Another sea area separated from the Panthalassic Ocean in the Norian, the Late Triassic: The lowest Sr isotopic composition of the Ishimaki limestone in central Japan

Chemie der Erde 72 (2012) 77–84

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Another sea area separated from the Panthalassic Ocean in the Norian, the Late Triassic: The lowest Sr isotopic composition of the Ishimaki limestone in central Japan Kazuhiro Suzuki a,b,∗ , Yoshihiro Asahara a , Koichi Mimura a , Tsuyoshi Tanaka c a

Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan Earth Science Laboratory, Aichi Prefectural Meiwa High School, Higashi-ku, Nagoya 461-0011, Japan c Center for Chronological Research, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan b

a r t i c l e

i n f o

Article history: Received 4 March 2011 Accepted 13 June 2011 Keywords: Sr isotopic composition Seamount-type limestone Late Triassic Norian Paleoceanic topography Norian conodont Seawater 87 Sr/86 Sr ratio Hydrothermal fluids

a b s t r a c t Mt. Ishimaki is the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan. The Ishimaki limestone is thought to be seamount-type limestone. The P1 elements of the conodonts Norigondolella navicula and Ancyrogondolella quadrata found in the limestone indicate it is of Norian age. The Sr isotopic compositions of 45 Ishimaki limestone samples ranged from 0.7055 to 0.7077. Eighteen of these samples had lower Sr isotopic compositions than the lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. The Sr isotopic compositions in the limestone block are generally lower at the base of the block and higher at the top. The present Sr isotopic compositions of the Ishimaki limestone are unlikely to have been reduced by post-depositional alteration because most of the limestone samples had a low amount of Mn (<300 ppm) or high Sr/Mn ratios (>2) and the conodont elements had low (1–2) CAI (conodont alteration index) values. Additionally, there was little acid-insoluble residue. Thus, the low Sr isotopic compositions are thought to represent the strontium of the past ambient seawater. The low Sr isotopic compositions are in complete disagreement with the generally recognised range of seawater Sr isotopic compositions in the Norian stage of the Late Triassic (0.7075–0.7078) because the depositional environment of the Ishimaki limestone was closed or semiclosed from the Panthalassic Ocean. Therefore, the Sr isotopic composition of the limestone differs from that of the Panthalassic seawater. The low Sr isotopic compositions were probably affected by Sr inflows from mafic oceanic crust by hydrothermal fluid circulation or from hinterlands surrounded by mafic rocks by river water circulation. © 2011 Elsevier GmbH. All rights reserved.

1. Introduction The Sr isotopic compositions (87 Sr/86 Sr) of seawater are generally homogeneous throughout the ocean (Faure et al., 1965; Hamilton, 1966; Murthy and Beiser, 1968), and little isotopic fractionation from seawater to marine biogenic carbonates occurs. The Sr isotopic compositions of marine biogenic carbonates are identical to those of the seawater at the time of deposition if they have not been modified by later alteration (Ito, 1993). Attempts to associate Sr isotopic compositions of past seawater with marine biogenic carbonates have been conducted by Peterman et al. (1970), Veizer and Compston (1974) and others. Burke et al. (1982) summarised the general trends of the variation curve of 87 Sr/86 Sr ratios in Phanero-

∗ Corresponding author at: Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan. Tel.: +81 533 76 2852; fax: +81 533 76 2852. E-mail address: [email protected] (K. Suzuki). 0009-2819/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2011.06.004

zoic seawater. Sr isotope stratigraphy using this variation curve to determine the age of marine biogenic carbonates has become an effective method for determining the ages of rocks without index fossils (Elderfield, 1986). Recently, the Sr isotopic composition of past marine carbonates has been regarded as a proxy method for global chemostratigraphy, dating and paleoenvironmental analysis (Veizer et al., 1999; Nishioka et al., 1991; Miura et al., 2004; McArthur and Howarth, 2004; Kani et al., 2008). Mt. Ishimaki, with a peak elevation of 358 m, is a geological block in the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan (Hori, 2008) (Fig. 1). The Ishimaki limestone is thought to be a seamount-type limestone because it is associated with greenstone and chert. The deposition age of the limestone has not been clarified because no index fossils have been found within it (Hori, 2008). Suzuki et al. (2009) recently found P1 elements of Norigondolella navicula (Huckriede, 1958) and Ancyrogondolella quadrata (Orchard, 1991) from the Ishimaki limestone, and these conodonts indicate that the limestone is from the Early Norian within the Late Triassic.

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Fig. 1. (a and b) An index map and (c) a simplified geological map of the area surrounding Mt. Ishimaki. The geological map was modified from Niwa (2004).

Fig. 3. Sampling points in the Ishimaki limestone block with east longitude vs. north latitude.

The distribution of Sr isotopic compositions in the 200-m wide and 60-m thick Ishimaki limestone block was investigated in this study. Extremely low Sr isotopic compositions were found. The aim of this study is to clarify why the Ishimaki limestone block, which was deposited by seawater during the Norian, had the lowest Sr isotopic composition throughout the Phanerozoic.

the latitude and longitude directions and approximately 60 m in the vertical direction on the northern slope of Mt. Ishimaki (Fig. 1). This block is surrounded by mixed rock and greenstone. Fortyfive limestone samples were collected from this block in vertical and horizontal succession. Samples from the recrystallised part of the block were not collected. The latitude and longitude of each sampling point were accurately determined using a GPS receiver (Fig. 3). The locations of the sampling points are listed in Table 1.

2. Geological setting of the Ishimaki limestone and sampling points

3. Age and thermal history of the Ishimaki limestone block The study area is located in Toyohashi City, southeast of Nagoya City in central Japan. The geological units of the Chichibu Belt are distributed in a belt shape from east to west. The Jurassic accretionary complex of the Chichibu Belt contacts Sambagawa metamorphic rock (Mikabu greenstone in Fig. 1(c)) along a high angle fault at the northern edge and is widely overlain by Quaternary sediments (Hori, 2008). The lower part of Mt. Ishimaki mainly consists of blocks of mixed rock and greenstone, but greenstone, limestone and chert are tectonically distributed in the upper part (Fig. 1(c)). The greenstone primarily consists of basaltic lava and tuff. The greyish to white limestone is mainly layered, massive and partly recrystallised. The greenstone is partly intercalated into the limestone (Niwa, 2004). The limestone samples for the Sr isotope analysis were collected from a lens-shaped block (Fig. 2) approximately 200 m wide in both

Fig. 2. Image of the Ishimaki limestone block. The thickness and width of the block are approximately 60 m and 200 m, respectively. Forty-five Ishimaki limestone samples were collected from the block. Two species of conodont elements were extracted from the IS50 site. Several fragmented pieces of conodonts were also extracted from the IS250, IS400 and IS950 sites.

Two species of conodonts were collected from the IS50 point (Fig. 2 and Table 1) (Suzuki et al., 2009): N. navicula (Huckriede,

Fig. 4. Two species of Norian conodonts at IS50. Upper: Norigondolella navicula (Huckriede) extracted from the IS50 sample. 1a is a binocular micrograph. 1b and 1c are SEM micrographs. 1a is an oblique upper view. This CAI value is 1.0–2.0. 1b, oblique lower view. 1c, lateral view. Lower: Ancyrogondolella quadrata (Orchard) extracted from the IS50 sample. 2a is a binocular micrograph. 2b and 2c are SEM micrographs. 2a, upper view. This CAI value is 1.0–2.0. 2b, lower view. 2c, lateral view.

Table 1 Data for the 45 Ishimaki limestone samples are shown. The latitude, longitude and elevation of the sampling points, the Sr isotopic compositions in the 10% acetic acid and 6 M HCl leachates and the weight percentage of residues are listed. Sampling point

Latitude N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

IS50 IS100 IS150 IS200 IS250 IS301 IS400 IS501 IS504 IS601 IS602 IS700 IS800 IS900 IS950 ISB01 ISB02 ISB03 ISB04 ISB05 ISB06 ISB07 ISB08 ISB09 ISB10 ISB11 ISB12 ISB13 ISB14 ISB15 ISB16 ISB17 ISB18 ISB19 ISB20 ISB21 ISB22 ISB23 ISB24 ISB25 ISB26 ISB27 ISB28 ISB29 ISB30

34◦ 47 Near IS50 34◦ 47 34◦ 47 34◦ 47 34◦ 47 34◦ 47 34◦ 47 Near IS501 Near ISB17 Near ISB17 34◦ 47 34◦ 47 34◦ 47 34◦ 47 34◦ 47 Near ISB01 34◦ 47 Near ISB03 34◦ 47 34◦ 47 34◦ 47 34◦ 47 34◦ 47 Near ISB09 34◦ 47 Near ISB11 Near ISB11 34◦ 47 34◦ 47 34◦ 47 34◦ 47 Near ISB17 34◦ 47 Near ISB19 34◦ 47 34◦ 47 Near ISB22 34◦ 47 Near ISB24 Near ISB24 34◦ 47 34◦ 47 Near ISB28 34◦ 47

Longitude E 49.59

137◦ 27 ◦

Elevation (m) 56.73



48.21 48.38 47.76 47.91 48.02 48.57





137 27 137◦ 27 137◦ 27 137◦ 27 137◦ 27 137◦ 27

57.12 57.23 56.84 56.13 55.74 54.92

49.89 50.52 50.82 49.22 49.86

137◦ 27 137◦ 27 137◦ 27 137◦ 27 137◦ 27

53.27 53.00 52.56 53.60 57.12

50.21

137◦ 27

56.62

50.88 52.32 47.24 47.36 47.28

137◦ 27 137◦ 27 137◦ 27 137◦ 27 137◦ 27

56.13 55.09 58.27 57.67 57.67

47.56

137◦ 27

57.01

48.56 48.67 48.83 48.98

137◦ 27 137◦ 27 137◦ 27 137◦ 27

54.59 54.37 54.32 54.04

49.22

137◦ 27

53.60

48.02 48.37

137◦ 27 137◦ 27

56.18 55.58

48.97

137◦ 27

54.98

49.70 50.60

137◦ 27 137◦ 27

53.99 53.27

51.41

137◦ 27

52.94

59 61 78 81 86 87 90 92 95 96 98 101 106 110 113 60 62 61 63 63 64 81 81 84 86 84 86 87 92 91 93 92 93 92 93 91 92 91 91 92 94 93 92 91 91

Leaching acid

87

Sr/86 Sr

HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc

0.707055 0.705543 0.706933 0.707612 0.706505 0.706705 0.706148 0.707290 0.707070 0.707369 0.707020 0.707259 0.707428 0.707723 0.707604 0.707362 0.706485 0.706533 0.706231 0.706477 0.706362 0.707333 0.707243 0.706625 0.706737 0.707473 0.706993 0.707343 0.706840 0.706772 0.707001 0.707137 0.707002 0.706945 0.706807 0.707060 0.706732 0.706624 0.706660 0.706717 0.706885 0.706172 0.706584 0.706933 0.707123

2SE

Residue (wt. %)

Leaching acid

87

Sr/86 Sr

0.000014 0.000014 0.000014 0.000014 0.000014 0.000016 0.000016 0.000015 0.000018 0.000016 0.000016 0.000014 0.000016 0.000016 0.000016 0.000014 0.000014 0.000016 0.000021 0.000016 0.000013 0.000016 0.000018 0.000016 0.000014 0.000016 0.000013 0.000016 0.000014 0.000014 0.000016 0.000016 0.000017 0.000014 0.000016 0.000014 0.000016 0.000013 0.000016 0.000016 0.000016 0.000016 0.000013 0.000016 0.000016

7.7 4.0 11.4 11.0 13.5 16.1 14.2 0.5 9.5 14.7 14.2 25.3 27.3 24.7 27.8 2.3 7.2 0.3 1.1 1.1 3.8 0.0 0.9 2.0 0.7 0.3 0.4 1.2 1.6 2.5 0.3 5.4 0.9 4.5 2.5 1.4 1.0 6.9 0.5 0.3 2.1 1.4 0.2 1.2 0.6

HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1 HC1

0.707020 0.705530 0.706910 0.707633 0.706492 0.706705 0.706127 0.707296 0.707106 0.707384 0.707012 0.707222 0.707434 0.707753 0.707551

2SE

Residue (wt. %)

0.000042 0.000016 0.000017 0.000017 0.000013 0.000018 0.000014 0.000011 0.000010 0.000014 0.000017 0.000014 0.000014 0.000014 0.000023

0.3 3.5 0.8 0.5 0.0 1.5 0.1 0.1 8.3 1.0 1.1 0.4 1.7 3.4 5.4

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1958) and A. quadrata (Orchard, 1991; Suzuki et al., 2009) (Fig. 4). Both N. navicula and A. quadrata show a quadrata–spatulata fossil stratigraphical band in the Lower Norian (Ishida and Hirsch, 2001). According to this fossil stratigraphy, the deposition age of the limestone at the IS50 point is Early Norian in the Late Triassic. Several fragmented pieces of conodonts were found at IS250, IS400 and IS950 in the Ishimaki limestone block (Fig. 2), but the species could not be precisely identified because the fragments were small. However, they were likely to be a conodont genera, probably Mockina and Ancyrogondolella from the Norian, and they were shown to be Early-Middle Norian (Suzuki, K., unpublished data). Therefore, the Ishimaki limestone block was deposited during the Early to Middle Norian. The conodont elements were pale brown and dark brown at all of the sampling points. Judging from these colours, the CAI values of the Ishimaki limestone at IS50 (Fig. 4), IS250, IS400 and IS950 are 1–2 (Epstein et al., 1977; Helsen et al., 1995).

