iron isotopes 11/ 15/ 12
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Iron Isotopes 11/ 15/ 12. Banded iron formation, 2.1Ga. Lecture outline: the basics abiotic and biotic fractionations in modern-day environments Fe isotopes in the geologic record. Closeup of BIF. The basics. Fe oxidation states : - PowerPoint PPT PresentationTRANSCRIPT
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Iron Isotopes 11/15/12
Lecture outline:1) the basics
2) abiotic and bioticfractionations inmodern-dayenvironments
3) Fe isotopesin the geologic record
Banded ironformation,2.1Ga
Closeup of BIF
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Possibly radioactive witht1/2 = 3.1 x 1021 yrs
The basicsFe oxidation states:+3 (“ferric”, insoluble, hematite Fe2O3)+2 (“ferrous”, soluble, pyrite FeS2)both (magnetite, Fe3O4)
Rt in ocean is 3-5yrs
Standard is the average composition of igneous rocks (Beard et al., 1999):54Fe/56Fe = 0.06368357Fe/56Fe = 0.02308758Fe/56Fe = 0.0030614
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A word on measuring Fe isotopes
Millet et al., 2012
Analyte Interference |Δ m| mR
52Cr = 52.94065 37Cl16O = 52.96081 0.02016 53 262956Fe = 55.93494 40Ar16O = 55.95729 0.02235 56 250540Ca = 39.96259 40Ar = 39.96238 0.00021 40 190476
Resolution:R=m/Dm
quad ICPMS = 1
HR-ICPMS = up to 8,000
±0.02-0.04‰ (2s)measured byisotope dilution(Johnson & Beard, 1999on TIMS,Millet et al., 2012 onMC-ICPMS)
±0.04-0.1‰ (2s)measured asnatural ratios(John & Adkins, 2010on MC-ICPMS)
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Natural range is ±2-3‰
Beard and Johnson, 2004
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Beard and Johnson, 2004
Rule of thumb:Ferric-bearing phases higher d56Fe thanferrous-bearing phases.Except pyrite, which has highest d56Fe.
3+
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Experimentally-derived equilibriumfractionations:temperature-dependentNo effect from [Cl]consistent between experimentssmall fractionations (2-3‰)
modified by Beard and Johnson, 2004from Welch et al., 2003
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modified by Beard and Johnson, 2004from Shuklan et al., 2002Relatively large kinetic fractionations:
these data can be modeled as aRaleigh distillation process withD56FeFeIII-Hem = +1.3‰
but equilibrium inferred value isD56FeFeIII-Hem = -0.14‰
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Low-T environments
Beard and Johnson, 2004
Some observations:- surficial processes that occur under
oxic conditions do not change d56Fe
- in order to see d56Fe changes, you needto mobilize Fe, make different pools
- in anoxic environments, redox cyclingof Fe results in large fractionations(via bacterial Fe reduction or interactionwith H2S)
Precipitation of sulfide minerals shiftd56Fe of residual vent fluids?
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Beard et al., 2003a
Sources of Fe to the modern oceans
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This model is confirmed byobserved Fe isotope anomaliesin Fe-Mn nodules from modernoceans
Beard et al., 2003a
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4 processes reflected in distribution of Fe isotopes in fluids:1. transport of dissolved or colloidal Fe in rivers2. oxidation of Fe2+
3. isotopic exchange with reactive S during BSR4. dissimilatory Fe reduction (DIR)
Johnson et al., 2008
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main point: Biotic and abiotic fractionations overlapbut DIR is contributing the largest, lowest d56Fe pool
Johnson et al., 2008
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Johnson et al., 2008
BIFs and associated d56Feanomalies signal presenceof large Fe3+ and Fe2+ poolssimultaneously; explainedby episodic O2 increasesfollowed by return tolow-O2 conditions?
Fe isotopes in theancient rock record
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Coupling betweenFe, S, and Cisotopes
Johnson et al., 2008
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grey bar =
rise of methanogenesis?(low d13C)
GOE = more SO42-
more BSR?
Johnson et al., 2008
Putting it all together…
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Johnson et al., 2008
grey bar =
GOE eventstops MIF signatransportto sediments
DIR increases as O2 increases,more Fe3+ available
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Christmas Creek Iron mine, Australia produces 6-7Mt per year of Fe ore!