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Geochimica et Cosmochimica Acta, Vol. 62, No. 21/22, pp. 3475–3497, 1998
Copyright © 1998 Elsevier Science Ltd
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PII S0016-7037(98)00253-1
Rare earth element variations in mid-Archean banded iron formations: Implications for
the chemistry of ocean and continent and plate tectonics
YASUHIRO KATO,1,2,* IZUMI OHTA,1 TOMOKI TSUNEMATSU,1 YOSHIO WATANABE,3 YUKIO ISOZAKI,4 SHIGENORI MARUYAMA,4
and NOBORU IMAI3
2
1
Department of Earth Sciences, Yamaguchi University, Yamaguchi 753, Japan
Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3
Geological Survey of Japan, Tsukuba 305, Japan
4
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 153, Japan
(Received November 26, 1997; accepted in revised form July 24, 1998)
Abstract—Abundances of major and rare earth elements (REEs) are reported for mid-Archean (3.3–3.2 Gyr)
sedimentary rocks including banded iron formations (BIFs) and ferruginous/siliceous mudstone from the
Cleaverville area in the Pilbara craton, Western Australia. Geological, lithological, and geochemical lines of
evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments,
ranging from that typical of mid-oceanic spreading centers to convergent plate boundary settings; a range of
environments most likely caused by plate movements. Except for the mudstone, the REE content of these
sedimentary rocks changes gradually from the lower to upper stratigraphic horizons. Europium anomalies
decrease up-section (Eu/Eu* values normalized to NASC change from 3.5 to 1.1) as the REE contents and
LREE/HREE ratios increase. The striking similarity in these REE signatures of BIFs and modern hydrothermal sediments leads us to propose that the BIFs were in situ hydrothermal precipitates near a mid-ocean ridge.
Significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the
uppermost horizon. The observation that the source of the sediments shifted from proximal hydrothermal
through distal hydrothermal to terrigenous suggests that plate tectonics, dominated by horizontal movement,
was already operating in the mid-Archean. Distal hydrothermal sediments without a Eu anomaly (when
normalized to NASC) suggest that mid-Archean seawater had already been strongly influenced by a riverine
flux from an upper continental crust and that this component bore no Eu anomaly (i.e, it had a negative Eu
anomaly when normalized to chondrite). In addition to an absence of a Eu anomaly, mid-Archean seawater
did not have a Ce anomaly, suggesting less oxic conditions in the mid-Archean than in the modern ocean.
Copyright © 1998 Elsevier Science Ltd
formations (BIFs) (e.g., Fryer, 1977; Dymek and Klein, 1988;
Barrett et al., 1988; Klein and Beukes, 1989; Derry and Jacobsen, 1990; Alibert and McCulloch, 1993; Manikyamba et al.,
1993; Gnaneshwar Rao and Naqvi, 1995; Khan et al., 1996)
and chert (Zhou et al., 1994). The ubiquitous existence of
positive Eu anomalies in Archean BIFs provides evidence that
the BIFs are of mostly hydrothermal origin. Furthermore, several researchers have attempted to derive the evolution of the
Precambrian ocean from the point of view of temporal changes
in their REE features, on the assumption that the BIFs represent
deposits in equilibrium with normal seawater (Fryer, 1977;
Fryer et al., 1979; Derry and Jacobsen, 1990; Danielson et al.,
1992; Bau and Möller, 1993). Generally, they concluded that
the Archean ocean was controlled by huge quantities of hydrothermal solutions on the grounds that Archean BIFs have a
conspicuous positive Eu anomaly. However, it should be kept
in mind that variations in REE patterns of BIFs should not be
attributed solely to changes in REE patterns of seawater, but
also to variable mixing ratios of other source materials such as
hydrothermal solutions and detrital phases (Graf, 1978). These
variations in REE patterns likely reflect the depositional environments and location of BIFs relative to hydrothermal vents.
Unfortunately, the depositional environments and geologic settings of BIFs have not been as fully investigated as those of
Phanerozoic marine sediments due to their sporadic exposure
and ambiguous geological settings. Better resolution of the
1. INTRODUCTION
The abundance of rare earth elements (REEs) and their relative
distribution in sediments and sedimentary rocks are powerful
tools for defining geologic environments and processes. The
REE behavior during the formation of modern marine sediments is well known, including pelagic sediments (e.g., Toyoda
et al., 1990; Fagel et al., 1997), coastal sediments (Elderfield
and Sholkovitz, 1987; Sholkovitz, 1988), and metalliferous
(Fe-Mn) sediments (e.g., Bender et al., 1971; Piper and Graef,
1974; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988). The
REE geochemistry of Phanerozoic sediments is also welldocumented from marine carbonates (Wang et al., 1986; Liu et
al., 1988), fossil apatite in sediments (Wright et al., 1987), and
terrestrially exposed chert (Rangin et al., 1981; Murray et al.,
1991a). On the basis of REE data for these sediments and
sedimentary rocks, their paleoceanographic depositional positions have been well reconstructed (e.g., Ruhlin and Owen,
1986; Olivarez and Owen, 1991; Murray et al., 1991a). Redox
conditions of seawater overlying sediments were inferred on
the basis of Ce anomalies (e.g., Wright et al., 1987; Liu et al.,
1988).
REE abundances have been used to clarify the origin and
genesis of Precambrian sedimentary rocks such as banded iron
*Author to whom correspondence should be addressed (yas@
po.cc.yamaguchi-u.ac.jp).
3475
3476
Y. Kato et al.
depositional settings of BIFs through more detailed and systematic geologic research is needed to extract information on
the REE geochemistry of the Archean ocean.
In addition to hydrothermal discharges, the upper continental
crustal composition is one of the most significant factors controlling ocean chemistry via the riverine flux. There are ongoing debates regarding the chemical composition of the Archean
continents. Studies on REEs in the Archean upper continental
crust suggest that intracrustal differentiation was less important
in the Archean, as indicated by the infrequent occurrence of
negative Eu anomalies (when normalized to chondrite) in finegrained sediments (Taylor and McLennan, 1985; Taylor et al.,
1986; McLennan and Taylor, 1991). In contrast, other researchers have noted that both Archean cratonic shales and granites
typically show sizable negative Eu anomalies, suggesting that
fractionated and differentiated upper continental crust already
existed in the Archean (Reimer et al., 1985; Boryta and Condie,
1990; Kröner and Layer, 1992; Condie, 1993).
There has also been much discussion about the style of
Archean tectonics, which controlled the depositional settings of
BIF, and the composition of the upper continental crust (e.g.,
Sleep and Windley, 1982; Windley, 1984; Hoffman and Ranalli, 1988; Condie, 1989; Goodwin, 1991; de Wit et al., 1992;
Kröner and Layer, 1992; Lowe, 1994). Recently, several greenstone successions were interpreted to be obducted Archean
ophiolites comparable to Phanerozoic counterparts, that is, as
remnants of Archean oceanic crust (tectonically emplaced allochthonous blocks) rather than as products of intracontinental
rift volcanism (Helmstaedt et al., 1986; de Wit et al., 1987;
Kusky and Kidd, 1992). The occurrence of horizontal tectonics
early in the evolution of the gneisses and greenstones of the
Shaw batholith, one of the major batholiths in the Pilbara craton
(Bickle et al., 1980), suggests that horizontal crustal shortening,
probably due to plate convergence, was a dominant process in
Archean tectonics (Kröner, 1991). However, sedimentary assemblages resembling those in Phanerozoic accretionary complexes have not been reported. Moreover, the timing of the
commencement of plate tectonics is subject to debate. Seismic
evidence demonstrated that plate tectonics was active in the late
Archean (2.69 Gyr ago) (Calvert et al., 1995).
Since 1990, we have been mapping the ca. 3.3–3.2 Gyr
Cleaverville Formation on the northern coast of Western Australia on a scale of 1:5000. Stratigraphic sections from the
Cleaverville Formation preserve a continuous spectrum of various marine depositional regimes and provide an excellent clue
to a variety of mid-Archean marine environments. Here we
report geochemical variations and trends in these mid-Archean
sediments. One of the main purposes of this study is to determine which marine sediments best reflect the REE signature of
seawater at the time of precipitation and, as a result, investigate
the geochemical nature of mid-Archean seawater. In addition,
these geochemical trends provide further constraints on the
REE composition of the upper continental crust and on midArchean tectonics.
REEs are particularly useful as geochemical tracers owing to
their lack of mobility during post-depositional processes (e.g.,
Taylor and McLennan, 1985; Elderfield and Sholkovitz, 1987;
McLennan and Taylor, 1991; Murray et al., 1991a). Several
recent studies have demonstrated REE redistribution during the
diagenesis of nearshore sediments (e.g., Milodowski and
Zalasiewicz, 1991; Murray et al., 1991b). However in general,
the distribution coefficients of REEs between sediments (sedimentary rocks) and diagenetic, metamorphic, and hydrothermal fluids are very large (approximately 105-106), and thus
REEs in these sediments and rocks are not expected to change
severely, except when the fluid-rock ratios are very high (e.g.,
Elderfield and Sholkovitz, 1987; Michard, 1989). Intense
weathering which causes a complete disappearance of original
rock textures modifies REE patterns, but the Eu anomaly,
which will be chiefly discussed in this paper, is unaffected
(Nesbitt, 1979). There is no evidence indicating strong postdepositional alterations in our samples. Therefore, it would be
valid to consider that REE indices, especially the Eu anomaly,
of these sedimentary rocks have inherited those of original
sediments.
2. GEOLOGICAL SETTING AND SAMPLE DESCRIPTIONS
Samples were collected from several continuous outcrops of
the Cleaverville Formation, along the northern coast of Western
Australia (Fig. 1a). The age of the Cleaverville Formation of
the Gorge Creek Group is estimated to be 3.3–3.2 Gyr on the
basis of U-Pb zircon dating (Hickman, 1990). The Formation is
composed of weakly metamorphosed greenstone, BIF, mudstone, sandstone, and minor amounts of conglomerate. The
greenstone and sedimentary rocks have NE-trending strikes
with dips of 50 –70oN. The Cleaverville Formation is considered to be a thick autochthonous unit, in which basaltic magma
erupted several times during background sediment deposition
(Hickman, 1983; 1990). Based on our 1:5000-scale geological
mapping, we determined that the section consists of thrustbounded piles of imbricated tectonic slices cut by many layerparallel faults. The repeating basalt-sediment sequence was the
result of tectonic overlapping of the same stratigraphic units
(Ohta et al., 1996; Fig. 1a). Each tectonic slice exhibits a
stratigraphic orientation to the north, as indicated by pillow
shapes, vesiculation within pillows, and the shape of vesicles.
The NE-trending layer-parallel faults are cut in some places by
secondary N-trending high-angle faults and diverge westward
indicating a top to the east sense of shear. These duplex
structures suggest a compressional-deformation caused layerparallel shortening, typically found at the leading edge of active
continental margins or foreland basins (Boyer and Elliott, 1982;
Butler, 1982; von Huene and Scholl, 1993). The presence of
layer-parallel thrusts characterized by duplex structures and the
reconstructed oceanic plate stratigraphy (Fig. 1b) resemble the
Phanerozoic accretionary complex of Japan (e.g., Taira et al.,
1988; Isozaki et al., 1990; Matsuda and Isozaki, 1991).
