Geochemical Journal, Vol. 37, pp. 163 to 180, 2003
Rare earth element and strontium isotopic study of seamount-type
limestones in Mesozoic accretionary complex of
Southern Chichibu Terrane, central Japan:
Implication for incorporation process of seawater REE into limestones
KAZUYA TANAKA,* NORIKO MIURA,** YOSHIHIRO A SAHARA and IWAO KAWABE
Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University,
Chikusa, Nagoya 464-8602, Japan
(Received March 6, 2002; Accepted October 10, 2002)
Ishimaki and Tahara limestones occur as exotic blocks juxtaposed in the Mesozoic (Jurassic) accretionary complex of Southern Chichibu Terrane in eastern Aichi Prefecture, central Japan. They are supposed to be of the seamount-type limestone, since they have no terrigenous materials and are intimately
associated with greenstones. REE (rare earth elements) and Sr isotopic studies for the limestones have
been made in order to know their geochemical characteristics, ages and origins. Their 87Sr/ 86Sr ratios,
when referred to the seawater 87Sr/86Sr curve and relevant geological data, suggest that Ishimaki and Tahara
limestones are the late Permian and the Carboniferous to the Early Permian, respectively. Two greenstone
fragments found inside the Ishimaki limestone block and one greenstone sample associated with Tahara
limestone block, resemble the Hawaiian alkali basalt in their REE and Y patterns. This is supporting the
idea that the limestone blocks may be parts of reef limestones on ancient volcanic seamounts. All the
limestone samples, except three unusual Tahara ones, show seawater REE and Y signatures in their chondritenormalized patterns. Their REE/Ca ratios, however, are 102–10 4 times as high as those ratios of modern
biogenic carbonates like corals and the seawater. Accordingly, seawater REE and Y were incorporated into
the limestones, when originally biogenic carbonates transformed into inorganic calcite and its secondary
growths occurred in diagenesis in contact with sufficient seawater. This view is favored by the reported
REE partition experiment between calcite overgrowths and seawater solution. The seawater Ce anomaly
as a function of water depth in the modern ocean is a key to infer the water depth of the REE and Y
incorporation. The Ce anomalies given by log(Ce/Ce*) for about a half of Ishimaki samples and most of
Tahara ones are between –0.5 and –0.2, which are compatible with the shallow water origin. Another half
of Ishimaki samples, however, have log(Ce/Ce*) values between –0.7 and –1.0, suggesting moderately
deep waters (ca. 500–1000 m deep or more). This may reflect such a situation that water depths of REE
incorporation into the seamount-type limestones are generally greater than the depositional water depths
of original biogenic carbonates because of the fate of limestone-capped volcanic seamounts decided by
the oceanic plate motion.
1990). Limestone blocks yielding CarboniferousPermian fusulinids and corals, and those blocks
bearing Triassic corals, ammonites and bivalves
are known in Southern Chichibu Terrane of
Kyushu and Shikoku Islands (Matsuoka and Yao,
1990). Such limestone blocks are also known in
I NTRODUCTION
Marine limestones occur as exotic blocks with
greenstones in Mesozoic (Jurassic) accretionary
clastic rock complexes of Southern Chichibu
Terrane in southwest Japan (Matsuoka and Yao,
*Corresponding author (e-mail: [email protected])
**Present address: Kabe-cho 6-28-6, Oume, Tokyo 198-0036, Japan
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K. Tanaka et al.
Fig. 1. The Southern Chichibu Terrane as a Mesozoic accretionary complex and sampling locations in this study.
Fourteen limestone samples (ISLS-01~-14) and two greenstone samples (ISGS-01, -02) from Mt. Ishimaki and
twelve limestone samples (THLS-01~-10, -20, -21) and one greenstone sample (THGS-01) from Tahara Mine have
been collected, respectively.
Ishimaki and Tahara areas, Aichi Prefecture, central Japan, an eastern part of Southern Chichibu
Terrane (Fig. 1). Triassic radiolarian fossils have
been reported from the cherts in a few km east of
Ishimaki limestone (Ieda and Sugiyama, 1998),
but the limestone itself does not yield index fossils such as fusulinids partly because of
recrystallization. The fossil age of Tahara limestone is also unknown, although Triassic and
Jurassic radiolarians have been reported from
cherts and shales in the western part of Atsumi
Peninsula, in the immediate west of Tahara limestone (Ohba, 1997).
For determining formation ages of marine
limestones without index fossils or for constraining such ages, the idea of Sr isotope stratigraphy
(Elderfield, 1986; McArthur, 1994) is useful. This
method is based on the secular variation of
seawater 87Sr/86Sr ratio determined by using marine carbonate fossils and limestones with known
geologic ages (Burke et al., 1982; Veizer et al.,
1999). The purpose of this study is to clarify
geochemical characteristics, ages and origins of
Ishimaki and Tahara limestones from their REE
and Sr isotopic analyses and relevant geological
information. Kawabe et al. (1991, 1994a) emphasized that Permian Japanese limestones of the coral
reef origin on volcanic seamounts show seawaterlike lanthanide tetrad effects and large Y/Ho ratios much greater than the ratio of average shales.
