HR: 16:15h
AN: B24A-02    [Abstracts]
TI: Determining the Origin of Magnetite in Sedimentary Rocks Using Fe Isotopes
AU: * Beard, B L
EM: beardb@geology.wisc.edu
AF: Univ of Wisconsin-Madison, Dept. Geology and Geophysics 1215 West Dayton Street, Madison, WI 53706 United States
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: Univ of Wisconsin-Madison, Dept. Geology and Geophysics 1215 West Dayton Street, Madison, WI 53706 United States
AB: Authigenic magnetite is largely produced by the reaction of aqueous Fe(II) with poorly crystalline iron oxides and hydroxides. The Fe isotope composition of authigenic magnetite will primarily be controlled by the Fe isotope composition of the aqueous Fe, and the proportion of aqueous Fe to reactive iron oxide. In a situation where the flux of aqueous Fe(II) to iron oxide is high, the dominant control on the isotopic composition of authigenic magnetite will be the equilibrium isotopic fractionation factor between aqueous Fe(II) and magnetite. Based on long-term (1 year) experiments, magnetite is 1.3 ‰ greater in ⁵⁶Fe/⁵⁴Fe as compared to aqueous Fe(II). In situations where the flux of Fe(II) to iron oxide is low, the composition of magnetite will be controlled by simple mixing between the isotope composition of the ferric oxide and aqueous Fe(II) according to magnetite stoichiometry. The Fe isotope composition of sedimentary pore fluids and reactive iron oxide are just beginning to be constrained. Sedimentary pore fluids from sites along the California margin have low δ⁵⁶Fe values (-3 to 1 ‰) where dissimilatory Fe reduction is active. Pore fluids have high δ⁵⁶Fe values (-0.5 to +0.5 ‰) when they appear to be associated with bacterial sulfate reduction. The Fe isotope composition of reactive Fe oxide is expected to cluster about δ⁵⁶Fe = 0 if the suspended load of rives are the major contributors. Based on this analysis, authigenic magnetite that has high δ⁵⁶Fe values (0.8 ‰ or greater) are diagnostic of an authigenic environment where the flux of Fe(II) to iron oxide is high and the fluid has a δ⁵⁶Fe value of -0.5 ‰ or greater. Magnetite that has low δ⁵⁶Fe values (less than -1.0 ‰) would appear to be associated with a high Fe(II) to iron oxide flux and fluids with low ⁵⁶Fe/⁵⁴Fe ratios. The origin of magnetite with intermediate compositions (+0.5 to -1 ‰) are less certain and may be produced in low Fe(II) to iron oxide environments over a wide range of fluid isotope compositions.
DE: 0424 Biosignatures and proxies
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0471 Oxidation/reduction reactions (4851)
SC: Biogeosciences [B]
MN: Fall Meeting 2005