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B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155
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Fig. 3. REY distributions in IF samples normalized to Al-rich IF7 (5.3% Al 2 O 3 ). Inset
displays REY distribution in Al-rich IF7 normalized to chondritic meteorite (Anders and
Grevesse, 1989) and continental crust as represented by Post Archean Australian Shale
(PAAS, McLennan, 1989). PAAS possesses a negative Eu anomaly compared to bulk
Earth, and PAAS-normalized data will correspondingly display positive Eu anomalies
when no Eu fractionation exists. This is the case for Al-rich IF7, for which no anomalous
behavior of Eu is observed. The remaining samples all display positive Eu anomalies
relative to IF7 that significantly vary in magnitude, with greater Eu enrichments in
samples of lower total REY abundance. Positive La, Gd, and Y anomalies are present in
many of the samples and, similar to Eu, the magnitude of these anomalies increases
with decreasing REY concentration. The effect of detrital contamination is observed in
samples IF3 and IF4, which have the second highest Al 2 O 3 contents (1.5%) after IF7, and
relatively flat REY patterns. However, IF4 also possesses greater total REY concentrations
that IF7, which coupled with the positive Eu anomaly in IF4 indicates that a significant
fraction of its REY budget was supplied by the chemical precipitate which produced the
IF. Sample IF7a illustrates the large range of REY concentrations (and REY fractionation)
observed on cm-scales within the drill core, as it was obtained from a homogenous band
adjacent to sample IF7. The range of concentrations and REY distributions observed
throughout the core argues against the widespread mobility of REY at any point
following deposition.
nickel (Ni), and scandium (Sc)), including relatively mobile rubidium
(Rb) (Fig. 2). These correlations remain even if the single sample with
the highest Ti and Al content (IF7) is not considered (Fig. 2).
The iron-formations possess REE and yttrium (REY) concentrations
that vary by an order of magnitude between the samples, and the
distribution patterns of these elements vary significantly as well. This
may be illustrated by normalizing REY data to IF7, which possesses the
highest Al 2 O 3 content (5.3%), a procedure that effectively removes the
influence of any detrital material that may be present (Fig. 3). The Alrich
IF7 itself is light rare earth element (LREE) enriched when
normalized to chondrite (subscript CN , Anders and Grevesse, 1989)
and shows gadolinium/ytterbium ratios ((Gd/Yb) CN ) of ~1 and no
REY anomalies (Fig. 3). The two samples with the next highest Al 2 O 3
concentrations (1.5%, IF3 and IF4) have higher REY abundances than
the remaining samples, and REY concentrations change significantly
on small scales (1–2 cm) throughout the core as a function of Al
content. This is illustrated by samples IF5 and IF5a, which represent
individual ≤1 cm thick bands sampled within 2–3 cm of each other,
and that contain Al, Ti, and REY contents that vary by a factor of ~3.
However, a significant fraction of the REY in most of the samples
must be derived from the chemical precipitate which produced the IF.
Sample IF4 has one-third the Al of sample IF7, yet IF4 has higher REY
concentrations (Fig. 3). In the remaining samples four of the REY
display anomalous behavior to varying degrees, with strong positive
europium (Eu) and Y anomalies and smaller positive lanthanum (La)
and Gd anomalies. The IFs also possess slight positive lutetium (Lu)
anomalies, and the magnitude of all anomalies generally increases as
Sm–Nd analytical quality was monitored using the basalt reference
standard BCR-2 (United States Geologic Survey), which yielded 143 Nd/
144 Nd=0.512628±0.000002. Additionally, the Isua iron-formation
standard IF-G, issued by IWG-GIT (International Working Group –
Groupe International de Travail) was analyzed to provide reference
data for subsequent Sm–Nd isotopic studies of iron-formations, and
these data match well with the single previously published Sm–Nd
isotopic analysis of IF-G (Stecher et al., 1986).
5. Results
Analytical data are presented in Tables 1 and 2. Excluding samples
IF2a and IF7, all samples are more than 91% silicon- and iron-oxides
(SiO 2 and Fe 2 O 3 ), with SiO 2 between 29 and 62%, and Fe 2 O 3 between
37 and 69%. Sample IF2a is enriched in calcium (Ca) and magnesium
(Mg) compared to most samples, and IF7 possesses significantly more
aluminum (Al) and Mg than any other sample. While Al contents are
typically very low (b0.8%), consistent with very low amounts of
detrital aluminosilicates, IF3 and IF4 each contain 1.5% Al 2 O 3 , and IF7
contains 5.3% Al 2 O 3 . Trace elements that are considered immobile,
such as zirconium (Zr), hafnium (Hf), and thorium (Th), correlate well
with Al and titanium (Ti), as do most trace metals (e.g., cobalt (Co),
Fig. 4. Nd concentrations and Є Nd (t) of Pietersburg IF samples as a function of Al
content. Top figure (a): Nd concentrations generally decrease with decreasing Al
concentrations. However, significant Nd must be derived from the chemical precipitate
that formed the IFs, as IF4 contains less than one-third the Al of IF7, but higher Nd
concentrations. Bottom figure (b): Iron-formation samples with Al 2 O 3 N0.5% possess
very consistent negative Є Nd (t), whereas positive Є Nd (t) values are only observed in
samples with the lowest Al concentrations, indicating that contamination with minor
detrital aluminosilicates drives Є Nd (t) to more negative values. This suggests that preexisting
crust that provided aluminosilicates to the IF depositional environment possessed
Є Nd (t) of approximately −0.5, and ambient Fe-rich seawater possessed Є Nd (t)
equal to or greater than +1. Error bars for Є Nd (t) calculated by assuming average errors
in 147 Sm/ 144 Nd ratios of 0.5%.