glasses of known Fe³⁺/ΣFe, independently determined by Mössbauer spectroscopy. XANES spectra were collected at station X26A at NSLS, Brookhaven Natl. Lab, in fluorescence mode (9 element Ge array detector) using both Si(111) and Si(311) monochromators. Generally, the energy position of the 1s→3d (pre-edge) transition centroid is the most sensitive monitor of Fe oxidation state using XANES. For the mixture of Fe oxidation states in these glasses and the resulting coordination geometries, the pre-edge spectra are best defined by two multiple 3d crystal field transitions. The Si(311) monochromator, with higher energy resolution, substantially improved spectral resolution for the 1s→3d transition. Dwell times of 5s at 0.1eV intervals across the pre-edge region yielded spectra with the 1s→3d transition peaks clearly resolved. The pre-edge centroid position is highly sensitive to the background subtraction and peak fitting procedures. Differences in fitting models result in small but significant differences in the calculated peak area of each pre-edge multiplet, and the relative contribution of each peak to the calculated centroid. We assessed several schemes and obtained robust centroid positions by simultaneously fitting the background with a damped harmonic oscillator (DHO) function and pre-edge features with two Gaussians over a sub-sample of the pre-edge region (7110-7120 eV). We found that the relation between Fe³⁺/ΣFe and the centroid energy is non-linear over this fO₂ range, which is expected if the coordination environment changes with oxidation state. ΔQFM is linearly related (R²=0.99) to the centroid position. This new calibration allows the oxidation states of natural mantle melts to be discriminated with high spatial resolution (9μm). We apply the new calibration to determination of Fe³⁺/ΣFe in natural basaltic glasses and olivine-hosted glass inclusions (Cottrell et al. & Kelley et al., this meeting).
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting