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Geochimica et Cosmochimica Acta, Vol. 62, No. 21/22, pp. 3475–3497, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 1 .00 Pergamon 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. REFERENCES Alibert C. and McCulloch M. T. (1993) Rare earth element and neodymium isotopic compositions of the banded iron-formations and associated shales from Hamersley, western Australia. Geochim. Cosmochim. Acta 57, 187–204. Allègre C. J. and Rousseau D. (1984) The growth of the continent 3496 Y. Kato et al. through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Lett. 67, 19 –34. Balistrieri L. S. and Murray J. W. (1982) The adsorption of Cu, Pb, Zn, and Cd on goethite from major ion seawater. Geochim. Cosmochim. Acta 46, 1253–1265. Balistrieri L. S., Brewer P. G., and Murray J. W. (1981) Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean. Deep-Sea Res. 28A, 101–121. Barrett T. J. and Jarvis I. (1988) Rare-earth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect. Chem. Geol. 67, 243–259. Barrett T. J., Fralick P. W., and Jarvis I. (1988) Rare-earth-element geochemistry of some Archean iron formations north of Lake Superior, Ontario. Canadian J. Earth Sci. 25, 570 –580. Bau M. and Möller P. (1993) Rare earth element systematics of the chemically precipitated component in early Precambrian iron formations and the evolution of the terrestrial atmosphere-hydrospherelithosphere system. Geochim. Cosmochim. Acta 57, 2239 –2249. Bender M., Broecker W., Gornitz V., Middle U., Kay R., Sun S., and Biscaye P. (1971) Geochemistry of three cores from the East Pacific Rise. Earth Planet. Sci. Lett. 12, 425– 433. Bickle M. J., Bettenay L. F., Boulter C. A., Groves D. I., and Morant P. (1980) Horizontal tectonic interaction of an Archean gneiss belt and greenstones, Pilbara block, Western Australia. Geology 8, 525– 529. Boryta M. and Condie K. C. (1990) Geochemistry and origin of the Archaean Beit Bridge complex, Limpopo Belt, South Africa. J. Geol. Soc. London 147, 229 –239. Boyer S. E. and Elliott D. (1982) Thrust systems. Bull. AAPG 66, 1196 –1230. Butler R. W. H. (1982) The terminology of structures in thrust belts. J. Struct. Geol. 4, 239 –245. Byrne R. H. and Kim K. (1990) Rare earth element scavenging in seawater. Geochim. Cosmochim. Acta 54, 2645–2656. Calvert A. J., Sawyer E. W., Davis W. J., and Ludden J. N. (1995) Archaean subduction inferred from seismic images of a mantle suture in the Superior Province. Nature 375, 670 – 674. Campbell A. C. et al. (1988) Chemistry of hot springs on the MidAtlantic Ridge. Nature 335, 514 –519. Condie K. C. (1989) Plate Tectonics and Crustal Evolution. Pergamon. Condie K. C. (1993) Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales. Chem. Geol. 104, 1–37. Danielson A., Möller P., and Dulski P. (1992) The europium anomalies in banded iron formations and the thermal history of the oceanic crust. Chem. Geol. 97, 89 –100. de Baar H. J. W., Bacon M. P., and Brewer P. G. (1985) Rare earth elements in the Pacific and Atlantic Oceans. Geochim. Cosmochim. Acta 49, 1943–1959. de Baar H. J. W., German C. R., Elderfield H., and van Gaans P. (1988) Rare earth element distributions in anoxic waters of the Cariaco Trench. Geochim. Cosmochim. Acta 52, 1203–1219. de Wit M. J., Hart R. A., and Hart R. J. (1987) The Jamestown Ophiolite Complex, Barberton mountain belt: A section through 3.5 Ga oceanic crust. J. Afr. Earth Sci. 6, 681–730. de Wit M. J. et al. (1992) Formation of an Archaean continent. Nature 357, 553–562. Derry L. A. and Jacobsen S. B. (1990) The chemical evolution of Precambrian seawater: Evidence from REEs in banded iron formations. Geochim. Cosmochim. Acta 54, 2965–2977. Dymek R. F. and Klein C. (1988) Chemistry, petrology and origin of banded iron-formation lithologies from the 3800 Ma Isua