The sedimentary rocks overlying the greenstone within the
accretionary packages are BIF-1 (alternating iron-rich rock,
hereafter designated BI-1, and chert), BIF-2 (alternating ironrich rock, defined as BI-2, and ferruginous/siliceous mudstone),
and turbidites (alternating sandstone, mudstone, and some conglomerate), in ascending stratigraphic order (Fig. 1b). This
paper discusses the BIF-1 and BIF-2 sequences. BIF-1 consists
of two alternating lithologies, BI-1 and chert, bands several
millimeters to several centimeters thick. BI-1 is composed
mainly of hematite, goethite, and quartz. Chert consists dominantly of quartz with minor amounts of goethite, hematite,
and/or kaolin minerals and illite. The overlying banded iron-
REE variations in mid-Archean BIFs
3477
Fig. 1. (a) Geological map of the Cleaverville area (Isozaki et al., in prep.). Only mappable BIF-1 are as BIF-1 shown
in this figure, and many fragmentary and disrupted BIF-1 occur in the clastic rock unit (BIF-2 and turbidite). Inset shows
the location of study area. (b) Schematic columnar section showing reconstructed oceanic plate stratigraphy. Letters on the
left (X, Y, Ylower and R) represent the relative stratigraphic position of each section.
rich rock (BI-2) is composed of hematite, quartz, and minor
goethite and alternates with mudstone. Siliceous and ferruginous mudstone increases in amount up-section. Siliceous mudstone consists of quartz (largely detrital), kaolin minerals, illite,
chlorite, and feldspar, in some cases small amounts of goethite
and hematite. Ferruginous mudstone is mineralogically a physical mixture of BI-2 and siliceous mudstone. Detrital quartz
generally increases in abundance and coarsens upward in the
section.
The analyzed samples were collected from four localities X-,
Y-, Ylower-, and R-sections (Fig. 1a). The X-, Ylower-, and
R-sections are composed of BIF-1. The Y-section consists of
BIF-2. Continuous samples were taken in 15-m and 23-m thick
sections of the X- and Y-sequences, respectively. Continuous
sampling of both the Ylower-section, which underlies the Ysection, and the R-section was not done because many layerparallel faults disrupt the continuity of stratigraphy. The lithology of BIF-1 of the X-section differs from that of the Ylowersection in that the latter has a thicker iron-rich band, with
clearer banding, and is higher in iron than the X-section. These
lithological features and mineralogy of the BI-1 in the Ylowersection continue well into the BI-2 iron-rich rock of the overlying Y-section. The BIF-1 of the R-section is similar in lithology and mineralogy to that of the Ylower-section, but is not
adjacent to mudstone. Thus the stratigraphic position of the
R-section is inferred to be between the X-section and Ylowersection.
3. ANALYTICAL METHODS AND REE TERMINOLOGY
Fresh rock fragments were carefully hand-picked under the binocular
microscope to avoid contaminations of altered and vein materials.
These hand-picked fragments were then pulverized in an agate mortar.
Major element abundances were determined by X-ray fluorescence
analysis on fused glass beads using a Rigaku 3270 spectrometer
equipped with Rh tube at the Ocean Research Institute, the University
of Tokyo. Loss on ignition (LOI) was calculated from the weight loss
during igniting at 1000°C for 8 h. A 0.4000-g ignited sample was well
mixed with 4.000 g of Li2B4O7 flux and fused at 1150 –1170°C for 7
min in a Pt crucible. Ignited samples containing Fe contents in excess
of approximately 50 wt% was diluted 1:1 by weight with ultra-pure
amorphous silica powder (99.999 wt% SiO2, Pure Chemistry Co., Ltd.)
in order to avoid the formation of an alloy of iron in the sample and
platinum in the crucible. JB-1a (Quaternary basalt), a geochemical
reference sample issued by the Geological Survey of Japan (GSJ), was
used as a calibration standard. The reproducibility (95% reliability) of
measurement on a relative scale is better than 1.0% for SiO2, TiO2,
Al2O3, Fe2O3, CaO, and K2O, 1.0% for MgO, 1.2% for P2O5, 1.4% for
MnO, and 1.6% for Na2O. Details of the XRF analytical procedures are
given in Irino (1996).
REEs were measured by Yokogawa Analytical Systems PMS-200
inductively-coupled plasma mass spectrometry (ICP-MS) at the Geological Survey of Japan. A 0.1000-g powdered rock sample was digested completely with 2 mL HNO3, 1.5 mL HClO4, and 2.5 mL HF in
a tightly sealed 7 or 12 mL Teflon PFA screw-cap beaker, heated for
12 h on a hot plate under 180°C, then evaporated nearly to dryness. The
residue was dissolved with 5 mL (1 1 1) HNO3 by heating and 5 mL
of 4 ppm indium solution was added as an internal standard. For
samples with more than approximately 40 wt% Fe2O3, 0.0500 g were
digested and evaporated in order to accomplish a complete dissolution
of its residue. The solution was diluted to 1:1000 in mass. Because the
preliminary measurements showed that the REE concentrations of chert
are low, the solution of the SiO2-dominant chert sample was diluted to
1:100 or 1:200 to raise the REE signal intensity of the ICP-MS, while
taking into consideration the major element matrix of the samples.
Sample solutions were divided into several batches by their Fe concentrations. For each batch, the solution of GSJ standard JB-1 (Quaternary basalt) with or without an addition of a suitable amount of Fe
standard solution was prepared as a standard to match the Fe matrix,
and a calibration line between the standard and the blank solution was
provided. The isotopes used for the determinations are 139La, 140Ce,
3478
Y. Kato et al.
141
Pr, 143Nd, 147Sm, 151Eu, 157Gd, 159Tb, 163Dy, 165Ho, 167Er, 169Tm,
Yb, and 175Lu. The Ba concentration in almost all samples determined by ICP-MS is low, and no serious overlap of 135Ba16O with
151
Eu occurred. A minor revision of Eu concentrations was completed
for five siliceous mudstone samples having Ba concentrations of
780 ; 1020 ppm. All the samples were analyzed in duplicate, and
average values are reported. Analytical run precision is often ,2% of
the reported values of the reference samples JB-1, JB-3 (basalt), JA-1
(andesite), and JG-1 (granite) of GSJ and mixed standards. Reproducibilities of REEs are ,6%. More detailed descriptions of the methods
for ICP-MS analysis are given in Imai (1990).
REE data of the North American Shale Composite (NASC) obtained
by Goldstein and Jacobsen (1988) are used as normalization values in
this paper. Properly, Archean marine sediments should be normalized
to contemporaneous Archean shale which is representative of average
upper continental crustal compositions. However, as stated in the
introduction, there is no consensus regarding standards of Archean
crustal compositions. Note that no Eu anomaly, when normalized to
NASC, corresponds to a negative Eu anomaly when normalized to
chondrite. The Eu anomaly is defined quantitatively as
173
Eu/Eu* 5 ~2Eu/Eu NASC!/~Sm/Sm NASC 1 Gd/Gd NASC!
where Eu* is the hypothetical concentration that strictly trivalent Eu
would have. When Eu is depleted, the Eu/Eu* ratio is less than one and
vice versa. The Ce anomaly is defined as
Ce/Ce* 5 ~3Ce/Ce NASC!/~2La/La NASC 1 Nd/Nd NASC!
The degree of light REE enrichment relative to heavy REE is presented
as the ratio of NASC-normalized La and Yb (Lan/Ybn).
4. RESULTS
Abundances of major elements and fourteen REEs for sixtyfive BI-1, forty-four chert, fifteen BI-2, ten ferruginous, and ten
siliceous mudstone samples are presented in Table 1. Average
values with standard deviations (61s) of each lithology in four
sections are given in Table 2.
4.1. Major Elements
BI-1, BI-2, and chert are mainly composed of SiO2 and
Fe2O3 (Fig. 2a). The average Fe2O3 contents of these three rock
types are 53.68 6 21.16 (mean value 6 1s), 55.54 6 8.66 and
7.34 6 7.49 wt%, respectively. The average Fe2O3 content of
all BI-1 samples from the X-, R-, and Ylower-sections is approximately the same as those of BI-2. However, a large
number of BI-1 samples in the lowest, or X-, section contain
considerably less Fe2O3 and correspondingly more SiO2 than
the BI-1 samples in the upper sections (R- and Ylower-sections).
The chert of the X-section also exhibits more Fe2O3 depletion
than the chert of the R- and Ylower-sections. Moreover, there is
not a definite compositional gap between BI-1 and chert in the
X-section (Fig. 2a). In contrast, a compositional discontinuity
exists between BI-1 and chert in the Ylower- and R-sections.
These differences are due to the fact that the BI-1 bands of the
X-section are thin and often show ambiguous banding with
SiO2-dominant chert, indicating that the banding separation
between BI-1 and chert bands in the X-section is not complete
relative to that in the R- and Ylower-sections. While BI-2 has the
same lithological and mineralogical nature as BI-1 of the
Ylower-section, the Fe2O3 content of BI-2 is generally lower
than that of BI-1 in the Ylower-section. The siliceous and
ferruginous mudstones in the upper Y-section are plotted along
another regression line in Fig. 2a. Most of the siliceous mudstones contain trace amounts of Fe2O3 and approximately 70
wt% SiO2. Almost all samples of ferruginous mudstone plot
between BI-2 and siliceous mudstone, supporting the suggestion that ferruginous mudstone is a simple mixture of BI-2 and
siliceous mudstone. The MnO contents of iron-rich rocks (BI-1
and BI-2) are very low compared to those of modern metalliferous sediments, but MnO decreases gradually from the X- to
Y-sections (Table 2).
A plot of TiO2 vs. Al2O3 content is given in Fig. 2b.
Siliceous and ferruginous mudstone contains considerable
amounts of Al2O3 (up to 21.5 wt%) and TiO2 (up to 1.70 wt%).
The TiO2 content correlates well with the Al2O3 content (correlation coefficient 5 0.95). Also, as Fig. 2b indicates, all
ferruginous mudstone samples occupy a position between BI-2
and siliceous mudstone. Besides Al2O3 and TiO2, the siliceous
and ferruginous mudstone contains a maximum of 3.26 wt%
K2O. Although the Al2O3, TiO2, and K2O contents in BI-1,
BI-2, and chert are small (Fig. 2b and Table 2), they show a
definite trend; they decrease in amount from the X-section to
the R-section and increase in amount from the R-section to the
Ylower- and Y-sections.
4.2. Rare-Earth Elements
With stratigraphy, NASC-normalized REE patterns of the
investigated samples show a considerable sequential change
(Fig. 3). The BI-1 and chert of the lowest section (X) have
HREE-enriched and relatively flat patterns, respectively, with
conspicuous positive Eu anomalies. The positive Eu anomalies
both in BI-1 and chert become smaller up-section (the R- and
Ylower-sections) and their patterns become flatter. In the upper
horizon (the Y-section), the BI-2 shows a generally flat REE
pattern with the smallest Eu anomaly. Moreover, normalized
values are inclined to become greater up-section (from the Xto the Y-sections). In the upper layers of the Y-section, the Eu
anomalies of ferruginous and siliceous mudstone are more
positive. Except for Eu, the siliceous mudstone shows an intermediate REE-depleted concave pattern.
The sequential change in Eu anomaly, which is the most
noticeable feature in the REE patterns, is described in more
detail in Fig. 4. This figure clearly shows that the Eu anomalies
of BI-1 and chert decrease gradually from the X-section (BI-1:
3.44 ; 2.16; chert: 3.55 ; 1.95) through the R-section (BI-1:
2.54 ; 1.36; chert: 3.14 ; 1.38) to the Ylower-section (BI-1:
1.83 ; 1.22; chert: 2.26 ; 1.20). The values of BI-2 vary narrowly from 1.60 to 1.09, which are close to the value of NASC.
Those of ferruginous and siliceous mudstone increase up-section, and vary from 2.23 to 1.29 and from 2.98 to 1.62,
respectively. Another noteworthy finding is that the Eu anomaly values of chert are larger than those of BI-1 in the same
horizon, although the chert anomalies vary considerably as
compared to the BI-1 samples.
Pronounced positive Eu anomalies are recognized both in
BI-1 and chert of the lower horizon, and in the mudstone of the
upper horizon. The Eu anomaly values are plotted against the
Al2O3 contents in order to discriminate causes of positive Eu
anomalies in BI-1, chert, and mudstone (Fig. 5). The magnitude
of the Eu anomalies decrease considerably from BI-1 and chert
of the X-section to BI-2 while their Al2O3 content is nearly
constant and less than 4 wt%. In contrast, Eu anomalies in
siliceous and ferruginous mudstone increase with Al2O3 con-
Table 1. Major and rare-earth element data
X-section
Sample No.