REE and Y data of Ishimaki and Tahara limestones
provide important clues to consider the incorporation of seawater REE and Y into marine limestones.
SAMPLES AND GEOLOGICAL BACKGROUNDS
Fourteen limestone samples (ISLS-01~-14)
and two greenstone samples (ISGS-01 and -02)
intercalated in the massive limestone have been
collected from Mt. Ishimaki in Toyohashi City.
The limestone sample number increases from the
top of Mt. Ishimaki to the lower elevations. The
greenstone sample (ISGS-01) was found at the
horizon between the limestone samples of ISLS09 and -10, and another greenstone sample (ISGS02) was between the samples of ISLS-12 and -13.
Twelve limestone samples (THLS-01~-10, -20 and
REE and Sr isotopic study of seamount-type limestones
-21) and one greenstone sample (THGS-01) associated with the limestone have also been collected
from Tahara Mine in Tahara Town, Atsumi Country. The locations of Mt. Ishimaki and Tahara Mine
are shown in Fig. 1.
The Ishimaki and Tahara areas belong to the
eastern part of Southern Chichibu Terrane in
southwest Japan (Matsuoka and Yao, 1990). Although no data of fossil geologic ages have been
reported from Ishimaki and Tahara limestones,
fragments of crinoids and calcareous algae are
observed in thin sections of Ishimaki and Tahara
limestones in this study under optical microscopes.
Such preservation of original and biogenic structures indicates that these limestones have not been
subjected to high temperature alteration or metamorphism. This also suggests that these limestones
possibly preserve chemical information of ancient
seawater. There have been reported several
stratigraphic studies on the adjacent areas: Ieda
and Sugiyama (1998) reported Triassic
radiolarians from cherts several km east of
Ishimaki limestone. From the northwest of
Hamana Lake, about 10 km east of Ishimaki limestone, Mizugaki (1985) reported Middle to Upper
Permian cherts, Upper Triassic cherts, and Lower
to Middle Jurassic pelitic rocks, together with a
Lower Permian limestone lens. Nagai and
Ishikawa (1995) found Middle Permian
radiolarians in a bedded chert in Atsumi Peninsula. Ohba (1997) also reported well-preserved
Triassic to Jurassic radiolarian fossils in cherts and
shales in the western part of Atsumi Peninsula,
immediate southwest of the Tahara area.
According to Matsuoka and Yao (1990), Carboniferous-Permian and Triassic limestones with
greenstones occur as exotic blocks in Jurassic clastic sequences in the Southern Chichibu Terrane of
Kyushu and Shikoku Islands. The limestones
sometimes yield shallow marine fossils like corals and megalodonts, so that the limestonegreenstone units are inferred to represent upper
parts of limestone-capped seamounts. They were
possibly scraped off a subducting oceanic crust
and juxtaposed into the Middle Mesozoic clastic
rocks in the continental margin at that time. Hence
165
geological studies of Southern Chichibu Terrane
imply that Ishimaki and Tahara limestones were
also formed as shallow marine carbonates on
seamounts in the Carboniferous to Triassic ocean.
EXPERIMENTAL PROCEDURES
In order to know amounts of non-carbonate
phases in limestone samples and what minerals
constitute non-carbonate phases, non-carbonate
residues after leaching aliquots (about 5 g) of selected limestone samples by CH 3 COOH, have
been weighed and examined by XRD.
The analytical method for determining REE
and Y in limestone samples by ICP-AES is the
same as reported elsewhere (Kawabe et al., 1991,
1994b). In brief, each pulverized sample (10–25
g) is dissolved in HCl. Insoluble materials are removed by filtration. REE and Y in the filtrate are
coprecipitated with Fe(OH) 3 twice in order to
separate them from a large amount of Ca. Fe(OH)3
with REE and Y is dissolved in HCl, and then REE
and Y are purified by cation-exchange chromatography. They are determined by an ICP-AES
spectrometer (Seiko SPS-1500R). Selected limestone samples were dissolved also by acetic acid,
and REE and Y were determined in the same way
as in HCl digestion. The results of replicate analyses for GSJ carbonate reference rocks are reported
by Kawabe et al. (1994b).
Minor elements (Mg, Fe, Mn, P, Al, Sr, Ba and
Na) in each limestone sample have also been determined by using another sample solution, which
was prepared by dissolving another aliquot (about
0.05–0.5 g) in HCl and filtering it to remove insoluble materials. They are determined by an ICPAES spectrometer (Seiko SPS-1500R).
Major element compositions of greenstone
samples have been determined by XRF
spectrometry. REE and Y analyses were also made
by the method described in Kawabe (1995). In
short, each pulverized sample weighed 1.2 g is
digested by HNO3-HClO4-HF and then dried completely. This process is repeated at least twice. The
final evaporation residue is dissolved in HCl, and
then filtered. The filtered insoluble materials are
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K. Tanaka et al.
fused with Na2CO3 plus H3BO3 in a Pt crucible.
The fusion product is dissolved in HCl and combined with the first filtrate. The following procedure is the same as that of limestones.