4. Measurement of Sr isotopic compositions The Sr isotopic compositions of the Ishimaki limestone were determined according to the method by Asahara et al. (1999, 2006). Forty-five Ishimaki limestone bulk samples were carefully washed with water and then dried. Each pulverised sample, weighing 0.1–0.2 g, was dissolved in 5 ml of 6 M HCl or 5 ml of 10% acetic acid at room temperature for 4 h. The residue was centrifuged for 10 min at 3500 rpm. After the residue was removed, the strontium was purified using cation-exchange chromatography (BioRad AG50WX8, 200–400 mesh) with a 2.4 M HCl eluent. The residue was dried at 95 ◦ C for 12 h and weighed, and the residue weight percentage was determined. The Sr isotopic compositions were measured using a thermal ionisation magnetic sector-type mass spectrometer (TIMS: VG Sector 54-30) at Nagoya University. The measured Sr isotopic compositions were normalised by 86 Sr/88 Sr = 0.1194. The 87 Sr/86 Sr ratio of NIST-SRM 987 during this study was 0.710254 ± 0.000015 (2␴, n = 12).The strontium and Mn concentrations of the Ishimaki limestone samples were determined using an atomic absorption spectrometry (AAS: Hitachi A2000) at Nagoya University. The 0.1 g limestone samples were dissolved in 6 M HCl, and the concentrations were measured. The rubidium concentrations were determined using the isotope dilution method using a Finnigan MAT thermal ionisation quadrupole mass spectrometer (THQ) at Nagoya University.

Fig. 5. (a) The Sr isotopic compositions (grey circles) of 45 Ishimaki limestone samples on the 87 Sr/86 Sr curve throughout the Phanerozoic seawater. The 87 Sr/86 Sr variation curve by Burke et al. (1982) was slightly modified. (b) Plot of the 87 Sr/86 Sr ratios of seawater vs. age from the Late Permian to Early Cretaceous. The variation curve by Koepnick et al. (1990) was slightly modified. Each dot is an analysed 87 Sr/86 Sr ratio. The upper and lower lines define a band that encloses 90% of the Triassic and Jurassic data.

mountain trail of Mt. Ishimaki was 0.7068 (Tanaka et al., 2003), which is higher than that of our work. Some limestone samples had more than 20 weight percentage of residue after 10% acetic acid leaching. However, after 6 M HCl leaching, all limestone samples had less than 10 weight percentage of residue, and only 4 among the 15 samples had more than 2 weight percentage of residue (Table 1). Fifteen samples (Nos. 1–15 in Table 1) had almost the same 87 Sr/86 Sr ratios with 10% acetic acid leaching or 6 M HCl leaching (Fig. 6), which shows that the Sr isotopic compositions in other chemical materials (e.g., phosphates, sulfides and hydroxides) that are leached with 6 M HCl instead of 10% acetic acid are identical with those of carbonates that are leached with 10% acetic acid; thus, the various acid-soluble

5. Results The latitude, longitude and elevation of the sampling points for the 45 Ishimaki limestone samples are listed in Table 1. The Sr isotopic compositions (87 Sr/86 Sr) leached with 10% acetic acid and 6 M HCl, and the weight percentages of the residues are listed in Table 1. The rubidium concentration at IS301 was 0.088 ppm. In this case, the Rb/Sr ratio is approximately 0.005, and the influence on 87 Sr/86 Sr by the radioactive decay of 87 Rb, which has a half life of 48.8 b.y., is smaller than 0.0001. Therefore, age correction for the measured Sr isotopic compositions was not conducted. The Sr isotopic compositions of the 45 samples had a range of 0.7055–0.7077, and almost all values were lower than the seawater 87 Sr/86 Sr ratios in the Norian, the Late Triassic reported by Burke et al. (1982). Although the lowest Sr isotopic compositions throughout the Phanerozoic were approximately 0.7068 at the end of the Permian and Jurassic (Burke et al., 1982) (Fig. 5(a)), 18 Ishimaki limestone samples had lower Sr isotopic compositions. The lowest Sr isotopic composition of the limestone along the southern

Fig. 6. Comparison of the Sr isotopic compositions of 15 Ishimaki limestone samples by 10% acetic acid leaching and 6 M HCl leaching. The 15 samples were collected in proportion to elevation (see Table 1, Nos. 1–15).

K. Suzuki et al. / Chemie der Erde 72 (2012) 77–84

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Table 2 Sr and Mn concentrations of the 15 Ishimaki limestone samples are shown.

Fig. 7. (a) Distribution map of the Sr isotopic compositions of 45 Ishimaki limestone samples leached with 10% acetic acid at a position of the north latitude and elevation as an xy coordinate and (b) distribution map of the Sr isotopic compositions of the same data at a position of the east longitude and elevation as an xy coordinate.

components in the Ishimaki limestone have almost the same Sr isotopic compositions. The distribution maps of the Sr isotopic compositions of the 45 Ishimaki limestone samples leached with 10% acetic acid were drawn at a position with the north latitude and elevation indicated as an xy coordinate in Fig. 7(a) and at a position with the east longitude and elevation indicated as an xy coordinate in Fig. 7(b). The blue areas in Fig. 7 correspond to 87 Sr/86 Sr < 0.7066, and the yellow and red areas correspond to 87 Sr/86 Sr > 0.7066. The green area corresponds to 87 Sr/86 Sr = 0.7066, which is the mean value between the lowest 87 Sr/86 Sr = 0.7055 and highest 87 Sr/86 Sr = 0.7077 of the Ishimaki limestone samples. Furthermore, the value is almost identical to the lowest Sr isotopic composition throughout the Phanerozoic reported by Burke et al. (1982). The Sr isotopic compositions of the Ishimaki limestone are generally lower at the base and higher at the top of the block. 6. Discussion 6.1. Preservation of the Sr isotopic compositions of the Ishimaki limestone Do the Sr isotopic compositions of Ishimaki limestone samples retain the original seawater 87 Sr/86 Sr ratios at the time of deposition? The Sr isotopic compositions of limestone samples are generally identical to the seawater 87 Sr/86 Sr ratios when the limestone was deposited. However, the original seawater 87 Sr/86 Sr ratios in the limestone can be increased or decreased by alteration. Before further discussing the fluctuation of the Sr isotopic compositions of past seawater using the Ishimaki limestone, we will

No.

Sampling point

Sr (ppm)

Mn (ppm)

Sr/Mn

Mn/Sr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

IS50 IS100 IS150 IS200 IS250 IS301 IS400 IS501 IS504 IS601 IS602 IS700 IS800 IS900 IS950

93.7 312 155 169 89.4 91.4 129 257 138 135 73.1 116 104 83.4 80.5

23.2 169 19.0 45.1 19.4 62.6 31.4 47.3 508 296 41.0 188 104 312 453

4.0 1.8 8.2 3.7 4.6 1.5 4.1 5.4 0.27 0.46 1.8 0.62 1.0 0.27 0.18

0.25 0.54 0.12 0.27 0.22 0.68 0.24 0.18 3.7 2.2 0.56 1.6 1.0 3.7 5.6

verify that the 87 Sr/86 Sr ratios of the limestone have not changed since deposition. Changes in the original limestone 87 Sr/86 Sr ratios can only occur by importing and fixing foreign Sr with a different isotopic composition. These changes are likely to be caused by alteration during diagenesis, dolomitisation, regional metamorphism and selective dissolution of carbonate minerals (Faure, 1986). In many of the alteration scenarios described above, the Mn contents are used as a proxy for intensifying the alteration. Denison et al. (1994) investigated the relation between the trace element content in limestone and the alteration after deposition. They found that limestone samples with a Sr/Mn > 2 or Mn < 300 ppm retained the original isotopic ratio. The criteria Sr/Mn > 2 or Mn < 300 ppm are based on the assumption that the alteration after deposition is in proportion to the Mn content. Meanwhile, the criterion of the Mn/Sr ratio is currently available for detecting alterations of the isotopic ratio. Various researchers have accepted Mn/Sr < 1–3 as an indicator of unaltered isotopic ratios (Jacobsen and Kaufman, 1999; Yoshioka et al., 2003). The Sr and Mn concentrations and the Sr/Mn and Mn/Sr ratios of 15 samples (Nos. 1–15 in Table 1) of the Ishimaki limestone are listed in Table 2. The Sr isotopic composition vs. the sampling elevation in the limestone block is plotted in Fig. 8(a), the Sr/Mn ratio vs. the sampling elevation is plotted in Fig. 8(b), and the Mn concentration vs. sampling elevation is plotted in Fig. 8(c). All samples from the lower positions of the Ishimaki limestone block with generally lower Sr isotopic compositions satisfy Denison’s criteria (Sr/Mn > 2 or Mn < 300 ppm) (Nos. 1–8, IS50–IS501 in Tables 1 and 2; Fig. 8(a)–(c)). Four of the seven samples at the upper positions with generally higher Sr isotopic compositions also satisfy the criteria (Sr/Mn > 2 or Mn < 300 ppm) (IS601, IS602, IS700 and IS800 in Tables 1 and 2; Fig. 8(a)–(c)). Furthermore, the Mn/Sr ratio vs. the sampling elevation is also plotted in Fig. 8(d). Overall, 12 of the 15 samples have a Mn/Sr < 3, and the remaining three having comparatively higher Sr isotopic compositions (IS504: 0.7070, IS900: 0.7077, IS950: 0.7076). These 12 samples also satisfy Denison’s criteria (Sr/Mn > 2 or Mn < 300 ppm). These results show that the Ishimaki limestone samples at lower positions of the limestone block, including sample IS100, which has the lowest 87 Sr/86 Sr ratio of 0.7055, completely retain the original seawater 87 Sr/86 Sr ratios. Provided that the entire Ishimaki limestone block followed the same geological history after deposition, the limestone is likely to have almost retained the original Sr isotopic compositions at the time of deposition. The major elemental component of the Ishimaki limestone from sampling point IS301 (87 Sr/86 Sr = 0.7067; Table 1) was measured by XRF at Nagoya University. The result was Fe2 O3 = 0.02% (weight percentage) and MgO = 0.65% (weight percentage). Furthermore, almost all the Ishimaki limestone samples have a low amount of

