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

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

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)

Natural range is ±2-3‰

Beard and Johnson, 2004

Beard and Johnson, 2004

Rule of thumb:Ferric-bearing phases higher d56Fe thanferrous-bearing phases.Except pyrite, which has highest d56Fe.

3+

Experimentally-derived equilibriumfractionations:temperature-dependentNo effect from [Cl]consistent between experimentssmall fractionations (2-3‰)

modified by Beard and Johnson, 2004from Welch et al., 2003

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‰

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?

Beard et al., 2003a

Sources of Fe to the modern oceans

This model is confirmed byobserved Fe isotope anomaliesin Fe-Mn nodules from modernoceans

Beard et al., 2003a

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

main point: Biotic and abiotic fractionations overlapbut DIR is contributing the largest, lowest d56Fe pool

Johnson et al., 2008

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

Coupling betweenFe, S, and Cisotopes

Johnson et al., 2008

grey bar =

rise of methanogenesis?(low d13C)

GOE = more SO42-

more BSR?

Johnson et al., 2008

Putting it all together…

Johnson et al., 2008

grey bar =

GOE eventstops MIF signatransportto sediments

DIR increases as O2 increases,more Fe3+ available

Christmas Creek Iron mine, Australia produces 6-7Mt per year of Fe ore!