Sr isotopic ratios of limestone samples have
been determined according to the method in
Asahara et al. (1995). Each pulverized sample
weighed 0.5 g was dissolved in HCl. After removal
of insolubles by filtration, Sr was purified by
cation-exchange chromatography. The 87Sr/ 86Sr
ratios were determined by a VG Sector 54-30
mass-spectrometer. The 87Sr/86Sr ratio of NIST
SRM 987 during the course of this study was
0.710252 ± 0.000007 (2σ m, n = 11).
RESULTS
carbonate contents of the two limestones are 96–
99%, indicating that they are the limestones with
very low impurity contents. Ishimaki and Tahara
limestones have low Mg contents of 0.3~1.5 mol%
as MgCO3 and are composed of low-Mg calcite
(Table 3). High-Mg calcite containing 4~20 mol%
MgCO3 and dolomite are unimportant for the limestones of the present study.
Chemical analyses of Ishimaki and Tahara
greenstone samples (ISGS-01, -02 and THGS-01)
are summarized in Table 2. The unusually high
CaO content of ISGS-01 is due to calcite veins in
it. Except for the CaO content, their major element compositions are of the alkali basalt. Their
chondrite-normalized REE patterns are compared
with those for Hawaiian alkali basalt and tholeiite
as well as for mid-ocean ridge basalts (Fig. 2).
Table 1 lists the contents of residues after
leaching aliquots (about 5 g) of selected limestone
samples by acetic acid. The minerals identified in
the respective residues by XRD have also been
listed. Quartz is the principal constituent mineral,
although chlorite and illite are identified. Because
no XRD peaks of carbonate minerals were found,
the residue contents are those of non-carbonate
impurities. They are from 0.8 to 4.2% in Ishimaki
samples and from 1.3 to 4.3% in Tahara ones. The
Table 1. Contents of non-carbonate residues after
acetic acid leaching and their constituent minerals
by XRD
Sample
Residue(wt.%)
Mineral
Mt. Ishimaki
ISLS-05
ISLS-07
ISLS-09
ISLS-10
ISLS-12
1.52
0.79
1.52
1.87
4.23
quartz, chlorite, illite
quartz, chlorite, illite
quartz, chlorite, illite
quartz, chlorite, illite
quartz, chlorite, illite
Tahara Mine
THLS-01
THLS-03
THLS-07
THLS-10
THLS-20
THLS-21
4.08
4.29
1.25
3.64
3.61
3.34
quartz, chlorite, illite
quartz
quartz, illite
quartz, chlorite, illite
quartz, chlorite, illite
quartz, chlorite, illite
Fig. 2. Comparison of chondrite-normalized REE and
Y abundance patterns for greenstone samples in this
study and for oceanic basaltic rocks. REE and Y data
of Hawaiian basalts and MORB are cited from Garcia
et al. (1993) and Sun et al. (1979), respectively. The
C1 chondrite values reported by Anders and Grevesse
(1989) are used for normalization.
REE and Sr isotopic study of seamount-type limestones
167
Fig. 3. Sr isotope stratigraphic diagram. Isotopic data by Denison et al. (1994) and Koepnick et al. (1990) are
used in order to reconstruct the seawater Sr isotopic variations from the late Paleozoic to the early Triassic.
Filled circle and diamond data outlined approximately by a solid curve are judged to be more likely to have
retained original seawater Sr isotopic compositions than open circle data as Denison et al. (1994) wrote. All the
cited data are normalized to NIST SRM 987 (87Sr/ 86Sr = 0.710250) in order to compare them with Ishimaki and
Tahara limestone samples. Ishimaki and Tahara limestone samples are plotted on the left and right sides, respectively. The limestone samples shown by filled triangles satisfy the chemical criteria (Sr/Mn > 2 and Mn < 300
ppm) by Denison et al. (1994). The samples shown by open triangles do not satisfy the criteria.
Fig. 4. Comparison of chondrite-normalized REE and Y abundance patterns for selected limestone samples by
two dissolution methods of HCl dissolution and acetic acid leaching. L denotes the data by acetic acid leaching.
The C1 chondrite values by Anders and Grevesse (1989) are used for normalization.
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K. Tanaka et al.
Fig. 5. Chondrite-normalized REE and Y abundance patterns for (a) Ishimaki and (b) Tahara limestone samples.
The C1 chondrite values by Anders and Grevesse (1989) are used for normalization.
Evidently these greenstone samples resemble the
Hawaiian alkali basalt of Loihi seamount with
respect to their REE patterns. These chemical
analyses of Ishimaki and Tahara greenstones support the possibility that Ishimaki and Tahara limestones were formed on the tops of volcanic
seamounts.
Sr isotopic ratios and minor element contents
of Ishimaki and Tahara limestones are summarized
in Table 3. Their Sr/Mn ratios are also listed in
the last column of Table 3, since Denison et al.
(1994) noted that whole-rock limestones with Sr/
Mn > 2 and Mn < 300 ppm yield the most consistent seawater 87Sr/ 86Sr curve of the late Paleozoic.
In Fig. 3, the Sr isotopic data in this study are compared with the late Paleozoic seawater 87Sr/86Sr
curve (Denison et al., 1994) and the early Triassic
one (Koepnick et al., 1990).