Lithology
Color
Horizon (m)
A208
BI-1
lgt br
1.06
A219
BI-1
rd
1.43
A690
BI-1
rd br
11.23
A720-1
BI-1
rd
11.62
A720-2
BI-1
rd br
11.64
A750
BI-1
rd br
12.20
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
86.29 59.05 61.46 78.02 62.65 61.86 64.06 27.98 60.30 41.32 69.03 65.59 40.45 33.67
0.16
0.03
0.02
0.18
0.08
0.01
0.02
0.05
0.01
0.05
0.01
0.01
0.05
0.05
1.3
0.8
1.4
2.4
4.0
0.2
0.3
0.6
0.2
0.7
0.2
0.2
0.4
1.1
8.83 37.54 33.21 15.17 27.49 32.25 31.14 62.32 32.81 49.94 24.87 28.46 51.90 56.75
0.026 0.047 0.026 0.016 0.021 0.506 0.247 0.081 0.665 0.313 1.804 0.802 0.078 0.362
0.19
0.15
0.10
0.39
0.21
0.09
0.17
0.24
0.38
0.29
0.12
0.21
0.25
0.34
0.08
0.11
0.13
0.11
0.17
0.08
0.13
0.24
0.24
0.20
0.07
0.07
0.23
0.23
0.04
0.05
0.07
0.04
0.03
0.03
0.01
0.03
0.03
0.03
0.11
0.46
0.69
0.01
0.059 0.036 0.213 0.198 0.229 0.186 0.037 0.087 0.121 0.186 0.047 0.084 0.083 0.106
3.04
2.14
3.28
3.03
4.43
4.81
3.92
8.44
5.22
7.04
3.82
4.59
6.50
7.39
83.06
0.02
0.6
14.39
0.026
0.14
0.08
0.01
0.09
0.047
1.57
43.75
0.05
1.0
47.90
0.117
0.33
0.20
63.35 59.80 71.73 60.55 75.24 68.71
0.03
0.04
0.09
0.04
0.10
0.06
0.6
0.8
0.8
0.4
1.1
2.0
31.76 36.17 23.11 32.99 21.40 23.23
0.051 0.030 0.193 0.104 0.044 1.682
0.19
0.22
0.12
0.11
0.16
0.29
0.14
0.22
0.11
0.08
0.08
0.12
0.04
0.04
0.06
0.07
0.07
0.06
0.01
0.16
0.33
0.138 0.086 0.066 0.081 0.039 0.120
3.78
2.59
3.70
5.53
1.57
3.41
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
7.11
15.3
1.87
7.79
1.90
1.01
1.64
0.34
2.10
0.39
1.35
0.25
1.68
0.27
8.46
18.3
2.18
8.98
1.78
0.92
1.53
0.25
1.21
0.23
0.77
0.12
0.72
0.12
7.32
15.1
1.94
8.62
2.11
1.07
1.66
0.31
1.67
0.33
1.04
0.17
1.28
0.21
4.22
9.14
1.45
8.17
2.30
1.03
1.64
0.29
1.43
0.22
0.58
0.10
0.67
0.10
A271
BI-1
rd br
3.35
7.73
18.1
2.40
11.3
3.76
1.91
2.75
0.48
2.47
0.45
1.31
0.22
1.40
0.21
A308
BI-1
rd
4.45
6.32
15.5
2.22
11.0
2.98
1.99
3.21
0.57
2.66
0.40
1.13
0.19
1.38
0.20
A390
BI-1
rd br
5.37
4.11
8.87
1.07
5.06
1.23
0.75
1.07
0.22
1.17
0.24
0.80
0.13
0.97
0.15
A430
BI-1
rd br
6.23
0.92
1.97
0.25
1.42
0.33
0.22
0.33
0.07
0.39
0.09
0.35
0.06
0.35
0.07
A465
BI-1
rd br
6.94
4.58
7.78
0.94
4.12
0.96
0.48
0.88
0.21
1.55
0.36
1.43
0.31
2.25
0.40
A500
BI-1
rd br
7.58
4.02
6.79
1.02
5.16
1.06
0.63
1.40
0.26
1.58
0.32
1.05
0.15
0.85
0.15
A540
BI-1
rd br
8.43
7.70
14.7
1.86
9.08
1.94
1.14
2.01
0.41
2.20
0.44
1.58
0.23
1.48
0.27
A550
BI-1
rd br
8.53
4.50
7.56
1.45
7.42
1.81
1.00
1.94
0.47
2.68
0.51
1.61
0.24
1.53
0.30
A590
BI-1
rd br
9.40
3.37
5.64
0.89
4.41
1.02
0.61
1.19
0.24
1.40
0.28
0.97
0.15
0.82
0.13
A630
BI-1
rd br
10.17
4.29
8.42
1.07
4.84
1.12
0.60
0.96
0.18
1.11
0.21
0.79
0.12
0.80
0.13
7.56
16.8
2.18
10.2
2.28
1.15
2.08
0.44
2.63
0.56
1.86
0.33
2.14
0.36
0.02
0.317
6.34
3.17
6.80
0.93
4.21
1.19
0.62
1.00
0.23
1.26
0.25
0.80
0.12
0.74
0.13
A770
BI-1
rd
12.66
3.67
7.32
0.99
4.36
1.06
0.57
1.00
0.19
1.16
0.24
0.75
0.13
0.76
0.12
A780
BI-1
rd br
13.02
5.86
12.7
1.57
6.63
1.57
0.71
1.36
0.27
1.47
0.32
1.05
0.19
1.19
0.21
A799
BI-1
rd br
13.17
2.79
5.54
0.68
3.34
0.81
0.49
0.92
0.22
1.27
0.28
0.92
0.14
1.06
0.19
A810
BI-1
rd
13.41
10.2
21.0
2.66
11.3
2.53
1.12
2.06
0.40
2.31
0.54
1.79
0.28
1.83
0.31
A840
BI-1
rd br
14.03
6.15
15.7
2.03
8.96
2.35
1.09
1.79
0.38
2.12
0.45
1.45
0.25
1.69
0.28
REE variations in mid-Archean BIFs
1.17
2.34
0.31
1.26
0.45
0.34
0.49
0.11
0.69
0.13
0.44
0.07
0.47
0.07
A260
BI-1
rd
3.05
Lithology: cht 5 chert, Fe ms 5 Ferruginous mudstone, Si ms 5 Siliceous mudstone
Color: rd 5 red(dish), br 5 brown(ish), wht 5 white, gr 5 gray(ish), yl 5 yellowish, gn 5 greenish, lgt 5 light, dk 5 dark
Fe2O3*: total iron as Fe2O3. No entry indicates data below detection limit.
3479
3480
Table 1.
(Continued)
X-section
Sample No.
Lithology
Color
Horizon (m)
A850
BI-1
rd br
14.38
A870
BI-1
rd
14.59
A455 A480
cht
cht
wht gr wht
6.80
7.30
A590
cht
wht
9.42
A680
cht
lgt gr
10.94
A720
cht
lgt gr
11.59
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
64.77 59.46 64.40 99.04 94.69 93.97 97.06 93.65 97.09 95.62 99.12 98.48 98.29 96.78
0.03
0.35
0.03
0.01
0.05
0.03
0.01
0.21
0.24
0.05
0.01
0.01
0.01
0.01
0.6
4.0
1.5
0.2
1.1
0.7
0.2
2.5
1.2
0.2
0.2
0.2
0.1
0.1
29.68 32.72 28.79
0.21
3.38
4.21
2.30
1.68
0.33
3.23
0.17
0.87
0.96
1.33
0.088 0.113 2.233 0.004 0.010 0.010 0.008 0.008 0.005 0.009 0.004 0.007 0.082 1.020
0.13
0.28
0.34
0.05
0.09
0.06
0.04
0.17
0.10
0.07
0.04
0.06
0.05
0.09
0.13
0.16
0.15
0.04
0.05
0.05
0.04
0.06
0.05
0.08
0.04
0.05
0.05
0.05
0.07
0.02
0.25
0.09
0.02
0.26
0.06
0.01
0.67
0.31
0.01
0.02
0.01
0.210 0.043 0.062 0.007 0.008 0.022 0.012 0.045 0.018 0.028 0.008 0.005 0.004 0.008
4.15
2.63
2.35
0.37
0.32
0.89
0.32
0.99
0.62
0.70
0.38
0.33
0.45
0.56
98.85
0.01
0.2
0.60
0.006
0.04
0.04
94.23
0.01
0.2
4.60
0.025
0.07
0.07
0.002
0.30
0.026
0.81
97.60 94.72 99.02 97.30 90.91 97.80
0.01
0.11
0.01
0.01
0.02
0.01
0.3
1.3
0.3
0.2
1.0
0.4
1.13
2.29
0.30
1.69
5.91
1.49
0.015 0.066 0.009 0.011 0.013 0.010
0.07
0.13
0.05
0.07
0.22
0.04
0.05
0.07
0.04
0.05
0.27
0.04
0.03
0.09
0.01
0.10
0.03
0.23
0.02
0.01
0.20
0.04
0.009 0.033 0.002 0.011 0.075 0.010
0.77
0.98
0.30
0.60
1.28
0.18
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
4.88
12.1
1.61
7.24
1.90
0.91
1.55
0.28
1.50
0.29
1.04
0.18
1.05
0.19
0.15
0.21
0.02
0.08
0.01
0.01
0.02
0.01
0.04
0.01
0.03
0.01
0.03
0.01
2.84
4.61
0.49
1.82
0.26
0.13
0.28
0.05
0.26
0.06
0.17
0.03
0.16
0.03
6.12
12.3
1.53
6.60
1.70
0.89
1.80
0.43
3.27
0.72
2.45
0.39
2.36
0.42
A176
cht
wht
0.17
0.85
1.75
0.22
0.97
0.18
0.11
0.17
0.04
0.20
0.04
0.13
0.02
0.13
0.02
A209
cht
lgt gr
1.12
2.07
4.06
0.48
1.62
0.30
0.17
0.28
0.06
0.36
0.08
0.29
0.06
0.40
0.05
A233
cht
br gr
2.00
3.59
7.79
1.12
5.43
1.23
0.74
0.75
0.13
0.62
0.13
0.45
0.07
0.48
0.08
A246 A273.5 A300
cht
cht
cht
wht
lgt gr lgt gr
2.58
3.42
4.22
0.96
1.66
0.20
0.82
0.20
0.11
0.18
0.04
0.24
0.05
0.16
0.02
0.16
0.02
3.23
6.12
0.76
2.93
0.87
0.57
0.86
0.20
1.05
0.22
0.64
0.13
0.73
0.12
3.60
6.81
0.85
3.42
0.78
0.48
0.73
0.16
0.99
0.20
0.70
0.13
0.84
0.16
A325
cht
rd gr
4.99
2.82
5.79
0.72
2.88
0.57
0.29
0.57
0.15
0.86
0.19
0.60
0.11
0.69
0.11
A331
cht
wht
5.16
0.82
1.83
0.26
1.22
0.34
0.26
0.35
0.07
0.39
0.09
0.29
0.04
0.28
0.05
A410
cht
wht
5.78
0.85
1.52
0.18
0.67
0.11
0.06
0.13
0.02
0.13
0.03
0.08
0.01
0.08
0.01
0.70
1.01
0.12
0.51
0.10
0.06
0.11
0.02
0.11
0.03
0.08
0.01
0.07
0.01
0.42
0.50
0.07
0.31
0.06
0.04
0.09
0.02
0.16
0.04
0.13
0.02
0.12
0.02
1.85
4.28
0.49
2.12
0.40
0.20
0.34
0.05
0.30
0.07
0.24
0.03
0.20
0.04
A780
cht
lgt gr
12.79
11.5
23.5
3.01
12.2
2.67
1.18
2.20
0.38
1.80
0.37
1.11
0.20
1.25
0.21
A799
cht
wht
13.18
5.08
11.4
1.49
6.53
1.44
0.71
1.40
0.21
1.07
0.21
0.74
0.10
0.56
0.08
A800
cht
lgt gr
13.24
7.78
17.3
2.43
12.7
4.95
2.53
4.17
0.68
2.44
0.40
1.01
0.16
1.03
0.14
A830 A870
cht
cht
br gr gr wht
13.78 14.58
5.43
12.0
1.53
6.35
1.41
0.63
1.14
0.21
1.12
0.24
0.75
0.13
0.70
0.11
1.19
2.71
0.36
1.26
0.29
0.11
0.23
0.03
0.15
0.02
0.06
0.01
0.09
0.01
Y. Kato et al.
11.4
45.6
3.59
20.9
7.24
3.11
6.30
1.61
10.0
1.85
6.02
1.02
6.62
1.17
A890
BI-1
rd
15.03
Table 1.
