1. Iron Isotopes 11/15/12 Lecture outline: the basics abiotic and biotic
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

2. The basics Fe oxidation states:
+3 (“ferric”, insoluble, hematite Fe2O3)
+2 (“ferrous”, soluble, pyrite FeS2)
both (magnetite, Fe3O4)
Rt in ocean is 3-5yrs
Possibly radioactive with
t1/2 = 3.1 x 1021 yrs
Standard is the average composition of igneous rocks (Beard et al., 1999):
54Fe/56Fe =
57Fe/56Fe =
58Fe/56Fe =

3. A word on measuring Fe isotopes
Resolution:
R=m/Dm
quad ICPMS = 1
HR-ICPMS = up to 8,000
± ‰ (2s)
measured by
isotope dilution
(Johnson & Beard, 1999
on TIMS,
Millet et al., 2012 on
MC-ICPMS)
± ‰ (2s)
measured as
natural ratios
(John & Adkins, 2010
on MC-ICPMS)
Millet et al., 2012
Analyte Interference |Δ m| m R
52Cr = Cl16O =
56Fe = Ar16O =
40Ca = Ar =

4. Natural range is ±2-3‰
Beard and Johnson, 2004

5. 3+
Rule of thumb:
Ferric-bearing phases higher d56Fe than
ferrous-bearing phases.
Except pyrite, which has highest d56Fe.
Beard and Johnson, 2004

6. Experimentally-derived equilibrium
fractionations:
temperature-dependent
No effect from [Cl]
consistent between experiments
small fractionations (2-3‰)
modified by Beard and Johnson, 2004
from Welch et al., 2003

7. modified by Beard and Johnson, 2004
from Shuklan et al., 2002
Relatively large kinetic fractionations:
these data can be modeled as a
Raleigh distillation process with
D56FeFeIII-Hem = +1.3‰
but equilibrium inferred value is
D56FeFeIII-Hem = -0.14‰

9. Low-T environments Some observations:
surficial processes that occur under
oxic conditions do not change d56Fe
in order to see d56Fe changes, you need
to mobilize Fe, make different pools
in anoxic environments, redox cycling
of Fe results in large fractionations
(via bacterial Fe reduction or interaction
with H2S)
Precipitation of sulfide minerals shift
d56Fe of residual vent fluids?
Beard and Johnson, 2004

10. Sources of Fe to the modern oceans
Beard et al., 2003a

11. This model is confirmed by
observed Fe isotope anomalies
in Fe-Mn nodules from modern
oceans
Beard et al., 2003a

12. 4 processes reflected in distribution of Fe isotopes in fluids:
transport of dissolved or colloidal Fe in rivers
oxidation of Fe2+
isotopic exchange with reactive S during BSR
dissimilatory Fe reduction (DIR)
Johnson et al., 2008

13. main point: Biotic and abiotic fractionations overlap
but DIR is contributing the largest, lowest d56Fe pool
Johnson et al., 2008

14. Fe isotopes in the ancient rock record BIFs and associated d56Fe
anomalies signal presence
of large Fe3+ and Fe2+ pools
simultaneously; explained
by episodic O2 increases
followed by return to
low-O2 conditions?
Johnson et al., 2008

15. Coupling between
Fe, S, and C
isotopes
Johnson et al., 2008

16. Putting it all together…
grey bar =
rise of
methanogenesis?
(low d13C)
GOE = more SO42-
more BSR?
Johnson et al., 2008

17. grey bar =
GOE event
stops MIF signa
transport
to sediments
DIR increases as O2 increases,
more Fe3+ available
Johnson et al., 2008

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