Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmgp51a0980k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #GP51A-0980
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1094 Instruments And Techniques, 3630 Experimental Mineralogy And Petrology, 3954 X Ray, Neutron, And Electron Spectroscopy And Diffraction
Scientific paper
Phase stability of mineral assemblages and their physical properties, especially transport properties, are influenced by oxygen fugacity. Redox effects in earth and planetary systems at high pressure include setting of ferric/ferrous iron ratios [controlling the electrical conductivity of crustal and mantle materials] and possible chemical reactions at the Earth's core-mantle boundary. Experimental controls of oxygen fugacity in high-pressure devices have been limited to discrete electrochemical potentials set by buffers such as C-CO, Ni-NiO, and QFM. By contrast, an electric field applied across a silicate sample inside a piston cylinder apparatus establishes a smoothly-varying electrochemical gradient that can be quantified and tied to the oxygen fugacity scale through synchrotron microXANES of polyvalent V and Fe within the silicate. Fugacity gradient samples were synthesized in a modified Boyd-England piston-cylinder configuration. Platinum electrodes were placed at both ends of a 2-mm cylinder of basaltic composition silicate glass containing ~5% Fe and ~2% V. The sample assembly was surrounded by MgO ceramic, sheathed within a Mo faraday sleeve to insulate the sample from the AC field of the heater, and placed within a 0.5 inch diameter pressure vessel. The assembly was sintered at 800°C for 72 hours to eliminate porosity in the MgO capsule, and then heated to 1400°C for 23 hrs at 10 kbar. At high temperature, a 1V potential difference was applied across the electrodes via an external power supply. The sample was then quenched, potted in epoxy, and polished to a thickness of ~30 μm, and analyzed via optical and scanning electron microscopy. Vanadium, with oxidation states of 0 and +II to +V, was used as a chemical marker to evaluate the absolute value of the fO2 conditions across the silicate sample. Synchrotron-based microXANES techniques at GSECARS at the Advanced Photon Source in Argonne, IL were used to measure the pre-edge peak height at the vanadium absorption edge, as a function of distance between the anodic and cathodic electrodes of the recovered piston cylinder experiments. The intensity of the pre-edge peaks varied greatly across the sample, from ~5% near the cathode end to ~70% of the absorption edge level adjacent to the anode. The systematic increase in the pre-edge peak was calibrated to the vanadium valence state and oxygen fugacity by comparison with vanadium microXANES spectra obtained for synthetic komatiite charges (known fO2; Canil 1997) and basaltic glasses (known fO2 and oxidation state; Schreiber 1987). The average vanadium oxidation state varies monotonically from +2.5 at the cathode (reducing) electrode to +4.5 at the anode (oxidizing) electrode, corresponding to an oxygen fugacity varying from -11 to -5 (log units) from cathode to anode. The sample appears reddish at the anode (oxidizing) end and grayer at the cathode (reducing) end, due in part to reducing the iron ferric/ferrous ratio from anode to cathode, in harmony with the V results. In summary, the application of an electric field creates an oxygen fugacity continuum in high-pressure apparati.
Kavner Abby
Newville Matthew
Sutton Sean
Walker Danielle
Wheeler Kathleen
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