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Fig. 8. (a) Plot of the Sr isotopic composition vs. sampling elevation for 15 Ishimaki limestone samples. 87 Sr/86 Sr = 0.7068 is the lowest ratio in Phanerozoic seawater according to Burke et al. (1982). (b) Plot of the Sr/Mn ratio vs. sampling elevation. Denison et al. (1994) suggested that the limestone sample with Sr/Mn > 2 preserved the Sr isotopic ratio of the ambient seawater at the time of deposition. (c) Plot of Mn concentration vs. sampling elevation. Denison et al. (1994) suggested that the limestone sample with a Mn concentration <300 ppm preserved the Sr isotopic ratio of the ambient seawater at the time of deposition. (d) Plot of Mn/Sr ratio vs. sampling elevation. Yoshioka et al. (2003) suggested that the limestone sample with Mn/Sr < 3 preserved the Sr isotopic ratio of the ambient seawater at the time of deposition.

acid-insoluble residues (Table 1). This data shows that dolomitisation with alteration never occurred in the Ishimaki limestone block. Palmer and Edmond (1989) showed that the Sr content and Sr isotopic composition of the hydrothermal fluid from the seafloor were 50–350 ␮M and 0.702–0.706, respectively. Hydrothermal fluid circulation is a mechanism for Sr inflow from the oceanic crust to seawater. Generally, hydrothermal fluids diffuse into the sea. This process partly regulates the Sr isotopic composition of seawater. If the present Sr isotopic compositions of the Ishimaki limestone became lower after deposition, then the local hydrothermal alteration would likely be one reason why the isotopic compositions are low. Here, the Mn in hydrothermal fluid generally has a 104 –105 times higher concentration than in seawater (Kawahata, 2008). As the experimental results indicate that the Mn content in the Ishimaki limestone is low (Table 2), they do not actively support the lowering of the Sr isotopic composition of the Ishimaki limestone. The CAI is applied to estimate the maximum temperature reached by limestone. The CAI value increases in accordance with the strength of thermal metamorphism. The colour of the conodonts changes from pale brown (CAI: 1.0–1.5) to brown and dark grey (CAI: 2.0–3.0) to black (CAI: 3.0–5.0) to grey (CAI: 6.0) and, finally, to opaque white (CAI: 7.0) (Epstein et al., 1977; Helsen et al., 1995). The Sr isotopic compositions of the conodont samples with CAI values <2.5 retain the original seawater 87 Sr/86 Sr ratios (Bertram et al., 1992; Cummins and Elderfield, 1994). The 87 Sr/86 Sr ratio of the conodont changes in proportion to the strength of the thermal metamorphism. Because the CAI values of the conodont samples at IS50, IS250, IS400 and IS950 in the Ishimaki limestone block are 1–2 (1a and 2a of Fig. 4), the paleotemperature can be estimated at 50–140 ◦ C, and the limestones are little metamorphosed after deposition. The Sr isotopic compositions of IS50, IS250, IS400 and IS950 are 0.7070, 0.7065, 0.7062 and 0.7076, respectively