Supracrustal Belt, West Greenland. Precamb. Res. 39, 247–302. Elderfield H. and Sholkovitz E. R. (1987) Rare earth elements in the pore waters of reducing nearshore sediments. Earth Planet. Sci. Lett. 82, 280 –288. Elderfield H., Hawkesworth C. J., Greaves M. J., and Calvert S. E. (1981) Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments. Geochim. Cosmochim. Acta 45, 513–528. Elderfield H., Upstill-Goddard R., and Sholkovitz E. R. (1990) The rare earth elements in rivers, estuaries, and coastal seas and their signif- icance to the composition of ocean waters. Geochim. Cosmochim. Acta 54, 971–991. Fagel N., André L., and Debrabant P. (1997) Multiple seawater-derived geochemical signatures in Indian oceanic pelagic clays. Geochim. Cosmochim. Acta 61, 989 –1008. Fryer B. J. (1977) Rare earth evidence in iron-formations for changing Precambrian oxidation states. Geochim. Cosmochim. Acta 41, 361– 367. Fryer B. J., Fyfe W. S., and Kerrich R. (1979) Archaean volcanogenic oceans. Chem. Geol. 24, 25–33. German C. R. and Elderfield H. (1989) Rare earth elements in Saanich Inlet, British Columbia, a seasonally anoxic basin. Geochim. Cosmochim. Acta 53, 2561–2571. German C. R., Klinkhammer G. P., Edmond J. M., Mitra A., and Elderfield H. (1990) Hydrothermal scavenging of rare-earth elements in the ocean. Nature 345, 516 –518. German C. R., Holliday B. P., and Elderfield H. (1991) Redox cycling of rare earth elements in the suboxic zone of the Black Sea. Geochim. Cosmochim. Acta 55, 3553–3558. Gnaneshwar Rao T. and Naqvi S. M. (1995) Geochemistry, depositional environment and tectonic setting of the BIF’s of the Late Archaean Chitradurga Schist Belt, India. Chem. Geol. 121, 217–243. Goldstein S. J. and Jacobsen S. B. (1988) Rare earth elements in river waters. Earth Planet. Sci. Lett. 89, 35– 47. Goodwin A. M. (1991) Precambrian Geology: The Dynamic Evolution of the Continental Crust. Academic Press. Graf Jr. J. L. (1978) Rare earth elements, iron formations and sea water. Geochim. Cosmochim. Acta 42, 1845–1850. Helmstaedt H., Padgham W. A., and Brophy J. A. (1986) Multiple dikes in Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading? Geology 14, 562–566. Herzig P. M., Becker K. P., Stoffers P., Bäker H., and Blum N. (1988) Hydrothermal silica chimney fields in the Galapagos Spreading Center at 86oW. Earth Planet. Sci. Lett. 89, 261–272. Hickman A. H. (1983) Geology of the Pilbara Block and its Environs. Geol. Surv. West. Aust. Bull. 127 Hickman A. H. (1990) Pilbara and Hamersley basin. In Third International Archaean Symposium Excursion Guidebook (ed. S. E. Ho et al.), pp. 1– 60. Hoffman P. F. and Ranalli G. (1988) Archean oceanic flake tectonics. Geophys. Res. Lett. 15, 1077–1080. Imai N. (1990) Multielement analysis of rocks with the use of geological certified reference material by inductively coupled plasma mass spectrometry. Anal. Sci. 6, 389 –395. Irino T. (1996) Quantification of Kosa (eolian dust) contribution to the sediments and reconstruction of its flux variation at ODP Site 797, the Japan Sea during the last 200 Ky. Ph.D. thesis, Univ. Tokyo. Isozaki Y., Maruyama S., and Furuoka F. (1990) Accreted oceanic materials in Japan. Tectonophysics 181, 179 –205. Jacobsen S. B. and Pimentel-Klose M. R. (1988) Neodymium isotopic variations in Precambrian banded iron formations. Geophys. Res. Lett. 15, 393–396. Khan R. M. K., Das Sharma S., Patil D. J., and Naqvi S. M. (1996) Trace, rare-earth element, and oxygen isotopic systematics for the genesis of banded iron-formations: Evidence from Kushtagi schist belt, Archaean Dharwar Craton, India. Geochim. Cosmochim. Acta 60, 3285–3294. Klein C. and Beukes N. (1989) Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa. Econ. Geol. 84, 1733–1774. Koeppenkastrop D. and De Carlo E. H. (1992) Sorption of rare-earth elements from seawater onto synthetic mineral particles: An experimental approach. Chem. Geol. 95, 251–263. Kröner A. (1991) Tectonic evolution in the Archaean and Proterozoic. Tectonophysics 187, 393– 410. Kröner A. and Layer P. W. (1992) Crust formation and plate motion in the early Archean. Science 256, 1405–1411. Kusky T. M. and Kidd W. S. F. (1992) Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe. Geology 20, 43– 46. Liu Y.