Table 4 summarizes REE and Y analyses of
Ishimaki and Tahara limestones, in which results
of CH3COOH dissolution for several samples are
also listed. Aliquots of limestone samples weighed
about 20 g were used even in the cases of acetic
acid leaching. Because of the large amounts of
carbonate samples, about 10% of carbonates remained undissolved. As the precise correction of
imperfect dissolution was unable to be made for
each sample, this gave REE and Y analyses
slightly lower than those by HCl leaching in Table 4. However, the REE patterns for the respective limestone samples treated by both acetic acid
and HCl are fairly parallel (Fig. 4). Accordingly,
REE and Y data by HCl dissolution represent their
concentrations in carbonates, and they are not affected by REE and Y enriched in non-carbonate
phases. Their chondrite-normalized REE and Y
abundance patterns for the results by HCl dissolution are shown in Figs. 5a and 5b.
REE and Sr isotopic study of seamount-type limestones
Sr isotope stratigraphic ages of Ishimaki and
Tahara limestones
Sr isotope stratigraphic ages can be almost
uniquely determined for marine carbonates formed
later than the Eocene, since the seawater 87Sr/86Sr
curve has changed almost monotonously for the
latest 40 Ma (Koepnick et al., 1985). However,
the curve repeats oscillations with time in the span
older than the middle Tertiary, and ages for the
time span cannot be determined immediately
(Burke et al., 1982; Veizer et al., 1999). Nevertheless, when stratigraphic data relevant to the
marine limestone of interest are available and its
possible age is limited to a narrow span older than
40 Ma, the Sr isotopic stratigraphic age can be
determined or constrained to be the further narrower time span. This situation meets the present
case of Ishimaki and Tahara limestones. As noted
above, Carboniferous-Permian and Triassic limestone blocks are known in Southern Chichibu
Terrane of Kyushu and Shikoku Islands (Matsuoka
and Yao, 1990). A limestone lens with Lower
Permian fusulinids has been reported from the area
about 10 km east of Ishimaki limestone (Mizugaki,
1985). Possible ages of Ishimaki and Tahara limestones, therefore, are in a range from the Carboniferous to the Triassic. In this context, the late
Paleozoic seawater 87Sr/ 86Sr curve by Denison et
al. (1994) and the Triassic one by Koepnick et al.
(1990) are compared with the Sr isotopic data of
Ishimaki and Tahara limestones in Fig. 3. The
variation curve of 87Sr/86Sr in the Phanerozoic
seawater (Veizer et al., 1999) is also referred to.
The 87Sr/ 86Sr ratios of Ishimaki and Tahara
limestones are plotted on the left and right sides
of Fig. 3, respectively. All the cited data in Fig. 3
are re-calculated by using that 87Sr/86Sr of NIST
SRM 987 = 0.710250 in the same way as our data.
The samples shown by filled triangles satisfy the
chemical criteria that Sr/Mn > 2 and Mn < 300
ppm for the whole-rock limestones giving the most
consistent seawater 87Sr/ 86Sr curve of the late
Paleozoic (Denison et al., 1994). Those samples
shown by open triangles do not satisfy the criteria.
Table 2. REE, Y and major element analyses of Ishimaki and Tahara greenstones
D ISCUSSION
169
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K. Tanaka et al.
Table 3. Minor element and Sr isotopic compositions of Ishimaki and Tahara limestones
All 87Sr/ 86Sr ratios are normalized to 86Sr/88Sr = 0.1194.
The 87Sr/ 86Sr ratio of NIST SRM 987 during the analysis is 0.710252 ± 7 (2σm, n = 11).
In the case of Ishimaki limestone, ten of total
fourteen ones satisfy the criteria by Denison et al.
(1994), and they make a cluster in the range of
87
Sr/ 86Sr =0.7070 ± 0.0004, which coincides with
the Late Permian seawater 87Sr/86Sr ratios. The age
of Ishimaki limestone is 245–260 Ma of the Late
Permian. However, only five of the total twelve
Tahara limestone samples meet the chemical criteria by Denison et al. (1994), and the acceptable
87
Sr/ 86Sr ratios are in 0.7081–0.7090. The three
samples (THLS-06, -20 and -21) are unusual because they can be distinguished from the others
not only by the much lower 87Sr/ 86Sr of 0.7073,
but also by REE and Y features, for example, they
show no Ce and Eu anomalies (Fig. 5b). Therefore, when the three unusual samples are excluded,
87
Sr/ 86Sr ratios of remaining nine Tahara samples
are in 0.7079–7090. This is almost the same range
of 87Sr/ 86Sr ratios for the five samples satisfying
the criteria by Denison et al. (1994). Hence the
average 87Sr/ 86Sr ratio of the nine Tahara limestone samples is 0.7084 ± 0.0003(σ), which means
the Carboniferous or the Early Permian. The Middle and Late Permian or the Triassic ages are less
likely for Tahara limestone, since the seawater
87
Sr/86Sr ratio is confined within the values around
0.7078 in those times.
Seawater-like REE and Y characteristics of marine limestones
The REE and Y abundance patterns for
Ishimaki and Tahara limestones normalized by C1
chondrite are shown in Figs. 5a and 5b, respectively. All the Ishimaki samples except for ISLS-
(b)
(a)
The data by acetic acid leaching.