(Table 1); thus, the total Ishimaki limestone block is not uniformly altered by thermal metamorphism. The limestone samples with conodont fossils are thought to have fluctuating, low Sr isotopic compositions with values identical to past seawater at the time of deposition. Geological evidence, the Sr/Mn ratio, the Mn concentration and the CAI value support the preservation of the Sr isotopic composition of past seawater at the deposition of the Ishimaki limestone. 6.2. Implication for the lowest Sr isotopic composition of the seamount-type limestone The Sr isotopic compositions of the 45 Ishimaki limestone samples have a range of 0.7055–0.7077. Why does the Ishimaki limestone have such low Sr isotopic compositions? Conodont elements from the bottom (IS50) and the small fragmented pieces of conodonts from the top (IS950) of the Ishimaki limestone block indicate the rocks are Early to Middle Norian in age. Therefore, the total Ishimaki limestone block shows the past seawater 87 Sr/86 Sr during the Early to Middle Norian in the Late Triassic. The typical 87 Sr/86 Sr variation curve of seawater throughout the Phanerozoic (Burke et al., 1982) is shown in Fig. 5(a). Each dot in Fig. 5(a) shows the measured 87 Sr/86 Sr ratio. When several ratios are plotted at a certain age, the curve seems to be drawn to connect the lower measured ratios. The Sr isotopic compositions of many limestone blocks throughout the world generally become higher because of post-depositional alteration. The many grey circles at approximately 210 Ma (Fig. 5(a)) show the distribution of the Sr isotopic compositions of the 45 Ishimaki limestone samples and that they are clearly separated from the seawater 87 Sr/86 Sr variation curve. Koepnick et al. (1990) re-evaluated limestone samples from the Triassic to Jurassic periods to determine the accuracy of the

K. Suzuki et al. / Chemie der Erde 72 (2012) 77–84 87 Sr/86 Sr

variation from Burke et al. (1982). They then presented a corrected variation curve (Fig. 5(b)), showing that the Sr isotopic compositions of all seawater samples everywhere in the Norian were limited to a range of 0.7075–0.7078. Additionally, the distribution of the Sr isotopic compositions in the Norian by McArthur and Howarth (2004) was limited to almost the same range. Only 4 (ISB11: 0.7075, IS200: 0.7076, IS950: 0.7076 and IS900: 0.7077) (Table 1) of the 45 samples of the Ishimaki limestone fall within this range. The other 41 samples have lower Sr isotopic compositions compared with the Sr isotopic compositions in the Norian range. As discussed in Section 6.1, it is thought that the total Ishimaki limestone block retains the original seawater 87 Sr/86 Sr ratios. The seawater 87 Sr/86 Sr ratios in the Panthalassic Ocean are quite different from those of the Ishimaki limestone. Where was the Ishimaki limestone deposited? Seamounts are generally made of mafic volcanic rock, such as oceanic basalt. Sr isotopic compositions of mafic volcanic rocks underlying seamount-type limestones are generally 0.704 ± 0.002 (Faure, 1986). These values coincide with the Sr isotopic compositions (0.703–0.707) of the Mikabu greenstone in Japan (Tanaka et al., 1979). It is suggested that the Ishimaki limestone was deposited on the top of the seamount and that it was accreted to the Japanese Islands during the Jurassic by plate movement (Tanaka et al., 2003). However, the distribution of lower 87 Sr/86 Sr ratios in the Ishimaki limestone block (Fig. 7) suggests that it was deposited in a closed or semi-closed sea area influenced by the lower Sr isotopic seawater originating from the oceanic crust through the circulation of hydrothermal fluid or river water. Therefore, the Sr isotopic compositions of Ishimaki limestone became lower than that of the Panthalassic Ocean in the Norian. A semi-closed sea currently exists in Europe. Andersson et al. (1992) reported Sr isotopic compositions of the modern seawater of the Baltic Sea and the Gulf of Bothnia. The Baltic Sea is a northern European intracontinental sea that is not completely closed from the Atlantic Ocean. River water with higher Sr isotopic compositions (e.g., the Kemi River into the Gulf of Bothnia at 0.7330 and the Kalix River into the Gulf of Bothnia at 0.7316) sourced from the Precambrian rock of the Baltic shield flow into the Gulf of Bothnia and the Baltic Sea. Water exchange with the North Sea only occurs in narrow and shallow transition zones and with the Atlantic Ocean. As a result, the Sr isotopic compositions of the modern seawater of the Gulf of Bothnia range from 0.70933 to 0.70972, and they are significantly higher than the 0.70915 of modern seawater (Andersson et al., 1992). Conversely, the Sr isotopic compositions of river water flowing into a basin of mafic rocks, such as oceanic basalt and greenstone metamorphosed from oceanic basalt are clearly lower than that of modern seawater. For example, Goldstein and Jacobsen (1987) reported that the Sr isotopic compositions of river waters in Japan and the Philippines (e.g., the Kitakami River in Japan at 0.7063, the Mogami River in Japan at 0.7071, the Agno River in the Philippines at 0.7045 and the Cagayan River in the Philippines at 0.7062) are significantly lower than that of modern seawater. Given that only river water with lower Sr isotopic composition could flow into the Gulf of Bothnia, the Sr isotopic composition of the seawater of the Gulf might be lower than the modern seawater. The Sr isotopic compositions of the atoll limestone at Kita-daitojima on the Philippine Sea plate were identical to the original seawater 87 Sr/86 Sr ratios at a range of 0.7082–0.7087 from 24 Ma to 15 Ma (Ohde and Elderfield, 1992). Thus, the closed or semi-closed sea area depositing the Ishimaki limestone is suggested to have been more global than general atolls in the oceans. According to the paleogeographic map by Scotese (2001), Pangea began to separate in the Late Triassic. A plateau called Kurosegawa-Ofunato Island is likely to have been in the Panthalas-