-G., Miah M. R. U., and Schmitt R. A. (1988) Cerium: A REE variations in mid-Archean BIFs chemical tracer for paleo-oceanic redox conditions. Geochim. Cosmochim. Acta 52, 1361–1371. Lowe D. R. (1994) Archean greenstone-related sedimentary rocks. In Archean Crustal Evolution (ed. K. C. Condie), pp. 121–169. Elsevier. Manikyamba C., Balaram V., and Naqvi S. M. (1993) Geochemical signatures of polygenetic origin of a banded iron formation (BIF) of the Archaean Sandur greenstone belt (schist belt) Karnataka nucleus, India. Precamb. Res. 61, 137–164. Marchig V. and Erzinger J. (1986) Chemical composition of Pacific sediments near 20oS: Changes with increasing distance from the East Pacific Rise. Initi. Repts. of the Deep Sea Drilling Project 92, 371–381. Marchig V., Erzinger J., and Heinze P. (1986) Sediment in the black smoker area of the East Pacific Rise (18.5oS). Earth Planet. Sci. Lett. 79, 93–106. Matsuda T. and Isozaki Y. (1991) Well-documented travel history of Mesozoic pelagic chert in Japan: From remote ocean to subduction zone. Tectonics 10, 475– 499. McLennan S. M. and Taylor S. R. (1991) Sedimentary rocks and crustal evolution: Tectonic setting and secular trends. J. Geol. 99, 1–21. Michard A. (1989) Rare earth element systematics in hydrothermal fluids. Geochim. Cosmochim. Acta 53, 745–750. Michard A. and Albarède F. (1986) The REE content of some hydrothermal fluids. Chem. Geol. 55, 51– 60. Miller R. G. and O’Nions R. K. (1985) Source of Precambrian chemical and clastic sediments. Nature 314, 325–330. Milodowski A. E. and Zalasiewicz J. A. (1991) Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudrock sequences of Llandovery age from central Wales. In Developments in Sedimentary Provenance Studies (ed. A. C. Morton et al.); Geol. Soc. Spec. Publ. 57, 101–124. Murray R. W., Buchholtz ten Brink M. R., Gerlach D. C., Russ G. P., III, and Jones D. L. (1991a) Rare earth, major, and trace elements in chert from the Franciscan Complex and Monterey Group, California: Assessing REE sources to fine-grained marine sediments. Geochim. Cosmochim. Acta 55, 1875–1895. Murray R. W., Buchholtz ten Brink M. R., Brumsack H. J., Gerlach D. C., and Russ G. P., III (1991b) Rare earth elements in Japan Sea sediments and diagenetic behavior of Ce/Ce*: Results from ODP Leg 127. Geochim. Cosmochim. Acta 55, 2453–2466. Nance W. B. and Taylor S. R. (1977) Rare earth element patterns and crustal evolution-II. Archean sedimentary rocks from Kalgoorlie, Australia. Geochim. Cosmochim. Acta 41, 225–231. Nesbitt H. W. (1979) Mobility and fractionation of rare earth elements during weathering of a granodiorite. Nature 279, 206 –210. Ohta H., Maruyama S., Takahashi E., Watanabe Y., and Kato Y. (1996) Field occurrence, geochemistry and petrogenesis of the Archean Mid-Oceanic Ridge Basalts (AMORBs) of the Cleaverville area, Pilbara Craton, Western Australia. Lithos 37, 199 –221. Olivarez A. M. and Owen R. M. (1989) REE/Fe variations in hydrothermal sediments: Implications for the REE content of seawater. Geochim. Cosmochim. Acta 53, 757–762. Olivarez A. M. and Owen R. M. (1991) The europium anomaly of seawater: Implications for fluvial versus hydrothermal REE inputs to the oceans. Chem. Geol. 92, 317–328. Östhols E. (1995) Thorium sorption on amorphous silica. Geochim. Cosmochim. Acta 59, 1235–1249. Oudin E. and Cocherie A. (1988) Fish debris record the hydrothermal activity in the Atlantis II Deep