(Continued)
R-section
R101
BI-1
dk rd
R124
BI-1
dk rd
R135
BI-1
dr rd
R146
BI-1
dk rd
R163
BI-1
dk rd
R165
BI-1
dk rd
R174
BI-1
dk rd
R178
BI-1
dk rd
R180
BI-1
dk rd
R182
BI-1
dk rd
R194
BI-1
dk rd
R203
BI-1
dk rd
R205
BI-1
dk rd
R210
BI-1
dk rd
R215
BI-1
br
R225
BI-1
dk rd
R229
BI-1
dk rd
R235
BI-1
dk rd
R249
BI-1
br
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
16.11
0.03
0.5
80.96
0.075
0.20
0.24
23.42
0.04
0.5
69.51
0.765
0.37
1.63
24.00
0.03
0.4
71.48
0.201
0.21
0.33
18.22
0.09
1.2
78.55
0.051
0.29
0.18
20.80
0.04
0.6
74.23
0.966
0.23
0.18
23.68
0.06
0.9
70.51
0.588
0.27
0.39
17.95
0.05
0.8
77.55
0.036
0.24
0.39
16.04
0.03
0.5
80.09
0.114
0.20
0.23
27.61
0.02
0.3
67.27
0.522
0.35
0.18
12.02
0.03
0.7
83.19
0.033
0.21
0.18
16.81
0.03
0.4
81.14
0.123
0.25
0.32
46.76
0.06
0.6
48.43
0.590
0.18
0.41
19.36
0.08
1.2
75.58
0.054
0.35
0.20
27.27
0.03
0.4
66.81
0.111
0.21
0.17
49.46
0.03
0.3
45.34
0.870
0.17
0.15
12.05
0.05
1.1
76.56
1.428
0.87
0.17
55.29
0.02
0.3
41.37
0.170
0.20
0.39
21.94
0.08
1.2
66.07
0.186
0.74
3.78
65.44
0.01
0.1
29.38
0.033
0.06
0.09
0.057
1.84
0.270
3.47
0.291
3.11
0.063
1.40
0.120
2.88
0.02
0.702
2.93
0.024
3.00
0.285
2.56
0.069
3.68
0.012
3.60
0.303
0.65
0.096
2.90
0.222
2.91
0.021
5.02
0.098
3.54
0.168
7.55
0.116
2.19
0.237
5.76
0.214
4.69
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
5.26
14.5
1.91
8.51
1.89
0.53
1.54
0.26
1.20
0.24
0.81
0.14
0.81
0.13
1.67
2.78
0.33
1.43
0.35
0.15
0.39
0.08
0.60
0.09
0.35
0.07
0.33
0.07
2.59
5.99
0.67
3.09
0.86
0.33
0.72
0.14
0.71
0.15
0.44
0.06
0.46
0.06
1.28
5.15
0.46
2.38
0.62
0.38
0.77
0.18
1.12
0.23
0.88
0.14
0.81
0.14
4.24
5.23
0.94
3.81
0.69
0.30
0.78
0.11
0.53
0.10
0.32
0.05
0.26
0.04
8.48
17.4
2.21
12.5
3.11
1.09
2.48
0.38
1.49
0.31
0.85
0.12
0.86
0.10
6.41
9.50
1.05
4.23
0.93
0.43
1.05
0.21
1.05
0.24
0.62
0.11
0.66
0.12
7.93
15.6
2.09
8.61
1.78
0.63
1.64
0.31
1.72
0.37
1.20
0.19
1.28
0.17
8.44
17.7
2.48
10.9
2.40
0.99
2.46
0.44
2.15
0.40
1.32
0.20
0.99
0.16
18.7
37.2
5.66
26.1
6.07
2.17
5.08
0.95
4.56
0.91
2.49
0.40
2.06
0.29
3.40
6.48
0.93
4.22
1.04
0.33
0.83
0.15
0.71
0.15
0.48
0.07
0.51
0.10
11.6
22.6
2.75
10.8
2.50
0.91
2.02
0.39
1.86
0.36
1.29
0.15
0.96
0.14
2.22
7.98
0.64
3.12
0.67
0.21
0.74
0.15
0.74
0.18
0.45
0.09
0.55
0.09
7.84
19.0
2.17
10.4
2.32
0.63
1.47
0.24
1.06
0.18
0.65
0.09
0.63
0.07
9.40
21.1
2.72
12.2
3.31
1.25
2.82
0.48
2.45
0.48
1.38
0.15
1.17
0.18
5.49
11.9
1.35
6.00
1.45
0.40
1.32
0.21
1.01
0.25
0.68
0.12
0.58
0.10
7.87
16.8
2.17
9.92
1.85
0.78
1.72
0.33
1.96
0.40
1.20
0.17
1.20
0.19
17.1
16.8
2.84
11.1
2.28
0.70
1.68
0.30
1.56
0.27
1.04
0.15
0.94
0.17
8.92
11.0
1.35
5.94
1.31
0.72
1.51
0.28
1.42
0.30
0.98
0.16
0.85
0.14
REE variations in mid-Archean BIFs
Sample No.
Lithology
Color
3481
3482
Table 1.
(Continued)
R-section
R255
BI-1
dk rd
R270
BI-1
dk rd
R281
BI-1
dk rd
R286
BI-1
dk rd
R292
BI-1
dk rd
R301
BI-1
br
R106
cht
br gr
R129
cht
wht
R133
cht
lgt rd
R139
cht
lgt rd
R149
cht
lgt rd
R1639
cht
rd gr
R1829
cht
yl gr
R200
cht
wht
R211
cht
yl gr
R245
cht
yl gr
R260
cht
lgt rd
R277
cht
wht
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
9.98
0.04
0.6
84.29
0.069
0.32
0.53
24.94
0.06
1.1
70.24
0.024
0.29
0.18
36.07
0.02
0.3
59.84
0.120
0.23
0.17
40.81
0.04
0.5
55.66
0.147
0.24
0.47
32.43
0.03
0.3
62.49
0.024
0.16
0.17
42.17
0.05
0.6
50.65
0.072
0.16
0.13
72.72
0.01
0.2
21.79
0.326
0.33
1.46
96.38
0.1
2.53
0.023
0.06
0.07
70.49
0.01
0.3
27.72
0.016
0.07
0.10
87.15
0.01
0.1
11.59
0.154
0.06
0.06
79.92
0.01
0.2
18.30
0.040
0.07
0.07
92.15
0.01
0.1
5.82
0.190
0.16
0.08
88.97
0.01
0.2
8.37
0.287
0.19
0.07
97.34
0.01
0.1
1.59
0.023
0.07
0.08
88.95
0.01
0.1
9.42
0.028
0.07
0.07
92.62
0.03
0.4
5.17
0.009
0.08
0.14
79.77
0.01
0.5
17.51
0.029
0.08
0.27
97.07
0.01
0.1
1.59
0.007
0.08
0.14
0.108
4.04
3.15
0.057
3.19
0.132
2.02
0.051
4.30
0.340
5.78
0.040
3.16
0.013
0.82
0.053
1.29
0.021
0.84
0.024
1.41
0.017
1.48
0.020
1.92
0.023
0.74
0.016
1.34
0.125
1.38
0.013
1.76
0.021
0.97
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
15.7
26.4
3.65
15.7
3.36
0.88
2.36
0.39
2.02
0.40
1.24
0.20
1.13
0.15
6.38
12.3
1.40
6.29
1.19
0.47
0.96
0.16
0.66
0.14
0.45
0.06
0.39
0.09
2.33
3.54
0.41
1.65
0.34
0.15
0.38
0.06
0.39
0.09
0.27
0.04
0.28
0.07
1.80
3.56
0.39
1.70
0.45
0.17
0.49
0.10
0.50
0.09
0.36
0.07
0.39
0.07
1.39
2.95
0.38
1.58
0.43
0.24
0.56
0.13
0.59
0.14
0.29
0.07
0.40
0.07
2.39
6.22
0.70
3.04
0.80
0.22
0.67
0.13
0.66
0.12
0.35
0.07
0.40
0.07
0.44
0.70
0.09
0.47
0.08
0.06
0.11
0.02
0.14
0.03
0.07
0.01
0.08
0.01
1.27
2.96
0.44
1.92
0.52
0.20
0.44
0.08
0.33
0.06
0.17
0.03
0.16
0.03
0.99
3.30
0.22
0.96
0.25
0.11
0.30
0.04
0.18
0.04
0.14
0.02
0.15
0.02
0.47
1.44
0.14
0.62
0.20
0.07
0.18
0.03
0.22
0.05
0.15
0.03
0.17
0.03
0.56
2.04
0.16
0.85
0.18
0.10
0.17
0.03
0.19
0.04
0.13
0.02
0.11
0.02
0.69
1.01
0.12
0.49
0.17
0.11
0.16
0.03
0.16
0.04
0.12
0.02
0.10
0.01
0.67
1.48
0.20
0.97
0.29
0.14
0.24
0.06
0.40
0.08
0.22
0.04
0.23
0.04
4.07
5.36
1.08
4.99
1.04
0.45
0.95
0.17
0.80
0.16
0.49
0.08
0.46
0.07
1.01
1.89
0.21
0.97
0.31
0.17
0.31
0.05
0.28
0.06
0.18
0.03
0.17
0.03
2.14
1.88
0.26
1.08
0.32
0.18
0.28
0.06
0.35
0.06
0.15
0.02
0.13
0.02
10.6
11.9
1.38
4.98
0.92
0.39
1.05
0.16
0.77
0.17
0.58
0.08
0.37
0.06
9.40
10.8
1.40
4.23
0.65
0.20
0.72
0.10
0.42
0.08
0.22
0.03
0.18
0.02
Y. Kato et al.
Sample No.
Lithology
Color
Table 1.
