HR: 11:35h
AN: V52A-06    [Abstracts]
TI: Dynamic Reduction of FeO-Bearing, Anhydrous Aluminosilicate Melts
AU: Everman, R L
EM: reverman@students.wisc.edu
AF: Univ. of Wisconsin-Madison, Materials Science Program, Madison, WI 53706 United States
AU: * Cooper, R F
EM: Reid_Cooper@Brown.edu
AF: Brown University, Dept. of Geological Sciences, Providence, RI 02912-1846 United States
AB: We have studied the reduction dynamics of FeO-bearing aluminosilicate melts at oxygen activities sufficiently low to form metallic iron. The experiments involved reacting droplets, suspended from refractory metal wires, with a high-temperature ($\sim$1400$^{o}$C), controlled-oxygen-activity environment maintained by a dynamic CO:CO$_{2}$ buffer. In the case of an FeO-doped magnesium aluminosilicate ("Fe-MAS") melt ($\sim$5 mol% FeO) exposed to an oxygen activity of 2x10$^{-13}$ ("QIF-2"; CO:CO$_{2}$=240:1), the reduction dynamic is rate-limited by chemical diffusion of Mg$^{2+}$: oxygen chemically ablates from the free surface and the network-modifying cations diffuse inward, charge-compensated by a counterflux of electron holes (the "semiconductor condition" holds for diffusion dynamics in this melt [e.g., Cook and Cooper, 2000]); nm-scale crystals of pure $\alpha$-Fe nucleate at an internal front. Diffusion of an oxygen species is not involved. In the case of FeO-doped calcium-magnesium aluminosilicate (Fe-CMAS) melt ($\sim$8 mol% FeO) exposed to an oxygen activity of 2x10$^{-15}$ (QIF-4; CO:CO$_{2}$=1750:1), the dynamic changes: molten Fe-C-Si alloy droplets form near the surface, and bubbles are seen to form internally, truncating at an internal front. Further, the reaction occurs more slowly than that seen for the similarly polymerized Fe-MAS melt. The results suggests that molecular CO diffuses inward, consuming electron holes so as to form carbonate ion species in the melt [cf. Brooker et al., 2001]. Quenching produces a driving potential to reverse the internal reduction reaction, so creating the bubbles. Consumption of the electron holes by reaction with the carbon species dramatically reduces the reduction-to-metal kinetics. Brooker RA, Kohn SC, Holloway JR, McMillan PF (2001) Chem Geol 174:241-254; Cook GB, Cooper RF (2000) Am Mineral 85:397-406
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3662 Meteorites
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting