HR: 10:20h
AN: V42A-01 INVITED     [Abstracts]
TI: Mantle Oxidation State and Oxygen Fugacity: Constraints on Mantle Chemistry, Structure and Dynamics
AU: * McCammon, C
EM: catherine.mccammon@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440 Germany
AB: Mantle oxidation state is reviewed based on M”ssbauer measurements of mantle xenoliths and synthetic high-pressure phases, and suggests relatively low Fe$^{3+}$/$\Sigma$Fe in upper mantle and transition zone phases, but high Fe$^{3+}$/$\Sigma$Fe in lower mantle (Mg,Fe)(Si,Al)O$_{3}$ perovskite, even under reducing conditions, with significant implications for physical and chemical properties. Whole rock Fe$_{2}$O$_{3}$ concentrations for pyrolite mantle are calculated to be low in the upper mantle and transition zone (ca. 0.3 wt% Fe$_{2}$O$_{3}$), but high in the lower mantle (ca. 4 wt% Fe$_{2}$O$_{3}$). High Fe$_{2}$O$_{3}$ concentrations may be balanced according to the iron disproportionation reaction 3 Fe$^{2+}$ = Fe$^{0}$ + 2 Fe$^{3+}$. Oxygen fugacity is relatively high at the top of the upper mantle (near the fayalite-magnetite-quartz buffer) due to the concentration of Fe$^{3+}$ in modally minor phases, but probably low in the transition zone and lower mantle (close to metal equilibrium). Material in subducting slabs may be more oxidised, however, and the expected increase in Fe$_{2}$O$_{3}$ concentration due to an increase in majorite garnet proportion and the garnet-perovskite transformation may be partly balanced by reduction of oxidised species such as carbonate in the subducting slab. Such coupled redox reactions may contribute to the genesis of diamonds in the lower mantle. Disproportionation of iron in the lower mantle to produce ca. 1 wt% Fe-rich metal may have occurred in the early Earth, with important consequences for mantle geochemistry. If the metal phase does not segregate appreciably, material can move across the lower mantle boundary without a net enrichment or depletion in oxygen.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3655 Major element composition
DE: 3924 High-pressure behavior
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting