School of Engineering and Science - Jacobs University

384 B.W. Alexander et al. / Geochimica et Cosmochimica Acta 72 (2008) 378–394
considerably lower (average 1.13). The REY patterns are
not correlated with major element chemistry, nor with
immobile trace element content, but the two groups of patterns
can be distinguished on the basis of relative stratigraphic
position; i.e., high (Dy/Yb) WMS corresponds to
samples collected at the base and at the top of the sequence
REY/WMS
REY/WMS
1
0.1
0.01
0.001
1
0.1
0.01
top of sequence
bottom of sequence
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
middle of sequence
WM42
WM132
WM131
WM62
WM61
WM52
WM51
WM42
WM41
WM122
WM121
WM112
WM111
WM102
WM101
WM72
WM71
studied, and low (Dy/Yb) WMS is found in samples collected
from the middle of the sequence.
The REY patterns of all the IF samples exhibit WMSnormalized
positive La, Eu, and Gd anomalies. Calculated
(La/La * ) WMS and (Gd/Gd * ) WMS are above unity for all 16
IF samples (Fig. 3), and (Eu/Eu * ) WMS ranges from 1.51 to
2.09 (average 1.69). The magnitude of the La, Eu, and Gd
anomalies are indistinguishable on the basis of stratigraphic
position. Y/Ho averages 42 for all samples, and higher Y/
Ho values are observed for samples collected from the middle
of the sequence, though the difference between the two
groups is small when samples WM121 and WM122 (Y/Ho
of 61 and 66, respectively) are not considered.
The REY pattern of sample WM42 is similar to samples
obtained stratigraphically above and below it, possessing
positive La, Gd, and Y anomalies. The RREY content is
the highest of the IF samples, and though most major element
concentrations for sample WM42 are comparable to
the other IF samples, Al 2 O 3 is enriched at 1.2% and MnO
is significantly higher (14.5%). WM42 also contains somewhat
greater concentrations of Rb, and significantly more
Zr, Hf and Th than any of the other 15 IF samples.
Seven iron-formation samples and three shale samples
were analyzed using TIMS, and Nd and Sm isotope data
are presented in Table 3. Six of the samples (one shale
and five IF samples) display similar 147 Sm/ 144 Nd between
0.1164 and 0.1486, values common to other Archean shales
and iron formation (e.g., Miller and O’Nions, 1985; Alibert
and McCulloch, 1993; Jahn and Condie, 1995; Bau et al.,
1997). Of the remaining four samples, two are iron formations
displaying the highest 147 Sm/ 144 Nd observed (>0.17),
whereas the lowest 147 Sm/ 144 Nd values are observed in the
remaining two shale samples (