(Continued)
Ylow-section
P35m
BI-1
dk rd
P40m
BI-1
dk rd
P118m
BI-1
dk rd
P132m
BI-1
dk rd
B91m
BI-1
dk rd
B81m
BI-1
dk rd
B70m
BI-1
dk rd
B64m
BI-1
dk rd
B56m
BI-1
dk rd
B40m
BI-1
dk rd
B35m
BI-1
dk rd
B28.5m
BI-1
dk rd
B24m
BI-1
dk rd
B14m
BI-1
rd
SiO2 (wt %)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
28.91
0.02
0.2
65.53
0.058
0.13
0.12
36.98
0.05
1.0
53.87
0.100
0.48
1.47
3.94
0.42
4.7
78.80
0.291
0.41
3.75
23.03
0.03
0.2
69.97
0.02
0.12
0.26
27.19
0.10
1.5
65.33
0.144
0.38
0.31
22.44
0.11
1.5
68.46
0.345
0.91
0.33
2.61
0.11
1.6
75.31
0.408
0.71
6.98
39.45
0.13
1.7
51.92
0.207
0.53
0.82
12.77
0.10
1.5
73.84
0.369
0.64
3.05
19.02
0.15
1.9
68.24
0.270
0.52
2.24
10.40
0.04
1.7
76.48
0.177
0.29
3.92
9.96
0.15
2.5
81.99
0.036
0.21
0.32
16.18
0.12
3.0
75.57
0.036
0.19
0.32
41.83
0.07
1.3
50.53
0.417
0.30
1.02
0.016
4.98
0.052
6.00
0.018
7.69
0.006
5.04
0.006
5.86
0.024
12.29
0.021
5.27
0.024
7.66
0.024
7.60
0.027
6.95
0.039
4.79
0.006
4.60
0.015
4.57
8.65
7.83
1.40
4.75
0.92
0.28
0.81
0.14
0.69
0.18
0.44
0.07
0.43
0.07
6.48
9.84
1.41
6.45
1.32
0.48
1.30
0.24
1.24
0.24
0.71
0.11
0.75
0.12
4.09
7.79
0.73
3.34
0.64
0.24
0.61
0.13
0.79
0.19
0.53
0.09
0.49
0.11
1.41
3.98
0.51
2.28
0.45
0.14
0.43
0.14
0.66
0.15
0.49
0.12
0.53
0.11
19.8
67.5
4.67
21.5
4.44
1.17
4.49
0.79
4.11
0.79
2.44
0.32
2.02
0.25
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
8.97
13.4
2.44
10.2
1.94
0.56
1.76
0.31
1.77
0.35
1.11
0.20
1.14
0.18
0.034
6.30
5.25
11.9
1.54
7.32
1.69
0.52
1.62
0.35
1.69
0.30
0.88
0.12
0.53
0.09
7.91
12.3
2.04
8.43
1.77
0.53
2.02
0.36
1.96
0.39
1.22
0.19
1.12
0.15
9.71
22.9
2.14
9.87
2.14
0.59
2.14
0.40
2.34
0.50
1.47
0.23
1.50
0.18
9.56
16.1
2.81
12.4
2.80
0.77
2.37
0.53
2.75
0.62
1.94
0.29
1.89
0.28
9.62
15.2
2.18
9.63
2.19
0.62
1.91
0.36
2.19
0.48
1.45
0.21
1.37
0.25
17.9
22.1
4.12
18.5
4.03
1.07
3.47
0.64
3.24
0.65
1.66
0.25
1.35
0.20
7.29
10.8
2.16
10.5
2.45
0.84
2.36
0.47
2.41
0.50
1.46
0.19
1.45
0.19
10.9
22.3
4.46
19.7
4.55
1.12
3.63
0.69
4.19
0.79
2.35
0.39
2.39
0.31
REE variations in mid-Archean BIFs
Sample No.
Lithology
Color
3483
3484
Table 1.
(Continued)
Ylow-section
B12m
BI-1
dk rd
P38m
cht
lgt rd
P87m
cht
wht
P110m
cht
rd gr
P164m
cht
lgt rd
B93m
cht
lgt rd
B89m
cht
lgt rd
B80m
cht
lgt rd
B72m
cht
lgt rd
B58m
cht
lgt rd
B48m
cht
lgt rd
B37m
cht
lgt rd
B22m
cht
lgt rd
B6m
cht
lgt rd
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
39.42
0.09
1.8
51.37
0.615
0.29
1.34
74.06
0.02
0.6
21.87
0.051
0.11
0.68
96.40
0.01
0.2
2.02
0.014
0.06
0.15
88.56
0.02
0.4
9.24
0.022
0.11
0.10
88.53
0.01
0.2
9.38
0.020
0.08
0.09
89.41
0.01
0.1
8.85
0.009
0.06
0.30
80.20
0.01
0.1
17.55
0.012
0.06
0.06
79.67
0.04
0.8
16.16
0.077
0.24
0.14
90.40
0.02
0.4
5.93
0.023
0.10
1.07
90.67
0.01
0.2
6.96
0.042
0.16
0.12
69.78
0.02
0.3
25.43
0.067
0.22
0.59
93.76
0.01
0.3
4.89
0.011
0.06
0.09
85.11
0.02
0.3
12.82
0.087
0.06
0.08
84.25
0.01
0.5
13.87
0.015
0.06
0.07
0.021
5.05
0.006
2.59
0.017
1.14
0.011
1.55
0.025
1.66
0.015
1.22
0.007
1.99
0.009
2.82
0.001
2.09
0.013
1.84
0.015
3.62
0.008
0.88
0.025
1.48
0.006
1.26
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
14.2
59.2
5.90
27.9
5.70
1.51
5.43
1.11
5.88
1.18
3.58
0.56
3.05
0.43
2.24
3.60
0.53
2.81
0.56
0.17
0.59
0.11
0.63
0.13
0.39
0.07
0.40
0.06
3.58
5.22
0.81
3.85
0.62
0.26
0.65
0.10
0.49
0.08
0.18
0.02
0.14
0.01
3.08
3.17
0.43
1.91
0.48
0.25
0.56
0.09
0.52
0.12
0.25
0.04
0.23
0.03
1.40
2.39
0.24
1.41
0.33
0.16
0.35
0.09
0.40
0.09
0.30
0.04
0.25
0.03
1.26
1.94
0.37
1.62
0.48
0.15
0.43
0.08
0.49
0.10
0.28
0.04
0.34
0.04
1.85
7.48
0.45
2.04
0.47
0.12
0.47
0.12
0.48
0.11
0.28
0.04
0.31
0.04
2.35
4.27
0.38
1.65
0.37
0.15
0.44
0.08
0.47
0.09
0.31
0.05
0.26
0.04
2.03
2.29
0.33
1.28
0.31
0.15
0.31
0.08
0.38
0.08
0.21
0.04
0.20
0.03
2.80
2.77
0.49
2.28
0.46
0.20
0.58
0.13
0.79
0.16
0.45
0.07
0.39
0.05
6.73
13.8
1.83
8.09
1.91
0.50
1.86
0.33
1.67
0.32
0.92
0.13
0.70
0.11
7.65
10.6
1.30
5.63
0.96
0.31
1.12
0.15
0.81
0.15
0.34
0.05
0.30
0.04
4.88
11.7
1.01
5.15
1.07
0.36
1.10
0.24
1.40
0.29
0.78
0.13
0.77
0.11
7.47
11.3
1.82
7.78
1.54
0.40
1.37
0.27
1.30
0.25
0.75
0.11
0.58
0.08
Y. Kato et al.
Sample No.
Lithology
Color
Table 1.
(Continued)
Y-section
B13
BI-2
dk rd
0.45
B35
BI-2
dk rd
1.37
B43
BI-2
dk rd
1.74
B50
BI-2
dk rd
2.16
B59
BI-2
dk rd
2.58
B64
BI-2
dk rd
2.82
B72
BI-2
dk rd
3.15
B89
BI-2
dk rd
3.98
B107
BI-2
dk rd
4.72
B121
BI-2
dk rd
5.28
B131
BI-2
dk rd
5.78
B148
BI-2
dk rd
6.53
B162
BI-2
dk rd
7.29
B178
BI-2
dk rd
8.13
B262
BI-2
dk rd
13.39
B2
Fe ms
rd
0.08
B23
Fe ms
rd
0.89
B31
Fe ms
rd
1.25
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
57.74
0.20
2.0
34.99
0.041
0.12
0.48
0.34
0.01
0.044
4.00
46.02
0.12
1.5
46.11
0.107
0.13
1.03
0.07
35.81
0.08
1.2
57.46
0.049
0.15
0.34
0.09
41.43
0.13
0.9
52.44
0.058
0.08
0.35
0.34
37.78
0.08
1.0
56.57
0.059
0.26
0.31
0.29
42.84
0.18
1.5
51.49
0.032
0.13
0.27
0.10
31.27
0.10
1.6
63.19
0.055
0.15
0.27
0.10
28.38
0.10
0.9
63.59
0.055
0.27
3.34
0.67
26.98
0.12
0.8
67.36
0.050
0.16
0.30
0.48
24.83
0.15
2.1
68.83
0.059
0.13
0.31
0.22
36.19
0.20
0.9
59.54
0.055
0.13
0.46
0.02
41.50
0.21
0.8
52.91
0.023
0.06
0.25
0.36
45.75
0.07
0.5
49.36
0.032
0.13
0.34
0.09
41.24
0.24
0.9
52.88
0.055
0.20
0.27
0.10
38.23
0.09
0.8
56.35
0.047
0.12
0.25
0.13
55.23
0.46
3.5
35.69
0.068
0.06
0.62
0.17
67.34
0.17
1.3
27.58
0.074
0.06
0.49
0.18
0.035
4.84
0.017
4.82
0.031
4.26
0.044
3.58
0.026
3.43
0.022
3.29
0.049
2.64
0.035
3.68
0.022
3.41
0.018
2.50
0.026
3.85
0.017
3.74
0.039
4.06
0.060
3.90
61.04
0.36
8.5
24.02
0.013
0.47
0.80
0.12
1.55
0.106
3.04
0.018
4.20
0.027
2.78
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
32.0
67.3
7.97
34.7
7.66
1.61
6.24
0.97
3.78
0.60
1.62
0.24
1.55
0.21
21.1
59.8
6.82
29.5
7.16
1.75
5.72
0.86
4.09
0.61
1.78
0.26
1.98
0.22
11.6
29.3
3.04
11.9
2.68
0.83
2.36
0.52
3.07
0.58
2.03
0.38
2.53
0.39
6.54
25.0
1.91
8.36
2.29
0.56
1.83
0.38
2.05
0.38
1.18
0.19
1.27
0.22
9.22
25.9
3.93
16.8
4.49
1.17
3.71
0.59
2.81
0.48
1.45
0.22
1.41
0.16
11.4
29.1
4.35
18.4
4.50
1.16
3.68
0.56
2.69
0.43
1.31
0.20
1.29
0.16
9.05
22.1
3.24
15.3
3.76
0.97
2.82
0.49
2.35
0.38
1.04
0.16
0.98
0.15
4.08
10.8
1.79
8.72
2.39
0.58
1.60
0.33
1.43
0.25
0.74
0.12
0.72
0.12
6.80
39.9
3.55
15.4
3.65
1.02
2.83
0.53
2.57
0.43
1.36
0.22
1.21
0.17
3.54
26.4
1.70
8.37
2.24
0.62
1.70
0.30
1.46
0.27
0.64
0.10
0.69
0.10
3.97
16.5
2.47
12.3
3.46
1.03
2.62
0.63
3.00
0.48
1.43
0.25
1.43
0.22
3.13
11.6
1.92
8.34
1.83
0.43
1.37
0.24
1.29
0.23
0.74
0.12
0.82
0.09
3.34
10.6
1.86
8.67
2.06
0.53
1.48
0.30
1.39
0.25
0.66
0.12
0.78
0.11
14.1
28.6
5.06
20.7
4.72
1.26
3.81
0.67
3.58
0.51
1.73
0.25
1.62
0.24
2.01
7.81
1.33
6.09
1.40
0.33
1.04
0.22
0.99
0.17
0.56
0.10
0.61
0.08
6.89
24.6
3.39
16.1
3.86
1.01
2.79
0.53
2.39
0.44
1.20
0.21
1.20
0.17
10.3
17.1
3.44
15.6
3.96
1.05
3.38
0.66
3.35
0.55
1.53
0.23
1.24
0.16
23.1
21.3
3.49
14.0
3.53
0.86
2.80
0.48
2.20
0.38
1.10
0.18
1.08
0.17
REE variations in mid-Archean BIFs
Sample No.
Lithology
Color
Horizon (m)
3485
3486
Table 1.
