HR: 15:10h
AN: DI43A-07    [Abstracts]
TI: New Micro-XANES Determinations of Fe Speciation as a Proxy for Oxidation State of Global MORB and Arc Magmas
AU: * Cottrell, E
EM: cottrelle@si.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, 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
AU: Fischer, R
EM: FischerRA@si.edu
AF: Dept. of Mineral Sciences, Smithsonian Inst., Washington, DC 20013, United States
AU: Fischer, R
EM: FischerRA@si.edu
AF: Northwestern Univ., 633 Clark St., Evanston, IL 60208, United States
AB: Erupted lavas may reflect their source oxidation state and offer insight into the relationship between tectonic environment, volatiles, and redox. The Fe³⁺/ΣFe of basaltic glasses provides one probe of oxidation state and can be determined at high spatial (9μm) and high fO₂ (±0.25 log units relative to the QFM buffer) resolution using a new μXANES calibration (Fischer et al., this meeting). We present new μXANES determinations of Fe³⁺/ΣFe for natural basalt glasses from mid-ocean ridges (n=18) and back-arc basins (n=2), and primitive, undegassed (up to 6.4 wt% H₂O), olivine-hosted melt inclusions from arc volcanoes (n=18). We used μXANES to analyze a small, global subset of MORB pillow glasses previously analyzed for Fe³⁺/ΣFe by colorimetry (Christie et al., 1986). Preliminary fO₂ derived from the spectra are significantly more oxidized (~QFM ±0.5) than those determined by Christie et al. (QFM -1.2 ±0.44), but are similar to wet-chemical determinations (Bezos and Humler, 2005; QFM –0.41 ±0.43) for global MORB and the JDFD2 standard. μXANES assures passage of the synchrotron X-ray beam through glass alone, distinguishing it from acid digestion techniques that necessarily sample larger aliquots, including microlites, or that may induce changes in the Fe oxidation state. Inclusion/exclusion of microlites, however, does not appear to account for the higher fO₂ measured using μXANES, nor does the X-ray beam have any oxidizing effect on the glass. The systematic offset between Fe³⁺/ΣFe determined by μXANES and colorimetry could reflect analytical bias or sampling bias due to the contrast in analyzed glass volume between these techniques, though Fe³⁺/ΣFe varied little within single glass chips (σ~0.02 but up to 0.5 log units). In the olivine-hosted glass inclusions, where small spatial sampling is required, these new data show Fe³⁺/ΣFe ratios consistently higher (>QFM) than in the global MORB samples, and a general increase in Fe³⁺/ΣFe with increasing dissolved H₂O content (Kelley et al., this meeting).
DE: 1030 Geochemical cycles (0330)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3621 Mantle processes (1038)
DE: 3640 Igneous petrology
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting