School of Engineering and Science - Jacobs University

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152 B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155
REY that can have valence states other than +3. The oxic nature of
modern oceans allows Ce to be readily oxidized at particle surfaces to
highly insoluble Ce(IV), preferentially removing Ce and producing the
very negative Ce anomaly observed in modern seawater. The fact that
negative Ce anomalies have not been observed in marine chemical
sediments (IF or carbonates) older than ~2.3 Ga indicates relatively
reducing oceans, and an atmosphere–hydrosphere redox state for the
early Earth that was lower than today (cf. Holland, 1984).
Whereas Ce fractionation and removal occurs in the water column
and at the sediment–water interface, Eu fractionation occurs during
water–rock interaction at high temperatures via reduction of Eu 3+ to
Eu 2+ . The alteration products of these water–rock reactions discriminate
against the relatively large Eu 2+ ion and produce a subsequent
enrichment of Eu in the fluid phase. This Eu enrichment is present
only in fluids which have generally exceeded ~200 °C, and has been
observed in numerous vent fluids from hydrothermal systems altering
oceanic crust (e.g., Michard et al., 1983; Schmidt et al., 2007, and
references therein). The large positive Eu anomalies in Archean–
Paleoproterozoic IFs and carbonates possessing seawater-like REY
distributions are fully consistent with significant high-T hydrothermal
fluid fluxes to Earth's early oceans. Unfortunately, Eu anomalies are
not suitable for mass fraction calculations regarding the mixing of
high-T hydrothermal fluids and ambient seawater, and only reveal
information concerning the temperature of the REY source (e.g., Bau
and Möller, 1993; Alexander et al., 2008).
6.3. Nd isotope ratios in Archean seawater
Generally, IFs older than 2.25 Ga that possess seawater-like REY
patterns have Є Nd (t) between −5 and +5 (Fig. 7), with much of the
data describing the ~2.5 Ga Hamersley IFs (Australia). The remaining
data are primarily from South Africa, though Frei et al. (2007) have
significantly expanded the Sm–Nd isotopic analyses of Archean IF
from North America; unfortunately, possible ages for these IF are
poorly constrained. Most Є Nd (t) values for IFs older than 2.25 Ga fall
between −1.5 and +2.5, similar to that observed in the PGB samples.
Data for IFs older than 3.0 Ga are dominated by studies of the
relatively few samples from Isua discussed previously. Considering the
wide range of Є Nd (t) values observed in the Isua IF and the few analyses
for samples clearly possessing seawater-like REY distributions, we
consider the IF-G Є Nd (3.8 Ga) value of approximately +2.5 as the best
estimate for average ~3.8 Ga seawater, similar to the conclusions of Frei
and Polat (2007). The Pietersburg data are similar to older IF, though
tending to more CHUR-like values, and support the interpretation that
Nd was significantly, if not dominantly, provided by ocean ridge
hydrothermal systems possessing a combination of radiogenic Nd
signatures and strong positive Eu anomalies. This trend seems consistent
until ~2.6 Ga, when negative Є Nd (t) values and smaller Eu
anomalies (Bau and Möller, 1993) become common in IFs, though in
only a few instances do Archean–Paleoproterozoic IFs display Є Nd (t)
lower than −1.5.
Focusing on the period near 3.0 Ga, the Pietersburg IF Nd isotopic
evolution (Fig. 8) is very similar to that observed for the 2.9 Ga
Mozaan IFs from South Africa (Alexander et al., 2008). The consistent
Nd isotopic evolution observed in the two groups of IF is expected due
to the similar Sm/Nd values observed in IFs with seawater-like REY
distributions. The reasonable range of potential depositional ages for
both the Pietersburg and the Mozaan IFs would be from ~2.84 to
3.1 Ga, and within this time frame the Nd isotopic ratios remain
distinctly different. As initial 143 Nd/ 144 Nd ratios in the Pietersburg IF
are negatively correlated with increasing aluminosilicate content
(Fig. 4), associated crustal rocks displayed Є Nd (2.95 Ga) of −0.5,
whereas ambient seawater during deposition of these IFs displayed
Є Nd (2.95 Ga) of approximately +1. This value is ~3–5 Є-units higher
than seawater contemporaneous with deposition of the Mozaan
samples IF (Alexander et al., 2008). These different Є Nd (t) values
indicated for seawater likely reflect different depositional environments,
as the previously discussed trace element data suggests that
the rocks associated with the Pietersburg IF were generated in an
Fig. 7. Neodymium isotopic signatures in oxide and silicate facies IFs older than 2.4 Ga. World IFs generally possess Є Nd (t) between −5 and +5, though most of the data are more
positive than −1.5. The dashed lines represent Є Nd (t) values predicted for simple Nd isotopic evolution in depleted mantle and continental crust with initial Є Nd (4.56 Ga)=0, and
possessing Є Nd (0) =+10 and −17, respectively. Also shown is the Nd evolution in a depleted mantle as modeled by Nägler and Kramers (1998). Data are screened to distinguish
samples which display seawater-like REY patterns similar to those shown in Fig. 6, and are from Miller and O'Nions (1985), Jacobsen and Pimentel-Klose (1988a, 1988b), 12 samples
considered pristine enough by Alibert and McCulloch (1993) for calculation of possible Archean seawater pH values, and Bau et al. (1997a). All IF samples of 3.7–3.8 Ga age are from
Isua (Greenland), with stratigraphic ages as reported in original datasets. All data have been calculated using a present day 143 Nd/ 144 Nd of 0.512638 in CHUR and normalized to
146 Nd/ 144 Nd=0.7219. The majority of the data fall between −1.5 and +2.5, and the Є Nd (t) of Archean–Paleoproterozoic seawater as suggested by IFs was generally between +1 and
+2 until ~2.7 Ga. Only the 2.9 Ga Mozaan IFs possess distinctly different Є Nd (t) values, which are similar to igneous rocks (rhyolite, andesite, basalt, and gabbro) from the South
Africa–Swaziland border area (Hegner et al., 1984, 1994).