(Continued)
Y-section
B95
Fe ms
gr rd
4.28
B171
Fe ms
rd
7.80
B209
Fe ms
rd
9.57
B290
Fe ms
rd
14.21
B338
Fe ms
gr rd
16.58
B380
Fe ms
gr rd
20.33
B400-3
Fe ms
gr rd
22.31
B28
Si ms
rd gr
1.12
B103
Si ms
rd gr
4.61
B118
Si ms
gr rd
5.17
B161
Si ms
gn gr
7.20
B188
Si ms
gn gr
8.43
B198
Si ms
gr
8.87
B203
Si ms
rd gr
9.27
B280
Si ms
gn gr
13.82
B311
Si ms
gn gr
14.69
B370-2
Si ms
gr
19.53
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
55.41
0.99
11.5
25.89
0.026
0.41
0.16
0.16
0.40
0.014
5.05
29.01
0.78
8.7
54.03
0.032
0.16
0.25
0.26
1.19
0.013
5.60
37.57
0.87
6.5
52.55
0.057
0.49
0.25
0.32
1.15
0.027
0.19
43.51
0.37
7.0
41.98
0.036
0.20
0.24
0.19
0.94
0.026
5.51
52.73
1.36
15.7
22.05
0.005
0.64
0.13
0.53
1.86
0.012
4.99
54.39
0.88
10.4
25.6
0.022
0.35
0.24
0.21
0.55
0.030
7.33
47.44
1.16
16.8
26.57
0.008
0.57
0.14
0.42
1.65
0.011
5.23
67.73
1.70
21.5
4.00
0.015
0.43
0.25
0.22
1.25
0.013
2.90
63.01
1.13
18.0
9.22
0.005
0.74
0.27
0.35
2.12
0.009
5.15
60.18
1.04
16.1
15.58
72.97
1.17
16.8
0.47
73.50
1.40
16.2
0.49
74.36
1.38
14.5
1.01
74.54
1.52
14.5
1.86
72.70
0.45
17.5
0.41
69.92
1.25
18.9
0.50
66.29
1.60
20.4
0.95
2.44
9.75
0.98
4.10
1.20
0.40
1.09
0.23
1.53
0.29
1.09
0.21
1.36
0.22
3.20
8.37
1.63
7.56
1.86
0.59
1.37
0.33
2.00
0.38
1.26
0.23
1.47
0.25
13.3
24.4
3.85
14.5
3.90
1.25
3.22
0.58
3.01
0.48
1.73
0.24
1.57
0.23
10.7
16.1
2.82
11.0
2.07
0.62
1.85
0.33
1.89
0.36
1.18
0.19
1.45
0.23
4.19
11.6
1.37
5.58
1.30
0.62
1.29
0.21
1.42
0.30
1.01
0.19
1.32
0.20
19.0
35.9
5.72
25.0
6.41
1.96
5.02
0.92
4.65
0.78
2.43
0.34
2.68
0.33
5.60
9.70
2.59
11.0
2.68
0.90
2.18
0.39
2.33
0.44
1.58
0.27
1.97
0.29
21.4
29.4
3.71
12.4
2.84
0.99
2.90
0.56
4.03
0.80
2.95
0.56
4.22
0.75
1.75
17.8
0.65
2.50
0.75
0.55
0.98
0.20
1.63
0.31
1.17
0.20
1.45
0.23
2.09
4.34
0.92
3.55
0.95
0.40
0.94
0.22
1.49
0.33
1.08
0.20
1.46
0.24
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
0.40
0.14
0.17
0.77
0.013
5.61
0.63
0.14
0.51
2.71
0.014
4.59
22.9
15.2
2.47
5.34
1.21
0.77
1.97
0.35
2.55
0.50
1.78
0.32
2.44
0.35
0.79
0.12
0.61
3.26
0.012
3.62
4.90
11.5
0.93
2.94
1.05
0.84
1.57
0.33
2.47
0.50
1.71
0.32
2.27
0.33
0.75
0.12
0.93
2.99
0.013
3.95
10.4
13.5
1.93
4.59
1.04
0.84
1.75
0.30
2.34
0.45
1.63
0.29
2.11
0.34
0.68
0.11
0.58
2.89
0.010
3.31
13.1
13.0
2.37
4.99
0.98
0.78
1.93
0.29
1.93
0.41
1.37
0.26
2.04
0.30
0.65
0.11
0.70
3.20
0.016
4.26
28.7
22.6
3.84
8.52
1.41
0.80
2.39
0.31
2.07
0.43
1.59
0.28
2.18
0.30
0.88
0.12
0.72
2.97
0.010
4.74
5.65
15.3
1.42
5.32
1.65
0.87
1.91
0.36
2.47
0.52
1.78
0.32
2.38
0.35
0.97
0.13
0.98
3.13
0.010
5.55
7.24
12.3
1.47
5.27
1.43
0.96
1.92
0.33
2.46
0.52
1.76
0.33
2.46
0.39
Y. Kato et al.
Sample No.
Lithology
Color
Horizon (m)
Table 2. Averages and standard deviations (61s) of each lithology
X-section
R-section
Ylower-section
Y-section
Siliceous
mudstone
0.032 6 0.013
3.73 6 0.67
50.35 6 11.28
0.74 6 0.39
9.0 6 4.9
33.60 6 11.96
0.034 6 0.025
0.34 6 0.21
0.33 6 0.23
0.25 6 0.13
0.93 6 0.67
0.028 6 0.028
4.40 6 1.99
69.52 6 5.05
1.26 6 0.36
17.4 6 2.3
3.45 6 5.07
0.002 6 0.005
0.69 6 0.18
0.15 6 0.06
0.58 6 0.28
2.53 6 0.87
0.012 6 0.002
4.36 6 0.91
3.64 6 2.29
6.18 6 4.21
0.77 6 0.56
3.50 6 2.41
0.73 6 0.50
0.24 6 0.12
0.76 6 0.47
0.14 6 0.08
0.76 6 0.43
0.15 6 0.08
0.42 6 0.24
0.07 6 0.04
0.37 6 0.19
0.05 6 0.03
9.40 6 8.07
26.5 6 17.4
3.52 6 1.92
15.7 6 8.00
3.81 6 1.80
0.97 6 0.41
2.99 6 1.52
0.52 6 0.22
2.48 6 0.99
0.41 6 0.14
1.19 6 0.42
0.19 6 0.06
1.17 6 0.40
0.16 6 0.05
9.97 6 6.96
19.1 6 9.49
2.74 6 1.40
11.3 6 5.89
2.79 6 1.54
0.86 6 0.45
2.30 6 1.17
0.44 6 0.21
2.42 6 0.96
0.44 6 0.15
1.46 6 0.47
0.24 6 0.07
1.67 6 0.55
0.25 6 0.07
11.8 6 9.47
15.5 6 6.74
1.97 6 1.13
5.54 6 2.92
1.33 6 0.60
0.78 6 0.18
1.82 6 0.58
0.33 6 0.10
2.34 6 0.70
0.48 6 0.14
1.68 6 0.51
0.31 6 0.10
2.30 6 0.77
0.36 6 0.15
17.8 6 11.1
0.85 6 0.36
1.67 6 0.38
0.85 6 0.56
69.0 6 39.9
1.18 6 0.45
1.37 6 0.12
0.57 6 0.35
56.0 6 26.6
1.01 6 0.39
1.62 6 0.27
0.50 6 0.45
46.5 6 21.3
1.19 6 1.13
2.39 6 0.46
0.39 6 0.30
chert
BI-1
chert
BI-1
chert
BI-2
61.06 6 14.25
0.06 6 0.07
1.1 6 1.0
32.60 6 12.98
0.387 6 0.615
0.22 6 0.09
0.14 6 0.06
0.03 6 0.03
0.09 6 0.17
0.115 6 0.075
4.21 6 1.84
96.54 6 2.29
0.04 6 0.07
0.6 6 0.6
1.93 6 1.64
0.069 6 0.231
0.08 6 0.05
0.06 6 0.05
0.01 6 0.03
0.10 6 0.17
0.017 6 0.018
0.59 6 0.30
28.03 6 14.54
0.04 6 0.02
0.6 6 0.3
66.69 6 14.42
0.295 6 0.372
0.28 6 0.17
0.45 6 0.76
86.96 6 9.25
0.01 6 0.01
0.2 6 0.1
10.95 6 8.59
0.094 6 0.115
0.11 6 0.08
0.22 6 0.40
22.28 6 13.12
0.11 6 0.09
1.7 6 1.1
67.15 6 10.61
0.233 6 0.175
0.41 6 0.23
1.75 6 1.94
85.45 6 7.71
0.01 6 0.01
0.3 6 0.2
11.92 6 6.87
0.035 6 0.027
0.11 6 0.06
0.27 6 0.31
38.4 6 8.49
0.14 6 0.06
1.2 6 0.5
55.54 6 8.66
0.052 6 0.019
0.15 6 0.06
0.57 6 0.79
0.23 6 0.18
0.162 6 0.152
3.45 6 1.49
0.032 6 0.032
1.42 6 0.66
0.022 6 0.013
6.31 6 2.00
0.012 6 0.007
1.86 6 0.77
La (ppm)
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
5.50 6 2.56
12.5 6 8.61
1.55 6 0.77
7.29 6 4.03
1.90 6 1.36
0.97 6 0.61
1.70 6 1.16
0.36 6 0.29
2.05 6 1.80
0.40 6 0.33
1.33 6 1.09
0.22 6 0.18
1.44 6 1.21
0.25 6 0.22
2.93 6 2.88
6.04 6 6.17
0.78 6 0.82
3.36 6 3.75
0.85 6 1.20
0.44 6 0.60
0.74 6 1.00
0.13 6 0.16
0.65 6 0.64
0.13 6 0.12
0.40 6 0.34
0.07 6 0.06
0.42 6 0.36
0.07 6 0.06
7.01 6 4.89
13.0 6 8.36
1.69 6 1.24
7.48 6 5.63
1.69 6 1.31
0.61 6 0.45
1.47 6 1.03
0.26 6 0.18
1.31 6 0.90
0.27 6 0.18
0.82 6 0.50
0.13 6 0.07
0.75 6 0.41
0.12 6 0.06
2.01 6 2.56
3.26 6 2.92
0.42 6 0.42
1.72 6 1.54
0.40 6 0.29
0.17 6 0.10
0.38 6 0.27
0.07 6 0.05
0.34 6 0.21
0.07 6 0.04
0.20 6 0.12
0.03 6 0.02
0.19 6 0.12
0.03 6 0.02
9.45 6 4.86
20.2 6 18.4
2.57 6 1.55
11.5 6 7.28
2.47 6 1.56
0.70 6 0.38
2.29 6 1.42
0.44 6 0.27
2.39 6 1.47
0.49 6 0.28
1.45 6 0.86
0.22 6 0.13
1.33 6 0.76
0.20 6 0.10
SREE
(Ce/Ce*)NASC
(Eu/Eu*)NASC
(La/Yb)NASC
37.4 6 23.1
0.94 6 0.15
2.63 6 0.29
0.35 6 0.15
17.0 6 17.6
0.93 6 0.11
2.66 6 0.44
0.59 6 0.29
36.6 6 24.4
0.94 6 0.22
1.85 6 0.29
0.75 6 0.46
9.28 6 8.04
0.99 6 0.36
2.25 6 0.58
0.84 6 1.06
55.7 6 37.2
0.91 6 0.32
1.44 6 0.18
0.63 6 0.33
SiO2 (wt%)
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
REE variations in mid-Archean BIFs
Ferruginous
mudstone
BI-1
3487
3488
Y. Kato et al.
Fig. 2. (a) SiO2 vs. Fe2O3. Regression line 1 was calculated from points of BI-1, BI-2, and chert samples. Regression line
2 was calculated from points of ferruginous and siliceous mudstone samples. (b) TiO2 vs. Al2O3. A regression line was
calculated from all sample points.
tents. These two trends demonstrate that the positive Eu anomalies of mudstone are completely different in origin from those
observed in BI-1 and chert, and are derived from aluminous
materials.