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sic Ocean during the Late Paleozoic to Mesozoic (Kimura et al., 1975; Kimura, 2002). There was possibly a seamount with the Ishimaki limestone in the sea near this island. Sugitani and Mimura (1998) have discussed the geochemical characteristics of the Triassic radiolarian bedded cherts in the Mino Belt, central Japan, and have summarised the paleoceanography as the deposition site isolated from the open sea. They thought that the change of sedimentary environment and diagenetic process was likely due to the fluctuating concentration of dissolved oxygen in the water of a semi-closed marginal ocean basin. Kunimaru et al. (1998) and Shimizu et al. (2001) also discussed the geochemical features of cherts in Japan and stated that the Triassic bedded chert in the Mino Belt and in the Chichibu Belt, central Japan, was deposited near the continent. Both the Mino Belt and the Chichibu Belt, which are 100 km apart, are Jurassic accretionary complexes. The Ishimaki limestone belongs to the Chichibu Belt. These studies support the idea that the Ishimaki limestone was deposited in a closed or shallow sea during the Late Triassic. The lower part of the Ishimaki limestone block has a lower Sr isotopic composition, while the upper part has a higher Sr isotopic composition, as shown in Fig. 7. The highest positions of the sampling points have the highest Sr isotopic compositions (IS900: 0.7077, IS950: 0.7076) (Table 1). Provided that these ratios were not modified by later alteration, they are in complete agreement with those of seawater in the Panthalassic Ocean during the Norian (Koepnick et al., 1990). After the Ishimaki limestone was deposited in a closed or semi-closed sea separated from the Panthalassic Ocean, the oceanic environment surrounding the seamount with the Ishimaki limestone might have become an open sea. The highest Sr isotopic composition at the sampling point at the top of the Ishimaki limestone block became identical with the Panthalassic seawater ratio during the Norian. This scenario can explain the relationship between the Sr isotopic variation of the Ishimaki limestone block and the paleoceanic topographical features. 7. Conclusions Our results are summarised as follows: (1) The conodont stratigraphy indicates that the Ishimaki limestone block is Early to Middle Norian in age. Therefore, the total Ishimaki limestone block was deposited during the Norian in the Late Triassic. (2) The low CAI values of the conodonts, the low Mn contents, the high Sr/Mn ratios, a low amount of acid insoluble residue and minor differences in the Sr isotopic compositions between the acetic acid-soluble materials and the HCl-soluble materials in the limestone suggest that no alteration of the Ishimaki limestone occurred after deposition. (3) The Sr isotopic compositions of the Ishimaki limestone range from 0.7055 to 0.7077. The Sr isotopic compositions of 18 of the 45 samples of Ishimaki limestone are lower than the generally recognised lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. (4) The Ishimaki limestone was deposited in a closed or semiclosed sea separated from the Panthalassic Ocean. The Sr isotopic compositions of seawater in the closed or semi-closed sea were mainly controlled by the Sr inflows from the mafic oceanic crust through hydrothermal fluid or from the hinterlands surrounded by mafic rock through river water. (5) The paleoceanic topography can be reconstructed by the Sr isotopic composition of seamount-type limestone. The Panthalassic Ocean that formed the Japanese Island seems to be more intricate than previously thought.

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