Ce anomalies are given by log(Ce/Ce*) = log{Ce n/(La n2 × Nd n)1/3}, where the suffix of “n” denotes chondrite-normalized abundances.
Table 4. REE and Y analyses of Ishimaki and Tahara limestones
REE and Sr isotopic study of seamount-type limestones
171
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K. Tanaka et al.
(a)
(b)
Fig. 6. (a) Comparison of chondrite-normalized REE and Y abundance patterns for the limestones in this study
and the Italian Cretaceous limestones (Bellanca et al., 1997). All the REE and Y data for Ishimaki and Tahara
limestone samples are averaged in the logarithmic scale. In averaging Tahara samples, the three samples (THLS06, -20 and -21) are excluded. The C1 chondrite values by Anders and Grevesse (1989) are used for normalization. (b) Chondrite-normalized REE and Y abundance patterns for Pacific seawaters. The analytical data for
water samples at different depths in Coral Sea (Zhang and Nozaki, 1996) are plotted. The C1 chondrite values by
Anders and Grevesse (1989) are used for normalization.
14 with no Eu anomaly show the following REE
and Y characteristics: 1) concave lanthanide tetrad effects of the W type, 2) fairly large negative
Ce anomalies, 3) small negative Eu anomalies, and
4) large Y/Ho ratios greater than the chondritic
one. Most of Tahara samples also show the characteristics as above, although magnitudes of their
negative Ce anomalies are smaller than those of
Ishimaki samples. The three samples of THLS06, -20 and -21, having the lowest 87Sr/86Sr ratios
of 0.7073, do not show negative Ce and Eu anomalies unlike the others. Their Y/Ho ratios are smaller
than those ratios of the other Tahara samples, suggesting that significant involvement of volcanic
materials in later recrystallization. Therefore, the
three Tahara samples are excluded in the subsequent discussion.
The REE and Y characteristics as above have
already been known for Permian Japanese coral
reef limestones in Ohnogahara, Akiyoshi,
Akasaka, Fujiwara and Kuzuu areas (Kawabe et
al., 1991, 1994a). Figure 6a compares REE patterns for the Cretaceous marine limestones in
Cismon area, northern Italy (Bellanca et al., 1997)
with the averaged patterns for Ishimaki and Tahara
limestones. All the REE and Y data for Ishimaki
samples are used in averaging in the logarithmic
scale, but the three unusual samples are excluded
in averaging the data for Tahara samples. It is evident that Cismon limestones also show the concave tetrad effect of the W type and negative
anomalies of Ce and Eu.
Figure 6b shows the chondrite-normalized
REE and Y patterns for water samples at different
depths in Coral Sea, the western South Pacific
(Zhang and Nozaki, 1996). The seawater REE and
Y patterns indicate the REE and Y characteristics
similar to those for marine limestones in Figs. 4,
REE and Sr isotopic study of seamount-type limestones
5a, 5b and 6a, but whole patterns for seawater samples differ from those of the marine limestones.
Seawater is relatively more enriched in heavy REE
than the limestones. This situation suggests the
REE and Y fractionation occurs significantly when
seawater REE and Y are incorporated into limestones.
It is very important to discuss what material is
suitable for normalization of geochemical samples. All the REE data for our greenstone and limestone samples (Figs. 2, 4, 5a, 5b and 6a) have been
normalized by C1 chondrite values by Anders and
Grevesse (1989) as well as those for Coral Sea
water samples cited in Fig. 6b. Recent REE studies on seawater and marine geochemical samples
sometimes report the REE abundance patterns
normalized by average shale like North American
shale composite (NASC) and Post-Archean Australian average shale (PAAS). Such average shales
may represent estimates of averaged crustal REE
abundances and then the shale-normalization in
that sense may be important in cases of REE studies of clastic sediments. Kawabe (1996) pointed
out, however, that PAAS shows a convex tetrad
effect variation when normalized by NASC. It is
quite natural that REE abundances are systematically different between particular average shales
because of a variety of crustal rocks. Originally
REE abundance patterns of geochemical samples,
i.e., Masuda-Coryell plots (Henderson, 1984), are
shown by plotting REE concentrations normalized
by chondrite on the logarithmic scale against the
atomic number of REE. Since the C1 chondrite is
the unique and most primitive solar system material, its REE abundances (Anders and Grevesse,
1989) are accepted as reliable data. Hence the
chondrite-normalization is useful to see fine structures of REE patterns like tetrad effects as exemplified in Figs. 4, 5a, 5b, 6a and 6b for marine
limestones and seawater, and in the REE patterns
for differentiated igneous rocks (Masuda and
Akagi, 1989; Bau, 1996; Irber, 1999).
In the case of REE studies of marine limestones, however, seawater is also suitable normalizing material, by which REE fractionation between marine carbonate and seawater can be ex-
173
Fig. 7. Seawater-normalized REE and Y abundance
patterns for Ishimaki and Tahara limestones. All the
REE and Y data for Ishimaki and Tahara limestone
samples, except THLS-06, -20 and -21, are averaged
in the logarithmic scale, respectively. The REE and Y
data for the water samples at depths of 50, 248 and
795 m in Coral Sea as cited in Fig. 6b are used for the
normalization.
amined. We will discuss the REE characteristics
of Ishimaki and Tahara limestones from this viewpoint in the next subsection.