As mentioned above, there is a general trend for the Eu
anomalies to change significantly from the lower to the upper
sections. However, only in the X-section do the Eu anomaly
values of BI-1 and chert exhibit considerable variation related
to their stratigraphic position and gradually decrease from 3.5
to 2.0 up the section (Fig. 6a). On the other hand, those of BI-2
in the Y-section are constant in the narrow range from 1.6 to
1.1. There is a negative correlation between Eu anomaly values
and total REE concentrations in BI-1, BI-2, and chert (Fig. 6b),
with the correlation coefficient of 0.46 which is statistically
significant on 99% confident level (Snedecor and Cochran,
1967). This means that the total REE concentrations become
higher up-section. The average SREE values (ppm) of iron-rich
rocks (BI-1 and BI-2) are 37.4 6 23.1 (the X-section),
36.6 6 24.4 (the R-section), 55.7 6 37.2 (the Ylower-section),
and 69.0 6 39.9 (the Y-section) up-section. The average SREE
value of BI-1 in the X-section is a little higher than that of the
overlying R-section, probably due to a minor REE contribution
of volcanic materials as discussed later. It should be noted that
the sample B13 (BI-2) with the lowest Eu anomaly value (1.09)
has the highest SREE value (166.5 ppm; Table 1). Although
the SREE values of chert are unlikely to show a clear sequential change, their range begins to narrow up-section.
There are also sequential and gradual changes in LREE/
REE variations in mid-Archean BIFs
3489
Fig. 3. NASC-normalized REE patterns of sedimentary rocks in stratigraphic order. Solid lines represent average REE
patterns. Shadowed areas represent ranges.
HREE ratios of the iron-rich rocks (Fig. 7). The average BI-1
in the lowest section (X) shows the most HREE-enriched
pattern. Lesser HREE-enriched patterns are recognized in the
R- and Ylower-sections; moreover, the average BI-2 is the least
HREE-enriched. Up-section, LREE are enriched compared to
HREE. However, the La/Yb ratios of the iron-rich rocks presented in Table 2 do not show a distinct trend because La
values fluctuate (BI-1 of the R-section is enriched in La; BI-2
is depleted).
Some portions of BI-1 samples, reddish brown in color,
3490
Y. Kato et al.
Fig. 4. Histogram of Eu/Eu* values in stratigraphic order. The Eu/Eu* value of chondrite is 1.49 when normalized to
NASC. Note that almost all Eu/Eu* values of BI-2 are below that of chondrite.
contain goethite, maybe formed by weathering. However, there
is no systematic difference between REE indices (SREE, Eu/
Eu*, Ce/Ce*, and La/Yb) of the BI-1 with goethite and those
without goethite. This suggests that REEs in the BI-1 are
unaffected by weathering.
5. DISCUSSION
5.1. Depositional Position of Sediments
The variations of the REE patterns in the BI-1, BI-2, and
chert are similar to the continuous changes in REE geochemistry of modern hydrothermal metalliferous sediments from the
East Pacific Rise (EPR). The Eu anomaly values of the drillcored, iron-rich sediments near the EPR decrease and become
more seawater-like up the core, that is, with increasing distance
from the paleo-rise crest, due to being transported away by
sea-floor spreading (Olivarez and Owen, 1991); the REE contents also increase with increasing distance (Ruhlin and Owen,
1986; Olivarez and Owen, 1989). Near the crest, rapid burial of
these hydrothermal sediments minimizes their exposure to seawater and continuous scavenging of REEs from seawater and
results in higher positive Eu anomalies (an original hydrothermal signature; Michard and Albarède, 1986) and lower REE
abundances. In contrast, the hydrothermal sediments deposited
away from the ridge crest are characterized by attenuated
positive Eu anomalies and enhanced REE concentrations due to
subsequent overprinting of seawater-derived REEs as suspended hydrothermal particles disperse away from the crest.
After settling, this REE overprinting continues for a long period
of time until the exposure of sediments to seawater ceases once
REE variations in mid-Archean BIFs
Fig. 5. (a) Relation between Eu/Eu* values and Al2O3 contents.
Symbols are the same as those in Fig. 2 (a). A shadowed array
represents the direction up the sections. (b) An enlarged figure of the
inset area in Fig. 5a.
sufficiently covered by subsequent sediment. The decrease in
the Eu anomaly values and the increase in REE concentrations
with increasing distance from the hydrothermal source were
also confirmed in suspended particulates (iron oxyhydroxides)
collected from the TAG hydrothermal vent field (German et al.,
1990).
An enrichment of HREEs in sediments near the rise crest and
no enrichment of HREEs in flank sediments, resulting in increasing LREE/HREE with increasing distance from the ridge
crest, were reported from the EPR by Piper and Graef (1974).
Similar results were reported by German et al. (1990), suggesting a continuing preferential uptake of LREEs compared to
HREEs. Moreover, experiments on REE scavenging in seawater demonstrated a general trend of preferential uptake of
LREEs (Byrne and Kim, 1990; Koeppenkastrop and De Carlo,
1992). In summary, modern hydrothermal metalliferous sediments are characterized by (1) a decrease in the Eu anomaly
3491
Fig. 6. Eu/Eu* values of chemical sediments (a) as a function of
stratigraphic horizon (BI-1 and chert of the X-section, and BI-2 of the
Y-section) and (b) as a function of SREE concentrations (ppm). Symbols are the same as those in Fig. 2 (a).
values, (2) an increase in total REE concentrations, and, (3) an
increase in LREE/HREE ratios up the section.
The striking similarity in REE signatures between these
modern hydrothermal sediments and the Archean sedimentary
rocks discussed here leads us to propose that BI-1 and chert
were in situ hydrothermal precipitates near a mid-ocean ridge.
The BI-1 and chert with larger positive Eu anomalies and lower
REE concentrations in the X-section are most likely to have
been deposited near the ridge crest. The HREE-enriched REE
patterns of BI-1 in the X-section resemble those of modern
iron-rich metalliferous sediments near the crest (Bender et al.,
1971; Piper and Graef, 1974; Barrett and Jarvis, 1988) with the
exception of the absence of large negative Ce anomalies. BI-1
and chert of the higher horizon (the R- and Ylower-sections),
which precipitated further away from the ridge crest, had their
positive Eu anomaly attenuated by subsequent overprinting of
seawater-derived REEs. As a result of this overprinting, the
SREE values also increased. Simultaneously, the REE patterns
became less HREE-enriched. The flat and most REE-enriched
patterns of BI-2 with diminishing or vanishing positive Eu
3492
Y. Kato et al.
Fig. 7. B13-normalized average REE patterns of iron-rich rocks. The sample B13 (BI-2) has the least Eu/Eu* value (1.09)
and the highest SREE value (166.5 ppm). Note Lu-normalization for convenient comparison.
anomalies are interpreted as the REE signature typical of distal
hydrothermal sediments, which most strongly reflect the REE
features of mid-Archean seawater.
REEs in BI-1, BI-2, and chert were predominantly derived
from aqueous solutions (hydrothermal solution and ambient
seawater), and were scavenged from the water column by
suspended hydrothermal particles discharging from the midocean ridge, during drift to the depositional positions and after
deposition in the same manner as the process occurs today.
Although the properties of suspended particles are not known
well, it seems likely that major scavenging materials were
iron-rich (e.g., Balistrieri et al., 1981; Balistrieri and Murray,
1982) and silica-rich (e.g., Schindler et al., 1976; Östhols,
1995) amorphous particles with large surface areas. Larger Eu
anomaly values of chert compared to BI-1 in the same horizon
(Fig. 4) may suggest that chert preserved an earlier and more
intense hydrothermal signature than BI-1 because the silicarich amorphous precipitates had a lower ability to scavenge
REEs than iron-rich ones. The iron-rich sediments (BI-1 and
BI-2) are likely to have had their positive Eu anomaly weakened by continuous overprinting of REEs because of a greater
ability to scavenge REEs. It is very difficult to demonstrate
precisely where REEs are incorporated in the very fine-grained
sedimentary rocks such as iron-rich rock and chert. It may be
plausible, however, that REEs scavenged and adsorbed on
suspended particles are mostly distributed at the grain-boundaries of iron minerals (e.g., hematite) and quartz, and are not
housed in crystallographic sites of minerals, considering that
considerable amounts of REEs are stored at the grain-boundaries of constituent minerals even in the igneous rock (Suzuki
et al., 1990).
In addition to REEs, the observation that amounts of lithogenous elements such as Al2O3, TiO2, and K2O in the BI-1 and
chert of the X-section are minor, but more abundant than those
of the upper R-section implies that minor amounts of the
hyaloclastic materials from ridge volcanism contributed to the
BI-1 and chert in the X-section and supports the suggestion that
the BI-1 and chert in the X-section were deposited near the
crest. Also in the modern sediments, it has been reported that
these lithogenous elements derived from ridge volcanics are
more enriched near the spreading center of the EPR relative to
those on the ridge flank (Marchig et al., 1986). Furthermore,
modern hydrothermal precipitates near the ridge are enriched in
silica and manganese; with increasing distance from the ridge,
the precipitates contain higher proportions of iron (Marchig and
Erzinger, 1986). These modern geochemical trends in SiO2,
MnO2, and Fe2O3 are in harmony with the present results on
mid-Archean rocks, although concentrations of SiO2 and MnO2
in modern sediments are much lower and higher, respectively.
In the modern ocean, hydrothermal silica precipitates are
poorly developed, because amorphous silica is undersaturated
owing to the prevalent development of siliceous microorganisms (Siever, 1957; Tréguer et al., 1995), although hydrothermal silica chimneys have been reported from some areas (e.g.,
Herzig et al., 1988). In contrast, the absence of these types of
microorganisms in the Archean ocean is likely to have promoted a much more silica-rich inorganic precipitation of near
the spreading centers. Little precipitation of manganese despite
intense hydrothermal activity suggests that the redox conditions
of the mid-Archean oceans were generally more reducing than
today. Although there are differences in the absolute concentrations of SiO2 and MnO2, the above-mentioned modern analogies in geochemical trends of REEs and major elements
support the suggestion that the depositional environment of the
X-section was near the ridge crest, and that depositional positions became more distant from the ridge in the order of the X-,
R- , Ylower-, and Y-sections.
Whereas the positive Eu anomalies of BI-1 and chert are
derived from the hydrothermal solutions emanating from the
mid-ocean ridge, those of siliceous and ferruginous mudstone
are inferred to be caused by detrital feldspar and feldsparderived clay minerals from the exposed upper continental crust
or island arcs. The same positive Eu anomaly was observed in
the Archean shale from Kalgoorlie, Western Australia, and was
attributed to a local accumulation of feldspar during sedimentation (Nance and Taylor, 1977). The Eu enrichment in the
siliceous and ferruginous mudstone of the present investigation
is probably due to the same cause, and may not be an indication
of the average chemical composition of exposed continental
crust on a global scale. It is certainly valid to deduce information about the average continental compositions from pelagic
argillaceous sediments, but the mudstone in the upper horizon
was deposited near a continental or island arc where terrigenous materials could be locally supplied. Consequently, the
sedimentary rocks of the present study show a wide variety of
depositional environments, ranging from that typical of midoceanic spreading centers to convergent plate boundary settings.
REE variations in mid-Archean BIFs
3493
Fig. 8. Comparison of Eu/Eu* values of mid-Archean hydrothermal sediments (this work) and modern sediments,
hydrothermal solution, and seawater. The solid circle, box, and horizontal line represent mean value, 61s and range,
respectively. Modern data are from the Pacific ocean, except fish debris in the hydrothermal sediment (Red Sea). BIF-1 of
the R- and Ylower-sections represents all samples of BI-1 and chert. The Eu/Eu* value of chondrite, 1.49, is represented as
a array, in addition to NASC. Modern* is defined here as including Quaternary time. Data sources: Hydrothermal solution
(Michard and Albarède, 1986); seawater (de Baar et al., 1985); fish debris in the Atlantis II Deep (Oudin and Cocherie,
1988); hydrothermal sediment (Ruhlin and Owen, 1986); ferromanganese nodules and associated sediment (Elderfield et al.,
1981). Note: Much higher values of Eu anomaly [Eu/Eu* 5 72.9 by Derry and Jacobsen (1990); Eu/Eu* 5 31.7 by
Campbell et al. (1988)] were reported from EPR hydrothermal solutions.
5.2. REE Signatures of Mid-Archean Seawater
Although our samples are from one area, this set of continuous samples is likely representing an excellent indicator of
mid-Archean marine environments. The hydrothermal sediments of this study deposited in the open and remote ocean, not
in the closed local ocean such as the Mediterranean Sea and the
Black Sea. As a consequence, our set of samples with a continuous spectrum of depositional positions is regarded to record
representative oceanic environments.