sediments (Red Sea). Geochim. Cosmochim. Acta 52, 177–184. Piper D. Z. and Graef P. A. (1974) Gold and rare-earth elements in sediments from the East Pacific Rise. Mar. Geol. 17, 287–297. Rangin C., Steinberg M., and Courtois-Bonnot C. (1981) Geochemistry 3497 of the Mesozoic bedded cherts of Central Baja California (VizcainoCedros-San Benito): Implications for paleogeographic reconstruction of an old oceanic basin. Earth Planet. Sci. Lett. 54, 313–322. Reimer T. O., Condie K. C., Schneider G., and Georgi A. (1985) Petrography and geochemistry of granitoid and metamorphite pebbles from the early Archaean Moodies Group, Barberton Mountainland/South Africa. Precamb. Res. 29, 383– 404. Ruhlin D. E. and Owen R. M. (1986) The rare earth element geochemistry of hydrothermal sediments from the East Pacific Rise: Examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta 50, 393– 400. Schijf J., de Baar H. J. W., Wijbrans J. R., and Landing W. M. (1991) Dissolved rare earth elements in the Black Sea. Deep-Sea Res. 38 (Suppl. 2), S805–S823. Schindler P. W., Furst B., Dick R., and Wolf P. U. (1976) Ligand properties of surface silanol groups. I. Surface complex formation with Fe31, Cu21, Cd21, and Pb21. J. Coll. Interf. Sci. 55, 469 – 475. Siever R. (1957) The silica budget in the sedimentary cycle. Amer. Mineral. 42, 821– 841. Sholkovitz E. R. (1988) Rare earth elements in the sediments of the North Atlantic Ocean, Amazon delta, and East China Sea: Reinterpretation of terrigenous input patterns to the oceans. Amer. J. Sci. 288, 236 –281. Sholkovitz E. R. and Schneider D. L. (1991) Cerium redox cycles and rare earth elements in the Sargasso Sea. Geochim. Cosmochim. Acta 55, 2737–2743. Sholkovitz E. R., Shaw T. J., and Schneider D. L. (1992) The geochemistry of rare earth elements in the seasonally anoxic water column and porewaters of Chesapeake Bay. Geochim. Cosmochim. Acta 56, 3389 –3402. Sleep N. H. and Windley B. F. (1982) Archean plate tectonics: Constraints and inferences. J. Geol. 90, 363–379. Snedecor G. W. and Cochran W. G. (1967) Statistical methods. Iowa State Univ. Press. Suzuki K., Adachi M., and Yamamoto K. (1990) Possible effects of grain-boundary REE on the REE distribution in felsic melts derived by partial melting. Geochem. J. 24, 57–74. Taira A., Katto J., Tashiro M., Okamura M., and Kodama K. (1988) The Shimanto belt in Shikoku, Japan– evolution of Cretaceous to Miocene accretionary prism. Modern Geology 12, 5– 46. Taylor S. R. and McLennan S. M. (1985) The Continental Crust: Its Composition and Evolution. Blackwell. Taylor S. R., Rudnick R. L., McLennan S. M., and Eriksson K. A. (1986) Rare earth element patterns in Archean high-grade metasediments and their tectonic significance. Geochim. Cosmochim. Acta 50, 2267–2279. Toyoda K., Nakamura Y., and Masuda A. (1990) Rare earth elements of Pacific pelagic sediments. Geochim. Cosmochim. Acta 54, 1093– 1103. Tréguer P., Nelson D. M., Van Bennekom A. J., DeMaster D. J., Laynaert A., and Quéguiner B. (1995) The silica balance in the world ocean: A reestimate. Science 268, 375–379. von Huene R. and Scholl D. W. (1993) The return of sialic material to the mantle indicated by terrigenous material subducted at convergent margins. Tectonophysics 219, 163–175. Wang Y. L., Liu Y.-G., and Schmitt R. A. (1986) Rare earth element geochemistry of South Atlantic deep sea sediments: Ce anomaly change at ;54 My. Geochim. Cosmochim. Acta 50, 1337–1355. Windley B. F. (1984) The Evolving Continents. Wiley. Wright J., Schrader H., and Holser W. T. (1987) Paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite. Geochim. Cosmochim. Acta 51, 631– 644. Zhou Y., Chown E. H., Guha J., Lu H., and Tu G. (1994) Hydrothermal origin of Late Proterozoic bedded chert at Gusui, Guangdong, China: Petrological and geochemical evidence. Sedimentology 41, 605– 619.