REE and Y fractionation between marine limestone and seawater
It is certain that seawater REE and Y were incorporated into calcite of Ishimaki and Tahara
limestones from the fact that the results by both
strong (HCl) and weak acid (CH 3COOH) dissolution show parallel REE abundance patterns with
the seawater REE and Y signatures (Fig. 4).
Whittaker and Kyser (1993) reported REE contents of Cretaceous molluscan shells from North
America, and they suggested the possibility that
seawater REE and Y were incorporated not into
impurities like Fe flocs but into molluscan shells
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K. Tanaka et al.
of carbonate minerals. Hence, when the averaged
REE and Y data for Ishimaki and Tahara limestones are normalized by seawater, the results must
be apparent REE and Y fractionation patterns between marine limestone and seawater. Figure 7
shows the empirical estimates for REE and Y
fractionation, in which the REE and Y data for
the ocean water samples at depths of 50, 248 and
795 m in Coral Sea as cited in Fig. 6b are used for
the normalization.
The apparent REE and Y fractionation patterns
do not indicate smooth curve or straight lines but
slightly convex tetrad curves of the M type (Fig.
7). Since the major seawater REE(III) species are
REECO3+(aq) and REE(CO 3)2–(aq), the convex tetrad curves can be readily expected, if the Racah
(E1 and E 3) parameters for REE3+ systematically
decrease among REECO3+(aq), REE(CO3)2–(aq) and
REE3+ in calcite in the following order:
REE3+(aq) > REECO3+(aq)
> REE(CO3)2–(aq) > REE3+ in calcite.
(1)
The relationship of (1) with respect to the first
three species has been proved by Kawabe (1999)
and Ohta and Kawabe (2000). The nephelauxetic
series is related to the systematically different
H2O/CO32– ratios in the first coordination spheres
of REE3+ ions among REE3+(aq), REECO3+(aq) and
REE(CO 3)2–(aq) as discussed in Kawabe (1999).
Possibly REE3+ ions occupy Ca-sites in calcite by
appropriate charge-balanced substitution like,
2Ca 2+ = REE3+ + Na+
(2)
as discussed in Zhong and Mucci (1995). The H2O/
CO 32– ratio in the first coordination spheres of
REE3+ in calcite is zero, and much smaller than
the ratio in REE(CO3)2–(aq). Hence the convex tetrad effects in the seawater-normalized REE patterns for the limestones in Fig. 7 are acceptable.
The empirical fractionation patterns of Fig. 7,
indeed, include the intrinsic REE and Y
fractionation between two phases, but they are also
affected possibly by the fact that modern seawater
samples for normalization are not really the ancient seawater coexistent with each marine limestone. In modern oceans, surface and deep waters
have substantially different REE and Y patterns.
In particular, their characteristics with respect to
heavy REE and Y are fairly contrasting. This can
be seen in the water samples in Coral Sea cited in
Fig. 6b, and reflected also in the apparent REE
and Y fractionation patterns of Fig. 7. REE and Y
data for the water sample at a depth of 50 m give
more conspicuous breaks in the heavy REE part
of each fractionation pattern than those at depths
of 248 and 795 m. In order to minimize such discrepancy of REE and Y features between the ancient seawater of interest and its substitute of a
modern water sample, it is primarily important to
know either surface or deep water supplied
seawater REE and Y to the marine limestones even
on the premise that the contrast between surface
and deep waters in the modern ocean with respect
to REE and Y is approximately the same as in the
ancient ocean.
An important clue to answer the question is a
comparison of Ce anomalies between the limestones and ocean waters. In Fig. 7, the averaged
REE data for Ishimaki and Tahara limestone show
negative Ce anomalies, when normalized by the
water samples at depths of 50 and 248 m in Coral
Sea. However, when normalized by the water sample at 795 m, the Ishimaki data show no Ce
anomaly, though the Tahara samples indicate a
small positive Ce anomaly. Thus the moderately
deep water appears to be an adequate substitute
for the ancient seawater coexistent with Ishimaki
limestone. The apparent REE and Y fractionation
pattern given by the pair of the Ishimaki limestone
and the water sample at 795 m deep may be closer
to the true one. The importance of Ce anomalies
of modern ocean waters and the limestones is discussed in more details below.
Depth profile of seawater Ce anomaly and its implications for limestone origin
Seawater Ce anomalies in water columns in
Pacific, Atlantic and Indian Oceans (German and
Elderfield, 1990; Piepgras and Jacobsen, 1992;
REE and Sr isotopic study of seamount-type limestones
Bertram and Elderfield, 1993; Sholkovitz et al.,
1994; German et al., 1995; Zhang and Nozaki,
1996) are plotted against the water depth (Fig. 8).
Despite of some scattering data points, there can
be found the seawater Ce anomaly as a function
of water depth: The Ce anomalies for the majority of surface waters shallower than 300 m are
within the range of log(Ce/Ce*)= –(0.45 ± 0.25),
and they rapidly become more negative with increasing water depth until 1,000 m, but they have
approximately constant values of log(Ce/Ce*) =
–(1.1 ± 0.1) at depths greater than 1,000 m.