At present, the predominant sources for REEs in the ocean
are river waters draining into it from the continents and hydrothermal solutions emanating from the mid-ocean ridge. The Eu
anomalies in mid-Archean hydrothermal sediments from this
study, modern seawater, hydrothermal solution, and modern
sediments including hydrothermal sediment, ferromanganese
nodules, and fish debris in hydrothermal sediment are presented
in Fig. 8. The Eu anomaly for modern seawater is almost
identical to that of NASC, ranging from 0.92 to 1.08 (mean
value: 0.99 6 0.04) and is completely different from that of
hydrothermal solutions which exhibit a significantly larger enrichment of Eu (Eu/Eu*: 6.3 ; 19.7). This indicates that REEs
in modern seawater are strongly controlled by those in exposed
continental crust via a riverine flux. The paleodistances of
depositional sites from Quaternary hydrothermal sediment vary
from 9 km (most proximal) to 1150 km (most distal) (Ruhlin
and Owen, 1986; Olivarez and Owen, 1991). Europium anom-
alies in even the most proximal sediments are close to that of
seawater because post-depositional scavenging from the overlying seawater attenuated the original hydrothermal Eu enrichment (Olivarez and Owen, 1991). Fish debris in the Atlantis II
Deep, accumulated in stagnant and dense hydrothermal brines,
have Eu anomalies close to that of pure hydrothermal solutions
(Oudin and Cocherie, 1988). The Eu anomalies of distal hydrothermal sediments (Olivarez and Owen, 1991), hydrogenous
ferromanganese nodules and associated sediments (Elderfield
et al., 1981) are similar to that of seawater. Like the modern
case, sequential and gradual changes in the Eu anomaly of
Archean sediments suggest that these sediments were deposited
within near-vent to off-ridge environments and that the Eu
anomaly of BI-2 (distal hydrothermal sediment) was possibly
closest to that of mid-Archean seawater. Among BI-2 samples,
the sample B13 with the lowest Eu anomaly (1.09) and the
highest SREE value (166.5 ppm) is most likely to have preserved the REE signature of mid-Archean seawater.
Based on the above arguments, the NASC-normalized REE
pattern of mid-Archean seawater, as a whole, was characterized
by a weak positive or null-value Eu anomaly. This conclusion
has important implications for the composition of both midArchean seawater and continents. First, our findings suggest
that mid-Archean seawater already had been strongly influenced by an influx of fluvial and/or eolian materials from the
upper continental crust that had a negative Eu anomaly relative
3494
Y. Kato et al.
to chondrite. This contrasts with the view of some researchers
that Archean seawater had a large positive Eu anomaly due to
significant hydrothermal activity at that time (Fryer, 1977;
Fryer et al., 1979; Derry and Jacobsen, 1990; Danielson et al.,
1992). While it is true that a significant positive Eu anomaly in
BI-1 and chert implies greater hydrothermal activity in the
Archean ocean than in modern ocean, it is reasonable to assume
that mid-Archean seawater, which had a composition controlled by river waters having a negative Eu anomaly relative to
chondrite, led to the gradual decrease of the Eu anomaly in
BI-1, BI-2, and chert. The initial «Nd value of the Cleaverville
BIF sample was reported to be 14.9 6 1.6 (Jacobsen and
Pimentel-Klose, 1988). This value is quite distinct from the «Nd
value of contemporaneous shale (23.3) from the Gorge Creek
(Allègre and Rousseau, 1984). These significant differences in
«Nd values led Derry and Jacobsen (1990) to conclude that the
REEs in the Cleaverville BIFs were mostly derived from hydrothermal solutions, not from continents. The Eu anomaly
value of this BIF sample is 1.85. This value corresponds to the
average Eu anomaly value of BI-1 in the R-section. In this
point, their conclusion is consistent with our inference that the
BI-1 in the R-section was a proximal hydrothermal precipitate.
Unfortunately, neodymium isotopes of the BI-2, a distal hydrothermal sediment, are not obtained. As neodymium isotopic
variations even in BIFs of one area have been demonstrated to
be fairly large (e.g., Miller and O’Nions, 1985; Alibert and
McCulloch, 1993), it seems invalid to consider that the isotopic
value of one BIF sample was representative of an ancient
global average seawater. A further study is needed on neodymium isotopic variations of proximal to distal hydrothermal
sediments deposited in a great variety of depositional sites in
order to place constraints on relative contributions of continental vs. hydrothermal fluxes into the Archean ocean.
Secondly, our finding that the mid-Archean seawater lacked
a Eu anomaly relative to NASC supports the inference that the
Archean upper continental crust had a negative chondritenormalized Eu anomaly and was already fractionated and differentiated (Reimer et al., 1985; Boryta and Condie, 1990;
Kröner and Layer, 1992; Condie, 1993). Moreover, it indicates
that the mid-Archean continental crust that experienced intracrustal differentiation was already so broadly exposed as to
control the chemistry of the contemporaneous ocean.
In addition to the Eu anomaly, the NASC-normalized REE
pattern of mid-Archean seawater, inferred from REE patterns
of BI-2, had a subtle negative or no Ce anomaly. Their modern
counterparts (distal hydrothermal metalliferous sediments) exhibit a conspicuous Ce depletion, and mimic that of overlying
oxic seawater (Bender et al., 1971; Ruhlin and Owen, 1986).
Apart from other strictly trivalent REEs, Ce is removed from
seawater after oxidation. The Ce anomaly of seawater corresponds to the redox conditions (e.g., de Baar et al., 1988;
German and Elderfield, 1989; Sholkovitz and Schneider, 1991;
German et al., 1991; Schijf et al., 1991) because the reduction
of Ce (IV) to Ce (III) and the oxidation of Ce (III) to Ce (IV)
are rapid enough to respond to changes in the redox conditions
(Sholkovitz et al., 1992). Therefore, the lack of a Ce anomaly,
although it fluctuates a little, may be caused by less oxic
conditions in the mid-Archean ocean than in the modern ocean.
Another plausible explanation is that the lower pH of the
Archean ocean may have been responsible for the absence of
the Ce anomaly, because a Ce anomaly depends not only on
pO2 but also on pH (de Baar et al., 1985; Liu et al., 1988;
Elderfield et al., 1990). Although, to date, there are no data
defining the pH value of the Archean ocean, the pH may have
been approximately 7 due to the combination of a higher pCO2
and considerable supersaturation with respect to aragonite in
the Archean ocean (H. D. Holland, pers. commun.). The pH of
seawater was estimated to be 7.5 6 0.3 at the time of the
Hamersley BIF precipitation (Alibert and McCulloch, 1993),
although the Hamersley BIFs deposited during the ArcheanProterozoic boundary and are much younger than the BIFs of
this study. In any case, around a pH of 7, a negative Ce
anomaly is observed in modern oxic river water (Elderfield et
al., 1990). Considering this observation, it is most likely that
the absence of a Ce anomaly in mid-Archean seawater is due to
the less oxic conditions of seawater rather than a lower pH.
5.3. Implication for Mid-Archean Plate Tectonics
Stratigraphic changes of REE geochemistry demonstrate that
the depositional positions were first truly oceanic and then
changed to near the continent or island arcs. From REE data
alone presented in this paper, it cannot be concluded that the
hydrothermal activity at mid-ocean ridge was responsible for
the proximal to distal hydrothermal sediments. The most probable candidate of the tectonic environment in which the proximal hydrothermal sediment was deposited is either mid-ocean
ridge or hotspot. Mantle plumes may have been more widespread than mid-ocean ridge in the Archean and, as a consequence, many hydrothermal vents on the seafloor may have
been related to plumes rather than mid-ocean ridge (Condie,
1989). It is, therefore, not reasonable to rule out the hotspot
which was related to the hydrothermal sediments. However,
geochemistry and petrogenesis of underlying greenstones revealed that they are Fe-rich low K tholeiites formed at midocean ridge and are not Archean OIB volcanics such as komatiites (Ohta et al., 1996). It is also suggested that their source
mantle is more Fe-rich than modern MORB source mantle, and
the estimated potential mantle temperature is 1400°C which is
120°C higher than the modern potential mantle temperature.
The present REE data in combination with these lines of
evidence suggest that the formation of the BIFs of this study
was related to the hydrothermal activity at mid-ocean ridge, and
the horizontal movement of the oceanic plate as the result of
seafloor spreading was responsible for a continuous spectrum
of various marine depositional regimes from the mid-ocean
ridge to convergent plate boundaries (Fig. 9). Horizontal shortening, resulting from plate convergence, caused the tectonic
repetition of chemical sediments with an affinity for mid-ocean
ridge hydrothermal solutions and clastic sediments with an
affinity for terrigenous contributions from the continental crust.
Therefore, it is very likely that plate tectonics was already
operating in the mid-Archean (3.3–3.2 Gyr ago). This supports
the conclusion, inferred from paleomagnetic data, that plate
tectonic styles beginning as early as 3.4 Gyr ago are essentially
comparable to those of the Phanerozoic plate-tectonic regime
(Kröner, 1991; Kröner and Layer, 1992).
REE variations in mid-Archean BIFs
3495
Fig. 9. Schematic diagram showing changes in depositional positions, columnar section, and REE patterns of
sediments.
6. CONCLUSIONS
Field relations and geochemical data indicate that the depositional environments of sedimentary rocks changed from midoceanic spreading centers to convergent plate boundary settings. The presence of layer-parallel thrusts and the
reconstructed oceanic plate stratigraphy suggest that the midArchean Cleaverville Formation is an accretionary complex.
There are remarkable similarities in geochemical features of
REEs and major elements between the BIFs and modern hydrothermal sediments from the EPR. The REE geochemical
trends are characterized by a decrease in the Eu anomaly and an
increase in SREE concentrations and in LREE/HREE ratios
up-section. The decrease in Al, Ti, and K derived from ridge
volcanics and the decrease in proportions of silica and inverse
increase in proportions of iron are observed in the lower sections. These similarities to modern hydrothermal sediments
suggest that the BIFs were in situ hydrothermal precipitates
near the mid-ocean ridge and that the origin of the BIFs shifted
from a proximal hydrothermal to a distal hydrothermal source.
These geological and geochemical observations have important
implications regarding the operation of plate tectonics before
the mid-Archean time. It seems likely that plate tectonics was
already operating in the mid-Archean (3.3–3.2 Gyr ago).
Distal hydrothermal sediments, BI-2, are of great significance in that their REE signatures resemble those of midArchean seawater. The BI-2 with negative Eu anomalies relative to chondrite suggests that mid-Archean seawater had
already been strongly influenced by a fluvial flux from the
upper continental crust, not by hydrothermal flux from the
mid-ocean ridge. In addition, this negative Eu anomaly in the
BI-2 implies that intracrustal differentiation and fractionation
were widespread in the mid-Archean continental crust. Moreover, the differentiated and fractionated upper continental crust
had already been subaerially exposed. The inferred REE pattern
of mid-Archean seawater was also characterized by the lack of
a Ce anomaly, most likely due to less oxic conditions in
seawater than that occurring today.
Acknowledgments—We thank H. D. Holland, S. B. Jacobsen, R. L.
Rudnick, C. G. Olson, J. Hedenquist, C. W. Mandeville, and N.
Shikazono for their critical review and helpful comments on the manuscript. Discussions with E. R. Sholkovitz, G. E. Ravizza, and K.
Ruttenberg were also of great assistance. Thanks also to T. Irino, T.
Ishii, and H. Yoshida for XRF analyses. The field collaboration with
A. H. Hickman and A. Thorne was helpful. We would also like to
acknowledge K. C. Condie and an anonymous reviewer for their
helpful comments. This research was supported by the Ministry of
Culture and Education of Japan (Intensified Study Area Program number 259, 1995 through 1997) and H. Hironaka, the president of
Yamaguchi University.
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