The characteristic depth profile must be produced by the elementary processes controlling Ce
redox chemistry in the present-day ocean (Byrne
and Sholkovitz, 1996): (i) photoinhibitation of
Ce(III) oxidation in surface water, (ii) oxidative
scavenging of Ce(III) as particulate Ce(IV) from
seawater solution in ocean water columns, and (iii)
circulation of oxygenated cold water being produced in polar regions across different ocean basins. It is important to comment on whether such
a depth profile of seawater Ce anomaly is expected
in any ocean in the geological past. The three elementary processes of (i), (ii) and (iii) are driven
more directly by physico-chemical principles
rather than geological settings. Therefore, it seems
conceivable that such a depth profile was present
in the past ocean after the present atmospheric PO 2
had been established, i.e., the end of the
Proterozoic (Kasting, 1993). However, the circulation pattern of ocean water mentioned in (iii)
was substantially different between the present and
past, since it depends on the continental distribution changing with the geologic time.
The average depth profile of seawater Ce
anomaly (Fig. 8) may be useful to infer the water
depth at which seawater REE were incorporated
into each ancient marine limestone, though qualitatively. The Ce anomalies of Ishimaki and Tahara
limestones are plotted on the upper part of Fig. 8,
but three unusual Tahara samples are not plotted.
About a half of Ishimaki samples and the Tahara
limestone samples show log(Ce/Ce*) values between –0.5 and –0.2. This appears to be compatible with the relatively shallow water origin of
175
Fig. 8. Seawater Ce anomaly plot against the water
depth. Ce anomalies of Ishimaki and Tahara limestones
are plotted for comparison with seawater Ce anomalies. Ce anomalies are given by log(Ce/Ce*) = log{Cen/
(La n 2 × Nd n ) 1/3 }, where the suffix of “n” denotes
chondrite-normalized abundances. REE values of
seawater are quoted from German and Elderfield
(1990), Piepgras and Jacobsen (1992), Bertram and
Elderfield (1993), Sholkovitz et al. (1994), German et
al. (1995) and Zhang and Nozaki (1996). Seawater Ce
anomalies are averaged at 50 m intervals for the depth
range 0–100 m, 100 m intervals for 50–250 m, 200 m
intervals for 200–800 m, 300 m interval for 700–1000
m, 400 m interval for 800–1200 m, 500 m intervals for
1000–2250 m, 1000 m intervals for 2000–5000 m and
1500 m interval for 4500–6000 m.
biogenic carbonates. However, several samples of
Ishimaki limestone have quite large negative Ce
anomalies of log(Ce/Ce*) = –0.7~–1.0, implying
that seawater REE were incorporated into the limestone from moderately deep waters at depths of
500–1000 m.
The depth profile of Fig. 8, however, must be
used carefully, since redox conditions of ocean
waters are variable locally. The variability seems
to be a cause of scattering data in Fig. 8, in addition to analytical errors and artifacts related to filtered or unfiltered water samples. Local waters
176
K. Tanaka et al.
Fig. 9. Apparent REE and Y partition coefficients (D = (REE/Ca)carbonate/(REE/Ca) water) between Ishimaki and
Tahara limestones and present-day seawater. The averaged REE and Y concentrations for the limestone samples
in Fig. 7 are used. Seawater data reported by Zhang and Nozaki (1996) at depths of 248 m and 795 m in Coral Sea
are used for the calculations. The experimental data of partition coefficients between calcite overgrowths and
seawater solution reported by Zhong and Mucci (1995) are plotted for comparison. Two sets of experimental Dvalues are distinguished because of their dependence on REE concentrations in seawater solutions: the REE
concentrations are 70 nM (filled circles) and minimum concentrations in their experiments (open circles). The
minimum concentrations are not constant across REE series: 10 nM in light REE and 120 nM in heavy REE.
Modern coral data by Sholkovitz and Shen (1995), Holocene microbialite data by Webb and Kamber (2000) and
aragonitic chimney data by Barrat et al. (2000) are also plotted.
more reducing than the normal one can be found
in stagnant water masses, for example, anoxic
waters in Cariaco Trench by De Baar et al. (1988),
but it is almost impossible to expect local waters
more oxygenated than normal seawater. This situation must be taken into account when considering Ce anomalies of marine limestones. By the
plot of Fig. 8, the log(Ce/Ce*) values between
–0.5 and –0.2 for the marine limestones, in fact,
suggest ocean waters at depth of 0–300 m or
greater depths than those. Similarly, in the case
that log(Ce/Ce*) = –0.7~–1.0, water depths of
500–1000 m or more are suggested.
Biogenic carbonates can be produced at water
depths transparent to sunlight, normally shallower
than about 250 m. Thus reef limestones on the tops
of seamounts can grow at water depth of 0–250
m. This depth range is apparently incompatible
with the water depths of 500–1000 m or more suggested by large negative Ce anomalies of the
Ishimaki samples. However, this can be reconciled
if the accumulation depth of original biogenic carbonates and incorporation depth of seawater REE
into the limestone are different due to the fate of
limestone-capped volcanic seamount (Bloom,
1974). This point relates also to the fact that ancient marine limestone like present ones are much
enriched in REE and Y than modern biogenic carbonates.
REE and Sr isotopic study of seamount-type limestones
Incorporation of seawater REE into seamounttype limestones
The average REE/Ca ratios for Ishimaki and
Tahara limestones are 10 2 to 104 times as high as
the seawater ratios, whereas modern biogenic carbonates like corals (Sholkovitz and Shen, 1995)
have approximately the same REE/Ca ratios as the
seawater, which is shown in Fig. 9. Apparent partition coefficients between aragonitic chimney and
water reported by Barrat et al. (2000) are as low
as those between corals and seawater by
Sholkovitz and Shen (1995). Accordingly,
seawater REE and Y may have been incorporated
into the marine limestones when originally biogenic carbonates transformed to inorganic calcite
in early diagenesis and secondary growths of calcite occurred in contact with sufficient seawater.
Our field data are qualitatively consistent with
large experimental partitioning coefficients of
REE between calcite overgrowths and synthetic
seawater solution reported by Zhong and Mucci
(1995), which is also cited in Fig. 9. Holocene
microbialite, composed of Mg-calcite, has also
large REE/Ca ratios (Webb and Kamber, 2000),
but still smaller than the averaged REE/Ca ratios
in the present limestones composed of “low-Mg”
calcite (Fig. 9).
Water depths at which biogenic carbonates
accumulated on summits of volcanic seamounts
do not always coincide with the water depths of
their diagenetic transformation into inorganic calcite. As limestone-capped volcanic seamounts
move away from a spreading center and are
cooled, their heights above sea-floor are reduced
and they submerges further as far as seawater level
changes are unimportant (Bloom, 1974;
Schmincke, 1981). Hence the water depth of REE
incorporation into calcite is generally deeper than
the accumulation depth of original biogenic carbonates. The large negative Ce anomalies and high
REE/Ca rations of Ishimaki limestone can be explained by this mechanism of submerging volcanic
seamounts on which transformation of original
biogenic carbonates to inorganic calcite and secondary growths of calcite occur. As the oceanic
crust with limestone-capped volcanic seamounts
177
approach the continental margin and begin the
subduction beneath it, the limestones and
seamounts are scraped off the oceanic crust and
parts of them are juxtaposed into the clastic sequences in the subduction zone. The Ishimaki and
Tahara limestone blocks may have experienced
such histories.
CONCLUSIONS
Ishimaki and Tahara limestones, which are
exotic blocks juxtaposed in the Mesozoic
(Jurassic) accretionary complex of Southern
Chichibu Terrane in eastern Aichi Prefecture, central Japan, have been studied by their REE and Y
analyses and Sr isotopic ratios. The following conclusions have been drawn:
1) Sr isotope ratios of the limestones, when
referred to the seawater 87Sr/ 86Sr curve and relevant geological data, suggest that the formation
ages are the Late Permian for Ishimaki limestone
and the Carboniferous to the Early Permian for
Tahara one.
2) Major element and REE analyses of two
greenstone samples found inside Ishimaki limestone and one greenstone sample associated with
Tahara limestone, indicate that they resemble the
Hawaiian alkali basalt. This is evidence for the
geological idea that such a limestone block may
be a part of a carbonate sequence formed on a
volcanic seamount.
3) All the limestone samples, except three
unusual Tahara ones, show seawater REE and Y
signatures in their chondrite-normalized patterns,
but their REE/Ca ratios are 102–104 times as high
as those ratios of modern biogenic carbonates like
corals and of the seawater. Hence it is conceivable that seawater REE and Y were incorporated
into the limestones when originally biogenic carbonates transformed into inorganic calcite in their
early diagenesis and secondary growths of calcite
in contact with sufficient seawater. This view is
supported by the reported REE partition experiment between calcite and seawater solution.
4) The seawater Ce anomaly as a function of
water depth found in the present-day ocean is use-
178
K. Tanaka et al.
ful to infer water depths of REE and Y incorporation into ancient limestones. The Ce anomalies
defined by log(Ce/Ce*) values for about a half of
Ishimaki samples and most of Tahara samples are
between –0.5 and –0.2, and they appear to be compatible with the shallow seawater origin. Another
half of Ishimaki samples, however, have log(Ce/
Ce*) values between –0.7 and –1.0, suggesting
moderately deep waters (500–1000 m deep or
more).
5) In view of the fate of limestone-capped
volcanic seamount, the moderately deep waters
must be a line of evidence to indicate that water
depths of REE incorporation into such a type of
limestones are generally greater than water depths
of accumulation of originally biogenic carbonates,
as far as seawater level changes are less important.
Acknowledgments—The authors would like to thank
T. Tanaka who allowed us to determine Sr isotopic ratios, and T. Ito and A. Ohta who helped us in the field
and laboratory works. They are also grateful to K.
Yamamoto, K. Suzuki and M. Tsuboi for their help in
XRF analysis, and to T. Ozawa, M. Adachi and M.
Takeuchi who gave us advice and suggestions as to the
stratigraphy and tectonics of the Southern Chichibu
Terrane. This work was supported in part by the grants
Nos. 0340218 and 06453007 from the Ministry of Education, Culture, Sports, Science and Technology,
Japan to I.K.
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